Water from some wells in the Regional aquifer in Mojave River groundwater basin have arsenic concentrations in excess of the U.S. EPA Maximum Contaminant Limit of 10 micrograms per liter (μg/L). Coupled well-bore flow and depth-dependent water-quality sample collection for public supply well 5N/4W-31A1S show arsenic concentrations less than the detection limit of 0.12 μg/L in the upper part of the well to a depth of 163 m below land surface (bls). Arsenic concentrations increase with depth to a maximum concentration of 17.6 μg/L 213 m bls. High arsenic concentration in the deeper part of the well are associated with pH’s greater than 9 and dissolved oxygen concentrations less than 0.2 milligrams per liter. An axially-symmetric, radial groundwater flow model, developed using the computer program AnalyzeHOLE, was used to simulate flow to the well under pumping conditions. Model simulations show that modifying the existing well design by eliminating the two deepest screened intervals from 189 to 213 m bls would reduce arsenic concentrations in the surface discharge of the well to 6.7 μg/L with a 30 percent reduction in yield. Well-modification could reduce the need for expensive arsenic removal or blending of waters from different sources to reduce arsenic concentrations in water delivered to consumers.