Natural Attenuation for Groundwater Remediation (2000) / Chapter Skim
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4 Approaches for Evaluating Natural Attenuation
4 Approaches for Evaluating Natural Attenuation
Pages 150-211
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From page 150... ...
This same principle applies to natural attenuation. This chapter describes a weight-of-evidence approach for demonstrating the mechanisms responsible for observed contaminant losses in natural attenuation.
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From page 151... ...
at the St. Joseph site, but the lack of COD removal indicated that reductive dechlorination of TCE was unlikely at Edwards Air Force Base.
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Sampling errors also can confound efforts to document footprints. Because of the possibility of confounding factors, a weight-of-evidence approach, measuring several footprints, generally must be used to document natural attenuation.
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Documentation of high NO3- and SO42- concentrations; demonstration that TCE moves with water Formation of degradation by-products (cis-1,2-DCE, 1,1-DCA, vinyl chloride, and ethene) ; CH4 and H2S formation; increase in C1- concentration Detection of breakdown products; detection of unique transient metabolites; observation of microbial metabolic adaptation and expressed biodegradation genes Depletion of O2; detection of unique metabolic by-product; detection of genes for degrading PAHs in site microorganisms; rapid PAH degradation in soils taken from site Observation of carbonate dissolution leading to pH increase coincident with metal precipitation; observation of manganese oxide precipitates in stream sediments Sorbed radionuclides observed in site samples Loss of O2, NO3-, and SO42-; formation of Fez+ and CH4; increase in inorganic carbon concentration; increase in alkalinity Loss of O2; formation of Fez+, Mn2+, and CH4; formation of intermediate metabolites; observation of selective degradation of petroleum hydrocarbons relative to more stable chemicals NOTE: BTEX = benzene, toluene, ethylbenzene, and xylene; DCA = dichloroethane; DCE = dichloroethene; MTBE = methyl tert-butyl ether; PAHs = polycylic aromatic hydrocarbons; PCBs = polychlorinated biphenyls; TCE = trichloroethene; TCA = trichloroethane.
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From page 154... ...
The conceptual model should include a description of the groundwater flow system, estimated locations of the contaminant source and plume, and a list of reactions that might contribute to natural attenuation. The second step is to analyze site measurements to quantify the attenuation processes.
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However, numerical answers at each stage of the process should be scrutinized carefully and not be overvalued. Characterizing the Grounc~water Flow System The foundation of a site conceptual model always is the site's hydrogeology.
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From page 156... ...
Measurements of hydraulic heads in all available wells should then be used to create maps in cross section and plan view showing the groundwater elevations at the site. Hydrologic boundaries to be shown in the conceptual model include surface water bodies, flow divides, recharge wells, pumping wells, and evaporation.
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Uncertainty in MocleIing the Flow System Although sophisticated equipment and analysis techniques are utilized to characterize the subsurface, uncertainty is inevitable in estimates of contaminant behavior because of temporal and spatial variability. The best approach to accounting for this uncertainty is the formulation of multiple conceptual models, each representing a different hypothesis about how the system behaves.
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The distributions of hydraulic conductivity in each of the realizations affect predictions of groundwater flow (and subsequent predictions of contaminant transport) in different ways.
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In each realization, a different set of numerical parameters is used for different parts of the equations. The foundation of a constructed model is mass balance, which is simply an accounting system to make sure that the mass of a material (such as a contaminant)
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Delineating the Contaminant Source After characterizing the groundwater flow system, identifying the sources of contamination is the next critical step in creating a conceptual model. As described in Chapter 3, the source is the subsurface volume
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Sometimes the source cannot be located, even when the plume it creates has been measured. These characteristics of the source create inherent uncertainty in delineating it, just as spatial and temporal variability in aquifer properties create uncertainty in modeling groundwater flow.
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Nonetheless, more work is necessary to refine and/or demonstrate methods for characterizing contaminant sources in order to predict the success and long-term performance of natural attenuation. Delineating the Plume As part of developing a conceptual model, the contours of the plume of contamination emanating from source areas must be delineated.
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From page 163... ...
Chapter 3 described destruction and immobilization reactions that can cause loss of a contaminant in a natural attenuation setting. The goal at this stage of evaluation is to develop a conceptual model of the reactions based on observations that are connected directly to possible destruction or immobilization mechanisms.
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· All of the reactions produce inorganic carbon, indicated by CO2 in the equations. Therefore, increases in CO2 concentration are a footprint for these reactions, although the amount is lower for methanogenesis.
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This example illustrates that sites with higher concentrations of petroleum hydrocarbons (e.g., greater than 3 mg/liter) and residual NAPL sources that will persist for many decades cannot be evaluated adequately with a simple, short-term criterion that does not account for the long-term sustainability of electron acceptors.
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From page 166... ...
In any case, the presence of electron donors is the foremost screening criterion used to determine the potential for reductive dechlorination of chlorinated solvents. An example of a reductive dechlorination reaction is given here for TCE (C2C13H)
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When petroleum hydrocarbons provide the electron donors required for chlorinated solvent degradation, natural attenuation of the solvents is tied strongly to the presence and longevity of the petroleum hydrocarbon
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Figures 4-3, 4-4, and 4-5 illustrate this broader view of the contaminant source. For petroleum hydrocarbons to catalyze reductive dechlorination, the petroleum hydrocarbon and chlorinated solvent plumes must occupy the same vertical interval of the aquifer.
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(B) Source zones partly interact since the strongly reduced zone created by petroleum hydrocarbons overlaps a portion of the solvent plume and supports natural attenuation of that portion.
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The three-dimensional locations of the two sources and their plumes must be delineated well enough to evaluate whether or not the solvents and petroleum hydrocarbons overlap in space and time. Complete and sustainable natural attenuation of a chlorinated solvent plume due to a plume of petroleum hydrocarbons should be considered the exception, rather than the rule.
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From page 171... ...
The strategy for evaluating natural attenuation of any contaminant is the same as that for evaluating petroleum hydrocarbons and chlorinated solvents: several footprints of the postulated reaction must be documented. As an example, immobilization of heavy metals, such as cadmium, can occur by precipitation of sulfide solids: Cd2+ + S2- ~ CdS To create the conditions for the precipitation of CdS(S)
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From page 172... ...
Implicit in the hierarchy of Table 4-3 is that whenever a detailed analysis is appropriate, the simpler analyses also are applied to the site. For example, if simple solute transport modeling is needed for a site, then graphical and statistical analyses and mass budgeting are performed to prepare for the transport modeling.
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From page 173... ...
MTBE) Simple flow, Graphical Graphical Mass Simple Mass uniform and and budgeting solute budgeting geochemistry, statistical statistical transport with simple and low analyses analyses model solute concentrations transport model Simple flow, Mass Simple Mass Simple Compre small-scale budgeting solute budgeting solute hensive physical or transport with simple transport flow and chemical model solute model solute heterogeneity, transport transport and model models medium-high concentrations Strongly Mass Mass Compre- Compre- Compre transient flow, budgeting budgeting hensive hensive hensive large-scale or simple or simple flow and flow and flow and physical or solute solute solute solute solute chemical transport transport transport transport transport heterogeneity, model model models models models or high concentrations NOTES: In the site descriptions given along the left-hand side, the recommended data analysis strategy applies when all of the conditions are satisfied unless the term "or" is used.
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Finally, the measurements of the various geochemical parameters should be consistent with the conceptual model of possible reactions. A common example is inconsistency between dissolved oxygen concentration and redox potential.
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For example, a domain can be defined in such a way that it encompasses a plume underneath a NAPL. Then, the rates at which electron acceptors and inorganic carbon transport into and out of the domain can be estimated.
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for a contaminant, electron acceptors, inorganic carbon, alkalinity, or any other material of interest, their stoichiometric ratios can be computed and compared to what ought to occur if natural attenuation is acting.
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Box 4-5 provides an idealized example of how a mass budget can provide evidence that biodegradation is causing the loss of contaminants. (The example illustrates the principles of mass budgeting but does not account for how other geochemical reactions might affect the calculations.)
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Box 4-5 shows the result of this computation: a depletion rate of 2,550 g of COHN per year. Long-term tracking of the source depletion rate
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(The example of Box 4-6 considers methanogenesis and concludes that it accounts for a small percentage of BTEX degradation.) Mass budgeting is a powerful tool for determining the relative importance of different processes and establishing their approximate rates.
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APPROACHES FOR EVALUATING NATURAL ATTENUATION 185 Solute Transport Models The highest level of analysis for natural attenuation site data uses mathematical equations to represent the full suite of processes that can affect the fate of contaminants and other important solutes in the groundwater. Table 4-3 identifies two levels of solute transport models: simple and comprehensive.
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From page 186... ...
Then, human intuition often is unable to make the connections among the processes and prioritize their importance based on geographical and statistical analyses or mass budgeting alone. Comprehensive models are powerful tools for developing and evaluating conceptual models, understanding why natural attenuation should (or should not)
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88 NATURAL ATTENUATION FOR GROUNDWATER REMEDIATION attenuation is incapable of controlling the contaminants (case By, in which case natural attenuation should be eliminated from consideration and other remediation measures pursued; and 3. the range of predicted fates is wide and does not lead to any clearcut decision about the viability of natural attenuation (case C)
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APPROACHES FOR EVALUATING NATURAL ATTENUATION Maximum Acceptable A Concentration C' IL C' Cat IL C' Cow IL 1 I ~ Concentration B 1 Concentration C Concentration 189 FIGURE 4-6 Various distributions of predicted concentration at a point in space and time resulting from analysis of a range of conceptual models lead to very different decisions about natural attenuation: (A) accept, (B)
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Simple solute transport models can be divided into two categories, according to the format of the solution. An analytical solution is comprised of one or a small number of mathematical equations.
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2. The natural attenuation process must be internally consistent with a site-specific conceptual model.
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Comprehensive Solute Transport MocleIs Comprehensive, computer-based models (known technically as "numerical models") can account for variations in hydrogeologic proper
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Situations for which comprehensive solute transport models are particularly useful include those in which · the reactive materials exist in different chemical forms, such as in a range of acid-base species or complexes; · the products of key reactions participate in other reactions (e.g., precipitation or complexation) that affect aqueous-phase concentrations; · the contaminant materials or products partition to other phases;
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From page 194... ...
However, the quantity of oil in the aquifer is very large, and the oil will continue to be a source of contaminants to the groundwater in the future. This case study provides a particularly good example of how a comprehensive solute transport model can be very useful in predicting the evolution of geochemical conditions and contaminant concentrations for a long-lasting source.
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From page 195... ...
Further, the modeler should view poor correspondence between model predictions and field measurements as an opportunity to improve understanding, not as a failure of the modeling or the modeler. Modeling is an iterative process, and the poor predictions of an early model open the door for improving the conceptual model, as well as the solute transport code.
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98 NATURAL ATTENUATION FOR GROUNDWATER REMEDIATION TABLE 4-4 Common Problems With Models Type of Problem Examples Solution Model Framework: Poor Assumptions or Input Applying an inappropriate model or concept to the problem Relying on parameter values taken from publications unrelated to the site Failing to meet conditions assumed in the model Weighting observations inappropriately in the calibration Using a first-order rate law for biodegradation; simulating reactions that do not occur at the site or assuming a reactant is present in excess Sorption coefficients, biodegradation coefficients, hydraulic conductivities Assuming that climatic conditions and anthropogenic effects will remain the same Errors associated with inaccuracy and imprecision of the measuring device and process or human error Model Application: Closed Mind During the Modeling Process Failing to consider alternate conceptual models Forcing the model to predict the expected outcome Model Use and Presentation Extrapolating beyond the model's capability Overstating accuracy or reliability Check that site geochemical data support the model formulations Use site-specific measurements to obtain reasonable values of parameters Evaluate the uncertainty associated with this assumption and its effect on the results Weight the observations using the inverse of the variance of the measurements that established the value of the observation Filling gaps in hydraulic conductivity measurements according to a single conceptual model Changing the input parameter values to match the data Using a flow model calibrated to steady-state conditions to predict transient flow fields Reporting only a single value for the prediction of interest, with numerous significant figures Use multiple realizations of conceptual models of a site; combine all available data types to reduce uncertainty Evaluate whether processes that control the fate of the plume may have been overlooked; constrain parameter values to reasonable ranges Collect new data for calibration of storage coefficient or other uncalibrated features Provide a range of possible outcomes, reflecting the range of uncertainty associated with input parameters
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The second category includes problems that arise when the model is applied to evaluate the field data. A common mistake is to accept prematurely only one conceptual model.
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For example, hydraulic conductivity or biodegradation rate coefficients cannot be orders of magnitude larger than normal and acceptable values. Again, unrealistic parameter values usually signal that the conceptual model is flawed.
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From page 201... ...
For example, the long-term supply rates of electron acceptors in the upgradient groundwater or from the soil gas often are predictable and reasonably steady. On the other hand, supply rates of electron donors for reductive reactions normally depend on the long-term existence of a hydrocarbon NAPL or a landfill.
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In summary, estimating the sustainability of natural attenuation requires identifying active attenuation mechanisms, distinguishing nonrenewable mechanisms from renewable mechanisms, and comparing release rates of contaminants to the potential rates of transformation and immobilization. Mass budgeting is an important tool for assessing sustainability.
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If the results from the conceptual model and data analysis lead to the decision that natural attenuation is protective, then long-term monitoring must provide assurance that the site's protective processes continue to operate over time. Monitoring within the plume
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develop a conceptual model of the site's hydrogeology and biogeochemical reactions; 2. analyze site measurements to quantify the attenuation processes (looking for changes in contaminants and their footprints)
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A conceptual model should show how the heterogeneities affect plume migration, and vertical and horizontal plots of concentration data should demonstrate that plume migration is consistent with the conceptual model. If the likelihood of success is not high or the sustainability is uncertain (for example, due to high concentrations from the source)
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From page 206... ...
In some cases, simple mass transport modeling may be needed to interpret whether or not concentrations are decreasing over distance and time. · A level-3 effort is needed when the site is highly heterogeneous, flow is strongly transient, the likelihood of success is moderate to low (according to Table 3-5)
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From page 207... ...
· Responsible parties should gather field data in order to evaluate the validity of the conceptual model and quantify the natural attenuation processes. At the beginning of the site investigation, multiple conceptual models will have to be created.
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From page 208... ...
At the next level, mass budgeting is a powerful tool for demonstrating whether or not the footprints of the reactions are commensurate with observed contaminant losses; it is valuable for handling sites with moderate levels of uncertainty. Finally, a solute transport model may be needed when uncertainty is high due to site complexity or poorly understood reactions; these models range in complexity from analytical models to comprehensive numerical models that account for variations in aquifer properties, groundwater flow rates, and contaminant reactions (see NRC, 1990, for details)
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1996. Conceptual models for chlorinated solvent plumes and their relevance to intrinsic remediation.
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1996b. Death Valley regional groundwater flow model calibration using optimal parameter estimation methods and geoscientific information systems.
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1993. Analysis of uncertainty in optimal groundwater transient, three-dimensional groundwater flow model using nonlinear regression.
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