Aquatic life water quality criteria derived via the UC Davis method: II. Pyrethroid insecticides.

Aquatic life water quality criteria were derived for five pyrethroids using a new methodology developed by the University of California, Davis (TenBrook et al.2010). This methodology was developed to provide an updated, flexible, and robust water quality criteria derivation methodology specifically for pesticides. To derive the acute criteria, log-logistic SSDs were fitted to the medium-sized bifenthrin,cyfluthrin, and cypermethrin acute toxicity data sets while the X-cyhalothrin and permethrin acute data sets were larger, and Burr Type III SSDs could be fitted to these data sets. A review of the cyfluthrin acute criterion revealed that it was not protective of the most sensitive species in the data set, H. azteca, so the acute value was adjusted downward to calculate a more protective criterion. Similarly, the cypermethrin criteria were adjusted downward to be protective of H. azteca.Criteria for bifenthrin, X-cyhalothrin, and permethrin were calculated using the median fifth percentile acute values while the cyfluthrin and cypermethrin criteria were calculated with the next lowest acute value (median first percentile). Chronic data sets were limited in all cases, so ACRs were used for chronic criteria calculations, instead of statistical distributions. Sufficient corresponding acute and chronic data were not available for bifenthrin, cypermethrin, or permethrin, so a default ACR was used to calculate these chronic criteria while measured ACRs were used for cyfluthrin and X-cyhalothrin. A numeric scoring system was used to sort the acute and chronic data, based on relevance and reliability, and the individual study scores are included in the Supporting Information.According to the USEPA (1985) method, the data sets gathered for these five pyrethroids would not be sufficient to calculate criteria because they were each missing at least one of the eight taxa required by that method. The USEPA (1985)method generates robust and reliable criteria, and the goal of creating the UCDM was to create a method that also yields statistically robust criteria, but with more flexible calculation methods to accommodate pesticide data sets of varied sizes and diversities. Using the UCDM, acute and chronic water quality criteria were derived for bifenthrin (4 and 0.6 ng/L, respectively), cyfluthrin (0.3 and 0.05 ng/L, respectively), cypermethrin (1 and 0.2 ng/L, respectively), X-cyhalothrin (1 and 0.5 ng/L,respectively), and permethrin (10 and 2 ng/L, respectively). Water quality criteria for these five pyrethroids can be used by environmental managers to control the increasing problem of surface water contamination by pesticides.