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Defining Equilibration Time of Spiked Sediments for Use in Whole Sediment Toxicity Testing: A Post Hoc Assessment

Spiked whole sediment toxicity testing in support of US pesticide registration typically utilizes a  rolonged aging period of spiked sediments prior to organism exposure, during which concentrations of the applied test compound presumably achieve a state of equilibrium between sediment and pore (interstitial) water. In recent years, regulators have requested that compound-specific equilibration periods be defined prior to organism exposure, as opposed to using default aging periods (e.g., 30-days) described in older test methods. A test design for determining a compound-specific equilibration period can be conceptualized relatively easily, however, interpretation of the results can be difficult due to the adsorptive qualities of the compound, organic carbon content of the sediment-pore water matrices, and the conventional analytical methods employed (e.g., liquid-liquid extraction). While partitioning of a compound with a relatively low adsorption coefficient can be interpreted rather clearly, compounds with a higher adsorption coefficient may result in more variable data, which also may not be considered realistic or relevant to anticipated aqueous concentrations (e.g., measured values exceeding solubility). While many data sets can appear to be random or ambiguous, equilibrium partitioning theory can aid in drawing objective conclusions of time to equilibration depending on compound physico-chemical characteristics and pore water organic carbon (OC) content.