National Water-Quality Assessment (NAWQA) Project
A reconnaissance survey of Hg in water, streambed sediment, and fish at 291 sites across the Nation was conducted from 1998 through 2005. The objectives of the survey were to 1) assess total mercury (THg) and methylmercury (MeHg) in water and streambed sediment, and THg in fish; 2) span regional and national gradients in mercury source strength (deposition, mining) and factors that govern mercury cycling; 3) identify ecosystem characteristics that favor the production and bioaccumulation of MeHg; and 4) explore environmental factors that may drive regional and national variations in fish mercury concentrations.
Site selection targeted several specific settings thought to be important to the source, transport, or speciation of mercury in aquatic ecosystems. Settings included
Land cover in reference settings included forest, rangeland, non-cultivated agriculture (hay, pasture, orchards), and wetland. Each site was sampled once, typically during summertime low-flow conditions, for mercury and related constituents in stream water, streambed sediment, and top predator (piscivorous) fish. Largemouth bass were targeted; at sites where bass were not available in sufficient numbers, alternative top predator fish species were selected.
Results from the complete study, from 1998-2005, are described by Scudder and others (2009).