Geohydrologic Effects on Drainwater Quality

Abstract

This study was conducted on the water-district scale and provides insight into the effects of percolating irrigation water, local ground water, and regional ground water on the quantity and quality of drainwater from on-farm drains in the western San Joaquin Valley, Calif. Ground-water flow is downward in the upslope areas of the water district, but is upward from depths greater than 25 m below land surface in some downslope areas. Transitional areas exist where ground water flows laterally for distances as great as 3.6 km prior to interception by the on-farm drainage systems. Model results indicate that about 89% of the annual drainflow during 1987–91 originated as recharge directly above the drainage systems, and 11% of the annual drainflow was lateraland upward-moving ground water that originated as recharge in areas upslope of the drainage systems. There is general correlation between drainage systems that discharge high concentrations of selenium and areas that intercept upward-moving ground water.