In the assessment design, ground water sampling sites (wells) were grouped together into different "networks." Each network was formed to contain wells in a particular setting -- shallow agricultural wells, shallow urban wells, or the deeper wells of major aquifers. All wells from agricultural and urban networks and major aquifer networks were used. Wells from other ground water networks, such as mining networks, were not included.
It is possible for one well to be in two networks, such as a major aquifer network and an agricultural network, or a major aquifer network and an urban network. In such cases, the well is included in both bars.
Surface water sites were used if enough samples were collected there. In general, all surface water sites were used for the nutrient graphs, but some were not used for the pesticide graphs because they were not sampled often enough to compare to data from other sites. In general, all sediment and tissue sites were used, except the very few sampled for particular, specialized purposes rather than for a general survey of occurrence and spatial distribution.
See these lists for actual sites used for surface water, sediment, and tissue: