Evaporation ponds as a drainwater disposal management option

Constructed evaporation ponds are being utilized for disposal of saline subsurface drainage waters in San Joaquin Valley, California. These terminal evaporation ponds are located in hydrologically closed basins and/or regions with no surface drainage out of the valley. The saline drainwaters disposed into the ponds are sodium-sulfate or sodium-sulfate-chloride type waters and upon desiccation produces mirabilite and halite. The drainwaters contain excessive levels of trace elements from geochemical origins. The trace element of most concern, is selenium because it bioaccumulates in the aquatic food chain and causes death and deformity of waterbirds attracted to the pond environment. At the present, the only economic drainwater disposal option in the southern, portion of the valley is evaporation ponds. The operation of these ponds is heavily regulated by waste discharge requirements to reduce and mitigate wildlife impacts. A case study on evaporation ponds and bird usage from a drainage district illustrates the extensive monitoring and mitigation required. The prognosis is evaporation basins will be needed for the foreseeable future unless breakthroughs occur in economic and effective drainwater treatment and drainwater reuse options.