September 27, 2010
On Thursday, September 23, the USGS National Water-Quality Assessment Program (NAWQA) released a comprehensive national analysis of the occurrence and distribution of nutrients in streams and groundwater based on water-quality data collected from 1992 through 2004. The report indicates that despite major federal, state, and local efforts to control point and non-point sources and transport of nutrients, concentrations of nutrients have remained the same or increased in many streams and aquifers across the nation since the early 1990s, and may pose a threat to human and animal health, particularly in agricultural areas.
The study found that nutrient concentrations in streams and groundwater in river basins with significant agricultural or urban development are substantially greater than naturally occurring levels. Median concentrations of total nitrogen and phosphorus in agricultural streams are nearly 6 times greater than background levels, and are routinely 2 to 10 times greater than the nutrient criteria recommended by the Environmental Protection Agency (EPA) to protect aquatic life.
The highest concentrations of nitrogen in streams are found in parts of the Midwest, Northern Plains, California Central Valley, and the Northwest, which have some of the most intense applications of fertilizer and manure in the nation. High concentrations are also evident in many streams in the Northeast and parts of the eastern Midwest, but are largely influenced by atmospheric deposition rather than fertilizer application.
Nitrate concentrations above the federal drinking-water standard of 10 milligrams per liter (mg/L, as nitrogen) were found in more than 20 percent of shallow (less than 100 feet below the water table) domestic wells in agricultural areas, raising concerns for human health in rural agricultural areas where shallow groundwater is used for domestic supply. Ingestion of nitrate via drinking water can lead to methemoglobinemia in infants, a condition affecting the oxygen-carrying capacity of the blood, and contributes to the formation of compounds that may have ties to cancer, diabetes, and adverse reproductive outcomes.
Contamination in these shallow wells may warn of future contamination of deeper groundwater, where much of the public water supply is sourced. Current models predict moderate to severe nitrate contamination in groundwater underlying the High Plains, northern Midwest, and areas of intensive agriculture in the East (such as in eastern Pennsylvania and the Delmarva Peninsula) and the West (such as in the Columbia Plateau in Washington, the San Joaquin Valley in California, and the Snake River Plain in Idaho).
The report suggests that significant reductions in sources of nutrients, as well as reductions in transport through the implementation of land and water management strategies, are needed to meet the EPA’s recommended nutrient criteria. Unfortunately, improvements in water quality as a result of reductions in nutrient inputs on the land surface may not be apparent in streams and deeper aquifers for decades because of the slow rate of groundwater movement from the land surface to the subsurface, and efforts to reduce nutrient loading will have to be both significant and sustained in order to gradually restore healthy nutrient levels in both streams and groundwater.
Categories: Conservation, Energy & Environment