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Typically, single default values for the organic carbon\u2013water (KOC) and dissolved organic carbon\u2013water (KDOC) partition coefficients are used to estimate the bioavailability of a given pyrethroid across sample sites. However, sorption capacities can vary across samples due to the varying quality of organic carbon among other factors. In this study, KOC and KDOC coefficients were experimentally measured for 6 pyrethroids (bifenthrin, cyfluthrin, cypermethrin, esfenvalerate, lambda-cyhalothrin, and permethrin) in 19 streambed sediment samples collected across California, USA. The purpose of this study was to examine the variability of site-specific coefficients, compare site-specific with established default coefficients, and use statewide sediment\/water chemistry and Hyalella azteca toxicity testing data to assess the variability in predicted pyrethroid toxic units and organism survival. Across all pyrethroids tested, measured site-specific log KOC values ranged from 5.32 to 7.11 and log KDOC values ranged from 5.28 to 7.41. Toxic units estimated using site-specific coefficients and default coefficients derived from the geometric means of measured values, explained a comparable amount of the variance in reported H. azteca survival and closely aligned with 50% organism survival. We recommend the use of the default coefficients derived from this study, which are reflective of 19 unique sediment samples. However, the uncertainties associated with site-specific considerations should be acknowledged.<\/jats:p>","DOI":"10.1093\/etojnl\/vgag182","type":"journal-article","created":{"date-parts":[[2026,7,21]],"date-time":"2026-07-21T16:13:44Z","timestamp":1784650424000},"page":"2840-2852","source":"Crossref","is-referenced-by-count":0,"title":["Variability in site-specific pyrethroid\n                    <i>K<\/i>\n                    OC and\n                    <i>K<\/i>\n                    DOC partition coefficients and implications for predicted aquatic toxicity"],"prefix":"10.1093","volume":"45","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-2554-1666","authenticated-orcid":false,"given":"Berkley N","family":"Anderson","sequence":"first","affiliation":[{"name":"Department of Civil and Environmental Engineering, University of California, Davis , Davis, CA,","place":["United States"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-7217-4753","authenticated-orcid":false,"given":"Thomas M","family":"Young","sequence":"additional","affiliation":[{"name":"Department of Civil and Environmental Engineering, University of California, Davis , Davis, CA,","place":["United States"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"286","published-online":{"date-parts":[[2026,7,21]]},"reference":[{"key":"2026100103520636600_vgag182-B1","doi-asserted-by":"publisher","first-page":"966","DOI":"10.1897\/04-146R1.1","article-title":"Use and toxicity of pyrethroid pesticides in the Central Valley, California, USA","volume":"24","author":"Amweg","year":"2005","journal-title":"Environmental Toxicology and Chemistry"},{"key":"2026100103520636600_vgag182-B2","doi-asserted-by":"publisher","first-page":"1700","DOI":"10.1021\/es051407c","article-title":"Pyrethroid insecticides and sediment toxicity in urban creeks from 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