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Background Specific Conductivity and Associated 5% Extirpation Estimates in Arkansas
Citation:
Cormier, S., C. Wharton, AND Y. Wang. Background Specific Conductivity and Associated 5% Extirpation Estimates in Arkansas. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-22/215, 2022.
Impact/Purpose:
The Arkansas Freshwater Explorer is an interactive map of a network of stream in Arkansas. The tool provides easy access to observed data for total ions [specific conductivity (SC)] and estimated background SC on a map of Arkansas. The EPA GeoPlatform tool also includes capabilities to extract, analyze, and overlay other data layers and precalculated SC levels estimated to result in the loss of 5% of native aquatic genera. These data can be used to assess the protectiveness of site-specific water quality criteria proposed by third parties. This product was developed for Region 6 and had been externally peer reviewed and cleared for external release based on Region 6’s discretion.
Description:
This report describes the analyses and models used in a weight of evidence for characterizing background specific conductivity (SC) in Arkansas. The ionic composition of waters in the state are described. Formulae for converting total dissolved solids (TDS) to SC are provided for each ecoregion. Stream background was estimated using observed and empirically modeled data by choosing among three options, the objective being to identify the best available estimate of minimally affected background for estimating 5% extirpation. The lowest of three values is recommended for estimating background: the median observed SC at the station, the median of stations within 5 km, or the estimated default background for the ecoregion. In the Mississippi Alluvial Plain, Mississippi Valley Loess Plains, Ouachita Mountains, Ozark Highlands, or South Central Plains, the default background is the stream segment empirically modeled background. In the Boston Mountains and Arkansas Valley, the default background is the station background or the ecoregional 75th centile. The ecoregional estimate is recommended rather than the predicted stream segment estimate because the empirical model consistently over-predicts background in these two ecoregions. These methods, data, and models may be used to assess the protectiveness of site-specific water quality criteria proposed by third parties.