USGS Groundwater Information
The Role of Saturated Flow in Artifical Recharge Projects
By Steven P. Phillips
The goal of most artificial recharge projects is to convey water to the saturated zone. Evaluation of the viability of proposed projects and of the effectiveness of existing projects requires an understanding and predictive capability of their hydraulic and chemical effects. This abstract focuses on the potential hydraulic consequences of altering the saturated flow system through artificial recharge, which are largely controlled by the geologic and hydrologic characteristics of the aquifer system. A combination of field, laboratory, analytical, and simulation methods generally are used to develop an understanding of the hydrogeologic system as a basis for predicting potential consequences. Optimization techniques may be coupled with predictive models of ground-water flow and other processes to create an effective tool for planning and management of artificial recharge projects. Pre-project and long-term monitoring of key aspects of a flow system is an essential part of a successful management plan.
Potential Hydraulic Effects of Artificial Recharge Projects
Artificial recharge projects are undertaken for many purposes in a variety of aquifer systems and associated hydraulic conditions. Regardless of the initial distribution and trend of hydraulic heads in these systems, artificial recharge will alter these heads and associated conditions. Depending on the purpose of the artificial recharge project, heads may show seasonal fluctuations or relatively long-term trends. Potential hydraulic effects of these changes, which may be positive, negative, or neutral include change in aquifer storage; changes in base flow in streams and on the rates of gain or loss from other surface-water bodies; changes in interbasin flow, evapotranspiration, and other sources of recharge and discharge; altered rates of land-surface deformation (subsidence and uplift) and saltwater intrusion; changes in interaquifer flow; induced seismicity; and several effects related to a shallow water table, including flooding of, and structural damage to basements and other surface and subsurface structures; surface flooding from flowing wells; increased susceptibility to liquefaction; and mobilization of salts or other near-surface contaminants. Mixing of water from different sources clearly has chemical and microbiological effects, which are discussed elsewhere in these proceedings.
Although prediction of many of these effects can be fairly straightforward in the vicinity of artificial recharge projects, experience has shown that more widespread and long-term effects are sometimes difficult to foresee. The city of San Bernardino in southern California, for example, is several miles down-gradient from multiple artificial recharge projects that have successfully raised water levels and reduced pumping costs for ground-water users near these projects. An unfortunate side effect of these operations, combined with above-average rainfall, was large increases in water levels underlying the city and associated severe structural and water-related damages (Danskin and Freckleton, 1992). This, as well as other examples, emphasize the importance of regional characterization of the saturated zone, which has the potential to transmit pressure rapidly over large distances and can support flow rates that may affect ground-water flow on a regional scale during the life of an artificial recharge project.
Geologic and Hydrologic Characterization
Characterization of the geology is important in determining the viability of an artificial recharge project, particularly where significant lateral and (or) vertical ground-water flow is required between recharge and discharge locations. Key features such as faults with significant offset, folds, and areally extensive coarse- or fine-grained sedimentary units can exert dominant controls on a flow system and on the fate of water from artificial recharge projects. This point is emphasized by experience in the Rialto-Colton basin in southern California, where imported surface water was recharged through surface ponds upgradient of a group of production wells intended to capture the recharge. Subsequent investigations revealed a low-permeability fault between the ponds and the wells, and geochemical evidence and predictive simulations suggest the wells will receive little hydraulic benefit from the artificial recharge project (Woolfenden and Koczot, 2001).
Methods for geologic characterization include surficial mapping; drilling and core analysis; borehole, surface, and aerial geophysics; sequence stratigraphy; geostatistical analysis of lithology; and others. The degree of geologic investigation required for a given project likely will depend on the availability of existing information, the complexity of the geology, and the scale of the project. Given the importance of understanding the continuity between the recharge and discharge locations, a tightly focused investigation is often warranted. For small-scale investigations, cross-hole tomography may be used to generate three-dimensional interpolations between boreholes of various physical characteristics. Advanced techniques for collecting and analyzing continuous core samples are among the other options for small-scale investigations.
Hydrologic considerations for the saturated-flow component of an artificial recharge project typically include the distribution of head and stress prior to and during project operations, hydraulic properties, the fate of artificially recharged water, and offsite effects. Determination of the initial and transient head distribution at key locations in the aquifer system commonly is accomplished using standard electronic monitoring equipment in some combination of wells and piezometers. Continuous (frequent sample rate, for example, hourly) data are far more useful than periodic data; collecting barometric data at the same high frequency is recommended, because it can be used to remove the barometric effect from the water-level record, or, in some cases, to estimate the vertical hydraulic diffusivity of the aquifer (Rojstaczer, 1988; Quilty and Roeloffs, 1991). Differential microgravity measurements have been used in surficial (Pool and Hatch, 1991) and injection-based artificial recharge projects (James F. Howle, U.S. Geological Survey, written commun., 2001) to estimate specific yield when coupled with water-level measurements, and to estimate changes in the position of the water table in the absence of wells. This and other geophysical techniques can be an effective means for increasing the sample density of a traditional water-level network.
The prediction of saturated flow during artificial recharge projects requires information on the distribution of stress, or recharge and discharge. These stresses can include a variety of natural and artificial processes that can be measured in a variety of ways. With respect to artificial recharge, surface applications of known volumes of water are converted to recharge rates over an area, which are determined through analysis of unsaturated-zone flow (see Alan Flint's abstract "The Role of Unsaturated Flow in Artificial Recharge Projects" in these proceedings) and (or) measured system responses. The vertical distribution of recharge through injection wells may be inferred using head and chemistry data from multi-level piezometers, and (or) measured directly using existing and recently developed methods for measuring vertical profiles of flow and chemistry in wells (Izbicki and others, 1998; Izbicki and others, 1999). Discharge through wells can be measured in the same way; most forms of surface discharge can be monitored using standard methods.
The hydraulic properties of an aquifer system, along with the distribution of stress, determine the direction and rate of saturated flow. The estimation of storage and transmissive properties of parts the saturated aquifer system generally is done through some combination of laboratory analysis of core samples, borehole geophysics and velocity logs, multi- or single-well aquifer tests, as well as other methods. Extrapolation of this local information to the region of interest generally involves incorporation of regional geologic information and simulation of the flow system, often iteratively. Powerful geostatistical methods have been developed and used to characterize the geology, quantify the range of possible conceptualizations, and express it in a hydrogeologic framework (Fogg and others, 2000).
Given the distribution of head, stress, and hydraulic properties, simulation models can be developed to help address the fate of artificially recharged water and offsite effects. The ability to recover some percentage of recharged water, often referred to as the recovery efficiency, varies widely and is sometimes an important criterion for project success. If recovery efficiency is important, tracer tests can be used as a direct measure of recovery, and can further constrain the hydraulic properties of the simulation model. Particle tracking or solute transport modeling can address the fate of recharged water that is not recovered. Monitoring and simulation are both used to address offsite effects; however, simulation can also be used to design an efficient monitoring network prior to full-scale implementation.
Optimization, Management, and Monitoring of Artificial Recharge Projects
Successful planning and management of an artificial recharge project often requires consideration of many water management objectives, water routing capabilities, economics, offsite effects, as well as other factors. Optimization techniques are designed to identify an optimal way to meet an objective given a set of constraints. The linkage of a predictive ground-water flow model with optimization techniques, or a simulation/optimization model, allows for simultaneous consideration of the flow system and physical and (or) economic constraints determined by water-resource managers.
Simulation/optimization models have been applied to ground-water problems for decades (Gorelick, 1983; Yeh, 1992; Wagner, 1995; Ahlfeld and Mulligan, 2000), and have been used to plan and manage artificial recharge projects (Danskin and Gorelick, 1985; Reichard, 1995; Mosch, 1998; Dreher and Gunatilaka, 1998; Basagaoglu and Marino, 1999). In Vienna, Austria, for example, infiltration of contaminants from the Danube River was severely degrading ground-water quality. A simulation/optimization model was developed to minimize river infiltration through real-time control of 21 pairs of injection/extraction wells on the basis of continuous measurements of water levels and water quality in 190 ground-water monitoring wells and 20 surface-water sites (Dreher and Gunatilaka, 1998). This complex application has been very successful.
Monitoring of hydraulic conditions prior to and during an artificial recharge project is an essential part of a management plan, and often is an integral part of project operations. Measurement of project performance is clearly one goal of a monitoring program. A second goal is to provide the information needed for future improvement of predictive modeling capabilities and adjustment of optimization constraints. Reduced uncertainty in model results translates directly to increased confidence in management decisions based on these models.
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