Skip to main content
U.S. flag

An official website of the United States government

Development of a new toxic-unit model for the bioassessment of metals in streams

January 1, 2010

Two toxic-unit models that estimate the toxicity of trace-metal mixtures to benthic communities were compared. The chronic criterion accumulation ratio (CCAR), a modification of biotic ligand model (BLM) outputs for use as a toxic-unit model, accounts for the modifying and competitive influences of major cations (Ca2+, Mg2+, Na+, K+, H+), anions (HCO3, CO32−,SO42−, Cl, S2−) and dissolved organic carbon (DOC) in determining the free metal ion available for accumulation on the biotic ligand. The cumulative criterion unit (CCU) model, an empirical statistical model of trace-metal toxicity, considers only the ameliorative properties of Ca2+ and Mg2+ (hardness) in determining the toxicity of total dissolved trace metals. Differences in the contribution of a metal (e.g., Cu, Cd, Zn) to toxic units as determined by CCAR or CCU were observed and attributed to how each model incorporates the influences of DOC, pH, and alkalinity. Akaike information criteria demonstrate that CCAR is an improved predictor of benthic macroinvertebrate community metrics as compared with CCU. Piecewise models depict great declines (thresholds) in benthic macroinvertebrate communities at CCAR of 1 or more, while negative changes in benthic communities were detected at a CCAR of less than 1. We observed a 7% reduction in total taxa richness and a 43% decrease in Heptageniid abundance between background (CCAR = 0.1) and the threshold of chronic toxicity on the basis of continuous chronic criteria (CCAR = 1). In this first application of the BLM as a toxic-unit model, we found it superior to CCU.

Publication Year 2010
Title Development of a new toxic-unit model for the bioassessment of metals in streams
DOI 10.1002/etc.302
Authors Travis S. Schmidt, W.H. Clements, K.A. Mitchell, Stanley E. Church, Richard B. Wanty, David L. Fey, Philip L. Verplanck, Carma A. San Juan
Publication Type Article
Publication Subtype Journal Article
Series Title Environmental Toxicology and Chemistry
Index ID 70037696
Record Source USGS Publications Warehouse
USGS Organization Fort Collins Science Center; Toxic Substances Hydrology Program