Long-term monitoring (10 years minimum) of the chemistry of wet deposition will be conducted at National Trends Network (NTN) sites across the United States. Precipitation samples will be collected at sites that represent broad regional characteristics. Design of the NTN considered four basic elements during construction of a model to distribute 50, 75, 100, 125 or 150 sites. The modeling oriented design was supplemented with guidance developed during the course of the site selection process. Ultimately, a network of 151 sites was proposed.
The basic elements of the design are:
(1) Assurance that all areas of the country are represented in the network on the basis of regional ecological properties (96 sites);
(2) Placement of additional sites east of the Rocky Mountains to better define high deposition gradients (27 sites);
(3) Placement of sites to assure that potentially sensitive regions are represented (15 sites);
(4) Placement of sites to allow for other considerations, such as urban area effects (5 sites), intercomparison with Canada (3 sites), and apparent disparities in regional coverage (5 sites).
Site selection stressed areas away from urban centers, large point sources, or ocean influences. Local factors, such as stable land ownership, nearby small emission sources (about 10 km), and close-by roads and fireplaces (about 0.5 km) were also considered. All proposed sites will be visited as part of the second phase of the study.