\ If United States Environmental Protection Agency Robert S. Kerr Environmental Research Laboratory Ada OK 74820 Research and Development EPA/600/S2-89/028 Sept. 1989 oEPA Project Summary Groundwater Modeling: An Overview and Status Report Paul K.M. van der Heijde, Aly I. El-Kadi, and Stan A.Williams This report focuses on groundwater models and their application in the management of water resource sys- tems. It reviews the kinds of models that have been developed and their specific and general role in water resource management The report begins with the intro- duction of system concepts ap- plicable to subsurface hydrology and presents groundwater modeling terminology, followed by a discus- sion of the role of modeling in groundwater management with spe- cial attention to the importance of spatial and temporal scales. The model development process is dis- cussed together with related issues such as model validation. A separate section provides information on model application procedures and is- sues. In addition to a review of the model application process, this chap- ter contains discussion of model selection and model calibration and provides information on specific aspects of pollution modeling. The report also contains an extensive overview of current model status. Here, the availability of the models, their specific characteristics, and the information, data, and technical expertise needed for their operation and use are discussed. Also discussed are quality assurance in groundwater modeling and manage- ment issues and concerns. The re- port concludes with a review of cur- rent limitations in modeling and of- fers recommendations for improve- ments in models and modeling procedures. This Project Summary was de- veloped by EPA's Robert S. Kerr Environmental Research Laboratory. Ada, OK, to announce key findings of the research project that is fully docu- mented in a separate report of the same title (see Project Report order- ing information at back). Background In the mid-1970s, by request of the Scientific Committee on Problems of the Environment (SCOPE), part of the Inter- national Council of Scientific Unions (ICSU), the Holcomb Research Institute (HRI) at Butler University, Indianapolis, Indiana, carried out a groundwater model- ing assessment. This international study, funded in large part by the U.S. En- vironmental Protection Agency (EPA) through its R.S. Kerr Environmental Re- search Laboratory in Oklahoma, resulted in a report published by the American Geophysical Union (AGU) in its series, Wafer Resources Monographs. In 1985 a second edition of this monograph was published, based on information collected at HRI through its International Ground Water Modeling Center (IGWMC) from its inception in 1978 until December 1983. The Center is an international clearing- house for groundwater models and a technology transfer center in groundwater modeling. Since 1983 the Center has been linked to the TNO Institute of Ap- plied Geosciences, Delft, The Nether- lands, which operates the European office of the IGWMC. Supported largely by the EPA and in part by HRI, the Center organizes and conducts short courses and seminars, and carries out a research program to advance the quality of modeling in groundwater management, in support of the Center's technology transfer functions. The report summarized herein presents results of research and information processing activities performed by the ------- IGWMC under a research and tech- nology transfer cooperative agreement initiated in 1985. The report serves three functions: (1) it provides an introduction to groundwater modeling and related is- sues for use as instruction material in short courses and for self study; (2) it provides an overview of the status of major types of groundwater models; and (3) it presents a discussion of problems related to the development and use of groundwater models. Introduction Groundwater modeling is a method- ology for the analysis of mechanisms and controls of groundwater systems and for the evaluation of policies, actions, and designs that may affect such systems. Models are useful tools for under- standing the mechanisms of groundwater systems and the processes that in- fluence their composition. Modeling serves as a means to ensure orderly in- terpretation of the data describing a groundwater system, and to ensure that this interpretation is a consistent repre- sentation of the system. Modeling can also provide a quantitative indicator for resource evaluation where financial re- sources for additional field data collection are limited. Finally, models can be used in what is often called the predictive mode by analyzing the response a sys- tem is expected to show when existing stresses vary and new ones are intro- duced. They can assist in screening al- ternative policies, in optimizing engineer- ing designs, and in assessing operative actions in order to determine their im- pacts on the groundwater system and ul- timately on the risks of these actions to human health and the environment. In managing water resources to meet long-term human and environmental needs, groundwater models have be- come important tools. The field of groundwater modeling is expanding and evolving as a result of: • Widespread detection of contami- nated groundwater systems • Enhanced scientific capability in modeling groundwater contamination in terms of the physical, biological, and chemical processes involved • Rapid advancement of computer software and hardware, and the marked reduction in the cost associ- ated with this technology. The rapid growth in the use of ground- water models has led to unforeseen problems in project management. Some of the projects in which these sophis- ticated tools have been used have even led to adversary legal procedures in which the model application or even the model's theoretical framework and coding have been contested. Often, the key issue is the validity of model-based predictions. Other issues of concern in- clude code availability and reliability, model selection and acceptance criteria, project review and procurement, data re- quirements, information exchange, and training. The Groundwater System Groundwater is a subsurface element of the hydrosphere, which is generally understood to encompass all the waters beneath, on, and above the earth's surface. Many solar-powered processes occur in the hydrosphere, resulting in a continuous movement of water. This dy- namic system is referred to as the hy- drologic cycle. Its major elements are at- mospheric water, surface water, water in the subsoil (shallow and deep vadose zone), groundwater, streams, lakes and ocean basins, and the water in the lithosphere. (Figure 1). Movement of water occurs both within each element of the hydrologic cycle and as exchanges between the elements, and results in the dynamic character of this relatively closed system. The exchange processes between the surface subsys- tem and the atmosphere include evapo- ration, precipitation (rainfall and snowfall), and plant transpiration. Infiltration, seep- age, groundwater recharge from streams, and subsurface discharge into lakes and streams (both interflow and baseflow) are interelement processes between the earth's surface and subsurface. Surface runoff forms the link between the earth's surface and the network of streams. In addition, interactions take place between the subsurface hydrosphere and ele- ments of the earth's biological environ- ment (e.g., consumptive use of water by plants). A groundwater system is an aggregate of rock in which water enters and moves, and which is bounded by rock that does not allow any water movement, and by zones of interaction with the earth's surf- ace and with surface water systems. In such a system, the water may transport solutes and biota; interactions of both water and dissolved constituents with the solid phase (rock) often occur. Water enters the groundwater system in recharge zones and leaves the system in discharge areas. In a humid climate, the major source of aquifer recharge is the infiltration of water and its subse- quent percolation through the soil into the groundwater subsystem. This type of recharge occurs in all in-stream areas except along streams and their adjoi floodplains, which are generally charge areas. In arid parts of the w recharge is often restricted to mour ranges, to alluvial fans bordering tt mountain ranges, and along the chan of major streams underlain by thick permeable alluvial deposits. In addition to these natural rechi processes, artificial or man-made charge can be significant. This type 01 charge includes injection wells, indui infiltration from surface water bodies, irrigation. Outflows from groundwater syst< are normally the result of a combina of inflows from various recharge soun Groundwater loss appears as interflov streams (rapid near-surface runoff); groundwater discharge into streams suiting in stream baseflow); as spri and small seeps in hillsides and va bottoms; as wetlands such as lakes • marshes fed by groundwater; as capill rise near the water table into a zone ft which evaporation and transpiration i occur; and as transpiration by phre; phytes (plants whose roots can live in saturated zone or can survive fluctuate of the water table). Other outflows are tificial or human-induced, as agricultt drainage (tile-drains, furrows, ditch and wells for water supply or dewater (e.g., excavations and mining). The unsaturated zone has a signific smoothing influence on the tempo characteristics of the recharge of groui water systems. High variable (houi precipitation and diurnal evapotranspi tion effects are dampened and seaso and long-term variations in flow rai become more prominent further from I soil surface. In this dampening t higher-frequency fluctuations are tered, a process that continues in I groundwater zone. Its ultimate effect c be observed in stream base flow, whi is characterized by seasonal and lor term components. Model Development In groundwater modeling, a distinct! is often made between two major cai gories of activities: model developrm and model use in management. Moc development consists of researching t quantitative description of the groun water system, a software developme component, and model testing. Moc development is closely related to the si entific process of increasing knowledg observing nature, posing hypotheses I the observed information, verifying tl proposed relationships, and thus esta lishing a credible theoretical framewo ------- and improving our understanding of na- ture. Model development is often driven by the short-term and less frequently by the long-term needs of natural re- sources management. The resulting, often-generic computer codes are used in model application as part of a larger set of activities which included data col- lection and interpretation, technical de- sign, economical evaluation, and so forth. The final report presents a complete, detailed discussion of the model devel- opment process, scenarios and data- bases as well as model applications and management issues including quality assurance. Groundwater Modeling and Management Groundwater management is con- cerned with the efficient utilization of groundwater resources in response to current and future demands, while pro- tecting the integrity of the resources to sustain general environmental needs. Groundwater modeling has become an important methodology in support of the planning and decision-making proc- esses involved in groundwater manage- ment. Groundwater modeling provides an an- alytical framework for understanding groundwater flow systems and the proc- esses and controls that influence their quality, particularly those processes influ- enced by human intervention in the hy- drogeologic system. Models can provide water resource managers with necessary support for planning and screening of al- ternative policies, making management decisions, and reviewing technical de- signs for groundwater remediation based on a risk analysis of benefits and costs. Such support is particularly advanta- geous when applied to development of groundwater supply, groundwater protec- tion, and aquifer restoration. / / / / / / ' / / / / \TMOSPHERE transpiration precip- itation evaporation -//Earth/Soil XT'/Surface //// Infil- tration surface runoll precipitation . Surface Water // Bodies // (rivers, lakes)/' // x /. seepage percolnllon recharge capillary rise Interflow seepage (wetlands) discharge (base (low) stream flow evaporation discharge recharge / / / ' ' ' / / /// S7 ' ff ' ' '/ s / / ' ' ' S^^s / / R 0 U NPW A T E R Z b N E / AQ U I ER saltwater Intrusion .LITHOSPHERE / / / / / s / / / / / / s / / Figure r. Elements of the hydrologic cycle and their interactions. ------- Successful utilization of modeling is possible only if the methodology is properly integrated with data collection, data processing, and other techniques and approaches for evaluation of hydro- geologic system characteristics. Further- more, frequent communication between managers and technical experts is es- sential to assure that management issues are adequately formulated and that the technical analysis using models is well targeted. Where precise aquifer and contami- nant characteristics have been reasona- bly well established, groundwater models may provide a viable, if not the only, method to predict contaminant transport and fate, locate areas of potential en- vironmental risk, identify pollution sources, and assess possible remedial actions. Some examples in which mathe- matical models have assisted in the man- agement of groundwater protection pro- grams are: • Determining or evaluating the need for regulation of specific waste dis- posal, agricultural, and industrial practices • Analyzing policy impacts, as in eval- uating the consequences of setting regulatory standards and rules • Assessing exposure, hazard, dam- age, and health risks • Evaluating reliability, technical feasi- bility and effectiveness, cost, opera- tion and maintenance, and other aspects of waste disposal facility de- signs and of alternative remedial actions • Providing guidance in siting new fa- cilities and in permit issuance and petitioning • Developing aquifer or well head pro- tection zones • Assessing liabilities such as post- closure liability for waste disposal sites Models generally applied to ground- water pollution problems can be divided into two broad categories: (1) flow mod- els describing hydraulic behavior of sin- gle or multiple fluids or fluid phases in porous soils, or porous or fractured rock, and (2) contaminant transport and fate models for analysis of movement, trans- formation, and degradation of chemicals present in the subsurface. In the context of groundwater protection programs, a distinction is often made between site- specific and generic modeling. However, generic modeling ap- proaches are being increasingly con- tested through public comment on draft regulations or in courtroom legal pro- cedures. An example is the recent court decision that EPA's VHS model (Vertical Horizontal Spread model) cannot be used to grant or deny a delisting petition under the RCRA permitting program. Site-Specific Modeling Whether for permit issuance, investiga- tion of potential problems, or remediation of proven contamination, site-specific modeling is required as a necessary in- strument for compliance under a number of major environmental statutes. The National Environmental Policy Act of 1970 (NEPA) stipulates a need to show the impact of major site-specific con- struction activities in Environmental Im- pact Statements; although not required by the regulations, potential impacts are often projected successfully by math- ematical models. Some of the most challenging site- specific problems involve hazardous waste sites falling under the purviews of RCRA (Resource Conservation and Re- covery Act of 1976) and CERCLA (Com- prehensive Environmental Response, Compensation, and Liability Act of 1980-Superfund), both administered by the U.S. Environmental Protection Agen- cy. Associated with most of these sites is an intricate array of chemical wastes and the presence of, or potential for, ground- water contamination. Furthermore, the hydrogeologic settings of such sites are usually complex. Under such conditions, groundwater models are useful instru- ments for analyzing compliance with RCRA and CERCLA legislation. Generic Modeling Where the results of environmental analysis must be applied to many sites, data availability is limited or other con- straints are present. In such cases, site- specific modeling is not feasible. As a result, many decisions are made by ap- plying models to generic management issues and hydrogeologic conditions. Models used for this type of analysis are more often analytical than numerical m their mathematical solutions, in contrast to models used for detailed analysis of site-specific conditions. Because of their limited data requirements, analytical models can be applied efficiently to a larger number of simple datasets or to statistical analyses representing a wide variety of field conditions. The cost of such exercises would often be prohibitive when using numerical models. Conclusions An EPA Study Group has identified a variety of new models and modeling ap- proaches as important to groundwater protection: • Simulation of flow and transpo multimedia (e.g., coupled model surface water/groundwater ir action) • Representation of stochastic p esses in predictive modeling, improving the applicability of get tistical models • Improved modeling of hydrochen speciation • Simulation of flow and transpoi fractured and dual-porosity me including diffusion in dead- pores • Simulation of flow and transpoi soils containing macropores • Determination of effects of com tration-dependent density groundwater flow and pollu transport • Determination of effects of altera of geologic media on hydrolog and chemical characteristics (c dehydration of clay when attac by solvents, change in sorptive Ce city of material when heated) • Representation of the three-dim sional effects of partially penetra wells on water table aquifers • Development of models for mane ment of groundwater contamina plumes • Development of expert systems (; ficial intelligence) for such tasks selecting appropriate submodels subroutines for specific problems • Application of parameter identifi tion models to be used with : studies • Further development of pre- j post-simulation data processors • Continued development of risk sessment and management mode • Modeling of volatilization, multiph; flow, and immiscible flow • Incorporation of economic factors improve estimation of cleanup cos • Development of generic and si specific parameter databases. Fundamental research support! groundwater modeling is consider necessary in such areas as: • Transient behavior of process [ rameters (e.g., retardation, hydrai conductivity) • Desorption for nonhydrophot chemicals • Multicomponent transport and che ical interaction • Enhanced transport mechanist (e.g., piggy-backing on more nr bile chemicals) • Transport of silt with sorbed che icals m aquifers ------- • Improved numerical accuracy, sta- bility, and efficiency. Modeling the transport and fate of chemicals in groundwater is a major sub- ject of several EPA and DOE research programs. These programs focus on im- miscible flow associated with organic and oil-like liquids. Other topics currently being studied include simulation of flow and transport in fractured and dual- porosity media, representation of sto- chastic processes in predictive modeling, multimedia risk assessment, incorpora- tion of volatilization in multiphase trans- port models, and simulation of density- dependent flow. Paul K.M. van der Heijde, Aly I. El-Kadi, and Stan A. Williams are with International Ground Water Modeling Center, Butler University, Indianapolis, Indiana 46208. Joe rt. Williams is the EPA Project Officer (see below). The complete report, entitled "Groundwater Modeling: An Overview and Status Report," (Order No. PB 89-224 497/AS; Cost: $28.95, subject to change) will be available only from: National Technical Information Service 5285 Port Royal Road Springfield, VA 22161 Telephone: 703-487-4650 The EPA Project Officer can be contacted at: Robert S. Kerr Environmental Research Laboratory U.S. Environmental Protection Agency Ada, OK 74820 ------- United States Center for Environmental Research BULK RATE Environmental Protection Information POSTAGE & FEES PAII Agency Cincinnati OH 45268 EPA PERMIT No. G-35 Official Business Penalty for Private Use $300 EPA/600/S2-89/028 ------- |