Evaporation ponds contain agricultural drainage water and are used when agricultural growers do not have access to rivers for drainage disposal.
Drainage water is the only source of water in many of these ponds, resulting in extremely high concentrations of salts. Concentrations of other trace elements such as selenium are also elevated in evaporation basins, with a wide degree of variability among basins.
Such ponds resemble wetland areas that birds use for nesting and feeding grounds and may pose risks to waterfowl and shorebirds.
For instance, if bird carcasses are not regularly removed from the ponds, diseases such as avian botulism or cholera can spread. Additionally, high salinity levels may harm the reproductive health of ducks and eared grebes. Also, physical hazards such as flooding, grading, levee maintenance and vegetation control harm established nests and cause bird deaths [see also Kesterson Reservoir].
Due to the impacts on wildlife, evaporation ponds have been a major concern to state officials. In 1993, the Central Valley Regional Water Quality Control Board adopted waste discharge requirements for evaporation ponds.
The California Department of Fish and Wildlife, U.S. Fish and Wildlife Service and evaporation pond operators have used the Tulare Lake Basin as a testing ground for new methods to avoid and mitigate the impacts of agricultural drainage. Resulting guidelines call for new nesting habitat to compensate for losses of embryos and young birds.
Meanwhile, the Central Valley Regional Board has required pond operators to modify evaporation ponds to avoid, minimize and mitigate wildlife impacts at evaporation ponds. However, a short supply of suitable water for new wetland habitat remains a problem, and no accelerated-rate evaporation systems have been developed.
Deterrent programs have been implemented but with mixed success. Some programs, such as firing blank guns to discourage birds from nesting, have had mixed success. The most successful programs have complete enclosure of pond or a mix of deterrents.