As a component of the ecological assessment for the Carson River Mercury Site, EPA conducted a survey of benthic macroinvertebrates to evaluate the potential adverse effects of mercury in surface water and sediment of the Carson River. Measures of benthic community response to mercury contamination derived from the benthic survey are presented in this the EcoRisk Assessment, Appendix C. A characterization of the existing environment at the sampling locations-including the habitat characteristics, fish and wildlife populations, and surrounding land use.

Rapid BioAssessment Protocol II (RBP) involves the standardized collection, enumeration, and identification of major benthic taxa to the family level in riffle/run habitats. Standard data sheets are used to record results at each sampling. ___location. Collections are compared to a reference ___location, and an evaluation is made as to the impairment, or lack thereof, of the benthic community from stressors within the watershed.

The biological condition of the community can be ranked according to three categories of impact: nonimpaired, moderately impaired, or severely impaired, where the degree of impairment implies less than optimal community structure, the loss of intolerant species, and dominance by a few tolerant taxa compared to background. RBP II also involves assessment of habitat characteristics such as stream flow, bottom conditions, and riparian vegetation. In addition, standard water quality parameters such as dissolved oxygen, temperature, pH, conductivity, color, and turbidity are determined. The RBP II was conducted on August 17 and 18, 1994 during a period of drought.