The Occurrence Survey is designed to provide an initial identification of important constituents in each Study Unit based on data from few sites. The Spatial Distribution Survey improves geographic coverage for priority constituents through broader areal sampling and improved resolution in priority areas. The design of the Spatial Distribution Survey is determined by results of the Occurrence Survey. Data from both phases are combined for assessing distribution. The primary objective of the Occurrence Survey is to determine which target constituents are important to water-quality conditions in each Study Unit.
Importance is determined by the magnitude of constituent levels and the extent of their occurrence.
The highest level of importance is assigned to constituents found at elevated levels over a wide geographic area or within many small areas, such that they are viewed as significant water-quality issues for a substantial part of the Study Unit. Site selection and sampling strategy are designed to maximize the probability of detecting important constituents in the Study Unit. The choice of additional Indicator Sites is a balance between locating sites where contamination is known to be probable and dispersing sites so that streams that drain each major Environmental Setting in the Study Unit are sampled. Integrator Sites are chosen to provide a coarse downstream network of sites where large-scale contaminant occurrences not detected at the Indicator Sites have a reasonable chance of being detected. The sampling design in each Study Unit thus includes all Basic and Intensive Fixed Sites and additional sites selected specifically to meet objectives for bed sediments and tissues.