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This page contains information that can be used to conduct screening and baseline ecological risk assessments at hazardous waste sites. Those assessments require that contaminants be screened to identify chemicals that may pose an ecological hazard. A Screening Assessment involves a comparison of the reported environmental levels of the contaminants with toxicological benchmarks derived from laboratory or field data for a particular species or group of organisms. If a chemical concentration or the estimated exposure level is lower than the lowest calculated benchmark, then the chemical is unlikely to represent an ecological risk. However, if the chemical concentration or the reported detection limit exceeds a benchmark, then further analysis is needed to determine what, if any, hazard is posed by that chemical. The more the chemical concentration exceeds the benchmark value, the more likely that the contaminant poses an ecological risk. Screening benchmarks, therefore, provide a quick way to prioritize contaminants at a particular waste site.