of these toxicants within the organism may lead to cumulative toxicity. Bioaccumulation tests use bioconcentration factors (BCF) to predict concentrations of hydrophobic contaminants in organisms. The BCF is the ratio of the average concentration of test chemical accumulated in the tissue of the test organism (under steady state conditions) to the average measured concentration in the water. Freshwater tests and saltwater tests have different standard methods, especially as set by the regulatory agencies. However, these tests generally include a control (negative and/or positive), a geometric dilution series or other appropriate logarithmic dilution series, test chambers and equal numbers of replicates, and a test organism. Exact exposure time and test duration will depend on type of test (acute vs. chronic) and organism type. Temperature, water quality parameters and light will depend on regulator requirements and organism type. In the US, many wastewater dischargers (e.g., factories, power plants, refineries, mines, municipal sewage treatment plants) are required to conduct periodic whole effluent toxicity (WET) tests under the National Pollutant Discharge Elimination System (NPDES) permit program, pursuant to the Clean Water Act. For facilities discharging to freshwater, effluent is used to perform static-acute multi-concentration toxicity tests with Ceriodaphnia dubia (water flea) and Pimephales promelas (fathead minnow), among other species. The test organisms are exposed for 48 hours under static conditions with five concentrations of the effluent. The major deviation in the short-term chronic effluent toxicity tests and the acute effluent toxicity tests is that the short-term chronic test lasts for seven days and the acute test lasts for 48 hours. For discharges to marine and estuarine waters, the test species used are sheepshead minnow (Cyprinodon variegatus), inland silverside (Menidia beryllina), Americamysis bahia, and purple sea urchin (Strongylocentrotus purpuratus). Sediment tests At some point most chemicals originating from both anthropogenic and natural sources accumulate in sediment. For this reason, sediment toxicity can play a major role in the adverse biological effects seen in aquatic organisms, especially those inhabiting benthic habitats. A recommended approach for sediment testing is to apply the sediment quality triad (SQT) which involves simultaneously examining sediment chemistry, toxicity, field alterations, bioaccumulation, and bioavailability assessments that can be used in a laboratory or in the field. Due to the expansion of SQTs, it is now more commonly referred to as "Sediment Assessment Framework." Collection, handling, and storage of sediment can have an effect on bioavailability and for this reason standard methods have been