Parametric and probabilistic analysis of historical chlorpyrifos surface water monitoring data from the San Joaquin River watershed: 1991-2001

Abstract

The goals of this study were to use both parametric and probabilistic procedures to: (1) analyze historical chlorpyrifos water column monitoring from the San Joaquin River watershed from 1991 to 2001 to assess possible spatial and temporal trends; and (2) determine the probability of exceeding Total Maximum Daily Load (TMDL) water quality targets for chlorpyrifos established by Calfornia's Central Regional Water Quality Control Board. Chlorpyrifos concentrations ranged from non-detected to 2,282 ng L-1 from 49 mainstem and tributary sites sampled from 1991 to 2001. Approximately 62% of the 2,819 chlorpyrifos measurements were below the detection limits, which ranged from 1 to 50 ng L-1 during this 10 yr period. Temporal analysis showed that the highest frequency of detected chlorpyrifos concentrations (44 to 58%) occurred from March through October with the lowest frequency occurring in December, January and February (3 to 13%). Monthly mean concentrations of chlorpyrifos were greater in March, April and May at selected tributary sites with the most extensive data. Results from probabilistic analysis showed that 90th centiles, which ranged from 0.09 to 337 ng L-1, were higher at tributary sites (in close proximity to the source) when compared to mainstem sites. The probability of exceeding the chlorpyrifos acute (25 ng L-1) and chronic (14 ng L-1) TMDL targets was greater at all the pooled tributary sites (22 to 30%) when compared to pooled mainstem sites (≤ 1.6%).