Sustainability of Ground-Water Resources--Circular 1186

MEETING THE CHALLENGES OF GROUND-WATER SUSTAINABILITY

As we have seen, the sustainability of ground-water resources is a function of many factors, including decreases in ground-water storage, reductions in streamflow and lake levels, loss of wetland and riparian ecosystems, land subsidence, saltwater intrusion, and changes in ground-water quality. Each ground-water system and development situation is unique and requires an analysis adjusted to the nature of the water issues faced, including the social, economic, and legal constraints that must be taken into account. A key challenge for achieving ground-water sustainability is to frame the hydrologic implications of various alternative management strategies in such a way that they can be properly evaluated.

Ground-water scientists have developed an expanding capability to address issues associated with the development and sustainability of ground-water resources. Early efforts focused on methods of evaluating the effects of ground-water pumping on an aquifer's long-term capacity to yield water to wells. Subsequently, methods were applied to evaluate various effects of ground-water development on surface-water bodies, land subsidence, and saltwater intrusion. Starting in the late 1970's, increasing concerns about contamination of ground water by human activities led to an awareness of the great difficulty and expense of cleaning up contaminated aquifers and drew attention to the importance of prevention of ground-water contamination. With time, it has become clear that the chemical, biological, and physical aspects of ground-water systems are interrelated and require an integrated analysis, and that many issues involving the quantity, quality, and ecological aspects of surface water are interrelated with ground water. Thus, ground-water hydrologists are challenged continually by the need to provide greater refinement to their analyses and to address new problems and issues as they arise.

A key challenge for achieving ground-water sustainability is to frame the hydrologic implications of various alternative management strategies in such a way that they can be properly evaluated.

The Importance of Ground-Water Data

The foundation of any good ground-water analysis, including those analyses whose objective is to propose and evaluate alternative management strategies, is the availability of high-quality data. Principal types of data commonly required are listed in Table 2. Some, such as precipitation data, are generally available and relatively easy to obtain at the time of a hydrologic analysis. Other data and information, such as geologic and hydrogeologic maps, can require years to develop. Still other data, such as a history of water levels in different parts of ground-water systems, require foresight in order to obtain measurements over time, if they are to be available at all. Thus, a key starting point for assuring a sustainable future for any ground-water system is development of a comprehensive hydrogeologic data base over time. As examples, these data would include depths and thicknesses of hydrogeologic units from lithologic and geophysical well logs, water-level measurements to allow construction of predevelopment water-level maps for major aquifers as well as water-level maps at various times during development, ground-water sampling to document pre- and post-development water quality, and simultaneous measurements of streamflow and stream quality during low flows to indicate possible contributions of discharging ground water to surface-water quality. Many of the types of data and data compilations listed in Table 2 need tobe viewed on maps. Thus, Geographic Information Systems (GIS) typically are an integral part of the data-base system to assist in organizing, storing, and displaying the substantial array of needed information.

The foundation of any good ground-water analysis, including those analyses whose objective is to propose and evaluate alternative management strategies, is the availability of high-quality data.

Table 2.--Principal types of data and data compilations required for analysis of ground-water systems

Physical Framework

Hydrologic Budgets and Stresses

Chemical Framework

Use of Ground-Water Computer Models

During the past several decades, computer simulation models for analyzing flow and solute transport in ground-water and surface-water systems have played an increasing role in the evaluation of alternative approaches to ground-water development and management. The use of these models has somewhat paralleled advances in computing systems. Ground-water models are an attempt to represent the essential features of the actual ground-water system by means of a mathematical counterpart. The underlying philosophy is that an understanding of the basic laws of physics, chemistry, and biology that describe ground-water flow and transport and an accurate description of the specific system under study will enable a quantitative representation of the cause and effect relationships for that system. Quantitative understanding of cause and effect relationships enables forecasts to be made for any defined set of conditions. However, such forecasts, which usually are outside the range of observed conditions, typically are limited by uncertainties due to sparse and inaccurate data, poor definition of stresses acting on the system, and errors in system conceptualization (Konikow and Bredehoeft, 1992). Although forecasts of future events that are based on model simulations are imprecise, they nevertheless may represent the best available decision-making information at a given time. Because of the usefulness of computer simulation for decision making, the basic construction of computer simulation models, as well as model forecasts, need to be updated periodically as the actual ground-water system continues to respond to the physical and chemical stresses imposed upon it and as new information on the ground-water system becomes available.

Although forecasts of future events that are based on model simulations are imprecise, they nevertheless may represent the best available decision-making information at a given time.

(BOX F)

Computer simulation models have value beyond their use as purely predictive tools. They commonly are used as learning tools to identify additional data that are required to better define and understand ground-water systems. Furthermore, computer simulation models have the capability to test and quantify the consequences of various errors and uncertainties in the information necessary to determine cause and effect relationships and related model-based forecasts. This capability, particularly as it relates to forecasts, may be the most important aspect of computer models in that information about the uncertainty of model forecasts can be defined, which in turn enables water managers to evaluate the significance, and possibly unexpected consequences, of their decisions.

If a model is used to address questions about the future responses of a ground-water system that are of continuing significance to society, then field monitoring of the ground-water system should continue and the model should be reevaluated periodically to incorporate new information or new insights.

Strategies for Sustainability

When broadly considered, alternative management strategies are composed of a small number of general approaches, as outlined below.

Use sources of water other than local ground water. The main possibilities are (1) shift the source of water, either completely or in part, from ground water to surface water, or (2) import water (usually, but not necessarily, surface water) from outside river-basin or ground-water system boundaries. In two previous examples given in the "Storage Changes" section--the Chicago metropolitan area and Kings County, Long Island--the ground-water systems were stressed sufficiently to cause undesirable effects, and surface-water sources were substituted for ground-water sources as a result. On the other hand, ground water currently is used or is being considered for use in many localities as a supplement for surface-water sources that are no longer adequate.

Change rates or spatial patterns of ground-water pumpage. Possibilities include (1) an increase in pumpage that results in a new equilibrium of the ground-water system, (2) a decrease in pumpage that results in a new equilibrium of the ground-water system, or (3) a change in the spatial distribution of pumpage to minimize its existing or potential unwanted effects. Management strategies might include varying combinations of these approaches.

Increase recharge to the ground-water system. Usual options include (1) pumpage designed to induce inflow from surface-water bodies, or (2) recharge of surface water or reused water (ground water or surface water) of good quality by surface spreading or injection through wells. Examples of several of these options are presented in Box G.

Decrease discharge from the ground-water system. Possibilities include pumpage that is designed to decrease discharge (1) to streams, lakes, or springs, or (2) from ground-water evapotranspiration. Both of these possibilities can have undesirable effects on surface-water bodies or on existing biological resources.

Change the volume of ground water in storage at different time scales. Possibilities include (1) managed short-term (time scale of months and years) increases and decreases in storage in the ground-water reservoir, which suggests that the ground-water reservoir might be managed at a time scale that is comparable to the management of surface-water reservoirs, or (2) a continuing long-term (possible time scales of decades and centuries) decrease in ground-water storage. Of course, complete or almost complete depletion of aquifer storage is not a strategy for sustainability, but an extreme approach that may be considered in some situations.

Consideration of these general approaches indicates that they are not mutually exclusive; that is, the various approaches overlap, or the implementation of one approach will inevitably involve or cause the implementation of another. For example, changing rates or patterns of ground-water pumpage will lead to changes in the spatial patterns of recharge to or discharge from ground-water systems.

The short list of general approaches may suggest that proposing and evaluating alternative management strategies is deceptively simple. On the contrary, ground water is withdrawn from complex, three-dimensional systems, and many possible combinations of these approaches typically should be considered in developing management strategies for a particular ground-water system.

Innovative approaches that have been undertaken to enhance the sustainability of ground-water resources typically involve some combination of use of aquifers as storage reservoirs, conjunctive use of surface water and ground water, artificial recharge of water through wells or surface spreading, and the use of recycled or reclaimed water.

(BOX G)

Concluding Remarks

In conclusion, we would like to emphasize the following interrelated facts and concepts:

• The most important and most extensively discussed concept in this report is that volumes of water pumped from a ground-water system must come from somewhere and must cause a change in the ground- water system. Possible sources of water for pumpage are (1) more water entering the ground-water system (increased recharge), (2) less water leaving the system (decreased discharge), and (3) removal of water that was stored in the system.
• One of the critical linkages in both unstressed and stressed ground-water systems is between ground water and surface water. Pumping water from aquifers that are hydraulically connected with surface-water bodies can have a significant effect on those bodies by reducing ground-water discharges to surface water and possibly causing outflow from those bodies into the ground-water system. Thus, an evaluation of ground-water management strategies needs to involve consideration of surface-water resources, including closely related biological resources.
• A key feature of some aquifers and ground-water systems is the large volume of ground water in storage, which allows the possibility of using aquifers for temporary storage, that is, managing inflow and outflow of ground water in storage in a manner similar to surface-water reservoirs.
• From the standpoint of water use and water management, all ground water is not equal--the suitability of water, as measured by its quality, is a key consideration in developing water-management strategies. Furthermore, determining water suitability (or unsuitability) requires detailed information on the three-dimensional distribution and concentrations of potential contaminants, both naturally occurring contaminants and those resulting from human activities.
• Continuing large withdrawals of water from an aquifer often result in undesirable consequences. The most common of these consequences have been discussed throughout this report. From a management standpoint, water managers, stakeholders, and the public must decide the specific conditions under which the undesirable consequences can no longer be tolerated.
• The effects of ground-water development may require many years to become evident. Thus, there is an unfortunate tendency to forego the data collection and analysis that is needed to support informed decision making until well after problems materialize.
• Evaluation of possible ground-water management approaches (a) depends on the continuing collection, archiving, and analysis of a broad range of different types of information, and (b) can be assisted by well-designed computer simulation models.