The National Atmospheric Deposition Program (NADP) National Trends Network (NTN) was initiated in 1978 by the Association of State Agricultural Experiment Stations to monitor long-term atmospheric chemistry and the effects pollutants have on aquatic and terrestrial systems (Nilles, 2000). The number of sites increased from 21 in 1978 to 261 in 2006. As of 2010, precipitation samples were collected from a total of 251 sites in the United States (including Alaska, Puerto Rico, U.S. Virgin Islands) and Canada.