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Human activities have increased nutrients above background concentrations

U.S. Geological Circular 1225--The Quality of Our Nation's Waters--Nutrients and Pesticides



The USEPA has established a Federal drinking-water standard or Maximum Contaminant Level (MCL) of 10 mg/L for nitrate.(28) An MCL is a concentration above which adverse human health effects may occur.

The USEPA(29) has established criteria for un-ionized ammonia in surface water because of its toxicity to fish. The chronic criteria vary from 0.07 to 2.1 mg/L of total ammonia for pHs of 6.5-9.0 and water temperatures of 0-30 C.

National criteria have not been established for concentrations of dissolved phosphates in streams or ground water.

National criteria have not been established for total phosphorus or total nitrogen in streams. The USEPA has established a desired goal of 0.1 mg/L total phosphorus for the prevention of nuisance plant growth in streams and other flowing waters not discharging directly to lakes or impoundments.(29)

Effects of human activities on nutrients were assessed by comparing concentrations in streams and ground water to national background concentrations. Waters with nutrient concentrations above background are referred to as "enriched" in this report. Fifty-seven percent of sampled streams were enriched with total phosphorus on the basis of average annual total phosphorus concentrations exceeding national background concentrations. Similarly, 61 percent of sampled streams were enriched with total nitrogen and nitrate, but only 23 percent of sampled streams were enriched with ammonia. Only 1 of 28 relatively undisturbed forested or rangeland streams had average annual concentrations of total phosphorus or total nitrogen above national background concentrations. Most of the streams that were enriched with nutrients drained areas of agricultural and (or) urban land.

In 53 percent of shallow ground-water studies in agricultural and urban areas, median nitrate concentrations were above the national background concentration. Median nitrate concentrations were above background in only 3 of 33 major aquifers studied. Those three aquifers were beneath agricultural areas in three different Study Units.

In most cases, enrichment of streams with nutrients occurred in small watersheds and (or) regions dominated by agricultural or urban land use. Effects of human activities were found in shallow ground beneath agricultural and urban areas throughout the Nation, but not in many of the major aquifers sampled.

Photo Jeffrey D. Stoner

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The Otter Tail River, Minnesota (above), which supports a healthy growth of wild rice, and Fir Creek, Oregon (right), which contributes to Portland's drinking-water supply, are examples of streams with low nutrient concentrations.

Photo of rushing river
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