level) sediment quality guidelines. They found that the ERL most accurately predicted no-effects toxicity in benthic organisms. Furthermore, it was shown that as the number of SQGs exceeded increased, the resulting toxicity of the sediment increased as well, providing strong evidence that SCGs are useful in predicting sediment toxicities. It was noted, however, that when chemicals existed in mixtures, the toxicity of sediments increased, possibly at concentrations lower than the ERL and TEL. Vidal and Bay showed that the ERM performed better than the AET (apparent effects threshold) and EqP (equilibrium partitioning) at predicting a non-toxic sediment concentration, and in general was more conservative in its estimates than the SQGQ1 (sediment quality guideline quotient) and MECq (moderate effects concentration). The purpose of their study was to compare common SQGs used for site assessments in California. Both of these studies suggested using multiple SQGs, and gave guidance on selecting the best method based on site characteristics and the contaminants of immediate concern. Drawbacks to the ERL/ERM approach ERLs and ERMs must be used with caution. Overestimating the ability of these values to signal whether or not sediment may be toxic can lead to poor decisionmaking. Certain considerations must be made, and the weaknesses of these values understood, so that ERL and ERM screening levels are used properly. The ERL and ERM are not threshold values to determine whether toxicity will occur - they are relationships between bulk chemical concentrations and toxicity effects that are expressed along a continuum. There is no concentration above which toxicity will occur and below which toxicity will not occur. This fact may be overlooked by some users of ERL/ERM's, and do so could mislead the decisionmaking process. The derivation of ERL and ERM can also cause further misconceptions - since only effects data is used in determining an ERL/ERM, there are also overlapping concentrations where there is no co-occurrence of toxicity. Concentrations that did not elicit a significant effect are left out of the calculation when determining the 10th and 50th percentile values (ERL and ERM respectively). Therefore, within the ranges delineated by the ERL and ERM values, concentrations exist that were found to not have a significant biological effect. Many substances that are found to be very toxic do not have SQGs associated with them. The ability of an SQG to predict toxicity when other substances, without SQGs, are present, is currently unknown. Particle size