Faulting in the Ocotillo-Coyote Wells basin has offset sediments so as to create a complex hydrogeologic environment. Older sediments containing saline connate water are superimposed next to younger sediments saturated with fresh water. Results from nine d.c. resistivity surveys using the Schlumberger sounding technique are evaluated to estimate changes in lithology and water quality with depth. Depth estimates from electrical surveys suggest the conductive saline horizons to be fairly shallow (0-20 meters) in the eastern port ion of the basin and fairly deep (100-900 meters) in the western basin.A finite difference code (UPCON2D) developed by the author simulates a fresh water pumping well in an inhomogeneous and anisotropic media underlain by a saline water portion. The purpose for developing UPCON2D is to assess the potential for vertical saltwater intrusion (upconing) in the Ocotillo-Coyote Wells basin. The code developed first solves the radial groundwater flow equation in terms of pressure and density. The velocity flow field calculated from the updated pressure and density distribution is used to solve a radial contaminant transport equation derived by the author. UPCON2D exhibits significant numerical dispersion when small dispersion coefficients (≤1.0 meters) are used in the solute transport equation, however; with larger dispersion coefficients (≥10 meters) numerical dispersion is negligible.For early time, UPCON2D simulations agree well with an approximate solution developed for upconing assumes that the solute is normally distributed about an abrupt fresh water/saltwater interface. However, for later time or when the amount of upconing becomes greater, the numerical solution exhibits a greater amount of upconing near the pumping well than the approximate solution. It is possible that UPCON2D yields a better representation of the problem for greater amounts of upconing.Three locations are chosen for UPCON2D simulations using parameters compiled from electrical surveys, aquifer tests, groundwater flow models, and previous studies. Simulation locations are chosen on the basis of the estimated fresh water aquifer thickness. The three simulation locations chosen are: Ocotillo, with an aquifer thickness of 790 meters; Yuha Estates, with an aquifer thickness of 220 meters; and Crucifician Thorn, with an aquifer thickness of 152 meters.