ABSTRACT
Nitrogen contamination of groundwater in the nitrate (NO3-N) or nitrite (NO2-N) form is of health concern for both humans and animals. Elevated NO3-N concentrations in drinking water have caused infant death from the disease methemoglobinemia. Formation of potentially carcinogenic nitrosamines in the soil from NO2-N and secondary amines is also a health concern. Both NO3-N and NO2-N have been shown to negatively affect the metabolism of domestic animals. Movement of NO3-N and NO2-N (generally referred to collectively as NO3-N) to groundwater is of particular concern in the Southeast because of the unique climatic and geohydrologic regimes of the region. The Southeast climatically is characterized by warm temperatures and relatively high rainfall amounts. Because of the length of the growing season multicropping, which requires multiple applications of N, is commonly practiced. Annual rainfall distribution often also requires use of supplemental irrigation. The combination of relatively high N inputs, high rainfall, and use of irrigation for crop production coupled with areas of permeable soils and geologic materials means that the southeastern US has high potential for groundwater NO3-N contamination.
Current information on NO3-N leaching to groundwater in the Southeast is rather sparse. This chapter discusses processes and factors affecting NO3-N leaching, drawing information both from studies conducted in the Southeast and other regions of the USA or world, and also summarizes available information from the Southeast. Topics discussed include principles governing NO3-N transport to groundwater, physical factors affecting water and NO3-N transport in the southeastern USA, effects of management on NO3-N movement, current and potential impact of NO3-N on groundwater in the southeastern USA, role of models in assessing future impact and for guiding management decisions, and future research needs.
Available research information on NO3-N movement to groundwater in the Southeast indicates that under presently used N application rates for agriculture, both root zone NO3-N concentrations and shallow groundwater NO3-N 304concentrations may exceed the 10 mg L−1 drinking water standard. Studies in both the Maryland and Georgia Coastal Plain have shown concentrations in excess of 10 mg in shallow groundwater. Much less is currently known about deeper groundwater in the southeastern US, but the research information available from the Midwest and other parts of the US and world clearly indicate the potential for NO3-N contamination of deeper groundwater. The main conclusion about the Southeast is that the weathered soils of the region require relatively high input of N for adequate crop production, and that this input coupled with high annual rainfall and irrigation results in significant probability for NO3-N movement to groundwater. Recommendations for minimizing NO3-N leaching to groundwater primarily are management and land use related. Management decisions include using minimum amounts of inorganic fertilizers, carefully controlling amounts of applied animal wastes, designing septic systems with adequate drain field areas, and minimizing percolation. Winter cover crops may be useful because they minimize nutrient availability for deeper movement during the seasonal wet periods of the winter-spring months, when most subsurface movement and percolation occurs. One major land use tool which may assist in limiting NO3-N leaching to groundwater is the use of riparian zones to remove NO3-N from water either by denitrification or by vegetative uptake. Future research needs for the Southeast include new information on the effects of currently used agricultural practices on NO3-N movement to groundwater, and field, laboratory; and modeling work to better understand factors and processes affecting NO3-N leaching for this region.
