Assessment of nonpoint-source runoff in a stream using in situ and laboratory approaches

Anthropogenic activities that change a watershed can cause adverse impacts to receiving water. Agricultural and urban runoff are the two leading cause s of surface-water impairment in the United Stares. When assessing pollutan t sources and their effects on aquatic ecosystems, and prior to implementin g source controls, it is necessary to define the systems stressors and rece ptors of exposure. Toxicity assays are a key component to integrative asses sments that include habitat (physical), chemical, and indigenous community characterization. Traditional toxicity assay methods and the use of water-q uality criteria are often inappropriate because of exposure design and effe ct assumptions. Hyalella azteca and Chironumus tentans were exposed in situ for varying time periods during both low- and high-flow conditions to dete rmine the effect of urban and agricultural runoff. Short-term chronic and a cute toxicity of urban and agricultural runoff was then measured in the lab oratory and related to in situ test results. Nonpoint-source (NPS) runoff f rom urban areas was often more acutely toxic to organisms in the laboratory as compared to in situ results. Conversely, toxicity to the organisms was greater at the agricultural site during in situ exposures when compared to laboratory. In situ assays were an essential and integral component of NPS runoff assessments. They provided unique information that complemented labo ratory toxicity, habitat, benthic community, and physicochemcial characteri zations.