Concentration and transport of chlordane and nonachlor associated with suspended sediment in the Mississippi River, May 1988 to June 1990.

Technical chlordane, a formerly widely used organochlorine pesticide, has become widespread in the environment. The distribution of technical chlordane in riverine environments may be due in part to resuspension and aqueous transport of contaminated bed sediment. To test this hypothesis, the Mississippi River was sampled for suspended sediment five times over a two-year period, at up to 17 sites from St. Louis to below New Orleans, including major tributaries. The ratio of chlordane to nonachlor concentrations averaged 3.6 during May-June 1988 for the Mississippi River below its confluence with the Ohio River. During March-April 1989, the ratio was 0.6, suggesting weathered technical chlordane contributions to the suspended sediment. During June 1989, the ratio averaged 1.1, indicating some input of less weathered technical chlordane. During February-March and May-June 1990, the ratios again shifted, from 0.8 to 1.3. This shifting ratio is likely due to resuspension of weathered technical chlordane associated with bed sediment during spring runoff. Annual transport by suspended sediment from the Mississippi River to the Gulf of Mexico was estimated to be 110 kg of chlordane and 100 kg of nonachlor.