The Occurrence of Nitrate in Groundwater and the Connection to Nutrient Loading to Surface Water

During the past 6 to 7 decades, nitrate contamination of groundwater has increased dramatically and is a widespread problem in numerous agricultural regions of the USA. The most common sources of nitrate in ground water are inorganic fertilizer, animal feeding operations, septic tanks and residual nitrogen in soil. Total nitrogen application to the land surface in the USA increased from less than 550 tons per year in 1945 to approximately 12,000 tons/year in 2001. There are more than 260,000 animal feeding operations within the USA, of which more than 20,000 are concentrated animal feeding operations.  Fertilizer use in the USA increased from 40 pounds per acre in 1965 to about 140 pounds per acre in 2005. The primary human health problem associated with nitrate is methemoglobenimia (blue baby). Possible links to bladder and ovarian cancer from long term exposure to nitrate concentrations of 2 to 4 mg/l has also been suggested. Nitrate in ground water also contributes to high nitrate and nitrogen concentrations in streams and lakes. Along gaining reaches of streams ground water discharge can deliver significant loads of nitrogen directly to the stream or to the stream bed. The net contribution to a stream from nitrogen in discharging ground water is controlled by complex geochemical and biological processes within the hyphoreic zone. Based on recent data from around the US it is clear that efforts to reduce nitrate contamination of groundwater have not been as successful as intended. Numerous, high yield, surficial, unconsolidated aquifers have been significantly impacted by nitrate contamination. A suite of best management practices (BMPs) have been developed to prevent or minimize leaching of fertilizers to the underlying water table and to streams; however the effectiveness of these BMPs is not clear.