Water Quality Assessment of the Lower Cache and the Bayou Deview Rivers, Arkansas: An Assessment of Nutrients, Sediment, Lead, and Acute Toxicity of Acephate and Thiamethoxam

摘要

The Cache River Watershed (CRW) is located in northeast Arkansas and flows through the Delta Ecoregion. The CRW has been modified for agricultural use, especially in the upper section where it was channelized decades ago. Its designated uses by Arkansas Department of Environmental Quality (ADEQ) are for fish and wildlife propagation, primary and secondary recreation, and as a water source for domestic, agricultural and industrial supply. Unfortunately, the standards for these designated uses are not met due to the presence of toxic metals, elevated chloride levels, and dissolved solids. Consequently, the CRW is cited on the 303d list of the ADEQ as being impaired and is included as part of the Mississippi River Basin Initiative (MRBI) project for contributing nutrients, suspended solids and the presence of metals including Pb.;The research goal of this project was to monitor the water quality of the lower section of the CRW for a three-year period. In the study, there were seven sampling sites and the parameters monitored were pH, turbidity, dissolved oxygen (DO), conductivity, temperature, total suspended solids (TSS), nutrients, and lead (Pb2+) concentrations. In addition to water quality monitoring, this project determined the toxicity of two insecticides (Thiamethoxam and Acephate in a laboratory setting) commonly used by farmers in the CRW. The results indicated turbidity, TSS, total and dissolved N increased through the three-year period in all sites with most being significantly different. Meanwhile, total P and dissolved P increased in six of the seven and three of the seven sites, respectively. Among the different factors that could influence these increases was the increased precipitation and a greater number of precipitation events. Obviously, heavy precipitation and the accompanying runoff has the potential to discharge large amounts of sediments and other pollutants into surface water bodies. Other factors that could have influenced the results are land cover, land use, presence or absence of buffer areas and direct farming practices. Decreases in TSS, turbidity, dissolved and total nutrients and Pb from the upstream to downstream at sites of the Bayou DeView River main channel coincided with the increase in the percentage of land dedicated to wetlands and forest. Results of toxicity testing of two commonly-used pesticides revealed no fish toxicity was measured to acephate, while for C. dubia the LC50 was 32.3 ppm. For thiamethoxam, the LC50 for C. dubia was 111.0 ppm while for fathead minnows was 229.3 ppm. The mixture toxicity results were varied with antagonist and additive effects. The water quality monitoring in this watershed provided evidence of the filtering effects of buffer zones and forested areas as well as indicated which sections of the watershed need additional monitoring; toxicity testing for the commonly-used insecticides in the CRW provided much needed information concerning co-occurring pesticides commonly used in agricultural watersheds.