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Approaches to Synthesis of Groundwater issues at the Regional Scale The many grounc~water issues summarized in Chapter ~ require an increased emphasis on regional-scare studies. Whereas past regional studies focused mostly on present and future groundwater availability, current issues involve complex interrelated processes that are not ade- quately characterized by traditional methods of inventorying water re- sources. How does one conduct a regional study in such a context? Many conceptual and instrumental advances are becoming available to support regional process studies; these are described in Chapter'4. Such acivances, however, will merely constitute answers in search of problems without a unified strategy that defines and advances the goals of regional investigations. What is meant by a regional study? Why is it that we need such stuffiest This chapter considers alternative approaches to regional studies of groundwater processes. It briefly reviews the groundwater programs and expertise of the U.S. Geological Survey (USGS) and summarizes tradi- tional groundwater resources studies. The customary description of groundwater as a resource is contrasted with a process-based approach to regional groundwater science and regional assessments. Finally, a new approach to regional studies is proposed, based on parallel and synthe- sized regional groundwater assessments (i.e., evaluation of the quantity and quality of available groundwater, recharge, extraction rates, etc.), and regional groundwater science (i.e., the study of critical processes of regional significance, systematically approached). 25
26 Investigating Groundwater Systems USGS GRO[JNDWATER PROGRAMS PAST AND PRESENT This section summarizes many of the groundwater programs of the USGS from the early nineteenth century onward, and how these pro- grams have evolved with the changing needs of society. The Changing Arena of Hydrogeology The public and scientific arena in which the USGS surface water and groundwater programs function is continually changing. The water re- sources programs of the Survey initially focused on the investigation of surface water. However, as the development and utilization of ground- water grew following World War T. groundwater investigations became an increasingly important component of its program. In fact, the devel- opment of hydrogeology as a scientific discipline is largely the result of work by USGS scientists. In particular, O. E. Meinzer, chief of the Sur- vey's former Division of Groundwater from 1912 to 1946, is generally regarded as the father of modem hydrogeology. USGS scientists have led many of the major scientific advances in the field over the last cen- tury (Table 2.1), and these advances have generally paralleled the social and economic needs of the day. In the early 1900s, bolstered by the growing demand for water and the technological capabilities developed to exploit petroleum resources, regional surveys for discovering, documenting, and developing ground- water became a core element of the Survey's water programs. The wide- spread availability of economical deep pumps and inexpensive energy made widespread exploitation of ground water resources feasible throughout the Great Plains states and CaTifornia's Central Valley. During the first half of the twentieth century, the public needed reli- able water supplies to serve a growing and westward-expanding popula- tion, with much of this water being consumed for irrigation. These early pressures on groundwater resources revealed many of the issues, in- cluding subsidence, groundwater depletion, groundwater contamination, and salt-water intrusion, that continue to define issue-based groundwater investigations conducted by the Survey today. However, most USGS work during this period focused on understanding regional geology and hydrogeology (e.g., Meinzer, 1923) and on an improved theoretical un-
Approaches to Synthesis of Groundwater Table 2.1. Advances In hydrogeology over the last century 27 Decade 1 880-1 920 Hydrogeologic advances -first use of term "hydrogeology' by USGS -understanding of artesian wells Chamberlin (1885) - early groundwater maps (King,1899) -advances in potential theory (e.g. Slichter, 1902) 1920-1940 -understanding of regional hydrogeology (Meinzer, 1 923) -pumping tests; analytical solutions for groundwa- ter flow (Theis, 1935; Hubbert, 1940) 1940-1960 -use of electrical resistance analog models -World War II -experiments with scaled physical model of ground- water flow -advances in well hydraulics 1960-1970 -mainframe computers available -first digital flow models (Pinder and Bredehoeft, 1 968) -regional geochemistry (Hem, 1959, Back and Hanshaw, 1965) -groundwater flow systems (Toth, 1962; Freeze and Witherspoon, 1967) 1970-1980 -geochemical speciation models (WATEQ) -first digital transport models (Bredehoeft and Pinder, 1973) -groundwater-surface water studies (Winter, 1976) -first practical transport codes (e.g. MOC; Konikow and Bredehoeft, 1978) Other significant events -World War I -Great Depression -first satellites program -Vietnam war -Moon landing 1980-1990 -widely-available digital codes (e.g. MODFLOW; McDonald and Harbaugh, 1988) -improved geochemical codes, e.g. PHREEQE -advances in contaminant hydrogeology -USGS RASA studies -practical particle tracking codes, e.g. PATH3D (Zheng, 1992, MODPATH (Pollock, 1989). -improved transport codes, e.g. SUTRA (doss, 1984); MT3D (Zheng, 1990) -use of natural tracers (tritium, 018) 1990-2000 -graphical interfaces for computer codes (e.g. Groundwater Vistas, GMS) -USGS NAWQA studies -increased use of analytic element methods (e.g. Strack, 1989) -inverse model codes (e.g. UCODE, PEST) -innovative groundwater tracers (CFC's) -advances in wetland hydrogeology -Earth Day; increased public concern for envi- ronment -USEPA established; Resource Conservation and Recovery Act (RCRA); Superfund (CERCLA) enacted -personal computers become practical and affordable -contamination events at Love Canal, NY and Wo- burn, MA -environmental consulting and groundwater reme- diation become major industries -significant interest in organic contaminants (VOC, NAPL) -internet and world-wide web available -wide use of internet for information transfer -popular book and movie A Civil Action describes Woburn events
28 Investigating Grounc~water Systems derstanding of wells and aquifers (e.g., Theis, 1935~. The regional groundwater investigations tended to mimic the Survey's mineral re- source investigations. The close structural association between the oc- currence of mineral deposits and groundwater, combined with the USGS expertise in mapping and geophysical characterization of resource avail- ability and abundance, provided a natural framework for groundwater investigations. The need for better predictions of basinwide groundwater flow in turn led to the development of mathematical models. The first practical groundwater models were electrical analogs based on networks of resis- tors and capacitors. As mainframe computers were developed, numeri- cal models came into use, providing the capacity to simulate flow in complex hydrogeologic settings. The USGS developed many of these programs, including MODFLOW (McDonald and Harbaugh, 1988), which may be the most widely used groundwater-flow modeling pro- gram in the world today. At the same time, the inorganic geochemistry of groundwater began to receive more attention. Most of the emphasis was placed on regional geochemistry and the suitability of subsurface water for municipal water use. Limited early investigations of contaminant transport emerged as issue-driven studies of specific incidents associated with industrial spills. Beginning in the late 1960s, there was increased public awareness of environmental degradation and an associated widespread sense of ur- gency to protect groundwater from contamination. The public also be- came aware of issues such as nuclear waste disposal and its related hy- drogeologic constraints. These pressures motivated the Survey to go beyond its earlier, relatively isolated district-level evaluations of con- taminant transport and to begin conducting fundamental research into the physical, chemical, and biological processes that affect the move- ment of dissolved constituents through the hydrologic system. In es- sence, a national demand was created for the technical expertise, data, and ability to synthesize local and regional issues to address national needs. One major outcome was the development in the 1960s, 1970s, and 1980s by USGS scientists of the first practical computer programs for modeling contaminant transport (e.g., Konikow and Bredehoeft, 1978) and geochemical reactions (e.g., WATEQ, Truesdell and Jones, 1974; PHREEQE, Parkburst et al., 1980~.
Approaches to Synthesis of Groundwater Regional Aquifer-System Analysis (RASA) Program 29 Regional studies earlier in the century described the occurrence of water-bearing fo~ations, their regional extent, and production potential. In the past 30 years, the USGS made more quantitative assessments of the extent, availability, and quality of groundwater resources. The most comprehensive of these was the Regional Aquifer-System Analysis (RASA) Program, which operated from 1978 to 1995. In this program, existing groundwater data were compiled and digital models to simulate regional groundwater flow and response to pumping were constructed for 25 of the most important aquifer systems in the United States. The program's objectives were to define the regional aspects of geology, hy- drology, and geochemistry of these systems (Sun and Johnston, 1994), within regions covering thousands to tens of thousands of square miles (Figure 2.11. The completed RASA studies, consisting of a series of re- ports and maps, constitute the best available synthesis of the regional hydrogeology of the nation. The program was eventually coupled to the National Ground-Water Atlas series begun in 1987 to provide easily ac- cessible regional groundwater resource data to the public. Sun et al. (1997) compiled a bibliography of RASA-related publications. Although these studies provided an excellent description of general information about the nation's major aquifers, they did not tie that in- formation to many of the issues of sustainability described in Chapter 1. Moreover, although the hydrostratigraphic, hydrogeologic, and geo- chemical information contained in these studies continues to be useful, demographic changes, population growth, new public attitudes and con- cerns, and corresponding changes in the regulatory climate drive a need for periodic (5- to 10-year) reassessments of conditions. National Water-Quality Assessment Program The National Water-QuaTity Assessment (NAWQA) Program, im- plemented in 1991 (after several years of planning) as the RASA Pro- gram was nearly completed, is ongoing. Its purpose was to assess water quality distributions and trends, relate these distributions and trends to controlling processes, and evaluate implications for water quality man- agement for a number of large-scale study units (thousands of square
30 ::D Investigating Groundwater Systems EXPLANATION Regional aquifer system study areas 1 Northem Great Plains 2 High Plains 3 Central Valley, California 4 Northem Midwest 5 Southwest alluvial basins 6 Floridan 7 Northem Atlantic Coastal Plain 8 Southeastern Coastal Plain 9 Snake River Plain 10 Central Midwest 11 Gulf Coastal Plain 12 Great Basin 13 Northeast glacial aquifers 14 Upper Colorado River Basin 15 Oahu, Hawaii 16 Caribbean Islands 17 Columbia Plateau 18 San Juan Basin 19 Michigan Basin 20 Edwards-Trinity 21 Midwestern basins and arches 22 Appalachian valleys and Piedmont 23 Puget-Willamette Lowland 24 Southem California alluvial basins 25 Northem Rocky Mountain Inte~ontane Basins FIGURE 2.1 Regions studied in the RASA Program. .~} :.: ....<s '/, ...'.: c: . ~ .~...~.... i.\ I. - .. .. `. . miles) around the nation (NRC, 1994). Typical NAWQA reports de- scribe the environmental settings of large regions, including their geol- ogy, soils, climate, land and water use, ecology, and hydrologic systems. The quality of both surface water and groundwater is inventoried for standard parameters, including dissolved solids, nutrients, trace ele- ments, radionuclides, pesticides, organic chemicals, and bacteria. A key
Approaches to Synthesis of Groundwater 31 component of the program has been the rigorous use of appropriate and consistent sampling and analytical techniques at all sites. These high standards have made NAWQA an especially valuable program for re- source assessment. The NAWQA Program differs significantly from RASA in its focus on water quality and in its inclusion of surface water and biological indi- cators in addition to groundwater. Because NAWQA is focused on indi- vidual study units, a national synthesis of the results is critical, and the National Research Council ~C) outlined a method to achieve such a synthesis (NRC, 1994~. In brief, that synthesis study includes aggrega- tion of data on regional and national levels and aggregation of geo- graphically widespread study units sampled with a consistent methodol- ogy; comparisons are then made for spatial and temporal trends and pat- terns. Groundwater Monitoring Networks Understanding the environmental effects of groundwater develop- ment on land and surface water resources, and evaluating the overall sustainability of pumping rates, requires long-term monitoring and as- sessments. The extended time periods are necessary to distinguish tran- sient responses of regional groundwater systems to groundwater with- drawals from other factors such as interannual- to decadal-scaTe vari- ability in climate and changing land use. For example, recent increases in groundwater levels in the northern High Plains aquifer appear to be attributable, in part, to an extended pe- riod of wet precipitation anomalies from 1980 to 1994 (Dugan and Sharpe, 1994~. Although the USGS has been monitoring water levels in the High Plains region for over 60 years, it was only in 1988 that the USGS instituted a foam monitoring program of 8,000 wells to assess annual changes in this important aquifer system (Dugan and Sharpe, 1994~. The USGS is the lead agency, since it can monitor or coordinate the monitoring of aquifer systems that underlie two or more states, but the monitoring is performed in cooperation with many other federal, state, and local agencies. A broad network of information is required, because baseline information even from areas that are not presently be- ing heavily exploited may be useful for groundwater resources planning and management.
32 Invesfigatir~g Groundwater Systems Other Current USGS Programs The Federal-State Cooperative Water (Coop) Program, in which state funds are matched by federal dollars (generally on a 50:50 basis) has provided incentive and support for numerous local groundwater re- sources projects. The cooperative nature of the program ensures stakeholder support on local issues, but the studies have limitations. They tend not to cross state boundaries, and they generally do not ad- dress the complex process-related issues that transcend these boundaries. Also, they usually cannot effectively marshal the innovative research appropriate to those process issues. The USGS Water Resources Division (WRD) also has been active in basic and applied hydrogeologic research through several other pro- grams. Such activities, generally involving the collection of new field data at long-te~ research sites, have been linked and funded through the WRD's National Research Program, Toxic Substances Hydrology Pro- gram, and Water, Energy, ant! Biogeochemical Budgets (WEBB) Pro- gram. The USGS also carries out projects through the Department of Defense (DOD) Environmental Conservation Hydrology program; this work is focused on individual DOD sites. Outside funding also comes from the Department of Energy (DOE), and the Death Valley Regional Ground-Water Flow System Project is one example of a large, regionally integrated study largely supported by DOE funds (see Box 2.1~. Ground-Water Resources Program Current USGS groundwater assessment is greatly reduced in mag- nitude from the peak of the RASA program (Figure 2.4~. However, a limited number of regional studies are being conducted through the Ground-Water Resources Program (GWRP). As described in Chapter 1, the GWRP was created to examine the sustainability of the nation's groundwater resources through scientific assessments, regional and na- tional overviews, and research and methods development. Its projects are regionally integrated groundwater assessments on a scale smaller than the RASA or NAWQA studies, but larger than most Coop studies. GWRP studies target critical basins or watersheds where important man- agement questions coincide with less-than-adequate technical informa- tion.
Approaches to Synthesis of Groundwater 33
34 Investigating Groundwater Systems
Approaches to Synthesis of Groundwater 35
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Approaches to Synthesis of Groundwater 37 FIGURE 2.3 Spring pool (center) in northern Death Valley. Increased pumping in the basin could eliminate such pools, which provide habitat for the endangered pupf~sh. View is looking north toward the Monte- zuma Range. (Photograph courtesy of Frank D'Agnese, U.S. Geological Survey). One GWRP project that illustrates current thinking regarding re- gional process-based studies is the Southwestern Groundwater Re- sources Project. Coordinated from the Arizona district, the project also has study sites in California, Nevada, Utah, and New Mexico, and USGS scientists from many districts and the National Research Program par- ticipate. The Southwest is a region of spectacular population growth in which groundwater is especially critical to the economy. In many areas, there is extensive groundwater overdraft, leading to declining ground- water levels, increased recovery costs, decreasing water quality with depth, migration of contaminated water into production areas, subsi- dence with permanent damage to aquifers, earth fissures, and degrada- tion of rip arian habitats. The water budgets associated with overdraft
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Approaches to Synthesis of Groundwater 39 are clifficult to estimate because of the high spatial and temporal vari- ability of recharge processes and because of climate variability. The key issues for the southwestern basins are not adequately ad- dressed by water inventories alone. These issues require research into the groundwater processes so that the complexity, variability, and dy- namic feedback that govern the systems can be quantified. For example, water use by riparian plants needs to be related to streamflow, which in turn needs to be related to both groundwater conditions and planning and management decisions affecting the sustainability of current and pro- jected resource use. Are the present riparian communities sustainable given anticipated changes in water use? What will be the impact of fu- ture changes on endangered species that inhabit the riparian communi- ties? New research approaches will need to be introduced to provide the information on the complex underlying processes that will be necessary to resolve such questions. --~---o r- The Middle Rio Grande Basin Study (Bartolino, 1997b) is another GWRP project. In this study, the USGS is cooperating with local gov- ernments and agencies, including the New Mexico Bureau of Mines and Mineral Resources and the city of Albuquerque, to characterize the hy- drogeology of the Albuquerque basin. The project brings together ex- perts in various aspects of hydrogeology and geoscience (geologic map- ping, environmental tracers, geophysics, cartography, groundwater mod- eling, and uncertainty analysis) to understand hydrogeologic problems in the basin. Other GWRP topics include the restoration of the Florida Ev- erglades, salt-water intrusion along the Atlantic coast, and development of a national groundwater database (USGS, 1998~. With the exception of the database, these GWRP projects are being viewed tentatively as prototypes and as a foundation for an expanded program of groundwater studies. An important question addressed in the remainder of this report is whether such efforts represent an appropriate model and scale for future WRD efforts and, if so, how such studies should be prioritized for support. NEW OPPORTUNITIES AND MANDATES How might regional needs for groundwater information be met, in view of existing capabilities and potential of the WRD? The breadth of expertise available in the National Mapping, Biological Resources,
40 Investigating Groundwater Systems Geologic, and Water Resources Divisions of the USGS, along with the Survey's historical role in data collection and management, uniquely positions the USGS to conduct and synthesize local, regional, and na- tional groundwater investigations. Director Charles Groat has pointed out that issues facing the USGS are increasingly complex and interdisci- plinary and wit! tap the capabilities of all four USGS divisions (Moinia, 1999~. The Survey's experience with aquifer-based studies (RASA) and place-based, issue-oriented studies (Coop) provides the foundation for the development of process-based scientific and regional assessments of complex problems involving groundwater use for broader purposes than immediate supply. An Emphasis on Sustainability New issues are driving a change in priorities in groundwater science and information needs. Whereas regional groundwater assessments tra- ditionally focused on water availability and extraction, new concerns emphasize a complex of issues related to sustainable uses and manage- ment. "Sustainability" may have somewhat different meanings, de- pending on the regional context. For example, a given area may have achieved a Tong-term, sustainable balance between annual groundwater withdrawals and natural and artificial recharge volumes. However, local areas of excessive withdrawal could well have deleterious effects on ri- parian habitat. The survival of certain ecosystems may well be con- trolled by decadal-scaTe Tong-term trends, rather than by mean annual conditions. The goal of providing sustainable conditions for future gen- erations may conflict with that of providing optimal supply to current users. Guaranteeing sustainability requires an improved understanding of hydrogeologic processes and the hydrologic budget, and it requires the means of projecting Tong-term change. Sustainability implies a basic change in focus from groundwater as an exploitable human resource (the "basin yield" view) to groundwater as a vital part of the complex interrelated processes governing ecosystem health and flow system stability. Such processes include groundwater- surface water interactions, aquifer depletion and associated water-level declines, subsidence and related sinkhole collapse in karstic terrains, and salt-water intrusion. Regions for study will be defined by the effective scale of the dominant process rather than by a physically arbitrary politi-
Approaches to Synthesis of Groundwater 41 cal boundary. Thus, subsidence issues will be associated with a par- ticular sedimentary basin or group of basins unified by the occurrence of subsidence. Salt-water intrusion will be associated with a broad zone of coastal aquifers, and riparian zone impacts will be studied in regions suffering from lowered water tables or near-surface contamination. In short, society has broadened its view of the value of groundwater to in- clude its future use as well as its present use for ecosystem functions. Protecting these values entails a better understanding of hydrogeologic processes in a wide variety of settings at regionally appropriate scales. Extrapolation to future conditions will require models that appropri- ately represent measured process rates and dynamics. The limitations of model extrapolations for complex environmental systems will need to be carefully assessed. Advanced measurement methods and advances in modeling need to be balanced with an integrated scientific approach to each study region. Studies will necessarily be interdisciplinary, involv- ing collaborative work with scientists from the other divisions of the USGS. Many of the studies will be of such scope and complexity that expertise from outside the USGS will be required. Collaboration may be facilitated by partnerships with other federal or state science agencies and with universities. This approach to regional investigations requires the integration of regional process-based science and requires regional groundwater assessments oriented to issues of sustainability of current and projected uses, as is discussed later in this chapter. Alternative Meanings of "Regional" Groundwater investigations are regional in either of two senses of the word. First, a region may be a hydrogeologically and geographically distinct area encompassing most of one or more states (e.g., the High Plains aquifer). This first type of region has already been defined for the RASA Program. Second, a region may be discontinuous but widespread, encompassing related nonadjacent aquifer systems such as surficial aqui- fers or coastal aquifers. The discontinuous aquifers of this type of "re- gion" share common processes (e.g., salt-water intrusion, fracture flow, or surface water influence). "Regional" studies in this sense would in- clude some WRD studies traditionally referred to as "topical investiga- tions." The distinction is that many topical investigations have purely local significance. Regional studies of this type may perhaps be viewed
42 Investigating Groundwater Systems as topical investigations that have been optimized for regional applica- bility. Regional projects of either kind would generally require the co- operation of two or more USGS district offices, and their results would have broad application for water management for a large population. Problem-based research on regional concerns in the second sense of the word "region" must emphasize processes rather than properties of a specific aquifer. Having characterized the RASA aquifers, the USGS needs to turn its attention to widespread problems having in common not so much geography as process. The USGS has identified the following as the most important problems: groundwater depletion, groundwater- surface water interactions, freshwater/saltwater relations, and ground- water processes in several widespread but complex geologic environ- ments, including karst and fractured rock (USGS, 19984. Information from the RASA studies will help to define the extents of these regions. Chapter 4 examines the appropriateness of these choices. PROPOSED FRAMEWORK FOR REGIONAL-SCALE GROUNDWATER STUDIES We propose that future regional-scale groundwater studies have two distinct but complementary components: groundwater assessment and groundwater science. Regional Groundwater Assessment An essential policy-relevant form of regional groundwater investi- gation should focus on regional groundwater assessments. In broad terms, these assessments would draw upon the knowledge of the scien- tific community and the expertise of the Survey to assess the adequacy and sustainability of the quality and quantity of groundwater for its val- ued uses. Beyond the historically static, descriptive assessment of char- acteristics such as depth to water table, regional potentiometric maps, and the concentrations of water quality parameters, regional groundwa- ter assessments must integrate process-oriented science to evaluate the sustainability of current and projected trends for hydrogeologically and geographically distinct regional groundwater systems. Of necessity, these regional assessments must be inclusive, incorporating regional
Approaches to Synthesis of Groundwater 43 patterns of water use, ecosystem needs, hydrogeologic and manmade interconnections with surface and subsurface systems, and patterns of growth and development. The demands for regional assessments will challenge the existing tools, information base, and scientific under- standing of dominant processes such as recharge, compaction, and groundwater-surface water interactions. Regional groundwater assess- ment will therefore drive the need for improved regional groundwater science, to refine the understanding of critical processes and feedbacks operating on regional scales. Regional Groundwater Science Scientific investigation of processes affecting the integrity of the nation's groundwater may occur in many different geographic regions. These scientific investigations should be undertaken and coordinated as process-based regionaTized studies, united by their shared dominant pro- cess (e.g., salt-water intrusion, conduit flow). The Survey's unique ca- pability to undertake the study of common processes, contrasted across different hydrocTimatic and hydrogeologic settings, provides a robust framework to expand the scientific understanding of the sensitivity and drivers of these processes. Assessment and science are not independent. Regional assessments quantify and project the impacts from man-made and natural stressors of hydrogeologic systems. These impacts motivate policy and decision- making dynamics and also drive the need for better regional groundwater science. Regional groundwater science builds on the process-based un- derstanding of groundwater systems. Improved process-based ground- water science on regional scales, in turn, supports improved management and policy-making on regional scales. Thus, regional groundwater as- sessments and regional process-based science form the foundation sup- porting the synthesis of groundwater investigations on regional and na- tional scales. Example: Middle Rio Grande The Middle Rio Grande basin study (http://rockyweb.cr.usgs.gov/- public/mrgb/), a GWRP project, is offered as an example of a regional
44 Investigating Groundwater Systems study that may be an appropriate model for future USGS efforts to fur- ther our understanding of groundwater at a scale relevant for regional decision-making. Middle Rio Grande Regional Assessment. The Middle Rio Grande basin study typifies the interdisciplinary scientific and policy issues inseparably woven throughout regional issues related to growth, development, and sustainability of groundwater resource systems. Groundwater withdrawal from the Santa Fe Group aquifer is the primary source of supply for about 40 percent of the population of New Mexico. The sustainability of current and projected patterns of development have far-reaching implications for the regional economy and the ecosystems critically linked through the regional groundwater flow system. More detailed modeling and field investigations have indicated that the avail- able resource, once thought to be an extensive, high-yielding aquifer system, is substantially more limited than had previously been thought (Hawley and Haase, 1992; Thorn et al., 1993~. Moreover, the complexity of the geologic setting and data limita- tions created considerable uncertainty regarding the interconnections in the aquifer system, the magnitude, extent, and timing of exchange with the riparian system, and the dominant sources of aquifer recharge. Re- gional groundwater models, calibrated with substantially different as- sumptions regarding the underlying geologic structure of the aquifer systems, were nevertheless capable of reproducing the observed water levels. However, the parameter estimates associated with these alternate calibrations differed significantly, resulting in dramatic differences in model-computed flow paths and rates of recharge. Critical information needed for regional assessment of the sustainability of current and pro- jected uses thus motivated interdisciplinary research activities to resolve these uncertainties and better understand the fundamental processes in- fluencing the regional groundwater system. For the Middle Rio Grande-Santa Fe Group aquifer system, the in- formation needed to formulate sustainable policies requires a reliable quantitative description of the regional groundwater flow system, in- cluding the hydrogeologic framework as well as patterns and trends in extraction and recharge. The demand for this understanding, in turn, clefines critical issues in regional groundwater science that have become the focus of process-oriented groundwater research of regional and na- tional significance.
Approaches to Synthesis of Ground~water 45 For example, mountain front recharge in the alluvial aquifers of the Middle Rio Grande basin represents a critical and uncertain control on the regional groundwater system. Research on the mountain front re- charge processes is important to this regional study, and leads to the identification of common process-based research on recharge in basin fill aquifers in regions with similar hydrogeologic settings. Middle Rio Grande Regional Science. The regional assessment of the Middle Rio Grande basin system motivates the need to reliably quantify the recharge, flow paths, surface water interconnections, resi- dence times, and hydrostratigraphic uncertainties in the regional groundwater flow system. One of the critical scientific issues for this regional assessment is accurately characterizing the complex hydrologic connections between the Rio Grande and Santa Fe Group. This need has generated a variety of regional scientific investigations employing gra- vimetric investigations, digital mapping and geophysical techniques, and the use of environmental tracers to better elucidate the structural controls and flow paths linking the alluvial and aquifer systems. Similar coordinated investigations have been formulated to clarify the critical processes governing the timing, spatial extent, and rates of mountain front recharge, as well as fluxes between the fluvial and groundwater systems. The indeterminate nature of hydrogeologic pa- rameter estimates derived through the calibration of groundwater flow models has been supplemented by a variety of techniques, including the use of environmental tracers, hydraulic measurement, and thermal pulse tracing, to resolve the streambed interactions. These scientific investi- gations draw upon and contribute to the scientific expertise and national information base found in the USGS national research program. Beyond the resulting understanding of aquifer processes, these sci- entific investigations must also be coordinated to allow their quasi- independent results to be optimally combined, resolving the key process- based questions framed by the regional assessment issues. This need to link scientific investigations to policy-relevant regional assessment is- sues imposes additional structures, defines critical scales, and requires additional constructs for uncertainty analysis in the scientific research design. in this way, regional assessments further define and shape proc- ess-based regional science.
46 Investigating Groundwater Systems Regionalization Although the process-based scientific investigations supporting re- gional assessments will be targeted to the specific hydrogeologic setting, a regional approach to process-based science will expand the applicabil- ity of these investigations across hydrogeologic settings. Rather than viewing these local differences as unique settings, a process-based pro- gram of regional groundwater science can exploit these differences as cross-sectional intercomparisons, united by their common dominant pro- cesses. For example, the detailed investigations of recharge to alluvial aquifers and streambed fluxes in the Middle Rio Grande basin study re- quire scientific understanding of the dominant processes governing the flow between fluvial and groundwater systems. Regional groundwater science focused on common dominant processes, studied across different hydrocTimatic and hydrogeologic settings, will expand the scientific un- derstanding of recharge processes common to alluvial aquifers. The USGS's capability to coordinate process-oriented research in different geographic and hydrogeologic settings maximizes the benefits of re- gional groundwater science. The domain of regional studies will differ for regional groundwater assessments and regional groundwater science. The scope and extent of regional groundwater assessments will be determined by the spatial do- main of the dominant processes and forcings influencing sustainability. The importance of groundwater-surface water interactions will naturally expand the extent of a regional groundwater assessment to include the intersection of the affected watershed and aquifer systems. For example, the growing importance of conjunctive use in regional resource sustain- ability may naturally extend the limits of a regional assessment to multi- ple basins, which may be coupled through man-made interconnections establishing interbasin transfers. The regional domain for groundwater science will be determined by the extent and occurrence of common dominant groundwater processes. For example, karst terrains represent a spatially noncontiguous domain for research on fate and transport of nutrients and microbes in such aqui- fers. Similarly, investigations of recharge of alluvial aquifers of the Santa Fe Group may best be conducted as part of a regional scientific initiative on recharge processes common to alluvial aquifers of the Lower Colorado (critical to meeting treaty obligations to Mexico), Platte, or Missouri Rivers.
Approaches to Synthesis of Groundwater CONCLUSIONS 47 The USGS is uniquely qualified to conduct and synthesize ground- water investigations on regional and national scales. These investiga- tions will be most effective when structured around process-based rather than resource-based studies. The elements to support these investiga- tions include policy-relevant regional assessments of the sustainability of current and projected patterns of resource use. And finally, the man- agement and policy questions that drive regional groundwater assess- ments will, in turn, drive the need for regional scientific investigations in fundamental process-oriented groundwater science common to regions with similar dominant processes.
