The Grasslands soils are derived largely from the erosion of the marine rocks that form the California Coast Ranges. They contain abundant salt and other trace elements such as arsenic, boron, selenium and molybdenum. Depth to water can be quite shallow, often inundating the root zone of agricultural fields.
The SLD is located in the west side of the San Joaquin Valley. Authorized by Congress in 1960, an 188-mile concrete drain was to connect the San Francisco Bay Delta to the west side of the valley for the purpose of conveying agricultural drainage water for disposal outside of the basin. The drainage water was to come primarily from tile drains which were installed in fields to remove elevated salts and shallow groundwater from production areas.
Approximately 85 miles of the SLD was completed when construction was halted in 1975. Drain water was conveyed from Westlands Water District south of the Grasslands Subarea through the completed section of the SLD to its terminus at Kesterson Reservoir between 1978 and 1986. Tile drain discharges from the alluvial fan lands on the west side were discovered to contain elevated levels of selenium and boron in addition to having high salinity. Selenium is a highly bioaccumulative trace element which, under certain conditions, can be mobilized through the food chain and cause both acute and chronic toxicity to fish and wildlife. Conditions at Kesterson were such that selenium in the Drain water bioaccumulated causing, most notably, deaths, and deformity in birds. With the closure of the SLD, the Westlands Water District does not export drainage water outside district boundaries.