The hydrologic cycle, which connects surface water, groundwater, and the atmosphere, provides many opportunities for contamination of drinking water, according to Mark Nilles of the U.S. Geological Survey. The influence of contamination by atmospheric deposition can occur by both wet deposition methods (when pollutants are purged from the atmosphere by rain, snow, sleet, or fog) and dry deposition methods, (e.g., gas and particle removal) in the absence of precipitation through gravity and uptake by vegetation. Three such chemicals—ammonia, nitrates, and mercury—can affect human health when nitrates and ammonia enrich aquatic ecosystems in excess and mercury bioaccumulates in aquatic food chains. With specific regard to these three contaminants, Nilles presented status and trend data derived from the National Atmospheric Deposition Program’s National Trends Network—a collection of monitoring sites spread across the nation—which is supported by numerous contributors both public and private, including a wide range of federal and state agencies. A long-term network with multiple sites, it enables researchers from many agencies to use the data to correlate emission trends with potential sources and human and ecological endpoints.


The U.S. Environmental Protection Agency’s (EPA’s) National Air Quality and Emissions Trends 1999 report revealed that ammonia emissions in the United States derive largely from agriculture, with approximately 72 percent from livestock and 16 percent from fertilizer application. This is atypical among dominant anthropogenic contaminants because the association or reported association is not from fossil fuel oxidation. Of 149 sites monitored for ammonia, 64 reported an increase in the rate of ammonium deposition, which was not associated with any particular region of the country. The status in 2002 was that ammonium ion wet deposition (see Figure 3.1) had a background deposition rate of approximately 0.2 kg per hectare. In primarily rural regions such as the Mississippi River basin, the level rose to approximately 5 kg per hectare.

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