For more than four decades, the federal Central Valley Project (CVP) and the State water Project (SWP) have supplied water to growers irrigating approximately 1.3 million acres of drainage-impaired lands on the west side of the San Joaquin Valley and the Tulare Lake Basin.
These lands are naturally contaminated. They formed as an ancient seabed, and as wetlands of period dried up, toxic elements such as selenium, boron, molybdenum, mercury and arsenic concentrated heavily in the soils and rocks.
When irrigation service provided by the CVP and SWP began in the western San Joaquin Valley and the Tulare Basin, agricultural drainage water concentrated and mobilized these contaminants. This ultimately led to the wholesale poisoning of Kesterson National Wildlife Refuge in 1983.
The toxicity issues associated with the cultivation of these impaired lands have been known since the 1950s, and the state of California initially committed to providing drainage service to the area as part of the electorate’s approval of the State Water Project in 1960. However, the state ultimately was unable to follow through with this plan. Subsequently in 1960 ,the federal Bureau of Reclamation was given responsibility for drainage service when Congress created the San Luis Unit of the Central Valley Project, a precinct that contains the gigantic Westlands Water District and several smaller irrigation districts.
The irrigation of these lands with water exported from the Delta adds enormous quantities of salt to the already-saline soils of the western Valley: as much as 4,000 tons of salt daily (the equivalent of 40 railroad cars). Only 1,700 tons of salts exit the basin daily in runoff and drainage to the San Joaquin River.
When plants utilize irrigation water, they leave salts behind, which build up in the soil. To maintain viable agriculture in impaired areas of the San Joaquin Valley, up to half an acre-foot of water (about 160,000 gallons) must be applied to each acre of land. This “pre-irrigation” process leaches salts out of the root zone — but it also mobilizes selenium, molybdenum, arsenic, chromium and other toxic elements in the soils.
Contaminated drainage water from San Joaquin Valley agriculture percolates into aquifers, toward wetlands, the San Joaquin River and its tributaries. Underlying much of the San Joaquin Valley is an impermeable stratum known as the Corcoran Clay. At this juncture, the percolating water collects, ultimately rising back into the root zone. Without periodic flushing, the land would permanently “salinize” unable to support most vegetation – including food crops.
But contaminated drainage water poses public health issues even in those regions of the San Joaquin Valley where the Corcoran Clay is absent. In such areas, contaminated drainage water percolates into the aquifers that provide drinking water to many Valley residents.
Initially the Bureau and the State planned to build a San Luis Master Drain to the Bay-Delta estuary near Antioch, but construction of the drain was stopped after 93 miles were completed to the Kesterson Reservoir near Los Banos. In 1983, birds and other wildlife were found to have been poisoned with high levels of selenium. Bird embryos were deformed and dead.
The US Geological Survey estimated in 2006 that even if the San Luis Drain were completed, irrigation of the San Luis Unit (including the Westlands Water District) of the Central Valley Project were halted, and 42,500 pounds of selenium (link) a year were discharged into the Delta, it would take 65 to 300 years to eliminate the selenium already built up in Valley groundwater.
Farmers and water districts throughout the western San Joaquin Valley try to reduce their drainage water. They recycle, blend, drip irrigate, and reuse their delivered water, and are successful in some cases in reducing selenium, salt and other discharges that have polluted the San Joaquin River. However, retiring these lands from irrigated agriculture remains by far the most cost-effective and reliable method to eliminate harmful drainage discharges to the river, the wetlands, and aquifers of the San Joaquin Valley.
The Westlands Water District has already retired 100,000 acres of land once irrigated with federal water. Any long-term solution to the western Valley’s drainage problem must focus on larger-scale land retirement from irrigated production, improved irrigation practices, and application of new technologies where appropriate. Any approach not founded on land retirement will ultimately continue to store and concentrate toxic selenium and salts in the shallow aquifers where they may be mobilized by flood events or groundwater percolation.