Groundwater Fluxes Across Interfaces (2004)

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Introduction Atmospheric, surface and subsurface portions ofthe hydrologic system (Figure IN are three dynam~- cally linked water reservoirs having distinctly different time and space scales. The challenge is in understand- ing and measuring the dynamic interchange among these reservoirs, especially for interchanges with the sub- surface. Most subsurface storage of water is in the groundwater reservoir, with a small amount of storage as soil moisture in the overlying unsaturated zone (see Basic Concepts Related to Groundwater Recharge/Dis- charge). While soil moisture is directly connected to the atmosphere and is an important storage reservoir for the energy associated with water (latent heat), simply because of its ease of exchange with the atmosphere, groundwater is the far more important storage reservoir for the water mass itself. Fluxes to and from groundwater are called, respectively, recharge and discharge. Natural groundwater recharge has several origins. The most important of these are the flux of water across the water table from pre- cipitation that percolates through the unsaturated zone, and the influx of water from a bounding or overlying surface water bodies including rivers, lakes, wetlands and the ocean. Natural groundwater discharge is the efflux of water Tom the groundwater reservoir to surface water, or to the land surface itself where, for exam- pie, it may return to the atmosphere through evaporation and transpiration. Natural recharge and discharge zones develop in response to the regional climatic and local topographic, hydrogeologic and biospheric condi- tions. Important anthropogenic sources of recharge and discharge include agricultural Ligation and drainage, respectively, infiltration basins and recharge/injection and pumping (e.g., water supply) wells. The various recharge and discharge fluxes can be measured or estimated at a wide range of temporal and spatial scales. Uncertainties in the measurement methods and the related understanding in the process, and disparities in measurement and modeling scales, prevent adequate closure of the transient water balance at the spatial and temporal scales of interest to scientists, engineers, and decision makers. For example, in situ point measurements are representative of the point value of a flux but are inadequate for mapping larger regions of interest to planners, while regional chemical tracers and modeling techniques can provide estimates representa- tive of larger areas but these are difficult to relate to local conditions at water supply and aquifer remediation sites. The challenge to closing the groundwater balance, and to estimating groundwater fluxes to or fiom other water reservoirs, is to integrate flux measurement and estimation techniques at multiple scales, with multiple types of data. These estimates would make effective use of models to map recharge and discharge fluxes over a wide range of spatial and temporal scales. Estimates of recharge/discharge fluxes are needed at many different spatial scales in water budget studies of natural hydrologic systems, and in studies of systems impacted by agriculture, water supply, or con- tamination. Estimation and understanding of recharge and discharge rates are of importance when evaluating 5

6 Groundwater Fluxes Across Interfaces 1 1 Atmosphere ~ ~ l O ~ a Saturated ~ Zone Unsaturated .~ . zone I Surface water lo Ci a n $ FIGURE 1-1 Global water reservoirs. Arrows show the interchange between reservoirs. The arrows connecting the satu- rated zone to the other reservoirs represent groundwater recharge and discharge and are the subject of this report. SOURCE: Guido Salvucci, Boston University, written communication, May 2002. the effects of plans for sustainable use and management of groundwater resources since these rates, and how they respond to change dynamically affect surface water levels and ecosystems. At a regional scale, the amounts of total annual average groundwater recharge and discharge are important components of the water budget used for in decisions about basin management. At a smaller spatial scale information on groundwater fluxes is used in addressing local water supply issues and groundwater contamination problems. Groundwater discharge is a major component of surface water generation in headwater streams, and is responsible for stream baseflow. It plays a role in mass wasting (landslides) and in fluv~al geomorphology (e.g., groundwater sapping). Groundwater fluxes affect the chemistry, biology, and ecology of the subsurface, in both the satu- rated and unsaturated zones, and in bounding surface waters. Basin scale biogeochem~cal cycles are influ- enced by recharge/discharge patterns and the physiology of vegetation is strongly linked to recharge/discharge zones. Discharge of groundwater to surface water bodies, including wetlands, carries nutrients important to biological communities. Recent work at the groundwater-stream interface ~yporheic zone) and at the groundwater/lake and wetland interfaces (hypolentic zone) has demonstrated that most of the chemical trans- formation during flow exchanged between surface water and groundwater occurs within a few inches of the interface between reservoirs. Improved measurements and estimates of groundwater fluxes and associated biogeochem~cal processes within these interfaces are needed. Of particular interest are processes involving pathogens and nutrients such as nitrogen. Thus, process understanding and accurate estimates of groundwater recharge and discharge play a role in groundwater sustainability, geomorphology, biogeochemis~y and the health of ecosystems. Despite the importance to water and chemical cycles, to ecosystems, and to water re- ources management, there are no universally applicable methods or established networks to measure recharge and discharge rates, and only limited understanding of recharge and discharge processes. Limited understanding and difficulties with measurements arise in part because of the distnbuted na- ture and spatially large extent of most groundwater recharge and discharge areas, as well as the many different

Introduction 7 environments where these fluxes occur, ranging from the ocean floor to mountain headwater streams. While there are instruments designed to make direct point measurements (see Table 1-1) of infiltration or recharge (e.g., infiItrometers, lysimeters) and discharge (e.g., seepage meters), in most cases these measurements meth- ods have uncharacterized uncertainties, and it is not clear whether or how the measurements might be extrapo- lated to larger areas. Indirect methods of estimating fluxes (e.g., using head measurements and Darcy's law; analysis of water-level fluctuations, ground-based grav~metry, temperature profiles, electromagnetic methods, and isotopes and solutes dissolved in groundwater) also suffer from the similar limitations and Dom non- uniqueness (Table 1-2), as do integrated estimates of flux (e.g., baseflow estimates used to approximate groundwater discharge to rivers, water balance methods and model calibration to estimate recharge). In summary, fluxes at groundwater interfaces interest hydrologists as well as researchers in related sci- ences Figure I-2~. While both the theory and measurement of groundwater fluxes at interfaces has received some attention, there are still significant gaps in process understanding, and many difficulties in obtaining ac- curate estimates of the spatial and temporal distribution of these fluxes, including fluxes to and from rivers and streams, reservoirs and lakes, wetlands, and Me ocean. Similarly, there are no standard procedures for measur- ing recharge to the groundwater system Tom precipitation or other sources. Need for new work on these issues is widely recognized. Internationally, the World Water Council's World Water Vision initiative has singled out closer investigation of processes at hydrologic interfaces, including un- derstanding surface-subsurface water interactions, as important to achieving its goal of developing a vision for water management in 2025 JIartmann et al., 2000~. The Hydrogeology Program Planning Group of the Tnte- grated Ocean Drilling Program (IODP) is focusing attention on submarine fluxes occurring in the deep ocean basins as well as in coastal zones (Ge et al., 2003~. The National Science Foundations LEXEN Wife in Ex- treme Environments) Program is sponsoring research focused on determining whether subsurface fluxes at the sea floor promote the growth of indigenous microbial communities (Johnson et al., 2003~. Nationally, NRC's Water Science and Technology Board recently identified mapping of groundwater recharge and discharge vulnerability as a priority area for research in environmental science (NRC, 2000~. The Water Cycle initiative of the U.S. Global Climate Research Program (USGCRP) involves multiple federal agencies in coordinated research on the water cycle. The USGCRP Water Cycle Study Group (2001) has iden- tified "quantifying fluxes between key hydrologic reservoirs" as one of three goals under their Science Ques- tion 2: "To what extent are variations In the global and regional water cycle predictable?" The newly formed Consortium of Universities for the Advancement of Hydrologic Science, Inc. (CUAHS10 has proposed that the greatest challenges and most fertile opportunities to advance the science are found at primary hydrologic inter- faces (www.cuahsi.org). Of the four interfaces they propose, one involves recharge (land surface - groundwa- ter), and other recharge and discharge (surface water - groundwater). Finally, most of the federal agencies sponsoring the Committee on Hydrologic Science (COHS; see Preface) have identified groundwater fluxes at interfaces as an important priority area for research (Appendix C). Interest by agencies is widespread. The U.S. Army Corps of Engineers and the South Florida Water Management District are proposing artificial re- charge by means of injection wells (so-called aquifer storage and recovery, ASR) engineering groundwater recharge and its recovery to help play an important role in the restoration of the Florida Everglades (NRC, 2002c), while the Department of Energy is concerned with the role of natural recharge in the performance of the proposed nuclear waste storage facility at Yucca Mountain, Nevada (Flint et al., 2002; Box 4-2~. Brief summaries of agency interest in the topics of the workshop are in Appendix C.

8 TABLE 1-1 Methods for Estimating Recharge and Discharge Grourldwater Fluxes Across Interfaces Hydrologic zone where actual measurement is made Surface water Method Arid and semi-arid climates Humid climates Channel water budget Base flow discharge Seepage meters Heat tracers Isotopic tracers Solute mass balance Watershed Modeling Unsaturated zone (measurable discharge would mainly be upward exfiltration to vegetation) Groundwater * Lysimeters In situ sensors (Neutron probes, TDR etc.) Zero-flux plane Darcy's Law Tracers "historical 36C1, 3H, 2H, i8O), environmental (C1)] Numerical Modeling Thermal analysis Surface geophysics (DC, EM, radar) Cross-hole geophysics (DC, EM, ra- dar) Gravity geophysics Elastic compression measurements (e.g., GIS, InSAR) Tracers "historical (CFCs, 3H/3He), environmental (C1, i4C)] Numerical modeling Channel water budget** Base flow discharge** Seepage meters Heat tracers Isotopic tracers Solute mass balance Watershed Modeling * Lysimeters situ sensors (Neutron probes, TDR, etc.) * Zero-flux plane Darcy's Law Tracers (applied) Numerical Modeling Surface geophysics (DC, EM, radar) Cross-hole geophysics (DC, EM, ra- dar) Water-table fluctuations (observations wells, geophysics) Darcy's law Tracers [historical (CFCs, 3H/3He)] Numerical modeling Most methods can be used for estimating either recharge and discharge, although a given method may be better for one than the other. Methods appropriate only for recharge and discharge estimation are indicated by astensks. SOURCE: Repnnted, win pemussion7 Tom Scanlon et al. (2002). ~ 2002 by Spnnger-Verlag Heidelberg. STATEMENT OF TASK Fluxes to and from groundwater systems are critical to most aspects of hvdrolo~c science. and there- ~ ~ , is 7 tore to its related sister sciences (Figure 1-2), but these fluxes are ~ad~itionally neglected or estimated by using simple and unverified assumptions (e.g., by assuming that recharge is equal to some Faction of precipitation). The purpose of this report is to call attention to the importance of groundwater fluxes, to explore the potential of new technologies to measure or estimate these fluxes and to identify research bans and the potential for interdisciplinary collaboration (Figure I-2~. ~ . . . . .. ,~ ~ . --a ----I--- c,—r~ ~~ rid Ants report Is an outgrowth oi earlier recommendations of the Committee on Hydrologic Sciences (COHS), outlined in "Hydrologic Science Priorities for the U.S. Global Change Research Program" (NRC, 1999~. Tt is intended to help develop the framework for assessing strategic directions in the hydrologic sci-

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10 Soil (` Groundwater science I, ) recharge W~;~ hydrology ~ Limnology ~ ~ / ~ An\ Groundwater discharge Geochemistry ~ ~ ecology J and geophysics ~ / - FIGURE 1-2 Disciplines involved in eshmation of groundwater recharge and discharge. Groundwater Fluxes Across Interfaces Oceanography> ences. The report reflects deliberations at a COHS sponsored workshop held in May 2002 (preface; Appendi- ces A, B). The discussions at the workshop focused on naturally occulting Groundwater recharge and dis- charge. However, many of the concepts related to natural groundwater fluxes also apply to anthropogenic fluxes such as those associated with agriculture, municipal water supply, and aquifer contamination. The re- port is also not meant to be a comprehensive analysis of all issues related to Groundwater fluxes but instead focuses on the following questions, drawn from a broader set of issues (see preface) that workshop participants were asked to consider: I. Diffuse vs. focused recharge and discharge . What is the relative importance of diffuse versus focused recharge/discharge in various hydro- geologic settings? Oceans? mate? · Is Mesh Groundwater discharge a significant source of fresh water recycling to estuaries and the 2. Interactions of Groundwater with Climate Do Groundwater recharge and discharge processes provide feedback mechanisms that affect cTi- . What are the important time scales for Groundwater reservoirs affecting continental and global water balances, and how are they controlled by fluxes and storage? · What is the magnitude of the effect of fluctuations of sea level and levels of large lakes (e.g., the Great Lakes) on Groundwater recharge/discharge?

Introduction 1 1 3. Spadal and temporal scales of recharge and discharge · How do estimates of groundwater recharge/discharge aggregate when averaged over different scales and what implications does this have for measurement scale? · How accurately can recharge/discharge patterns/rates be estimated at a regional or national scale, and how might uncertainty in these patterns/rates vain with spatial and temporal scale and geographic loca- tion? BASIC CONCEPTS RELATED TO GROUNDWATER RECHARGE/DISCHARGE The zone of subsurface water can be divided into the unsaturated zone above the water table and the saturated zone below the water table. The unsaturated zone includes the zone of soil moisture, which extends ~ .. ~ ~ ~ ~ . ~ ~

12 Groundwater Fluxes Across Interfaces mation about the water budget. Development of conceptual models is fundamental to the science of hydro- geology (e.g., Davis and DeWeist, 1966~. Several major efforts have been made to organize He U.S. into re- gions of similar hydrogeologic charactenstics. Recent attempts include the Regional Aquifer-System Analysis (RASA) Program of the U.S. Geological Survey (Sun and Johnston, 1994) and a volume of papers on the hydrogeology of North America 03ack et al., 1988~. Recently, Winter (2001) introduced the concept of the Fundamental Hydrologic Landscape Unit (TUTU) to help guide the formulation of conceptual models. The FHLU is based on the concept that a single generic flow cell (Figure 1-3) forms a template for conceptualizing hydrologic landscapes (see Chapter 2~. Conceptual models built upon the concept of the FHEU can be useful in identifying pasterns of recharge and discharge in any given generic hydrogeologic setting, allowing hypothe- ses to be developed and tested independently of scale. The factors that influence the development of a concep- tual model based on the FHEU concept include land surface form, geologic framework and climatic setting, especially the net of annual precipitation minus evapotranspiration. The FHLU framework is used in this re- port to guide discussion. OVERVIEW OF METHODS FOR ESTIMATING RECHARGE AND DISCHARGE Scanlon et al. (2002) recently reviewed and summarized methods for estimating recharge (Table I-~) and noted that these techniques are dependent on both spatial (Figure 14) and temporal Figure 1-5) scales. They concluded that: "Recharge estimation is an iterative process that includes refinement of estimates as ad- ditional data are gathered. A wide variety of approaches should be applied in estimating recharge in order to reduce uncertainties and increase confidence in recharge estimates." Other review papers on recharge estima- tion are included in Scanlon and Cook (2002~. Similar statements about methods and space/time scales can be made about the estimation of discharge, which uses many of the same methods for estimation (Table 1-~. Whether for recharge or discharge, the methods of estimation draw upon expertise resident in a variety of dis- ciplines (Figure 1-2~. Estimates of recharge are dependent on the scale of measurement. Lysimeters, neutron probes, time- domain reflectometry (TDR) probes and (other) in situ sensors, and cross-borehole radar (Binley et al., 2001) oilier 'point" or "small-scale" data that can be used to estimate net infiltration. Translating estimates of infiltra- tion into recharge is not straightforward, however, and the results after this transformation are not necessarily 'point" estimates. Tracer studies, subsurface and in-stream thermal analyses, cross-borehole and surface DC, electromagnetic geophysics (e.g., Cook et al., 1992), gravity geophysics, and elastic compression measure- ments might be thought of as giving "mesoscale" estimates of recharge. The water-table fluctuation method (Healy and Cook 2002), mass-balance calculations, measurements of thermocline alteration (\eat tracer stud- ies), and isotopic tracer analyses might be considered "basin-scale" or "macro-scale" measurements. Infer- ences based on inverse groundwater modeling in conjunction with measurements of the elevation of the poten- tiometric surface (Sanford, 2002) are related to the scale of the model and may range from mesoscale to basin- scale. Discharge estimates, sometimes using the same methods, are also dependent on measurement scale. The baseflow discharge method (Table 1-1) measures basin-scale groundwater discharge to streams, which is then used as a proxy for recharge estimates under the assumption that, under steady state conditions on the wa- tershed scale, recharge equals discharge. Darcy's law is used to estimate point discharge rates by measunog hydraulic gradients in piezometers installed in a discharge area. Mini-piezometers are installed in river and lake beds to measure near shore gradients. According to Darcy's law, hydraulic gradient multiplied by hydrau- lic conductivity equals flux. The channel-water budget method can also be used to measure gains or losses of water from a stream by calculating the net gain or loss of water between two streamflow gaging stations, with the scale depending on the distance between the gages.

Introduction PRECIPITATION EVAPOTRANSPIRATION Direction of surface-water flow Surface-water LOWLAND UPLAND VALLEY Sly, ~W2'2 22.~,1 · e ~ ~ ~ ~ ! ~ ~e ~ ~ # ~ e~ ~ ~~e - ~ ~ - · Direction of ground-water flow ;.;. .~.. :: . ~ : ·. .. ; ~ ·· ·e · --;- ~~ e ·; · ··~~e·~·; ~ e~.~e~.~ · A e · ~e L' 5.'.2 ;': . ' .: ..~ . . '.:; . ~ : ~ -, ~ 13 FIGURE 1-3 The Fundamental Hydrologic Landscape Unit (FHLU). SOURCE: Reprinted, with permission, from Win- ter (2001~. ~ 2001 by American Water Resources Association. . - i: ~ ~ ~ EnYim~mer,.~l Hi ~ ~ HI ~m ~ ~ >~ HIGHS, (;FOs u, *a Water-ski - lion al En~nronr~ental (;1 T4is~rita1 311,"~' Splint Charm Law ins - lure p14~e Cy~imetea~ Heart trains _ tOw d'=ha~e imps me18= ~~ 1 i ~ 11300 ~ ~ ~ ~ —~ 1 ~ . . 4:: :0 -:D Do FIGURE 1~ Spatial scales for recharge estimation. SOURCE: Reprinted, with permission, from Scanlon et al. (2002~. ~ 2002 by Springer-Verlag Heidelberg.

14 Groundwater Fluxes Across Interfaces ·a: io .m :O LD .= ~ ~~.mnronn,0~l ~ ARC. Histoncal _ 3~3He~ CPC:s I:|| fluC=~;if~n E:~onmental C1 Hi:;-~oncal 31i,~1 _ ,~ppli~ It ~ a,'`. itasro-fi:.iu~; I. L',$im~. |8 -~ ~ 'I Pleat Or. m ~ Bas - ~w disel1argO vat ~ ~~ ~;3~ melees Hi, 1 1 Ad_ ~~ Or *~-¢ ~~;~~~>~ ~~_~ , _ 1040~ ~ ................... ... ....... ~ ! .. .... . ~ :::: .:::.:.:::: ::::: ::::,::::.:: :: ~ I 1 1 i Z ,.,, t - ~ ''''''''''' A- ~ arm_ By. ~~=v A- _—. - .- - —T. m. ~~.~ ~- ~5 ,, 1 i. 1 ~ ~ I ~ I, a, ~ I 1 O'O' 't jeep "~0j=.D - ~1- FIGURE 1-5 Temporal scales for recharge estimation. SOURCE: Repnuted' wit permission, from Scanlon et al. (2002~. 2002 by Springer-Verlag Heidelberg. Seepage meters, theoretically, can be used to make direct point measurements of either recharge or discharge of water between a surface water body and the groundwater system, but have been used more suc- cessfully to estimate discharge. Seepage meters for discharge estimation are designed to collect groundwater flux. The original design (Lee, 1977) was intended for use in lakes and estuaries and required manual collec- tion of the water sample from which an estimation of flux was calculated based on the elapsed time. Newer designs (e.g., Taniguchi and Fukuo, 1993; Paulsen et al., 2001) allow for automated measurements. Meso- scaTe discharge can be estimated with the help of heat tracers by relying on the difference in temperature be- tween groundwater and surface water (e.g., Lapham, 1989; Constantz et al., 1994; Hunt et al., 1996~. Solute mass balance techniques have been successfully used to estimate mesoscaTe or macroscaTe dis- charge to lakes and estuaries (e.g., Krabbenhoft and Webster. 1995; Burnett et al., 2002~. Watershed models and numerical groundwater flow models are also helpful in estimating mesoscaTe or macroscaTe discharge. Estimates of recharge and discharge are plagued by errors and uncertainties, especially biases. The errors depend on the scale, quantity, quality and type of data. Errors are often very large and are seldom quan- tified because available data are seldom adequate to give reliable assessments of the uncertainty in re- charge/discharge estimates, and many of the measurement tools are not amenable to calibration. Furthermore, traditional hydrogeologic evaluations are deterministic, and lack assessments of error and uncertainty. Estimates of recharge and discharge also suffer from non-uniqueness. For example, the base flow es- timation method assumes steady flow, so that recharge balances discharge. But estimates of both recharge and discharge will be inaccurate if the flow is sufficiently transient, and the effects of storage changes are inappro- priately assigned to one or the other, or missed entirely. Some of the methods (e.g., gravity, EM, and radar geophysical methods, elastic compression, water-table fluctuation, mass-balance calculations, inverse model- ing3 actually measure water in storage and use the water budget to estimate recharge or discharge indirectly. Moreover, these methods are sensitive to subsurface geological heterogeneity. The observed storage change may be due to reasons other than those assumed, making the recharge or discharge estimate non-unique, and dependent on assumptions. Many methods for recharge assume vertical infiltration, whereas lateral flow may likely be significant. Others suffer Dom poor space-time resolution of storm events. Flux estimates calculated using Darcy's law require an estimate of hydraulic conductivity, which can be difficult to measure accurately.

Introduction 15 Hence, closing the water budget requires a reduction of uncertainty and non-uniqueness In the measurement of the relevant fluxes. Flint et al. (2002) evaluated venous methods to estimate net infiltration and recharge at Yucca Moun- tain, Nevada (Box 4-2~. A summary of the methods, including general approach, scale of application, and strengths and limitations, is presented in Table I-2 and illustrates some of the issues in estimating groundwater fluxes. The methods in the table produce estimates of flux that reflect different spatial and temporal scales, they have different data requirements, strengths and limitations; and have varying sensitivity to water flux in Eactures, a major pathway through the unsaturated zone at Yucca Mountain. At this site, recharge varies spa- tially owing to variations in precipitation, surface microclimates, thickness of alluvial deposits, faults and frac- tures, and thickness and hydrologic properties of geologic strata in the unsaturated zone. Recharge also varies temporally due to weather and the climate vanability. SCOPE The following three chapters examine processes and measurements of fluxes at groundwater inter- faces, through the perspective provided by the three major themes of the workshop: diffuse vs. focused fluxes, interactions with climate, and spatial and temporal scales. Challenges In the understanding and estimation of fluxes are outlined within this context, and suggestions are made for future research. The themes are inter- twined. For example, highly variable and focused recharge may limit our ability to upscale or downscaTe measurements, and changes In climate may change a temperate zone with diffuse recharge to an arid zone with focused recharge. In Chapter 2, we also discuss the benefits that would be derived Tom the establishment of a network of experimental benchmark sites for recharge and discharge measurements. These sites would be integrated wherever possible with existing and planned experimental watersheds, such as those fi~nded by the National Science Foundation (e.g., the Long Term Ecological Research tETER] sites, or CUAHST's proposed Hydro- logic Observatories), or run by federal agencies (e.g., U.S. Department of Agnculture, the Forestry Service, and the U. S. Geological Survey), in order to leverage related hydrologic and ecological research.