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Ground Water Recharge Using Waters of Impaired Quality (1994)

Chapter: 3 Soil and Aquifer Processes

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Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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3
Soil and Aquifer Processes

The desired role of the unsaturated soil zone (vadose zone) in a recharge system is a straightforward one: remove or reduce chemical and biological constituents that pose a potential health risk before the recharge water enters the ground water. Unfortunately, the processes by which removal occurs are not completely efficient in a natural setting, and not all constituents are retained or degraded to the same extent. Moreover, management strategies that may enhance the removal of one chemical or pathogen may actually decrease the efficiency of removal of another.

This chapter describes the major processes by which soils and aquifers can remove chemicals and pathogens. The processes that occur in soils and aquifers are chemical-, pathogen-, and soil-specific, depending on a number of conditions that can vary significantly from site to site and from one compound to another. Thus this chapter reviews the principal processes governing transport and fate in sod first in a generic fashion, and later with respect to the behavior of specific chemical or pathogen groups. The soil properties that are important in a properly functioning soil-aquifer treatment (SAT) system are reviewed, and properties or processes that can create difficulties for chemical and pathogen removal processes are identified. Three separate issues of concern are addressed: (1) the overall effectiveness of the SAT system and its ability to ensure that the quality of the underlying water resources will not be impaired, (2) the long-term sustainability of the system, and (3) the feasibility of monitoring to determine both the performance and the safety of the operation.

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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CONDITIONS INFLUENCING PRETREATMENT

The soil and aquifer properties, recharge method, type of wastewater, and ultimate destination and intended use of the recovered ground water collectively dictate the degree of pretreatment required before recharge. In addition, recharge rates, regardless of the method used, depend to some degree on the quality of the source water that is recharged. Cost-effective recharge operations are achieved through tradeoffs between maximizing recharge rates and minimizing treatment costs.

The selection of a wastewater treatment process depends on the characteristics of the wastewater, the required effluent quality, and the cost of the selected treatment option. When considering a water source as a potential for recharge, the likely variety and concentration of contaminants also need to be considered. The use of waters of impaired quality as sources for recharge has raised questions about the level of pretreatment necessary prior to recharge. The most conservative approach is to assume that passage through the soil to the aquifer and through the aquifer to the withdrawal location provides no treatment and that pretreatment processes therefore should improve the source water to the quality level needed by the end user. This approach, however, can lead to expensive systems. Soil-aquifer processes can be counted on to provide treatment benefits.

Soil Properties

The ideal porous medium for an SAT operation is one that allows rapid infiltration and complete removal of all constituents of concern. Unfortunately, no such medium exists because the attributes required to achieve one goal hamper the achievement of the other. In surface soil, coarse-textured materials are desirable for infiltration because they transmit water readily; however, the large pores in these soils are inefficient at filtering out contaminants, and the solid surfaces adjacent to the main flow paths are relatively nonreactive. In contrast, fine-textured soils are efficient at contaminant adsorption and filtration, but they have low permeability and their small pores clog easily. Structured soils con-mining biological channels (e.g. worm holes or root holes) or cracks are permeable, but the large flow paths completely dominate the movement of material and much of the matrix is bypassed. The best choice for an SAT soil is therefore a compromise, such as a fine sand or a sandy loam with relatively little structure (Bouwer, 1985).

Nature of Recharge Operation

Of the two basic methods of artificial recharge—surface infiltration and well recharge—well recharge requires water of much higher quality. This is particularly true where an aquifer composed of granular rocks is to be recharged.

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

The flux per unit area of rock surface at the point of recharge is generally much greater for injection wells than for surface infiltration systems. Consequently, equivalent amounts of clogging material and nutrients for biological growth result in more severe operational problems in recharge wells. Surface infiltration systems can function effectively over a broad range of water quality and can readily tolerate variations in quality, although, in general, higher quality results in higher infiltration rates. Injection well operations, on the other hand, are much more sensitive to quality variations, except in the case of conduit-flow rocks such as solution-riddled limestone or fractured rock.

Recharge wells require water that is virtually free of suspended matter, especially where granular aquifers are to be used. Any injected suspended matter accumulates at and near the well-aquifer interface, and because this circumferential area is limited and the flux through it is large, rapid hydraulic head loss and reduction in injection capacity occur quickly. The clogging caused by the accumulation of the suspended material and biological growth must be remedied by pumping the well for backflushing, by surging or jetting, or at times by dosing the well with chemicals to loosen and/or dissolve the accumulated clogging materials. Removal of suspended solids (to very low levels, i.e., less than 1 milligram per liter) in the recharge water is required for successful operation of recharge wells, except where karstic or fractured rock aquifers are to be recharged. Wells recharging solution-riddled or fractured rock aquifers can tolerate water having higher levels of suspended solids without experiencing severe operational problems.

Wastewater Composition

Quality parameters of concern in the operation of surface infiltration systems are the suspended solids (SS) and total dissolved solids (TDS) content as well as the concentrations of nutrients that stimulate biological growth and of major cations such as calcium, magnesium, and sodium, which determine the sodium adsorption ratio (SAR).

The suspended solids concentration of the recharge water is the most important factor. Suspended solids settle out or are filtered from the water and accumulate on the soil and/or at a short distance below the water-soil interface. The accumulation reduces the permeability of the soil and retards movement of water into the subsurface.

Swelling and deflocculation of clay minerals contained in the aquifer can occur if the recharge water contains a higher ratio of monovalent to divalent cations (higher SAR) than does the native water. Clay dispersal can also occur if freshwater is recharged into a saline aquifer. The combination of low TDS with high SAR causes clays to disperse. If clays disperse, infiltration rates drop. Chemical reactions between the recharge water and the native ground water can cause precipitates to form and these can clog pores and reduce injection capac-

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

ity. Compounds of carbonates, phosphates, or iron oxides are the most likely to cause such problems. To avoid problems caused by clay dispersal or chemical precipitates, an evaluation of the chemical compatibility of the recharge water with the aquifer materials and the native ground water should be conducted. The evaluation may indicate the need to chemically modify the recharge water to avoid clogging problems caused by chemical incompatibility.

Water recharged by wells must be free of entrained or dissolved gases that may evolve when cold recharge water is injected into warmer ground water. Entrained air or gases that come out of solution will reduce the aquifer's hydraulic conductivity and, consequently, the injection capacity of the well.

Bacterial growth is yet another concern with regard to clogging of recharge wells. Removal of nutrients and biodegradable matter from the recharge well and disinfection of the water minimize the potential for bacterial growth in the immediate vicinity of the recharge well. When the recharge water contains a chlorine residual, well clogging is slower (Vecchioli et al., 1980). An alternative to continuous chlorination of the recharge water is to backpump the well frequently (about once a day) or to dose the well heavily with chlorine periodically to destroy the bacterial growth and then backpump the well to remove the spent chlorine solution and organic residue.

Recharge wells ending in the vadose zone (dry wells) cannot be redeveloped readily because it is not possible to pump water from them to remove accumulated suspended matter or other clogging materials. Therefore, water recharged to dry wells must be free of suspended solids and not cause clay dispersal or bacterial growth. On the other hand, because of the generally shallow depths of dry wells, replacing clogged wells is considerably less costly than for wells injecting water below the water table.

GENERAL DESCRIPTION OF SUBSURFACE PROCESSES

Vadose Zone Processes

The ultimate goal of a ground water recharge project is to resupply the subsurface with water that does not impair the quality of the underlying resource. Thus, the role of the unsaturated or vadose zone in recharge systems is to help filter out or transform harmful constituents in the soil solution as recharge water moves through the soil matrix en route to the aquifer.

The vadose zone is a much more complex transport medium than an aquifer, for several reasons. Because only part of the void space is failed with water, chemicals with a significant vapor pressure can move in the gas phase as well as in solution. The water flow rate can vary significantly. The resistance offered by the vadose zone to the flow of water through a given local soil volume is a nonlinear function of the water content, whereas in the saturated zone, it is a constant. The temperature varies in the surface regime in response to the cyclic

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

inputs of radiant energy, and the composition of the air, solid, and solution phases of the soil is also dynamic, causing spatial and temporal variations in the chemical and biological reactions that transform chemicals in the vadose zone. Also, the amount of water retained against gravity varies significantly with soil texture. Coarse-textured, sandy soils may hold as little as 10 to 20 percent of water-saturation after drainage becomes insignificant, while fine-textured silts or clays may hold as much as 90 percent. Restricting layers comprised of clay lenses or cementing agents can retard drainage greatly, even in otherwise permeable media. Soils that retain extensive water are prone to aeration problems.

There are many different processes that can remove chemicals or pathogens from the recharge water as it flows through the vadose zone. Some chemicals volatilize and escape to the atmosphere. They can be chemically or biologically transformed to a new form that may or may not be toxic. They can attach to stationary soil mineral or organic surfaces or precipitate out of solution. They can form complexes with dissolved constituents or particulate matter in solution, thereby reducing their attraction to the soil solid phase and enhancing their mobility in solution. Large pathogens such as parasites, some bacteria, and colloidal material containing contaminants can be filtered out of solution by narrow soil pores, a process that slowly clogs the medium and eventually reduces its permeability if the contaminants are not biodegraded. Viruses can be retained by soil solid phases and inactivated by reactions occurring in the soil.

Aquifer Processes

Chemicals or pathogens that are still present in solution when the recharge water reaches the aquifer are subject to many of the same processes that occur in the vadose zone, with several exceptions. The biological activity in ground water is much slower than in the near-surface zone, so degradation is greatly reduced. Water fills all of the pore spaces within the ground water zone, so the only place where volatilization can occur is in the capillary fringe above the water table interface. Aquifers used for ground water recharge projects are generally much more coarse textured than soils, so colloids and large pathogens, should they still be present in ground water, are not as easily filtered out of solution as they axe in surface soil.

Volatilization

Volatilization refers to the evaporation of chemical vapor from soil or water bodies and its subsequent loss to the atmosphere. Many organic compounds are volatile in water, as are some nitrogen compounds (e.g., ammonia, nitrous oxide) generated from biological transformations. In addition, some inorganic chemicals (e.g., selenium compounds) may be rendered volatile through biological reactions.

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

The chemical characteristic that is most indicative of a compound's volatility is its Henry's constant, which is the ratio between the vapor pressure and the solubility of the pure chemical in water. It may also be expressed in dimensionless form as the ratio between the chemical concentration in the gaseous and aqueous phases,

where Cg is the mass of chemical vapor per unit volume of air and Cw is the mass of chemical dissolved per unit volume of aqueous solution.

For a given chemical, the extent of volatilization loss is very dependent on the soil and atmospheric conditions. The primary factor determining loss is the air phase concentration maintained at the interface with the atmosphere. In general, volatilization is greatly reduced in soil compared to water because the soil solid phase retains the chemical mass, thereby reducing its vapor pressure. In addition, the soil can offer substantial resistance to the transport of chemical from the soil profile to the surface, particularly if the soil is wet and little upward flow of water is occurring. Therefore, in a typical SAT process where recharge water is ponded over the surface for prolonged periods of time, the primary route for volatilization loss will be from the surface of the standing water. During the drainage cycle when the soft becomes unsaturated, volatile constituents in solution near the surface can also evaporate and escape to the atmosphere.

Volatilization from Surface Water

Volatilization from standing water can be represented conceptually as a two-film resistance model, in which the dissolved compound moves from the bulk fluid through a liquid film to the evaporating surface, and then diffuses through a stagnant air film to the well mixed atmosphere above (Liss and Slater, 1974). The two-film model assumes that the chemical is well mixed in the bulk solution below the liquid film and that mass transfer across each film is proportional to the concentration difference. With these assumptions, a chemical in the water body volatilizes at a rate proportional to the bulk concentration, so that the entire loss process can be characterized by an effective ''half-life," defining the mount of time required to reduce the mass in solution by 50 percent. Thomas (1982) reviewed volatilization loss models for chemicals present in water bodies and performed model calculations for a number of compounds. The film thicknesses depend on specific conditions within the water and air, but may be crudely estimated from default values given in Thomas (1982) when no actual data are available. Table 3.1 summarizes effective volatilization half lives calculated for a range of Henry's constant values.

As seen from Table 3.1, the effective half-life varies widely, depending on

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

TABLE 3.1 Effective Volatilization Half-Life Ranges as a Function of Dimensionless Henry's Constant Values for a Stagnant Water Body of 1-m Depth

Henry's Constant KH

Example

Half life (days)

10-8 - 10-7

Bromacil

104 - 105

10-7 - 10-6

Amine

103 - 104

10-6 - 10-5

Phenol

102 - 103

10-5 - 10-4

Diazinon

10-100

10-4 - 10-3

EPTC

1-10

10-3 - 10-2

Bromobenzene

˜ 1

10-2 - 10-1

Benzene

˜ 1

10-1 - 1

Methyl bromide

˜ 1

1-∞

Vinyl chloride

˜ 1

 

Source: Default values for film transfer coefficients are taken from Thomas (1982), and KH values are taken from Jury et al. (1984b).

the value of the Henry's constant. Clearly, compounds with half lives that are considerably less than the detention time of the water on the surface will not enter the soil. For instance, volatilization losses of 22 to 73 percent were found for a wide spectrum of hydrocarbons in sewage effluent infiltration basins in Phoenix, Arizona (Bouwer et al., 1986).

Volatilization from Soil

The volatilization loss rates of chemicals from soil are generally smaller than those from standing water for several reasons. First, the diffusion resistance of soil is greater than that of flee air because of the solid and liquid barriers to gas movement. Second, adsorption of chemical to soil solids reduces the vapor pressure of the compound by removing mass from solution. Because the transport pathways from the soil to the surface are much more complex than in free water, the two-film resistance model of volatilization is not applicable to soil, and more sophisticated estimation methods must be used.

Jury et al. (1983, 1984a,b,c, 1990) developed a comprehensive screening

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

model for evaluating volatilization losses of chemicals after their incorporation into a soil layer of arbitrary thickness. They performed calculations covering a range of initial conditions on a large group of organic compounds, allowing the volatilization losses of different chemicals to be compared and grouped. In general, they found that compounds with dimensionless Henry's constant values less than 10-4 were not prone to significant volatilization after deposition in the soil, but that extremely volatile compounds could move upward to the surface from substantial distances if they were not rapidly degraded. Under certain conditions, water evaporation can greatly enhance volatilization from soil by concentrating the chemical mass at the surface and raising its vapor pressure.

There are several good references (see Howard, 1990; Howard et at., 1991) containing compendia of chemical properties and soil-chemical interaction coefficients (such as sorption coefficients and degradation rate coefficients). These are more reliable Sources of information on chemical properties than earlier references, which frequently contain outdated or inaccurate data.

Volatilization During Ground Water Recharge

Chemicals likely to volatilize during ground water recharge operations include nitrogen compounds and organics with high Henry's constant values. Operations conducted under high water saturation, such as ponding, will minimize the loss of organics from soil by volatilization because these will be blocked from entering the gas phase and reaching the soil surface if the soil air space is filled with water. However, the presence of standing water on the surface enhances the loss from water, so that any chemicals with volatilization half-lives that are significantly less than the total detention time in surface water Will probably evaporate out of solution before they ever reach the soil.

Recharge water containing fertilizer or sewage contributions may contain nitrogen compounds that can be transformed to volatile species. Ammonia is very volatile and vaporizes from anhydrous form immediately upon exposure to the air. The ammonium ion NH4+, which is a constituent of many fertilizers, partitions into a volatile gas when exposed to air; therefore, any dissolved NH4+ near the atmospheric interface will lose nitrogen to the atmosphere. NH4+ is readily transformed by soil bacteria to nitrate (NO3-), which is very mobile in soil and is nonvolatile. However, NO3- may be transformed anaerobically by biological denitrification to several gaseous species (primarily N2O and N2) when soil water content is high and a source of organic carbon is present. Ground water recharge by ponding may therefore enhance removal of residual nitrogen through this process.

There is evidence that potentially harmful inorganic compounds of selenium and other trace metals may be removed from contaminated soil by volatilization after the compounds have been methylated through biological transformation (Karlson and Frankenberger, 1989).

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

Transport of Dissolved Chemicals

Unless a compound is very volatile, it moves primarily in the aqueous solution phase. At the scale of the soil pore, there are two transport mechanisms that can move solutes through the medium: convection and diffusion. Convection is the transport of a dissolved chemical by virtue of bulk movement of the host water phase, while diffusion is the random mixing caused by collisions at the molecular scale. The local water flux describes three-dimensional flow around the solid and gaseous portions of the medium and is not measurable. Instead, the local quantifies are volume averaged to produce a larger-scale representation of the system properties. The averaging volume must be large enough that the statistical distribution of geometric obstacles is the same from place to place. If the porous medium contains the same material and density throughout, then the mean value produced by this averaging is macroscopically homogeneous over the new transport volume containing the averaged elements. The large-scale convective solute flux is then the product of the average water flux and the average solution concentration Cw.

However, in the process of averaging, some of the solute motion is lost. The small-scale migration of solution through tortuous pathways within the porous medium no longer appears in the water flux expression after volume averaging, because the fluctuations about the mean motion do not contribute to the net water transport. Dissolved chemicals convected along these tortuous flow paths do contribute to solute transport and cannot be neglected. The motion of solute due to small-scale convective fluctuations about the mean motion is called mechanical dispersion (Bear, 1972). The combined mixing associated with diffusion and mechanical dispersion is called hydrodynamic dispersion. Development of models to describe the hydrodynamic dispersion flux is one of the most active research areas in soil physics and hydrology today (Dagan, 1986). The most widely used representation of dissolved chemical movement is the convection-dispersion model, which assumes that the dispersive mixing is random or diffusionlike within the moving fluid.

Solute Velocity

The water flux (or Darcy velocity) q is the volume of water flowing per unit time per unit area of soil. Because the water flows only through the water-filled regions of the porous medium, the actual average velocity of the water is equal to

where θ is the volumetric water content (volume of water per volume of soil).

Equation (2) describes the average linear velocity of the water and therefore also represents the movement of a chemical that travels freely with water and

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

does not interact with solid surfaces in the soil. At normal pH levels, negatively charged ionic species such as NO3-are not attracted to soil mineral or organic surfaces and move relatively freely with flowing solution. In fact, the negatively charged clay lattice actually has a repulsive effect on the unions in solution, causing them to avoid the solution region that is closest to the surfaces, in which case (2) will somewhat underestimate the solute velocity. This effect is minor unless the soil is high in clay.

Therefore, a chemical moving in solution at a water flux q that flows through a water-filled volume fraction θ without interacting with the solid phase will move with a velocity given by (2) and hence will require a time

to move a distance L through the soil or aquifer. Because of diffusion and dispersion, not all of the solute molecules will move at the same speed, but will spread out around the average arrival time given in (3).

Solute Dispersion During Transport

The effect of dispersion is to produce chemical spreading during transport. It is greatly affected by soil structure and depends also on the scale over which the water flux is volume averaged. Soils with a pronounced macrostructure can transmit chemicals rapidly, leading to early arrival that greatly precedes the main pulse or front. In addition, aggregated regions of the medium containing water that is not flowing can act as repositories for diffusing chemicals, causing portions of a front or pulse to lag far behind the average flow.

Dispersion modeling is poorly developed in unsaturated field soils. Near the soil surface of an alluvial soil, it is likely that dispersion will be dominated by differences in water velocity at different points in the soil. In this case, the common convection-dispersion model assuming random solute spreading is not likely to be accurate, and a stochastic convective model assuming parallel flow of solute in isolated stream tubes may represent the solute mixing process better (Butters and Jury, 1989).

Solute Sorption During Transport

Chemicals that are hydrophobic or positively charged do not travel at the speed of the flowing water, but rather are slowed by their attraction to stationary solid sorption sites. Although the sorption process is very dynamic at the molecular scale, it is useful to conceptualize an adsorbed molecule as existing in a distinct phase that is temporarily immobilized by virtue of its attachment to stationary solid matter.

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

There are several different types of sorption reactions, distinguished primarily by the nature of the sorbing surface and the charge characteristics of the sorbing molecule. Positively charged ions in solution are attracted to negatively charged clay mineral surfaces and are temporarily immobilized by the process known as cation exchange. This is a partitioning reaction that divides the chemical mass between solution and adsorbed phases; it does not completely strip a compound from solution, nor is it permanent. The reaction depends on the nature of the molecule and also on the composition of the soil solution (Sposito, 1981). In addition, some positively charged species, notably the trace metals, appear to be specifically sorbed strongly to certain oxide surfaces (Chang and Page, 1985).

Anions are repelled from clay mineral surfaces that are negatively charged, but are attracted to positively charged broken end faces of minerals and also to free oxides in the soil. These surfaces have charges that are strongly pH dependent, and attract anions most strongly under acidic conditions.

Neutral organic molecules such as nonionic pesticides sorb primarily to organic matter surfaces in a reaction that can be approximated by a partition coefficient. The form of this relation is denoted through the linear sorption model

where Cs is the sorbed chemical concentration (mass of chemical sorbed per mass of soil), Cw is the chemical concentration in the soil solution (mass of chemical per mass of water), and the proportionality coefficient Kd is called the distribution coefficient (Hamaker and Thompson, 1972). The distribution coefficient has units of volume per mass. Because most of the sorption occurs on organic matter surfaces, the distribution coefficient may be subdivided into

where ƒoc is the fraction of soil organic carbon content (mass per mass of soil) and Koc is the distribution coefficient per unit organic carbon, called the organic carbon partition coefficient. Hamaker and Thompson (1972) showed that the Koc of a given compound varied significantly less between soils than the Kd. In that sense, it represents an intrinsic sorption affinity for a chemical that is soil independent. The organic carbon partition coefficient for chemicals generally decreases with increasing water solubility.

The retardation factor R is another measure of the relative partitioning of a chemical between the soil and the aqueous phases:

where ρb is the soil bulk density. R is the ratio between the total mass density and the mass per soil volume θCw in the dissolved phase. A simple derivation of

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

equation (6) was presented by Bouwer (1991). Because this relation applies instantaneously at equilibrium, it may be interpreted in probabilistic terms as describing that the sorbing solute spends 1/R as much time in the dissolved phase as an identical solute that does not sorb; hence, it will take R times as long to travel a given distance as the mobile one. Therefore, the solute velocity VR of an sorbing chemical with a linear sorption isotherm is equal to

Thus, a sorbing molecule moves R times slower than a nonsorbing one under the same conditions.

Equations (2) and (7) may be used to predict average migration of mobile and sorbing chemicals, provided that the assumptions of complete mobility of the water in soil and linear, equilibrium sorption of the chemical are met. Table 3.2 shows travel times required for different chemicals to move 50 meters (m) (160 ft) in ground water under the conditions given. The predictions apply only to the center of the pulse or front and do not address the extreme movement of the leading or trailing edges. The retardation factors in this table show a variability of nearly 4 orders of magnitude in the solute velocity and in the retention times for chemicals in the subsurface environment.

Although the travel time model describing average movement is simple, it is useful. Pratt et al. (1972) applied it to interpret deep cores taken underneath citrus groves throughout California. They found nitrate fronts moving very slowly underneath these fields and estimated that as much as 50 years would be required for the chemicals to reach deep ground water aquifers under the fields. Pesticides or other contaminants that also adsorb to soil solids would have considerably longer travel times under the same conditions. In contrast, SAT systems with relatively high flow rates and smaller distances from the soil surface to ground water may have much smaller travel times. For example, an SAT system recharging 50 m (160 ft) of water per year to ground water at a depth of 4 m (13 ft) through a soil layer with the same properties as the model aquifer in Table 3.2 would produce a travel time of only 11.7 days for mobile chemicals such as .

Other Attenuation Mechanisms

Chemical and biological reactions occurring during passage through an SAT system are specific to the compounds moving through them and are discussed in detail in the next section. Pathogens such as helminth cysts, protozoa, and bacteria are large enough to be filtered by the smaller soil pores and can be permanently immobilized near the surface, where they die and decompose, unless the

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

TABLE 3-2 Travel Times Required to Move 50 Meters Calculated for Different Chemicals in Ground Water.

 

Koc (m3/kg)

R

Travel Time (years)

Atrazine

1.6 × 10-1

5.8

2.3

Benzene

8.3 × 10-2

3.5

1.4

Bromide

 

1.0

0.4

DBCP

7.0 × 10-2

3.1

1.2

DDT

2.4 × 10+2

7,200

2,900

Ethylene dibromide

4.4 × 10-20

2.3

0.9

Toluene

9.8 × 10-2

3.9

1.6

Trichlororethylene

1.4 × 10-1

5.2

2.1

Trifluralin

7.3

220

88

Vinyl chloride

4.0 × 10-1

13

5.2

a Saturated Water Content θ = 0.4; bulk density ρb = 1500 Kilograms per Cubic Meter; and Organic Carbon Fraction ƒoc = 0.008, which is flowing at a Rate q = 50 meters per year.

Source: Jury et al. (1984c, 1990).

soil is very coarse textured. Colloidal complexes formed with mineral or organic matter suspended in solution are subject to the same filtration action.

IMPORTANT SOIL-AQUIFER PROPERTIES

Vadose Zone Properties

The effect of the soil properties in ground water recharge depends on the method of recharge used. For unconfined aquifers recharged by surface application to the soil above in an SAT system, knowledge of the physical, chemical, and biological properties of the soil materials is critical to the design of the operation. In contrast, for confined aquifers, where reclaimed water is injected directly into the ground water strata, the properties of the overlying soil layer are irrelevant to the operation.

The most critical soil properties for surface application are texture, permeability, presence of clay, iron, or hardpan, depth of soil profile, presence of

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

organic matter, and compaction characteristics (American Society of Civil Engineering, 1961). For a spreading basin operation, a number of factors must be considered during evaluation of a potential facility. Foremost among these are the quantity of recharge water available for application to the facility, the rate at which the recharge water will be applied, the quality of the recharge water, and available subsurface storage (Asano, 1985).

When this information has been assembled, the suitability of the site can be assessed by evaluating its capacity for accepting recharge water and transferring it to the aquifer. At this point, the infiltration capacity and uniformity are evaluated and a verification is made that the profile is free of subsurface barriers to downward flow, free of factors promoting clogging (such as free layers near the surface), and free of conditions that might cause cracks to form during dry periods. Basically, the potential infiltration rate for clear water (tap water) and no clogging is equal to the "rewet" or "resaturated hydraulic conductivity." The water content after drainage (the so-called field capacity) should also be relatively low for aeration purposes. Vadose zones should also be free of undesirable chemicals (anthropogenic or natural) that can be leached out. Aquifers should be free of pollution zones that could be moved to undesirable places with the recharge flow.

The vadose zone must function as a water purification system as well as a conduit to the aquifer. Depending on the operation and the constituents of the recharge water and soil, the following physical, chemical, or biological transformations of contaminants within the water may occur: filtration of suspended solids, parasites, and bacteria; sorption of trace elements, bacteria, and viruses; precipitation of phosphates and trace metals; biodegradation of organics; recarbonation of high pH effluents; and denitrification (Asano, 1985).

To assess the soil potential for water purification, certain chemical characteristics of the soil properties, such as organic matter content, pH, and cation exchange capacity (CEC), must also be characterized.

The soil properties desirable for the full range of operations are a compromise between the optimal properties required for portions of the full operation. Come-textured soils are highly desirable for rapid infiltration, but perform poorly in filtration and chemical transformation of undesirable constituents of the water (Nellor, 1980). Therefore, a compromise texture (sandy loam, fine sand) with sufficient clay for sorption and filtration but high enough permeability to accept high water infiltration rates is generally the best choice for an SAT system (Bouwer, 1985).

The final choices for site characteristics are dictated by the economics of the operation and availability of suitable land. The susceptibility of physical features, such as slope or terrain, to excessive runoff during intense rainstorms should also be considered.

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

Control of Ground Water Flow

If recharge water is to be extracted and reused, then the ground water flow must be controlled. Such control of subsurface hydraulics usually is maintained through injection or production wells. The purpose of such control is not only to segregate the recharge water from the native aquifer water, but also to achieve a minimum desired detention time for recharge water in the subsurface. For instance, the recharge system at El Paso, Texas, was designed for a residence time of 2 years; the residence time determined from ground water monitoring was actually three times longer.

If water is recharged to take advantage of the assimilative capacity of the subsurface as part of a treatment train, then it is generally desirable to minimize or control the mixing between recharge and native aquifer waters. The recharge water, once collected, may undergo further treatment before reuse. In addition, in coastal areas and in certain arid regions, recharge waters may be cycled through aquifers containing waters of poor quality, and here it is also desirable to segregate the recharge and native waters so as not to impair the quality of the recharge water prior to extraction.

To a large extent, the detention time of recharge water during subsurface flow determines the improvement in water quality. In designing recharge systems, it is desirable to maintain a minimum residence time in the subsurface as well as a variation of residence times for different parcels of recharge water. If the control system is designed to achieve a linear variation of residence times over a specified period, then arbitrary variations in quality of recharge water over the same period may be averaged out during subsurface flow (Huisman and Olsthoorn, 1983).

UNDESIRABLE SOIL CHARACTERISTICS

No field soil possesses ideal properties for all SAT operations. The extreme diversity of the soil environment ensures that there will be extremes in the local values of the hydraulic or retention properties of the medium that can cause problems in the operation or monitoring of recharge systems. The most important of these characteristics are discussed below.

Spatial Variability of Soil Properties

Soil properties, particularly those that influence transport, vary significantly over space, as shown in Table 3.3. For example, the saturated hydraulic conductivity, a reciprocal measure of the resistance of saturated soil to the flow of water, can have a coefficient of variability of several hundred percent on the scale of an agricultural field (Jury, 1985). Such variability creates enormous data requirements and also places extreme demands on model description of

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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TABLE 3.3 Sample Coefficient of Variation (CV) Measured for Different Soil Water and Solute Properties in Unsaturated Fields

 

Number of Studies

Range of CV (%)

Porosity

4

7-11

Bulk density

8

3-26

Water content at 0.1 bar

4

4-20

Percentage sand or clay

5

3-55

pH

4

2-15

Saturated hydraulic conductivity

12

48-320

Infiltration rate

5

23-97

Unsaturated hydraulic conductivity

4

41-343

Local solute velocity

 

 

Steady unsaturated water flow

3

36-75

Ponded water flow

2

78-194

extreme flow events. For example, in a field that has a coefficient of variability of 200 percent in its infiltration rate, nearly half of the water entering the soil will move through the 10 percent of the field having the highest infiltration rates (Jury et al., 1991). Prediction of extreme flow under conditions of such high variability often is not feasible.

Moreover, when variability of infiltration rate is high, basic calculations of the mean travel time and assimilative capacity of the soil are greatly in error. For the example described above, the 10 percent of the field receiving half of the water will have a travel time for mobile chemicals that is about one-fifth of the average and will exhaust its exchange complex in one-fifth the time calculated for the field based on average soil properties.

Modem theories of transport through media having spatially variable properties have addressed this problem by developing stochastic models, whose predictions are expressed in probabilistic rather than deterministic terms. Although stochastic modeling is highly developed for ground water flow (Dagan, 1986), little work has been done in unsaturated soils.

Spatial variability is primarily important where the recharge water is very clean and infiltration rates are controlled by soil hydraulic conductivity. Where

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

the water contains suspended solids, organic carbon, and/or nutrients, a clogging layer will be formed on top of the soil surface. Such clogging layers have a high hydraulic impedance, and they become the controlling factor of infiltration rates, which, of course, are much lower in clogging layers than in clean soils. Clogging develops faster where infiltration rates are high. Hence, clogging may well be the great equalizer in the infiltration process, and it may well render infiltration to be much more uniformly distributed than indicated by the spatial variability of the soil.

Preferential Flow of Chemicals

Preferential flow refers to the faster-than-average movement of contaminants or water, by whatever means, through part of the soil volume. It differs from the extremes in flow caused by property heterogeneity in that the nature of the flow is different from that in the bulk soil matrix. A major cause of preferential flow is the presence of numerous geometric voids in a structured or biologically diverse soil. For example, Ritchie et al. (1972) used a visible dye to demonstrate that much of the water moving through a swelling clay soil was migrating through vertical cracks. Omoti and Wild (1979) used fluorescent dyes in a weakly structured loamy sand to determine that earthworm channels, fissures with apertures between 0.05 and 0.10 millimeters (mm) (0.002 and 0.0039 inches), and loosely packed soil were all acting as conduits for the rapid transport of the adsorbing dyes they used in their field study. Scotter and Kanchanasut (1981) reported movement of chloride to a mole drain at 0.4 m (16 inches) under continuous ponding within 5 minutes after introduction of the chemical into the infiltrating water. Dye tracing revealed that root and worm channels and occasional fracture planes were carrying the flow.

Structureless sandy soils have also been found to exhibit preferential flow, arising from a variety of causes. Kung (1990) excavated a field plot on Plainfield sand after dye application, finding that the water funneled into increasingly narrow zones at greater depths, eventually carrying the bulk of the flow in a small fraction of the cross-sectional area. He postulated that the funneling mechanism was discrete coarse sand lenses that acted as barriers to downward movement, causing the water to flow around them and focus at the edges. A similar phenomenon was reported by Ghodrati and Jury (1990), who observed that preferential flow regions constituting a small fraction of the cross section were moving more than twice as deep as the main front under both ponder and sprinkler irrigation in undisturbed field plots on a loamy sand.

Preferential flow may also originate from unstable fluid flow through the soil. This phenomenon has a number of possible causes, including density and viscosity differences between the antecedent and incoming water, air entrapment during infiltration, and downward flow from a fine-textured to a coarse-textured soil layer (Hillel, 1987). Preferential flow has serious implications for chemical

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

and pathogen movement because it is desirable to keep these compounds from. migrating below the surface zone. Based on the information from studies conducted under controlled conditions in the field, preferential flow is widespread and often significant and also has been observed to occur in the structureless fine sandy soils that are favored in SAT systems (Jury and Fluhler, 1992). However, the presence of clogging material in SAT systems may act to prevent funnel flow from occurring to any great extent in SAT because this mechanism depends on very high permeability in the portion of the matrix that is active in flow (Kung, 1990). A more likely circumstance that could cause preferential flow in an SAT system is the presence of a more permeable zone located beneath a less conducting one, which can create fingering of fluid through narrow channels within a small part of the pore space of the more coarse-textured soil (Hillel, 1987).

Reviews of the prevalence of preferential flow and its importance in soil are given elsewhere (Beven and Germann, 1982; Jury and Roth, 1990; Jury and Fluhler, 1992).

TRANSPORT AND FATE OF SPECIFIC CONSTITUENTS OF RECHARGE WATER

The principles discussed in the previous sections are useful to understanding the chemical and pathogen removal processes that occur during transport through soil and aquifer material. This section looks at categories of solution constituents that correspond to groupings used elsewhere in this report.

Nitrogen

Nitrogen is a common constituent of wastewater and agricultural return flow. In the former, it is mostly present in the form of after conventional primary and secondary treatment, as it first makes contact with the soil surface. Because partitions to gaseous ammonia at the air-water interface, some volatilization occurs during the detention time and later during drainage cycles that expose soil remaining near the surface.

After enters the soil, it is biologically transformed to by a two-stage process called nitrification. In contrast to which is positively charged, is an anion and is quite mobile in soil. However, can be biologically transformed under conditions of high water content and sufficient organic carbon availability into nitrogen and nitrous gases, which escape into the atmosphere. This reaction occurs when nitrogen substitutes for oxygen as a terminal electron acceptor under conditions of limiting aeration, moderated by the availability of a carbon source. Since organic carbon is normally low in aquifers, denitrification is generally confined to the near-surface regime. Crites (1985)

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

TABLE 3.4 Nitrogen Removal Rates at SAT Sites

 

Loading Rate (feet/year)

Flooding: Drying Time

BOD:Na

Percentage Nitrogen Removal

Hollister, California

50

1:14

5.5:1

93

Brookings, South Dakota

40

1:2

2:1

80

Calumet, Michigan

56

1:2

3.6:1

75

Phoenix, Arizona

200

9:12

1:1

65

Ft. Devons, Massachusetts

100

2:12

2.4:1

60

Lake George, New York

190

1:4

2:1

50

Disney World, Florida

180

150:14

0.3:1

12

a BOD:N = ratio of biochemical oxygen demand to nitrogen.

Source: Crites, 1985.

states that carbon is required at a BOD (biochemical oxygen demand) to nitrogen ratio of about 3:1 for maximum denitrification.

Under SAT conditions the removal rates of nitrogen can be quite high if denitrification is optimized. Bouwer (1985) found that flooding and drying cycles allowed to adsorb to soil mineral surfaces by cation exchange, while the drying cycle permitted oxygen diffusion that promoted nitrification to the form, which subsequently denitrified after diffusion into anaerobic microsites. Table 3.4 shows nitrogen removal efficiencies measured at a number of SAT sites throughout the United States. The primary removal mechanism in these operations is denitrification, with perhaps some ammonia volatilization losses during drying cycles.

Phosphorus

Phosphates in recharge water are removed by precipitation to amorphous or crystalline forms with iron, aluminum, or calcium. At low pH, precipitation with iron and aluminum is favored, whereas under alkaline conditions, calcium phosphate is controlling. Phosphate mobility is greatest under neutral conditions (Bouwer, 1985).

Phosphorus removal is achieved through a fast sorption reaction and slow precipitation reactions. The final phosphorus concentration in the effluent water after passage through the SAT system is controlled by the solubility products of

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

the solution constituents; for that reason, removal is not complete and depends on soil properties and loading rates. The duration of the flooding cycle can be varied to promote optimal levels of denitrification for nitrogen-rich recharge waters.

Inorganics and Trace Metals

The organic constituents found in municipal wastewater tend to have been added during water use, while inorganic chemical concentrations tend to be characteristic of the source water. Moreover, they are not removed to any extent during use, but rather tend to accumulate as water is extracted from the source stream (Chang and Page, 1985). For that reason, mineral concentrations in municipal wastewater vary widely depending on the genesis of the water and its use prior to arrival at the SAT system. In contrast, inorganic chemical increases, especially in heavy metals, are more typical of stormwater runoff.

Trace elements present in suspended matter generally are removed during SAT by filtration and do not migrate. However, they do accumulate in colloidal material trapped in the clogging layer, which eventually must be removed from soil to restore its infiltration rate. Smaller suspended particulates that can move through soil pores without becoming trapped are also attenuated by sorption to mineral surfaces in the soil matrix. Chang and Page (1985) state that filtration and associated colloidal sorption are the primary means of removal of trace elements in soil.

Not all of the trace element mass is associated with suspended material in wastewater, it is also present in a dissolved form that is not affected by filtration. The dissolved constituents are affected by various chemical reactions in soil, some of which act to attenuate their movement through soil. Aerobic conditions and a high pH also decrease trace metal mobility.

Positively charged trace metal ions can be attenuated by ion exchange with negatively charged clay mineral surfaces. This process is a partition reaction affecting all cations in soil and therefore is not a significant removal mechanism for trace elements at low values. Moreover, soil cation exchange capacity is finite and therefore can be exhausted after many pore volumes of material have passed through the profile.

A more specific sorption reaction appears to occur with trace elements, which are strongly attracted to amorphous and crystalline oxides of iron, aluminum, and magnesium. This reaction is not easily reversed by other ions in solution and may become nonexchangeable over time (Jenne, 1968; Sharpless et al., 1969).

Precipitation reactions of trace metals also occur with other constituents of wastewater, which causes the concentrations in solution to be regulated by the solubility products of the species present. This reaction is difficult to separate from specific sorption.

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

Metals can also form complexes with dissolved organic matter (chelation), increasing their mobility in the process. Because of the complexity of solution chemistry when organic ligands are present, little is known about the behavior of specific complexes.

Because trace metal removal is incomplete and the soil sorption capacity is finite, SAT is not completely effective in preventing migration of all constituents within this group. There are also significant variations in trace element mobility, ranging from strongly attenuated (zinc and copper), to more mobile (cadmium and lead), to extremely mobile (boron). Ground water concentrations of silver, barium, cadmium, cobalt, and chromium below the treatment site at Hollister, California, have been unaffected by the additions of wastewater to the overlying soil. However, manganese, nickel, iron, zinc, lead, and copper were above background levels (Crites, 1985). Soil samples taken in the Whittier Narrows recharge plant after over 20 years of operation showed elevated levels of cadmium, chromium, copper, nickel, lead, and zinc in the top 60 cm (2 ft), but not below that depth, suggesting that the soil had the capacity to remove metals for many more years of operation before ground water would be affected (Chang and Page, 1985).

Organic Chemicals

Organic chemicals vary enormously in their mobility, volatility, and persistence in soil. In an SAT system, volatile compounds volatilize prior to application, and only the soluble organic constituents enter the soil. The organic removal efficiency of an SAT system depends on the degree to which a given compound can be chemically or biologically transformed to an innocuous state during its time Of passage through the system. Organic compounds degrade chemically by hydrolysis, photodecomposition, or redox reactions to varying degrees depending on their structure (Armstrong and Konrad, 1974). Microbial conversion occurs chiefly in the surface zone of soil, where bacterial populations and organic carbon levels are high. Low organic carbon levels generally limit microbial action in deeper regions of the vadose zone and in aquifers.

As discussed in a previous section, the travel time of an organic chemical may be roughly designated by its retardation factor in a given soil, or more intrinsically by its organic carbon partition coefficient Koc. In a similar manner, the overall action of the chemical and microbiological processes transforming an organic chemical moving in soil may be crudely expressed as a half-life or degradation rate constant. Then, the length of the half life compared to the travel time can be used as an index of the potential for the compound to survive its passage through the soil. This approach has been used to develop pesticide screening models to determine (in a relative sense) the potential of a compound to reach ground water (Rao et al., 1985; Jury et al., 1987).

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

Pathogens

The extent to which soil can remove pathogens depends on a variety of factors, including the physical, chemical, and biological characteristics of the soil, the size and nature of the organism, and the environmental conditions. The largest organisms, such as protozoa and helminths, are removed effectively by filtration unless the soil contains large pores or continuous voids. Bacteria are also filtered, but in addition may adsorb to soil solid material. Viruses are too small to be filtered by most soil pores and are attenuated only by sorption. The latter mechanism is much more pronounced in unsaturated soil than under saturated conditions, for reasons that are not clear at the present time. Speculation about explanations for the increase in sorption under unsaturated conditions have centered on the role of the air-water interface, either because it forces vital particles nearer to the solid surfaces (Lance and Gerba, 1984) or because the interface itself can trap the particle and perhaps deform it enough to cause inactivation (Powelson et al., 1990).

The most important factor in microorganism survival in soil and ground water is temperature. Below 4ºC, microorganisms can survive for long periods of time, but they die off rapidly with increasing temperature. The bacterial die-off rate approximately doubles with each 10ºC increase in temperature, but viruses have been observed to follow a more linear relation between inactivation rate and temperature (Yates and Yates, 1987). The soil pH affects survival, which shortens under acidic conditions. There is evidence that viral sorption increases at low pH, because the isoelectric points of soil minerals generally are lower than those of viruses, so that the vital particles become more electropositive than the soil at low pH. Soil moisture affects survival of all microorganisms, with inactivation decreasing significantly as water content is increased above air dry levels. Vital movement is not sufficiently well understood to model at the present time, in part because independent measurements of sorption in the laboratory seriously underestimate the extent of attenuation during transport through unsaturated soil. Gerba and Goyal (1985) discuss the major factors affecting vital movement in soil.

Evidence from monitoring at SAT sites suggests that the larger pathogens are mostly removed by filtration and do not reach ground water during their lifetime in soil, unless a continuous come-textured or void pathway is present through the surface soil zone. The greatest removal occurs in the surface mat of suspended particles that forms on the top few millimeters of soil and strains the entering solution.

Field measurements of vital movement at ground water recharge sites are limited. At the Flushing Meadows surface spreading site in Phoenix, Arizona, viruses were not detected after 10 to 30 m (33 to 98 ft) of travel through the fine sandy soil of the system, while more sensitive measurements at the Sweetwater surface spreading site in Tucson, Arizona, detected migration of viruses through

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

the soil system (Powelson and Gerba, 1993). A review of 6 land application (irrigation) sites and 11 rapid infiltration sites by Gerba and Goyal (1985) indicated that viruses have been isolated underground after various migration distances. Vertical migration ranged from 1.4 to 30.5 m (4.6 to 100 ft) at spray irrigation sites and from 2.4 to 67 m (8 to 220 ft) at rapid infiltration sites. At the rapid infiltration sites reviewed by Gerba and Goyal (1985), horizontal migration of viruses in the underground ranged from 3 to over 400 m (10 to over 1,300 ft). Disinfection of the water prior to recharge by surface spreading or injection can minimize or eliminate microorganisms underground, as can proper site selection and management of surface spreading systems.

Disinfection By-products

In most cases, source water intended for recharge will be disinfected with chlorine, chloramine, ozone, or a combination of these disinfectants. Table 3.5 lists the major potentially problematic by-product compounds that would be most likely to be produced from these disinfectants (Bull and Kopfler, 1991). Because bromide is present in many ground water systems, many brominated derivatives are included although data on occurrence frequency and concentrations for these compounds is limited. In addition, Table 3.5 presents only examples of DBP categories by structural class; many others could be listed in most cases.

Nature of Reduced Carbon Species

A major uncertainty arises from the fact that most of the product identification work has been done on standardized humic or fulvic acid fractions isolated from surface water sources of sufficient quality (low organic carbon levels) to be considered as a traditional water supply source. The chemical composition of alternative water sources, however, may be drastically different from that of traditional water sources. In the case of drainage from low-lying peat areas, it might be reasonable to assume that this water source differs only in the higher level of humic carbon present. However, Amy et al., (1984) found that higher levels of trihalomethanes (THMs) are characteristic of more than 200 agricultural drains in the Sacramento River delta and that the larger amount of dissolved organic carbon (DOC) in these water sources is characterized by a higher molecular weight and greater THM reactivity than that found in delta tributaries. Some research showed THM formation potential values as high as 3,580 micrograms per liter (µg/l) and di-and trichloracetic acid values of 1,650 and 1,990 µg/1, respectively (Amy et al., 1984). Clearly, carbon level is the largest single parameter controlling DBP levels, although chlorine to carbon dose ratio, pH, temperature, and contact time are also important. However, it remains uncertain whether the carbon levels from non-traditional sources are more or less sensitive

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

TABLE 3.5 Examples of Problematica Disinfection By-products in Water Prior to Recharge

DBP

Disinfectants

Estimated Fate During Recharge

Chloroform

Chlorine, chloramines

Volatilization, sorption

Bromodichloromethane

Chlorine, ozone

Volatilization, sorption

Trichloroacetic acid

Chlorine

Mineral sorption, ion exchange

Dibromoacetonitrile

Chlorine

?

Formaldehyde

Ozone, chlorine

?

1,1,1-Trichloropropanone

Chlorine

?

MXb

Chlorine

Sorption, degradation

Bromate

Ozone

Reaction with soil

a Listed DBPs are problematic the sense that the compound commonly has been found with the indicated disinfectant and is of toxicological significance.

b MX=3-Chloro-4(d-chloromethyl-5-hydroxy-2(5H)-furanone.

to disinfectant dose, and whether unique by-products are produced from some sources and not others.

The nature of the remaining DOC is also affected by the exposure to disinfectant oxidants. Liao (1983) has shown that dissolved carbon made from purified humic preparations is convened to smaller and/or more polar sizes by exposure to permanganate, and to some degree by hydrolysis. Anderson et al. (1985) also showed that remaining humic materials after low dosages of ozone are more polar and of lower molecular weight than untreated humic material. The effect of such changes on soil retention, metal binding capacity, interactions with other organics, and so on, is unknown. However, one might speculate that this partially oxidized carbonaceous material might be less retentive on soil organic matter because of decreased hydrophobicity and more retentive on soil mineral phases because of increased charge density. In addition to the known DBPs, residual halogen is present when chlorine is used. This material may then appear as adsorbable organic halogen (AOX) and be more biodegradable in anaerobic underground environments than nonhalogenated organic material.

Stability of DBPs in the Soil and Underground Environment

If ultraviolet disinfection is not used or if chlorine is used in conjunction

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

with ultraviolet radiation, the question of the stability of DBPs in ground water systems becomes important. Changes in the DBP chemical composition of recharge water will occur in the infiltration basin and in the soil vadose zone for surface recharge operations, and in the aquifer during transport and storage, regardless of the input procedure. In surface recharge operations, the water to be recharged will undergo some chemical change due to microbiological activity in the infiltration basin. As organisms respond to the DOC and begin growth cycles, total organic carbon (TOC) may increase or decrease slightly, depending on retention time and the acclimation phase (Bouwer, 1984). In the vadose zone, hydrophobic compounds will be retarded by the soil organic material and subjected to microbial decomposition. Halogenated organic compounds, including the nonvolatile DBPs, will have greater hydrophilicity and correspondingly less retention on soft organic matter and more rapid transport to the water table. Some DBPs, such as the halogenated acetic acids, will behave as fully ionized organic anions at the pH values of water in the vadose zone and may be expected to move at the same speed as the subsurface water. Thus, it might be expected that the smaller halogenated materials will persist longer and penetrate more deeply into the soil horizons than the larger and more hydrophobic molecules. The degree of microbial utilization is most difficult to predict and will probably vary from site to site.

The bioremediation literature notes that trichloroacetic acid is an intermediate in the oxidative degradation of vinyl chloride produced from anaerobic degradation of trichloroethylene and that it does not build up in the soil. This suggests that complete mineralization is possible for this DBP, at least in systems with fully adapted microorganisms. The same is probably true for the other haloacetic acids. Thus, control of the redox environment in the vadose zone through optimizing application rates is probably an important management criterion for these DBPs.

Of the list of DBPs in Table 3.5 one might expect significant losses of formaldehyde and chloroform in the infiltration basin due to volatilization, with somewhat less significant losses of any bromodichloromethane and 1,1,1-trichloropropanone, owing to lower volatilities. Losses of trichloroacetic acid may be expected in the vadose zone.

MX presents an unusual case because its polarity depends on whether the molecule is in the open form, which is a substituted botanic acid, or in the closed form, which is a substituted hydroxyfuranone. A pH value of less than 5 would give the closed form, which is substantially less polar and therefore more interactive with soil organic matter and more likely to be retained for microbial utilization. In the open acid form, it might be expected to behave like trichloroacetic acid. In addition, MX and its congeners are probably reactive with reduced forms of sulfur in the subsurface environment, but their reaction products are probably of less toxicological significance.

Singer et al. (1993) have studied the fate of chlorinated DBPs when treated

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

and disinfected drinking water was injected and stored in an aquifer at five sites. These authors concluded that THMs and haloacetic acids (HAAs) are removed from chlorinated drinking water during aquifer storage. HAA removal apparently precedes THM removal, and the more highly brominated species tend to be eliminated earliest. Removal of DBPs during aquifer storage was accompanied by a decrease in dissolved oxygen, implicating anaerobic biological mechanisms. In addition to DBP removal, reductions in the concentrations of organic precursor material were observed and correlated with measured reductions in THM formation potential and HAA formation potential. No effort was made to establish the identities of THM or HAA degradation products.

The results from Singer et al. (1993) concur with those from Roberts (1985) who evaluated organic contaminant behavior during a recharge project in Palo Alto, California. He also found that THM losses were correlated with losses of dissolved oxygen. In addition, he found that the loss rate for THMs was nearly 10 times as rapid as the rate of concentration decrease for compounds containing two carbon atoms, represented by trichloroethylene, tetrachloroethylene, and 1,1,1-trichloroethane.

For these reasons, disinfection to control viruses or other pathogens prior to infiltration may not necessarily result in an increase of DBPs in the ground water aquifer. However, it is possible to overload the removal mechanisms, and this is driven by TOC levels in the source water primarily. To provide disinfection and prevent DBP buildup in the ground water it would be necessary to pretreat source water in order to reduce levels of TOC. On the other hand, if the pathogen content of the ground water is not of primary concern, then disinfection (and DBP formation) may be avoided prior to recharge.

SUSTAINABILITY OF THE SAT SYSTEM

Sustainability refers to the long-term viability of an SAT operation, and specifically to the ability of the soil and aquifer to receive recharge water indefinitely without suffering deleterious side effects. Within the vadose zone, clogging is an inevitable side effect of SAT procedures, but is largely manageable with periodic drying to oxidize the accumulated organic material and restore the infiltration rates, along with periodic physical removal of the clogging layer by scraping, raking, or other techniques (Bouwer, 1985).

Of the material that is not completely removed from the recharge water before reaching the aquifer, viruses are of special concern because they may cause disease or pose an unacceptably high risk even when present in low concentrations. Sustainability must therefore include considerable travel distance between the receiving basin of the aquifer and any route to drinking water supplies to allow sufficient vital inactivation to occur (Yates et al., 1986).

Nitrates that enter the aquifer should be regarded as nondegrading, and therefore the recharge operation must operate very efficiently with respect to nitrogen

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
×

RAPID INFILTRATION-EXTRACTION PROJECT IN COLTON, CALIFORNIA

The cites of San Bernardino and Colton, California, are required to filter and disinfect their secondary effluent prior to discharge to the Santa Ana River, which is a source of drinking water and is used for body contact recreation. The two cities joined under the auspices of the Santa Ana Watershed Project Authority (SAWPA) to seek a regional solution to their wastewater treatment requirements. They hoped to develop a cost-effective alternative to conventional tertiary treatment (chemical coagulation, filtration, and disinfection) that would still result in an effluent that was essentially free of measurable levels of pathogens. SAWPA conducted a one-year demonstration project to examine the feasibility of a rapid infiltration-extraction (RIX) process to treat unchlorinated secondary effluent prior to discharge to the river and determine whether or not the RIX process is equivalent—in terms of treatment reliability and quality of the water produced—to the conventional tertiary treatment processes specified in the California Department of Health Services Wastewater Reclamation Criteria.

The demonstration was conducted on a site in Colton, California, adjacent to the Santa Ana River bed. Infiltration basins were built at two sites on the property to allow testing the RIX system under a variety of operating conditions. The soils were coarse sands with clean water infiltration rates of about 15 m/day (50 ft/day). Forty four monitoring wells were sampled for a number of organic, inorganic, microbiological, and physical measurements.

The study indicated that the optical filtration rate was 2 m/day (6.6 ft/day) with a wet to day ratio of 1:1 (1 day of flooding to 1 day of drying). Mounding beneath the infiltration basins ranged from 0.6 to 0.9 m (2 to 3 ft), and infiltrated wastewater migrated up to 24.4 m (80 ft), vertically and over 39.5 m (100 ft) laterally before it was extracted and had an aquifer residence time of 20 to 45 days, depending on the recharge site. Extraction of 110 percent of the volume of infiltrated effluent by downgradient extraction wells effectively contained the effluent on the RIX site and minimized mixing with regional ground water outside the project area.

The soil-aquifer treatment reduced the concentration of total coliform organisms 99 to 99.9 percent in samples collected approximately 7.6 m (25 ft) below the infiltration basins, and, generally, water from the extraction wells prior to disinfection contained less than 2.2 total coliforms per 100 ml. While viral levels were as high as 316 viruses per 2001 in the unchlorinated secondary effluent applied to the infiltration basins, only one sample of extracted water prior to disinfection was found to contain detectable levels of viruses. In addition, the turbidity of the extracted water generally averaged less than 0.04 NTU (Foreman et al. 1993).

Although the RIX process greatly reduced microorganism levels in the wastewater, disinfection of the extracted wastewater proved to still be necessary prior to its discharge to the Santa Ana River. Ultraviolet radiation was evaluated as an alternative to chlorination for disinfection. Due, in part, to the high quality of the extracted water, ultraviolet radiation was shown to be effective for the destruction of both bacteria and viruses and will likely be used instead of chlorine in the fullscale project (CH2M Hill, 1992).

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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removal in the vadose zone where it is transformed. Because proper management of an SAT system has been able to achieve as high as 80 percent removal of nitrogen from the recharge water, control of this chemical may be manageable in long-term operations.

Trace element accumulation in the vadose zone during operation may occur so slowly that exhaustion of the assimilative capacity of the surface zone may not occur in a time that will limit the economic viability of the operation (Chang and Page, 1985); however, the capacity of the soil to remove metal cations is not infinite, and their buildup should be monitored. Site closure at the termination of operation of an SAT system should be regulated to ensure that the metals residing in the surface soil are not disturbed by operations that might change the chemical characteristics of the soil solution and mobilize the contaminants.

Pesticides and other organic chemicals present in recharge water vary significantly in their mobility, persistence, and suspected health effects. Water that contains pesticides known to be resistant to microbial or chemical degradation in soil may pose problems for SAT systems in the long run, because refractory organics are only slowed by solid-phase sorption and not permanently removed from solution.

Ground water recharge operations, either from surface spreading or injection wells, introduce microorganisms to the subsurface environment in addition to organic and inorganic chemicals. Microbial activity associated with artificial recharge may have three principal effects: (1) bacteria may grow near the recharge facility and cause clogging, with a gradual reduction of soil and aquifer hydraulic conductivity; (2) microbial activity may produce substances that adversely affect the taste and odor of recovered water; and (3) pathogenic organisms in the recharge water may travel through the aquifer and cause illness when the water is later recovered for use without disinfection (Ehrlich et al., 1979a). During tests with injection of highly treated sewage into a sand aquifer on Long Island, chlorination of the injectant to 2.5 mg/1 suppressed bacterial growth and clogging (Ehrlich et al., 1979b). Similar tests at another injection site showed that clogging could be controlled with a chlorine residual maintained between 0.2 and 1.5 mg/l at the treatment plant (Schneider et al., 1987). The tests of Ehrlich et al. (1979b) also showed that movement of bacteria from the injection well into the aquifer was not extensive. Pathogenic bacteria are generally incapable of competing with soil bacteria for nutrients, while viruses are incapable of reproducing outside of a living cell. They do not multiply and eventually they die and decompose (Bouwer, 1978).

PERFORMANCE AND COMPLIANCE MONITORING

Vadose Zone Monitoring

Ideally, monitoring should be instituted near the point of application in the

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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soil to determine whether the SAT system is functioning in the desired manner. Vadose zone monitoring is difficult, however, for a number of reasons. First, there are essentially only two monitoring methods (soil sampling and vacuum extraction of solution), both of which have drawbacks. Soil sampling is destructive and very localized, so many samples must be taken to give an accurate picture of the average and extreme behavior of the SAT system. This causes substantial land disturbance. Vacuum extraction through porous soil solution samplers permanently installed in the soil is less destructive than soil sampling, but also can produce preferential flow pathways along the sides of the sampling tube if special care is not taken to produce and maintain a tight contact with the surrounding soil. Vacuum extractors also sample an unknown volume of soil and may be very inefficient at intercepting rapidly flowing solution such as might be present in preferential flow channels (Roth et al., 1991). They may also ''filter out" microorganisms and chemicals. For these reasons, the contaminant transport characteristics revealed by solution samplers, particularly those located near the surface, might not be representative of the true behavior in the field (Roth et al., 1991).

A second limitation to monitoring near the surface is that the transformation processes occurring within this zone may not have reached completion at the point of monitoring, so interpretation of the information recorded is problematic. Spatial variations in water velocity, chemical concentration, and soil structure are most pronounced near the surface, so the monitoring density required for accurate assessment of average and extreme behavior is greatest in this zone. For these reasons, the best zone of monitoring is generally farther from the surface, such as in the top of the saturated zone. Vadose zone monitoring is generally confined to periodic sampling of the soil and solution to determine general trends.

Tracing of Recharge Water

To follow the migration and fate of recharge water, water quality changes may be monitored by using water samples from wells and lysimeters. Chemical analyses usually focus on indicator chemicals rather than attempting to identify all recharge water chemical species. If a sufficient contrast exists between recharge and ambient waters, then TDS or conductivity may serve as indicators of general migration. Other mobile chemical species that may be used as indicators of recharge waters include chlorides, sulfates, and nitrates. Migration of organic species may be followed by grouping chemicals based on their potential mobility through R values. Lumped parameters such as TOC may serve as a surrogate for wastewater, with values less than 1 mg/1 TOC at the wellhead showing adequate dilution and TOC losses for potable use.

Tracing of recharge waters is more difficult if the recharge water quality is similar to that of native formation waters. At the Fred Hervey water reclamation

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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plant in E1 Paso, Texas, the recharge and formation water are very similar, and a, combination of chlorine, nitrogen, and 18O concentrations are used to determine breakthrough of recharge waters at monitoring and recovery wells.

Mixing of Recharge and Ambient Ground Water

The extent of mixing between recharge and ambient ground waters is important in determining whether potentially toxic chemicals in the recharge source waters exceed concentrations that may adversely affect human health when extracted waters are used for potable purposes. The degree to which recharge water has mixed with ambient ground water at a site downgradient from the recharge facility may often be estimated from water quality parameters. If chemical constituents behave conservatively during subsurface migration and mixing, then a linear weighting of concentrations may be used to estimate the degree of mixing (Reeder et al., 1966). For example, chloride and sodium concentrations were used to determine mixing of reclaimed water recharged at East Meadow, Long Island, with the ambient ground water (Schneider et al., 1987). The median sodium and chloride concentrations in the recharge water were approximately 115 and 157 mg/l, respectively, whereas those in ambient ground water were 22 and 27 mg/l, respectively. If the chloride concentration in the ambient ground water is approximately 30 mg/l, in the recharged water is 160 mg/l, and in the observation well sample is Co mg/l, then the fraction F of recharge water in samples from an observation well can be calculated from the following equation:

An observation well sample concentration of Co = 100 mg/l would correspond to a recharge water fraction of 0.54, or 54 percent in the water sampled.

California is developing regulations that place restrictions on the percentage of recharged and reclaimed water to be included in water produced by supply wells for potable use. A supply well can produce water from strata above and/or below as well as from the zone in which subsurface migration of the recharge water is occurring. Also, as a result of radial flow toward the supply well during pumping episodes, part of the water produced by the well may be from parts of the aquifer containing ambient ground water. Under either of these conditions, the water produced by the supply well may be a mixture of recharge water and ambient ground water. Equation (8) can be used to estimate the recharge water fraction in the produced supply water.

Not all chemical constituents allow a simple linear weighting of concentrations to be used to estimate the degree of mixing, especially reactive chemicals and those contributing to the buffering capacity of the system. As an example,

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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calcium may not serve as a good indicator of mixing because Of its affinity for clay minerals and because of its role in the calcium-carbon dioxide system. If ambient and recharge waters are mixed within the formation, then calcium carbonate may precipitate, remain in equilibrium with the resulting water, or dissolve, depending on the ratios of carbonate and bicarbonate in the two source waters (Huisman and Olsthoom, 1983).

Posttreatment of Recharge Waters

For many recharge systems the recharge water tends to lose its chemical identity while in the subsurface, and recovered waters have traditionally required little treatment before use. Even where potable reuse is contemplated, if pretreatment of recharge waters is sufficient, little posttreatment is required. For example, recharge waters in E1 Paso, Texas, are planned for potable reuse. The moderately weak domestic sewage receives advanced wastewater treatment before injection, and the recovered ground water receives only disinfection before use.

Where treatment is required, posttreatment of recharge waters is no different from ground water treatment. Tertiary treatment can remove taste and odor problems. For removal of organics, aeration (air stripping) and sorption using activated carbon are particularly effective, and neither is inordinately expensive nor difficult to install in a water treatment system. Activated carbon may not remove DOC to sufficiently low values. In that case, membrane filtration or reverse osmosis can be used. Point-of-use water treatment systems may include chlorination, ozonation, or ultraviolet disinfection, ion exchange for water softening, and filtration with activated carbon or membrane filtration for control of organics as well as taste and odor.

SUMMARY

The soil and underlying aquifer have a great capacity to remove chemical contaminants and pathogens from recharge water. The assumption that passage through the soil to the aquifer and through the aquifer to the point of withdrawal provides no treatment and that an treatment must be provided before recharge or after extraction is overly conservative when applied to most chemicals and microorganisms. Ground water recharge and recovery systems can provide significant treatment benefits at relatively low costs and in appropriate circumstances can make use of water of impaired quality attractive.

The ideal soil for an SAT system balances the need for a high recharge rate, which occurs in come-textured soils, with the need for efficient contaminant adsorption and removal, which are better in free textured soils. Because structured soils are undesirable for obvious reasons, the best choice of soil texture for SAT is a structureless fine sand or sandy loam.

Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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The vadose zone has the capacity to remove many constituents of concern during passage of recharge water toward the underlying aquifer. Nitrogen, for example, quickly transforms to nitrate, which is very mobile under normal conditions in the soil but can be removed only by denitrification under anaerobic conditions. This reaction can be enhanced by the proper combination of alternate wetting and drying cycles. Phosphorus levels are reduced by sorption and precipitation, but not completely, Trace metals, with the exception of boron and arsenic, are strongly attenuated and precipitated in the soil. There is some concern about their eventual passage through a soil that has been under SAT for many years or after closure of a site.

Organic chemicals are removed to varying extents by volatilization or chemical or biological degradation during passage through the vadose zone. Some pathogen removal in soil occurs by filtration in the surface clogging mat for the largest organisms and by sorption for bacteria and viruses. Viruses are considerably more mobile in soil than the larger pathogens, although they inactivate in soil eventually. Traditional disinfection by chlorination produces DBPs, which are mobile in soil and persistent to varying degrees.

With proper management and pretreatment, an SAT operation employing surface wastewater spreading with periodic drying to reduce clogging should be sustainable indefinitely. Slow trace element migration remains a concern, however, that requires monitoring during SAT use and regulation after closure. Although near-surface monitoring is desirable for proper vigilance, soil variability makes it difficult to achieve complete coverage with existing devices. Therefore, a combination of periodic near-surface and distant monitoring is important. With adequate management and monitoring, SAT systems can reduce pretreatment costs.

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Suggested Citation:"3 Soil and Aquifer Processes." National Research Council. 1994. Ground Water Recharge Using Waters of Impaired Quality. Washington, DC: The National Academies Press. doi: 10.17226/4780.
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Page 131
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As demand for water increases, water managers and planners will need to look widely for ways to improve water management and augment water supplies. This book concludes that artificial recharge can be one option in an integrated strategy to optimize total water resource management and that in some cases impaired-quality water can be used effectively as a source for artificial recharge of ground water aquifers. Source water quality characteristics, pretreatment and recharge technologies, transformations during transport through the soil and aquifer, public health issues, economic feasibility, and legal and institutional considerations are addressed. The book evaluates three main types of impaired quality water sources—treated municipal wastewater, stormwater runoff, and irrigation return flow—and describes which is the most consistent in terms of quality and quantity. Also included are descriptions of seven recharge projects.

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