Peak centiles of chlorpyrifos surface-water concentrations in the NAWQA and NASQAN programs.

We provide upper bound estimates for peak centiles of surface water chlorpyrifos concentration readings within spatial, temporal, and land-use domains of the United States Geological Survey (USGS) National Water-Quality Assessment (NAWQA) and National Stream Quality Accounting Network (NASQAN) programs. These datasets have large overall sample sizes but variable sampling frequencies and, for chlorpyrifos, extremely high levels of non-detections. Point and interval estimates are provided for the 90th, 95th, 99th, and the 99.9th centiles, given sufficient sample size. Overall upper bound estimates for the NAWQA program over the period 1992-2011 for the 90th, 95th, 99th, and 99.9th centiles are <0.005, 0.0066, 0.0214, and 0.0852 ug/L, respectively. The estimation method is based on a survey sampling approach, finding centiles of pooled data across aggregates of site-years. Although the population quantity estimated by a pooled data centile is not the easily interpretable average of population site-year centiles, we provide strong support that it bounds this average by a combination of theory, comparison of NAWQA aggregate and direct estimates, and using modeled populations.

A spatially and temporally explicit risk assessment for salmon from a prey base exposed to agricultural insecticides.

This risk assessment applied a framework for determining probable co-occurrence of juvenile spring Chinook salmon (Oncorhynchus tshawytscha) with agricultural pesticides in the Willamette Basin, Oregon (Teply et al. this issue) to characterize risk to the threatened population. The assessment accounted for spatial and temporal distribution of 6 acetylcholinesterase-inhibiting insecticides in salmonid habitat within the basin and their relative contributions to mixture toxicity estimated from chemical monitoring data. The 6 insecticides were chlorpyrifos, diazinon, malathion, carbaryl, carbofuran, and methomyl. Seasonal distributions of the juvenile salmon prey base across the basin were determined and compared to co-occurrence with the insecticide mixture to determine the probability of prey reduction and reduced production of juvenile fish. Probability of effect on freshwater aquatic invertebrates was based on acute toxicity species sensitivity distributions (normalized to the most potent compound, chlorpyrifos) using a novel approach to apply the toxicological concept of concentration addition to species sensitivity distributions with differing slopes. The chlorpyrifos distribution was then used to determine relative sensitivity among various species tested within the important taxa making up the prey base. A prey base index was devised, incorporating diet composition and prey availability, to evaluate the indirect effects of the insecticide mixture on juvenile salmon production occurring as a result of a reduction in the prey base. Our analysis targeted fish use of backwater and off-channel habitat units, because they generally coincide with agricultural lands in lowlands and represent shallow habitat with limited water exchange. The percentage of agricultural land use within 300 m of critical habitat stream reaches was used to scale chemical measurement data from a site with high agricultural land use across the full extent of the basin to provide estimates of chemical exposure in each reach. Seasonal impacts were evaluated from mean monthly concentrations. Stressor impact on 5 key taxa was evaluated at each time step and for each reach, and the outcome was compared to a conservation threshold assigned to the prey base index. Only 13% of juveniles reared in backwater, off-channel habitat within 300 m of agricultural land. Percent reduction of carrying capacity as a consequence of reduced prey was estimated to be 5% over the entire brood year. This can be considered lost capacity that is probably compensated elsewhere via increased occupancy (emigration to other habitat units within the reach), which is not accounted for in the model.

A spatially and temporally explicit model for determining the exposure of juvenile salmon to agricultural pesticides in freshwater.

In this paper, we present a novel approach for determining the probable co-occurrence of juvenile salmon or steelhead with agricultural pesticides and apply it to spring Chinook (Oncorhynchus tshawytscha) salmon in the Willamette Basin, Oregon. We adapted a published exposure analysis framework by explicitly considering fish migration among habitat units and assuming that habitat use is proportional to habitat quality. Temporal variability in habitat use was accounted for via biweekly time steps over the entire period when a single brood was expected to spawn until the last juvenile migrated to sea. Spatial variability was accounted for at the watershed and reach scale. Exposure to 6 acetylcholinesterase-inhibiting insecticides at any life stage was expressed in terms of the future adults (adult-equivalents; AEQ). Several datasets were available to inform our framework with input values on extent of spring Chinook fish use, habitat quality preferred by juvenile spring Chinook, choice of juvenile life-history pathways, timing of emergence, and timing of migration either in-stream or to sea. We used insecticide concentration profiles constructed from available monitoring data to demonstrate the effect of accounting for variation in space and time on predicted exposure to chemical residues. In contrast to the assumption commonly used in screening-level risk assessments that the entire population in a watershed is exposed, available data applied to our model framework indicate that a small fraction of AEQ juveniles in the Willamette Basin would co-occur with detectable concentrations of the 6 insecticides. Overall, our results indicated that the use of a spatially and temporally explicit framework yields a better understanding of the proportion of organisms potentially impacted by agricultural pesticides.

An ecological risk assessment for chlorpyrifos in an agriculturally dominated tributary of the San Joaquin River.

Single-species toxicity testing of ambient water samples and national-scale probabilistic risk assessment have implicated the organophosphorous (OP) insecticide chlorpyrifos (O, O-diethyl O-(3,5,6-trichloro-2-pyridyl)-phosphorothioate) as a potential chemical stressor of aquatic organisms residing in the lower San Joaquin River basin. This site-specific aquatic ecological risk assessment was conducted to determine the probability of adverse effects occurring from exposure to chlorpyrifos in an agriculturally dominated tributary of the San Joaquin River and to assess the ecological significance of such effects. Assessment endpoints were fish population persistence and invertebrate community productivity. Daily chemical measurements collected over a period of one year were analyzed temporally for frequency, duration, and spacing between events for acute and chronic exposure episodes. Effects thresholds for fish and freshwater lotic invertebrates were determined from single-species laboratory toxicity tests. Potential risk was characterized by the degree of overlap of distributions of exposure events and effects, with consideration given to additive toxicity of other OP insecticides, recovery periods, and duration of chronic exposure (> or = 21 d). Ecological significance was determined by analysis of fish assemblage dietary and reproductive habits in relation to the surrogate invertebrate taxa judged at risk. Results of analysis indicated no direct effects on fish, and indirect effects on fish through elimination of invertebrate food items were considered unlikely. Biological survey information will be necessary to address uncertainty in this risk conclusion, especially as it relates to the benthic invertebrate community. Results of this site-specific risk analysis suggest that fish population persistence and invertebrate community productivity were not adversely affected by measured chlorpyrifos residues during a year-long monitoring period.