Ground-water flow and solute movement to drain laterals, western San Joaquin Valley, California; II, Quantitative hydrologic assessment

Abstract

Ground-water-flow modeling was used to estimate ground-water-flow paths and traveltimes to quantitatively assess the hydrologic processes affecting ground water and solute movement to drain laterals. Modeling results were used to calculate the depth distribution of ground water flowing into drain laterals at 1.8 meters (drain lateral 1) and 2.7 meters (drain lateral 2) below land surface. The simulations indicated that under nonirrigated conditions about 89 percent of the flow in drain lateral 2 was from ground water originating from depths greater than 6 meters below land surface. The deep ground water has higher selenium concentrations than shallow ground water. Simulation of irrigated conditions indicates that as recharge increases, the proportion of deep ground water entering the drain laterals decreases. Ground-water-flow modeling also was used to estimate selenium loads in drain laterals for varying drain-lateral flow rates. Simulated loads are in general agreement with measured loads and increase with increasing drain-lateral flow. Simulations further indicate ground water that contains high-selenium concentrations probably will continue to enter drain lateral 2 for more than 8 years.