Analysis of diazinon monitoring data from the Sacramento and Feather River watersheds: 1991-2001.

Abstract

The objectives of this study were to: (1) analyze historical diazinon water column monitoring data from inconsistent monitoring programs in mainstem and tributary sites in the Sacramento and Feather River watersheds from 1991 to 2001 to assess possible spatial and temporal trends and (2) determine the probability of measured diazinon concentrations by site or similar pooled sites exceeding various proposed effects benchmarks such as Water Quality Criteria and 10th centiles derived from species sensitivity distributions proposed as target concentrations for Total Maximum Daily Loads (TMDLs). An analysis of diazinon monitoring data from both fixed and rain event sampling from the Sacramento/Feather River watersheds from 1991 to 2001 showed that 90th centiles for 27 different mainstem and tributary sites ranged from 12 to 14,897 ng L(-1). The 90th centiles were generally higher at tributary sites (as compared to mainstem sites) during rain event sampling prior to 1995. A comparison of rain event samples for similar sites sampled in 1994 and 2000 showed that 90th centiles were lower in seven of eight sites in 2000. A comparison of pooled mainstem sites between 1994 and 2000 for rain event data showed a lower 90th centile value for 2000; 90th centiles were also lower in 2000 at all pooled tributary sites and all sites when data from a highly influential site was removed. For various site designations (all sites, pooled mainstem sites etc.) the probability of exceeding the acute and chronic diazinon targets developed by California Department of Fish and Game decreased from 1994 to 2000. These data clearly show progress in the 6 yr period in reducing environmental concentrations of diazinon. Probability of exceeding the 10th centile targets based on species sensitivity distributions for arthropods (the most sensitive taxa to diazinon exposure) was similar and fairly low between years; the highest percent probability of exceedance for any site designation was 20%. Results from a two-way ANOVA using individual measurements from all sites sampled showed a significant decrease during rain events between 1994 and 2000, although the decrease was not equivalent for all sites. Sources of uncertainty identified in the analysis of rain event data from 1994 and 2000 were inconsistent frequency of sampling during rain events for each year, unknown definition of rain events between the two years and non-defined measurement point within the hydrograph of rain events sampled in each year. Analysis of diazinon trends from fixed sampling was limited due to lack of yearly data by site; therefore, only parametric analysis could be conducted. Based on parametric analysis of diazinon monitoring data from fixed sampling sites, the percent detected concentrations were greater than 20% for 12 tributary sites and 5 mainstem sites from samples collected during January-March. On the average over all sites and months, diazinon concentrations have decreased at fixed sampling sites in the Sacramento/Feather River watershed from 1991 to 2001.