National Water-Quality Assessment (NAWQA) Program
Wong, C.S., Capel, P.D., and Nowell, L.H.
The occurrence and distribution of 33 chlorinated organic contaminants in stream bed sediments and biota (fish and freshwater clams) sampled between 1991 and 1994 are summarized. These contaminants include past-use organochlorine insecticides (e.g., DDT and chlordane), some currently used pesticides (e.g., permethrin, dacthal), and industrial chemicals and byproducts (e.g., PCBs, HCB). Samples were collected as part of the U.S. Geological Survey's National Water Quality Assessment (NAWQA) at about 500 stream sites in 20 NAWQA study areas throughout the United States. Sites were located within agricultural and urban watersheds and in associated major rivers draining large basins with mixed land uses. Nationally, detection frequencies are highest in sediments and biota for the more persistent organochlorine compounds (e.g., DDT and metabolites, chlordanes, and PCBs) than for currently used pesticides (e.g., lindane, permethrin, dacthal). Organochlorine compounds were detected more frequently in whole fish than in clams and sediments. Concentrations were highest in urban areas and in agricultural regions with a high use history. PCB and DDT concentration levels in both sediment and biota generally were highest in the northeast. Some organochlorine concentrations exceeded guidelines for the protection of benthic organisms and(or) human health. Comparison with previous studies of this scope suggests a gradual decrease in organochlorine contaminant levels in sediments and fish.