Water-quality assessment of the L'Anguille River basin, Arkansas

For several years, dissolved oxygen in the L'Anguille River has been reduced to concentrations of less than 5.0 milligrams per liter during the summer and fall. The dissolved-oxygen reduction is due only in part to the municipal-waste discharges which enter the river. In addition, concentrations of pesticides have been reported consistently at one long-term station on the river, and trace metals have been reported at two long-term monitoring sites.

The U.S. Geological Survey conducted an intensive study of the L'Anguille River basin during the summer and fall of 1978. This study was done in cooperation with the Arkansas Department of Pollution Control and Ecology to fulfill the requirements of section 208 of Public Law 92-500. An assessment of the general water quality was made, the causes of stream-dissolved-oxygen reductions were determined, and the occurrence of pesticides and trace metals in the basin was documented. A steady-state, segmented, dissolved-oxygen model was calibrated and used to project simulated dissolved-oxygen profiles.

Pesticides are used extensively in the basin and their occurrence in streams throughout the basin is documented. Concentrations of DDT from the river were as high as 110 micrograms per kilogram in streambed material, whereas 1,600 micrograms per kilogram of DDE and 530 micrograms per kilogram of DDD were found in bottom-feeding fish. In addition, toxaphene concentrations of 45 micrograms per kilogram were found in streambed material and concentrations of 3,400 micrograms per kilogram were found in fish.

Concentrations of iron and manganese, at times, exceeded recommended limits for human consumption. Also, dissolved solids, chloride, and sulfate occasionally exceeded the water-quality standards set by the State.

Streambed materials consist of deposited sand, silt, clay, and organic matter. The respiration of bacteria, fungi, and benthic invertebrates, which feed on the organic matter, accounts for most of the dissolved-oxygen reduction in the river. The sources of the streambed materials include municipal wastes, agricultural fertilizers, fluvial sediment, and natural organic matter. Model projections indicate that a reduction of 60 percent in the streambed-oxygen demand would allow the stream-dissolved-oxygen concentrations to remain at or greater than 5.0 milligrams per liter.