Selection of target taxa for each Study Unit is based on information from the Retrospective Analysis and water-quality reconnaissance. If this taxon is not present at a site, then whole fish from a single species of fish on the NATT List would be the second-priority taxon to collect. If adequate numbers of these species cannot be collected, then no sample is submitted for organics analysis, except where a nontarget taxon may be of local importance. For trace elements, Corbicula fluminea is again the first-priority taxon for collection. If a sufficient sample cannot be collected at a site, then the second-priority target organisms are aquatic insects from the NATT List.
Stoneflies are lower priority than the caddisflies on the list.
Samples for each taxon are composited for a given site to average individual variability and to meet a minimum mass requirement for the intended analyses. The analytical strategy for bed sediments and tissues is listed in table 9 . The list for tissues is a subset of the list for bed sediments because compounds known to be metabolized by target taxa have been eliminated from the target analyte list for tissues. Analytical strategy for bed-sediment and tissue Occurrence Survey. The Spatial Distribution Survey adds improved geographic coverage with particular emphasis on the assessment of priority constituents identified in the Occurrence Survey.