The Role of Unsaturated Flow in Artifical Recharge Projects

By Alan L. Flint
U.S. Geological Survey, Placer Hall, 6000 J Street, Sacramento, California 95819-6129

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Introduction

Artificial recharging an aquifer may be achieved by either surface spreading, injection in wells, or altering the natural conditions of stream channels to increase infiltration. Except for recharge using injection wells directly into an aquifer, artificially recharged water must first move through the unsaturated zone. For the most part, the unsaturated zone provides the underground storage space for recharge, although the amount of storage is dependent on the water retention characteristics and the natural recharge occurring at the site.  The greater the natural recharge at a site, the greater the percent of porosity occupied by antecedent water moving through the unsaturated zone resulting in a smaller amount of available space for the artificially recharged water.

The hydrologic properties of an unsaturated zone help determine the suitability of a particular location for artificial recharge.  Optimally, areas used for artificial recharge should have high permeability soils, the capacity for horizontal movement of water in the unsaturated zone and in the receiving aquifer, a lack of impeding layers, and a thick unsaturateded zone.  Under optimal conditions, water should reach the top of the saturated zone and spread laterally rather than building up a column of water toward the surface, which would greatly reduce recharge (Freeze and Cherry, 1979).  The suitability of a site is often determined by field and laboratory measurements of soil properties, field experiments, and numerical modeling.

Several direct methods of artificial recharge commonly are used (Environmental and Water Resources Institute, 2001), including spreading basins and ditches for near-surface recharge applications, and pits and shafts for penetrating below near-surface restrictive layers.  A third method, direct well injection into the unsaturated zone, is often used to penetrate below deeper restrictive layers.  To highlight issues relating to the role of the unsaturated zone and unsaturated flow in recharging an aquifer, the following section discusses near-surface spreading basins being studied in the San Gorgonio Pass area in southern California.

Site Analysis

In 1991, spreading basins were used to test the feasibility of artificially recharging an aquifer in alluvial fans in Cherry Valley, which is within the San Gorgonio Pass area in southern California, (Shaikh and others, 1995).  In 1997, the U.S. Geological Survey (USGS) was asked to evaluate the suitability of the unsaturated zone for artificial recharge and to develop models of the unsaturated and saturated zones of the San Gorgonio Pass area.  Although well-organized guidelines are available for developing recharge spreading basins (Environmental and Water Resources Institute, 2001), spreading basins at this site were established in the 1960's prior to full analysis of subsurface hydrogeologic conditions and properties.  Hydrogeologic data are essential in siting recharge spreading basins, particularly in alluvial basins, where soils are highly stratified and contain continuous and discontinuous clay layers interbedded with sands and gravels (Flanigan, and others, 1995).

As part of the USGS evaluation, several test wells were cored in the unsaturated zone and instrumented with deep tensiometers, heat-dissipation matric-potential sensors, temperature sensors, and suction lysimeters.  Core samples and cuttings were analyzed in the USGS laboratory to determine particle-size distribution, water content, permeability, and lithology.  An interpretation of these data suggests that there are several alternating high and low permeability layers between the surface and the water table (approximately 600 feet deep). A perched water table is present above a very low permeability layer, 250 feet below the surface.  Results of inverse modeling of borehole temperatures and water-level measurements, which show a slow decline in the water levels in the perched zone, indicates that the vertical hydraulic conductivity of the layer is less than 1 foot per year.  Data from other boreholes in the area suggest that this perched layer is the top of an old, laterally extensive, geologic formation.

Surface-seismic and surface-resistivity measurements were used to develop a conceptual model of the layering and faulting in the area (fig. 1A).  The existence of a shallow water table north of the Banning Fault suggests that the fault is a barrier to lateral flow.  Temperature data from several boreholes in the area indicate that the coldest water in the unsaturated zone is the perched water.  Temperature measurements made directly from flowing water in a nearby stream (San Gorgonio Creek, fig. 1) suggest that water in the perched zone is from local stream recharge, and not from the shallow water table north of the fault, which supports the hypothesis that the Banning Fault is a barrier to flow.

Figure 1. Conceptual cross section of the layered stratigraphy (A) and the relative location of the cross section and near-surface spreading basins to features of the San Gorgonio Pass area, California (B).

Numerical Modeling

The conceptual model of the unsaturated zone at San Gorgonio Pass was used for a numerical model of the unsaturated zone to further analyze existing data and to develop workable scenarios for artificial recharge.  TOUGH2, an integrated finite-difference numerical code (Pruess, 1991), was used to develop the three-dimensional model.  This code simulates the flow of heat, air, water, and nitrate (assumed to be present in septic tank leach fields) in three dimensions under saturated or unsaturated conditions.  The geometry of the site requires a three-dimensional approach because of down-dip migration of recharged water through the alluvial fan deposits (north to south), as well as lateral flow of natural recharge (generally east to west) from the nearby stream.  The modeling domain is approximately 1.6 miles (east to west) by 0.8 miles by 600 feet deep and contains more than 50,000 grid elements.  The north and south lateral boundaries of the model are located along faults and are assumed no-flow boundaries.  The east and west boundaries represent the edges of the alluvial basin where they encounter the mountain block.  The bottom boundary is the water table and the upper boundary is specified flux.  The surface flux is temporally and spatially variable depending on the artificial recharge scenario, and the location and amount of streamflow, septic tank return flow, and natural recharge from precipitation.

The model was initially developed using the hydrologic properties measured or estimated from the laboratory data.  The model was further refined and calibrated by matching borehole temperature, matric potential data, and the occurrence of perched water.  The model was successfully used to simulate the artificial recharge experiment conducted in 1991 and described by Shaikh and others (1995).  The model simulated the measured temperature profiles by adding cold-water infiltration along the stream channel.  Once calibrated, the model was run to steady-state conditions assuming natural recharge from precipitation and streamflow based on the results of the calibration of the saturated zone flow model. The model was then used to simulate historical and future artificial recharge conditions from 1950-2005. The model simulated septic tank return flows that have 80 milligrams per liter nitrate-nitrogen (nitrate reported as nitrogen), the 1991 artificial experiment, and a proposed artificial recharge scenario.  For the proposed artificial recharge scenario, 1,000 acre-ft of recharge was applied over a 50-day period each year from 2001 through 2005.  The model simulation allowed comparison of measured and simulated data from 1991 to 2001, and predicted the response of the system to the proposed recharge scenario.

Before the application of artificial recharge in 1991, the simulated travel time from the surface to the water table was approximately 50 years for locations directly beneath the stream, increasing to more than 250 years for locations away from the stream.  The simulated addition of artificial recharge from 2001-2005 decreased the unsaturated-zone travel time to less than 10 years directly beneath the spreading basins, but the amount of applied water that recharged the regional aquifer was less than 5 feet per year.  The simulations suggest that little recharge will reach the regional water table under the spreading basin: further most of the artificially recharged water will remain above the perching layer at 250 ft below land surface, and will mound against the down-gradient no-flow boundary located about 4,000 feet south of the spreading basins.  Although the recharged water intercepts nitrates from septic tank leach fields as it spreads laterally and vertically through the unsaturated zone, the simulated nitrate-nitrogen concentration of water in the perched water layer is less than 10 milligrams per liter.

Summary

Generally, artificial recharge projects apply water in surface and near-surface spreading basins, pits, and trenches, using the unsaturated zone to transport and store water.  The hydrogeology of the unsaturated zone plays a critical role in transporting and storing artificially recharged water. Evaluating this zone will determine if the area is suitable for artificial recharge, as well as the most effective methods of surface or subsurface application of water.  Field and laboratory data, measured data, and field experiments were used to develop a conceptual and a numerical model of the unsaturated zone at San Gorgonio Pass in southern California. The results of the model simulations were used to refine the conceptual model and to test scenarios for artificial recharge.  Results of the numerical model simulations of this site indicate that little recharge will reach the regional aquifer beneath the spreading basins, and that most of the water will remain above a perching layer at 250 feet below land surface, and will mound along the assumed no-flow fault boundary located about 4,000 feet south of the spreading basins.  Further work on the characteristics of the fault and extension of the modeling domain further down gradient of the fault are required to provide more conclusive results for the characterization of the site for the application of artificial recharge.

References Cited

Environmental and Water Resources Institute, 2001, Standard guidelines for artificial recharge of ground water: Reston, Va, American Society of Civil Engineers, 106 p.

Flanigan, J.B., Sorensen, P.A., and Tucker, M.A., 1995, Use of hydrogeologic data in recharge pond design, vol. II in A.I. Johnson, and R.D. Pyne, eds., Artificial recharge of ground water;: New York, American Society of Civil Engineers, p.139-148.

Freeze, R.A. and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, New Jersey, Prentice-Hall, p. 367-370.

Pruess, K., 1991, TOUGH2-A general-purpose numerical simulator for multiphase fluid and heat flow: Lawrence Berkeley National Laboratory Rep. LBL-29400, 102 p.

Shaikh, A., Bell, R.B., Ford, M.E., and Stockton, S.P., 1995, Feasibility of recharge by surface spreading, vol. II in A.I. Johnson, and R.D. Pyne, eds., Artificial recharge of ground water: New York, American Society of Civil Engineers, p. 159-167.

In George R. Aiken and Eve L. Kuniansky, editors, 2002, U.S. Geological Survey Artificial Recharge Workshop Proceedings, Sacramento, California, April 2-4, 2002: USGS Open-File Report 02-89

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