Nitrate: The Persistence

Although nitrate contamination isn’t limited to agricultural activity, the fate and transport of nitrate affects domestic single family style well drinking-water delivery systems. Aquifer vulnerability depends on soil-drainage characteristics—the ease with which water and chemicals can seep to ground water—and the extent of cropland versus woodland in agricultural areas. Denitrification and plant uptake can occur beneath forests bordering streams near cropland, and precipitation seeping through forest soils to ground water contains less nitrogen than seepage beneath an agricultural field 1. Areas with a high risk of ground water contamination by nitrate generally have high nitrogen inputs from fertilizers, animal manure, atmospheric deposition of nitrogen compounds, well-drained soils, and less extensive woodland relative to cropland. Other sources of nitrate contamination can occur closer to household wells to include leaking septic tanks and ineffective septic leach fields. Wastewater treatment facilities that do not specifically remove nitrogen can lead to excess levels of nitrate in surface or groundwater. Nitrate that enters from the atmosphere carries nitrogen-containing compounds derived from automobiles, coal-fired generating units and other combustion sources 2.

Ground water harvested in the United States from domestic single family style well drinking-water delivery systems may be contaminated by nitrate. Areas within the United States that are at risk for ground water nitrate contamination are shown in Figure 1. The Midwest has a high risk of ground water contamination by nitrate, but parts of the western and northeastern United States also are high-risk. These areas have high nitrogen input and vulnerable aquifers. Nitrate can persist in ground water for decades and accumulate to high levels as more nitrogen is applied to the land surface every year. The national risk map indicates contamination potential, but does not show actual nitrate contamination of ground water. Water-quality testing is necessary to determine actual nitrate concentration. Compared with public-supply wells, most privately owned wells are not routinely monitored for water quality 3.

Approximately 15% of Americans who live in the United States rely on a personal water supply to provide drinking water for their household. The majority of domestic single family-style wells draw from ground water as their primary source of drinking water. The USGS estimated 98% of self-supplied drinking water is from ground water wells 4. Furthermore, the USGS documented nitrate levels above maximum contaminant level (MCL) of 10 mg/L in over 20% of shallow drinking water wells in agriculture areas of the United States 5. Meaning, nitrate contamination is more of a problem in shallow aquifers less than 100 feet below the ground surface. Therefore, the necessary task for homeowners is to be aware of aquifers in the United States that would be more at risk for nitrate contamination.

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