 That is, when initial screening indicates metals concentrations in excess of guidelines, [SEM]/[AVS] is calculated to determine if sufficient sulfide is present to mitigate metal bioavailability.In a 2002 guidance manual for assessment of contaminated freshwater sediment, the US Environmental Protection Agency (EPA) listed [SEM] - [AVS] (a variant of the approach in which data are normalized by subtraction rather than division) as a "Moderate Priority" metric of ecosystem health.Since 2005, EPA Equilibrium Partitioning Sediment Benchmarks (ESBs) for metal mixtures have been derived from a combination of Water Quality Criteria (WQC) and a variant of [SEM]/[AVS] corrected for the organic carbon content of the sediment.