OCCURRENCE OF NITRATE IN GROUNDWATER - A REVIEW

The results of federal, state, and local surveys, which included more than 200 000 NO3-N data points, are summarized in this review of NO3 in groundwater in the USA. The levels of NO3-N are associated with source availability and regional environmental factors. In regions where well-drained soils are dominated by irrigated cropland, there is a strong propensity toward the development of large areas with groundwater that exceeds the maximum contaminant level of 10 mg/L NO3-N. Most of these areas are west of the Missouri River where irrigation is a necessity. Aquifers in highly agricultural areas in the southeastern USA reportedly are not contaminated. Vegetative uptake and denitrification in this warm, wet, C-rich environment are responsible for the natural remediation of NO3 in shallow aquifers. In the Middle Atlantic states and the Delmarva Peninsula, localized contamination occurs beneath cropped, well-drained soils that receive excessive applications of manure and commercial fertilizer. Extensive tile drainage has for the most part prevented a NO3 problem in the groundwater of the Corn Belt states. Throughout the USA there are recurring themes. They include a decrease in NO3-N levels with depth; lower NO3-N levels in shallow wells (<8 m); and a significant increase in NO3-N in older wells and in wells with poor construction. The factors affecting the distribution of NO, in aquifers are complex and poorly understood. Interdisciplinary studies using discrete depth sampling, hydrogeological indicators, isotopic tracers, and microbiological techniques are necessary to unravel the complex dynamics.