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Se~B'cav~ ~" • 5.0 SCREENING LEVEL RISK ASSESSMENT A screening level risk assessment (SERA) was conducted to evaluate potential human and ecological risks associated with the West Pit and the Rito Seco (Appendix D). The SERA addressed: (1) human health risks associated with domestic and recreational use; (2) agricultural irrigation and livestock watering; and (3) potential affects associated with aquatic organisms (i.e., benthic invertebrates and fish) and wildlife (i.e., aquatic and terrestrial birds and mammals). The SERA was conducted in general accordance with USEPA guidance for human health and ecological risk assessment (U.S. EPA, 1989 and 1998). The task structure reflects a human health risk pazadigm including: (1) hazazd identification, (2) toxicity assessment, (3) exposure assessment, and (4) risk chazacterization. Available site data were used to chazacterize conditions in the stream and surrounding habitat, the appropriate chemical constituents, media and mode of constituent transport, and potential human and ecological receptor populations. • A three-step process was used to identify and evaluate the potential effects of constituents transported in groundwater from the West Pit. This screening process included eliminating: (1) all chemicals monitored but not detected in the Rito Seco, (2) all chemical and physical parameters for which the Colorado Department of Public Health and Environment (CDPHE) and USEPA has not developed a water quality criterion, and (3) chemicals for which the mean and maximum concentration at each stream sampling station for the period December 1997 to January 1999 were in complianc licable criteria. The results of these comparisons are presented i ables 5 and 6 of Appendix D. The constituents identified in the three-step process that may affect aquatic life protection were further evaluated in regazd to the number of detections, detection limits, and the applicable USEPA and CDPHE water quality criteria. In addition, the water quality database for the Ri[o Seco was evaluated using the three-step process and TDS, sulfates, and manganese were subsequently identified as in relation to agricultural and domestic water use. Additional screening analyses were conducted for TDS, sulfate, and manganese with the result that TDS and sulfate were removed from further consideration because the concentrations of TDS and sulfate Battle Mountain Resources, lnc. pal00?671reporlslmnrthrptlv16w1rmngtmarch.doc 60 Mnrch?2. /999