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Dara A. Goldrath, John A. Izbicki and Kathryn W. Thorbjarnarson
Water from well 5N/4W-31A1 in the regional aquifer in the Mojave River groundwater basin 97 kilometers north east of Los Angeles, California, occasionally exceeds the U.S. EPA Maximum Contaminant Limit for arsenic of 10 micrograms per liter (μg/L). Coupled well-bore flow and depth-dependent water-quality sampling for this well show arsenic concentrations less than 0.12 μg/L entering the well from the aquifer in the upper 163 meters (m) below land surface (bls). Arsenic concentration increase with depth to a maximum of 17.6 μg/L at 213 m bls. High arsenic in the deeper part of the well are associated with pH greater than 9 and dissolved oxygen concentrations less than 0.2 milligrams per liter. An axially-symmetric, radial groundwater flow simulation, developed using the computer program AnalyzeHOLE, was used to simulate flow to the well under pumping conditions. Simulations show that modifying the existing well design by eliminating the two deepest screened intervals below 189 m bls would reduce arsenic concentrations in the surface discharge of the well about 25 percent with a 30 percent reduction in yield. Such well modification may reduce or eliminate the need for costly arsenic treatment or blending of waters from different sources to reduce arsenic concentrations in water delivered to consumers.