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Historically, the city of Palmdale and vicinity have relied on groundwater as the primary source of water, owing, in large part, to the scarcity of surface water in the region. Despite recent importing of surface water, groundwater withdrawal for municipal, industrial, and agricultural use has resulted in groundwater-level declines near the city of Palmdale in excess of 200 feet since the early 1900s. To meet the growing water demand in the area, the city of Palmdale has proposed the Amargosa Creek Recharge Project (ACRP), which has a footprint of about 150 acres along the Amargosa Creek 2 miles west of Palmdale, California. The objective of this study was to evaluate the long-term feasibility of recharging the Antelope Valley aquifer system by using infiltration of imported surface water from the California State Water Project in percolation basins at the ACRP. Three monitoring sites were constructed, and geophysical surveys (gravity, seismic, and resistivity) were completed to define the thickness of valley-fill deposits, depth to water, and location of faults that could influence groundwater flow. Data collected at the monitoring sites, and results from the geophysical surveys, were used to identify three northwestsoutheast trending faults in the vicinity of the proposed recharge facility; these faults are probably related to the nearby San Andreas fault zone. Water levels collected from wells at the monitoring sites showed water-level altitude differences as much as 230 feet between the upgradient and downgradient sides of the faults, indicating that these faults are barriers to groundwater flow. Lithologic and geophysical logs indicated the presence of a coarse gravel and sand unit extending from land surface to about 150 feet below land surface that did not appear to be disrupted by faulting. Water samples collected from the monitoring wells were analyzed for major ions, nutrients, trace elements, dissolved organic carbon, volatile organic compounds, stable isotopes of oxygen (oxygen-18) and hydrogen (hydrogen-2, or deuterium), and the radioactive isotopes of hydrogen (hydrogen-3, or tritium) and carbon (carbon-14, or 14C) to determine the water quality of the aquifer system and to help determine the source and age of the groundwater. Results of the water-quality analysis indicated that the source of natural recharge is Amargosa Creek near the ACRP, but that the creek does not provide modern-day recharge downstream of the ACRP. Potential effects of artificial recharge at the ACRP were evaluated by using a local-scale model of groundwater flow. On the basis of geologic samples collected during drilling, the hydraulic conductivity of the sand and gravel unit in the upper 150 feet was assumed to range from 10 to 100 feet per day. To address the goal of minimizing the potential for liquefaction during an earthquake from water-table rise associated with groundwater recharge at the ACRP, simulated water levels were constrained to remain at least 50 feet below land surface, except beneath the proposed artificial-recharge facility. The hydraulic conductivities of faults were estimated on the basis of water-level data and an estimate of natural recharge along Amargosa Creek. With assumed horizontal hydraulic conductivities of 10 and 100 feet per day in the upper 150 feet, the simulated maximum artificial recharge rates to the regional flow system at the ACRP were 3,400 and 9,400 acre-feet per year, respectively. These maximum recharge rates were limited primarily by the horizontal hydraulic conductivity in the upper 150 feet and by the liquefaction constraint. Future monitoring of water-level and soil-water content changes during the proposed project would allow improved estimation of aquifer hydraulic properties, the effect of the faults on groundwater movement, and the overall recharge capacity of the ACRP. Introduction Historically, the city of Palmdale and vicinity have relied on groundwater as the primary source of water, owing, in large part, to the scarcity of surface water in the region. Since 1972, supplemental surface water has been imported from the California State Water Project (SWP) through the California Aqueduct to help meet the demand for water. Despite the importation of surface water, groundwater withdrawal for municipal, industrial, and agricultural use has resulted in groundwater-level declines in excess of 200 feet (ft) near the city of Palmdale since the early 1900s (Christensen, 1999). Feasibility and Potential Effects of the Proposed Amargosa Creek Recharge Project, Palmdale, California By Allen H. Christensen, Adam J. Siade, Peter Martin, Victoria E. Langeheim, Rufus D. Catchings, and Matthew K. Burgess 2 Feasibility and Potential Effects of the Proposed Amargosa Creek Recharge Project To meet the growing demand for water in the area, the city of Palmdale has proposed the Amargosa Creek Recharge Project (ACRP). The ACRP plans to use the SWP water to artificially recharge the Antelope Valley groundwater system through percolation basins. Purpose and Scope In 2008, the U.S. Geological Survey (USGS) and the city of Palmdale entered into a cooperative agreement to investigate the feasibility and potential hydrologic effects of artificially recharging the groundwater system by infiltrating surface water from the SWP into percolation basins near Amargosa Creek. The purpose of this study was to evaluate the long-term feasibility of the proposed artificial recharge program by improving the understanding of the hydrogeology and geochemistry of the aquifer system underlying the study area and by simulating the potential hydrologic effects of artificial-recharge for a range of aquifer properties. Specifically, the study seeks to estimate a plausible range of artificial recharges rates by using a newly developed groundwater model and various aquifer property scenarios. Several methods were used to help characterize the thickness of valley-fill deposits, the basement geometry, and geologic features that could influence groundwater flow, including regional gravity surveys, seismic refraction and reflection surveys, resistivity surveys, and test borings completed with monitoring wells. Data collected as part of this study and previous studies were used to develop a local-scale numerical groundwater-flow model to simulate changes in water levels and flow resulting from artificial recharge. The calibrated model was used to evaluate the potential effects of two longterm recharge water-management scenarios. General Description of Study Area The ACRP is in the southwestern part of the Antelope Valley, approximately 4 miles (mi) west of Palmdale, in Los Angeles County, California (fig. 1). The proposed ACRP site comprises approximately 150 acres along Amargosa Creek near the San Gabriel Mountain front where the Creek exits Leona Valley (fig. 2). The Amargosa Creek drainage basin covers approximately 30 square miles (mi2) in the Leona Valley along the San Andreas fault zone in the eastern San Gabriel Mountains with land surface elevation varying from 2,750 to 5,200 ft. Amargosa Creek provides drainage for the Leona Valley, then extends southeast from Leona Valley across the San Andreas fault zone on the west side of the city of Palmdale, before turning north near Lancaster and terminating at Rosamond Dry Lake (fig. 2). Average annual precipitation at local weather stations (fig. 2) ranged from about 8 inches (in.) at Palmdale and at Vincent, south of Palmdale, to about 18 in. at Pine Canyon (Western Regional Climate Center, 2011a, 2011b, 2011c). Average annual discharge measurements along Amargosa Creek were not available, but estimates varied between 170 (L.E. Flint, U.S. Geological Survey, written commun., 2011) and 800 acre-feet per year (acre-ft/yr; Los Angeles County Department of Public Works, 1991). Flows in Amargosa Creek are intermittent and usually only run during the winter rainy season or, infrequently, as a result of late summer monsoonal storms (Bloyd, 1967). Although the study focus was near the proposed ACRP, the study area was expanded to include western Palmdale, southern Lancaster, and the area west of Little Rock Wash to simulate the surrounding basement geometry better and to help reduce boundary effects in the groundwater-flow model simulations (fig. 2). Site Characterization The geohydrology of the study area was characterized by using previously published reports and maps; collecting and interpreting gravity, seismic, and resistivity surveys; and correlating geophysical and geologic logs from existing wells with those from the monitoring wells constructed as part of this study. Previous Geohydrologic Studies Several previous investigators have completed regional geohydrologic studies in the Antelope Valley area. Early surveys by Johnson (1911) and Thompson (1929) documented historic hydrologic conditions in the valley, describing the rapid development in the 1920s and the associated regional decline in the water table. Johnson (1911) also described the importance of recharge in the valley from the three main streams on the southwest side of the valley, Big Rock, Little Rock, and Amargosa Creeks. Dibblee (1960) mapped major faults in the valley, including the San Andreas fault zone, which forms the southern boundary of the Antelope Valley (fig. 1). Regional gravity surveying and areal mapping by Mabey (1960) indicated the presence of faults offsetting the basement complex and valley-fill deposits and of many northwest-trending faults that parallel the main strand of the San Andreas fault zone. Bloyd (1967) noted large differences in groundwater-level elevations along the southern boundary of the Antelope Valley near the San Andreas fault zone. Using water-level data, Bloyd (1967) also mapped faults in the valley that trend sub-parallel to the main strand of the San Andreas Fault. These faults form the southern boundary of the Antelope Valley groundwater basin (Bloyd, 1967) and the sout

Feasibility and potential effects of the proposed Amargosa Creek Recharge Project, Palmdale, California