Organochlorine insecticides, such as DDT and toxaphene, were used extensively in the San Joaquin Valley to control agricultural pests. The use of such compounds was banned in the 1970s in the United States because of detrimental effects on wildlife, such as the bald eagle and peregrine falcon. These chemicals are persistent in the environment because they degrade slowly and are tightly bound to soil particles. Contaminated soils from agricultural and urban areas containing these compounds are still entering streams because of soil erosion. Once contaminated soil has entered a stream as sediment, it becomes available to a variety of small aquatic organisms, such as insects that obtain food from the water or bed sediment. These organisms then are eaten by larger organisms, resulting in the contaminants being passed up the food chain in processes known as bioaccumulation. This process can result in concentrations of organochlorine compounds in fish and other biota that are harmful to wildlife and humans that consume them.
Studies done during the 1970s and 1980s documented contamination of both stream bed sediments and aquatic organisms in the San Joaquin River system. In those studies, levels of organochlorine insecticides in the San Joaquin Valley were high compared with other parts of the Nation, and levels in aquatic organisms exceeded guidelines for the protection of fish-eating wildlife in several areas (Brown, 1997). In October 1992, samples of tissue of aquatic organisms and fine-grained bed sediment were collected at 18 sites and analyzed to determine whether the distribution or concentrations of organochlorine insecticides had changed from the earlier studies.
Concentrations of organochlorine insecticides in aquatic organisms and bed sediment were highest in the small western tributaries to the San Joaquin River and in the lower part of the San Joaquin River (Brown, 1997). Concentrations in these areas were still high compared to national values from the 1970s and 1980s. Concentrations in tissue and sediment at the west-side sites were among the highest encountered at NAWQA Study Units. Comparison of 1992 data with data that were available for some sites showed evidence of a decline in concentrations in tissue at those sites. Bed-sediment concentrations appeared similar to historical data, but the historical data were collected using different methods, making direct comparisons difficult. There was a strong correlation between concentrations of DDT in tissue (of clams and fish) and in bed sediment, suggesting that bioaccumulation was taking place (Brown, 1997).
The concentration and variety of organochlorine insecticides in tissue and bed sediment were highest in west-side sites, intermediate in the San Joaquin River sites, and lowest in east-side sites. (Red values - exceeded guidelines for the protection of fish-eating wildlife [National Academy of Sciences and National Academy of Engineering, 1973]. NA - not analyzed; tissue and sediment values in micrograms per kilogram, wet and dry weight, respectively; ND - not detected).
The results of these comparisons indicate that, though these insecticide concentrations might be declining, they may adversely impact aquatic organisms, and hence other wildlife, in the San Joaquin Valley for years to come. An additional potential impact of these compounds has been revealed by recent studies that suggest that organochlorine insecticides can be harmful to the hormone (endocrine) and immune systems of wildlife and humans at much lower concentrations than was previously thought (Colborn and Clement, 1992).
NAWQA did studies on the west-side tributaries and main stem of the San Joaquin River to determine the processes controlling transport of sediment-bound pesticides. Samples of suspended sediment were analyzed for 15 organochlorine insecticides to compare transport during the irrigation season (June 1994) with transport during winter storm runoff (January 1995) (Kratzer, 1998).
Orestimba Creek during a winter storm (left) and irrigation season (right) (photographs by Sylvia V. Stork and Charles R. Kratzer, U.S. Geological Survey, respectively).
The most frequently detected organochlorine insecticides during both the winter storm runoff and irrigation season were p,p´-DDE, p,p´-DDT, p,p´-DDD, dieldrin, toxaphene, and chlordane. Aldrin, endrin, mirex, and lindane also were detected during the winter storm runoff; lindane was also detected during the irrigation season.
Median concentrations of total DDT, chlordane, dieldrin, and toxaphene on suspended sediment were slightly greater during the irrigation season than during winter storm runoff. However, streamflows, suspended-sediment concentrations, and instantaneous loads were substantially greater during the winter storm runoff.
Loads of total DDT were substantially greater in samples collected during winter runoff compared with samples collected during irrigation season at all sites except Hospital Creek.
Most of the calculated whole-water concentrations of p,p´-DDT, chlordane, dieldrin, and toxaphene exceeded the USEPA chronic criteria for the protection of freshwater aquatic life (U.S. Environmental Protection Agency, 1986). In addition, 6 of 16 toxaphene and 1 of 16 p,p´-DDT concentrations exceeded USEPA acute criteria for the protection of freshwater aquatic life (U.S. Environmental Protection Agency, 1986), and 5 of 16 chlordane and 1 of 16 toxaphene concentrations exceeded California drinking-water standards (California Department of Water Resources, 1995).
Most whole-water concentrations of toxaphene exceeded the USEPA chronic criterion for protection of freshwater aquatic life.
Estimated loads of organochlorine insecticides for the entire irrigation season exceeded estimated loads for the January 1995 storm by about 2 to 4 times for suspended transport and about 3 to 11 times for total transport. However, because the average winter runoff is 2 to 4 times the runoff during the January 1995 storm, average winter transport of organochlorine insecticides may be similar to irrigation season transport. The average winter transport is also dependent on long-term seasonal variations in suspended-sediment and organochlorine insecticide concentrations, both of which are unknown. Nevertheless, these findings indicate that runoff from winter storms will continue to deliver a significant load of sediment-bound organochlorine insecticides to the San Joaquin River for an indeterminate amount of time, even if irrigation-induced soil erosion is reduced.