GLOBAL IMPLICATIONS OF NITRATE IN GROUNDWATER ASSOCIATED WITH AGRICULTURAL SYSTEMS

Research from many regions of the world provide anecdotal information on hydrologic and agricultural factors useful for defining global trends in groundwater vulnerability to nitrate contamination. Data from the U.S.G.S. NAWQA program confirm these hypotheses for U.S. agricultural systems. Under agricultural systems, unconfined aquifers are most vulnerable to nitrate contamination with unconsolidated aquifers more vulnerable than carbonate aquifers. Where overlain by permeable soils the probability of contamination increases. Irrigation increases contamination by increasing recharge and nitrogen applications. The corn/soybeans/hogs system produced significantly larger nitrate concentrations than others. Nitrate concentrations under dairy, poultry, cattle and grains, and horticulture systems were similar. Irrigation, particularly in corn/soybean/hogs and horticulture systems were found to have consistently larger nitrate concentrations in the NAWQA data as well as in studies from outside the U. S. Substantial time-lags exist in groundwater responses to changes in agricultural inputs at the surface. If trends in the relation between increased fertilizer use and groundwater nitrate in the U. S. are repeated in other regions of the world, Asia may experience increasing problems because of very large increases in fertilizer use. Inorganic-fertilizer use continues to increase in both American continents, albeit at rates less than those prior to the ‘80s and those in Asia. Groundwater monitoring in Western and Eastern Europe as well as Russia over the next decade may determine if the increased nitrate contamination trend can be reversed in regions where fertilizer use has dropped since the early ‘90s. If the concentrated livestock trend in the United States is global, it may be accompanied by increasing groundwater contamination. Concentrated livestock provide point sources and intense non-point sources of nitrogen in manure disposal areas. A major contributor to groundwater vulnerability is the distribution of irrigation, a practice that is expanding throughout the world, particularly in Asia. Expanded monitoring and research in Asia is warranted to determine if water-quality patterns observed in the U.S. irrigated areas are repeated there.