The objective of the Resident Species Study is to determine whether the EPA’s freshwater or saltwater criteria are appropriate for regulating effluent released into the Santa Clara River Estuary (SCRE) by the Ventura Water Reclamation Facility. The study uses the taxonomic composition of benthic macroinvertebrates (invertebrates) living in the SCRE as the best way to characterize the salinity tolerance ranges of resident species in the estuary. Species composition is the EPAs preferred method, as described in the California Toxic Rule (CTR). In order to use the species composition data to determine the appropriate standard, two determinations are made: 1) comparison of the taxa found in the SCRE with those used by EPA in establishing the ambient water quality criteria for copper; and 2) the salinity tolerances of the taxa found in the SCRE.