Aquatic life water quality criteria derived via the UC Davis method: I. Organophosphate insecticides.

A new methodology for deriving freshwater aquatic life water quality criteria,developed by the University of California Davis, was used to derive criteria for three organophosphate insecticides. The UC Davis methodology resulted in similar criteria to other accepted methods, and incorporated new approaches that enable criteria generation in cases where the existing USEPA guidance cannot be used.Acute and chronic water quality criteria were derived for chlorpyrifos (10 and 10 ng/L, respectively), diazinon (200 and 70 ng/L, respectively), and malathion(170 and 28 ng/L, respectively). For acute criteria derivation, Burr Type III SSDs were fitted to the chlorpyrifos and diazinon acute toxicity data sets while an alternative assessment factor procedure was used for malathion because that acute data set did not contain adequate species diversity to use a distribution.ACRs were used to calculate chronic criteria because there was a dearth of chronic data in all cases, especially for malathion, for which there was a lack of paired acute and chronic invertebrate data. Another alternate procedure enabled calculation of the malathion chronic criterion by combining a default ratio with the experimentally derived ratios. A review of the diazinon chronic criterion found it to be under protective of cladoceran species, so a more protective criterion was calculated using a lower distributional estimate. The acute and chronic data sets were assembled using a transparent and consistent system for judging the relevance and reliability of studies, and the individual study review notes are included.The resulting criteria are unique in that they were reviewed to ensure particular protection of sensitive and threatened and endangered species, and mixture toxicity is incorporated into criteria compliance for all three compounds.For chlorpyrifos and diazinon, the UCDM generated criteria similar to the long-standing USEPA (1985) method, with less taxa requirements, a more statistically robust distribution, and the incorporation of new advances in risk assessment and ecotoxicology. According to the USEPA (1985) method, the data set gathered for malathion would not be sufficient to calculate criteria because it did not contain data for a benthic crustacean. Benthic crustacean data is also required to use a distributional calculation method by the UCDM, but when data is lacking the UCDM provides an alternate calculation method." The resulting criteria are associated with higher, unquantifiable uncertainty, but they are likely more accurate than values generated using static safety factors, which are currently common in risk assessment.

Comparing linear free energy relationships for organic chemicals in soils: effects of soil and solute properties.

Sorption isotherm data were determined for a set of 8 aromatic organic compounds with varying physical chemical properties in three soils with organic matter of differing quantity and composition. The primary goals of this study were to test single and multiparameter linear free energy relationships on their ability to predict the observed sorption behavior on different types of natural sorbents across a range of solutes and concentrations and to relate the accuracy of the predictions to sorbent and solute structural descriptors. Organic carbon normalized sorption coefficients (K(OC)) predicted using both single and multiparameter LFERs were in good agreement with experimental data obtained at the highest tested aqueous concentrations (average deviation less than 0.1 log units over all solutes and sorbents), but deviations were more substantial (0.59-0.65 log units) at the lowest tested concentrations. For chlorinated benzenes there was a significant correlation between experiment-prediction discrepancies and the aromatic content of the soil organic matter measured by (13)C NMR, and the magnitude of the effect was similar to that observed previously for dissolved organic matter.

Modeling and predicting competitive sorption of organic compounds in soil.

Binary systems consisting of 1,2-dichlorobenzene (12DCB) + competitor were investigated over a range of concentrations of competitor in three natural sorbents with distinct characteristics. Two models, the ideal adsorbed solution theory (IAST) and the potential theory (Polanyi-based multisolute model), widely used in the prediction of multisolute sorption equilibrium from single-solute data, were used to simulate competitive sorption in our systems. The goal was to determine which multisolute model best represented the experimentally obtained multisolute data in natural sorbents of varied properties. Results suggested that for the sorbents and sorbates studied, the IAST model provided much better results. On average, the IAST model provided lower errors (23%) than the potential model (45%). The effect of competitor structure on the degree of competition was also investigated to identify any relationships between competition and structure using molecular descriptors. The competitors chlorobenzene, naphthalene, 1,4-dichlorobenzene, 1,2,4-trichlorobenzene all showed very similar degrees of competition, while benzene, phenanthrene, and pyrene were the least effective competitors toward 12DCB across all sorbents. Different sorption sites or sorption mechanisms might be involved in the sorption of these molecules leading to a lack of competitive behavior. A significant relationship between competitor structure and the degree of competition was observed at environmentally relevant sorbed competitor concentrations for the soil containing the highest fraction of hard carbon (Forbes soil).