Use of Regional Geochemical Survey Data and exploratory statistics to estimate natural condition concentrations for trace metals in stream sediments: a case study for the Cook Inlet watershed,Alaska

The United States and other nations use the term natural conditions or reference conditions in legislation providing for the protection of aquatic ecosystems. Natural or reference conditions are generally used to describe environmental conditions in an aquatic resource before any human introduced pollution. The natural or reference conditions are used in establishing water quality criteria for protection of aquatic resources, helping classify aquatic resources within a regulatory category, and for assessing effectiveness of aquatic resource restoration actions. Regional scale geochemical mapping of stream sediments is available for many countries that when combined with exploratory data analysis and geospatial data can be used to establish screening level natural conditions for trace metals in stream sediments. This approach is applied to the Cook Inlet watershed in Alaska using the US Geological Survey National Geochemical Survey stream sediment trace metals data. Upper concentration limits are developed for As, Cr, Cu, Pb, Hg, Ni, and Zn in the Cook Inlet watershed to aid in screening sediments for potential anthropogenic contamination.