How frequently do nutrient concentrations exceed human-health benchmarks in source-water samples from streams?
Concentrations of nitrate in streams seldom exceeded the USEPA drinking-water standard of 10 mg/L as nitrogen or Maximum Contaminant Level (MCL). Concentrations of nitrate exceeded the MCL in only 2 percent of 27,555 samples, and in one or more samples collected at 50 of 499 streams. No samples exceeded the MCL in samples from undeveloped watersheds. Nearly 30 percent of agricultural streams had one or more samples with a nitrate concentration greater than the MCL. Most streams sites with nitrate concentrations above the MCL drain agricultural watersheds in the upper Midwest, where the use of fertilizer and (or) manure is relatively high. Concentrations exceeded the MCL at fewer streams draining urban land, which most likely reflects lower use of fertilizers on residential lands.
Related Questions
- How frequently do nitrate concentrations exceed human-health benchmarks in samples collected from shallow wells, domestic wells, and public-supply wells?
- Do nutrient concentrations typically exceed naturally occurring (or "background") concentrations in streams and groundwater?
- What are the benchmarks in the Writing and Speaking Scoring Guides? How are they different from the samples?
