Although USEPA drinking-water standards were not exceeded, criteria for the protection of freshwater aquatic life were exceeded in 37 percent of the stream samples (Panshin and others, in press). Concentrations of seven pesticides exceeded criteria for aquatic life; these are the herbicides diuron and trifluralin; and the insecticides azinphos-methyl, carbaryl, chlorpyrifos, diazinon, and malathion. Forty percent of these exceedances are attributed solely to diazinon.
The exceedance of a water-quality criteria indicates a strong probability that aquatic species are being adversely affected. Aquatic life criteria are determined by exposing test organisms to water containing only one pesticide at a time. Most of the samples tested in this study contained mixtures of more than 7, and as many as 22, different pesticides. The toxicity of combinations of pesticides is largely unknown, but there is some potential for additive or interactive effects.
Pesticides were detected in all but one of the 143 surface-water samples collected during calendar year 1993 from four sites--Orestimba Creek, Salt Slough, Merced River, and San Joaquin River near Vernalis. These sites were selected to evaluate how the concentrations of dissolved pesticides vary in contrasting parts of the basin and during different seasons (Panshin and others, in press).
Selected subbasins sampled in the San Joaquin River Basin.
Forty-nine of the 83 pesticides analyzed for were detected. The most commonly occurring ones were the herbicides simazine, dacthal, metolachlor, and EPTC, and the insecticides diazinon and chlorpyrifos. Concentrations of the detected pesticides usually were low, but highly variable: median concentrations of the six most frequently detected pesticides ranged from 0.004 µg/L for dacthal to 0.050 µg/L for simazine, and 10 pesticides had maximum concentrations greater than 1 µg/L. Over half of the pesticides detected have no established aquatic-life criteria, and the potential for these compounds to induce toxicity, endocrine disruption, or impaired immune response is not well known.
The California Department of Pesticide Regulation maintains detailed information on pesticide application. This information includes type of compound, location, date, amount applied, and target crop for each pesticide application. The vast majority of pesticide application in the Study Unit is for agricultural use.
The diversity of crops and pesticides applied is large in the San Joaquin River Basin (California Department of Pesticide Regulation, 1994).
Seventy percent (38 of 54) of the pesticides with known application were detected. Detection frequency is also related to the amount of pesticide applied; 4 of the 6 most commonly detected pesticides were among the 10 most heavily applied of the pesticides analyzed: chlorpyrifos, diazinon, EPTC, and simazine.
There is often a correspondence between the time a pesticide was applied and when, and at what concentration, it was detected (Panshin and others, in press). The maximum application and occurrence generally coincided for 19 pesticides (for example, EPTC), usually during the summer irrigation season. In contrast, several pesticides (for example, chlorpyrifos) attained their maximum concentration in streams during winter runoff rather than at the time of maximum application. This indicates that, in some cases, winter runoff was more efficient than irrigation return flows at transporting pesticides from the site of application to a stream. During the autumn there is neither rainfall nor irrigation, resulting in relatively few detections.
Data for monthly pesticide application in the San Joaquin River Basin and concen-tration in samples in the San Joaquin River near Vernalis show that the occurrence and application are often related in the summer, whereas the concentration may peak in the winter in spite of heavier application in the summer.
Orestimba Creek is typical of the small western tributaries to the San Joaquin River where streamflow is predominantly agricultural runoff during the summer, but may also include large amounts of runoff from the nonagricultural Coast Ranges during the winter. A greater variety of pesticides were detected here (28 herbicides and 12 insecticides) compared with the other sites. During the winter, high concentrations of some pesticides occur for brief periods because of transport by rainfall runoff (see the following section and Domagalski and others, 1997). During the irrigation season, a large number of pesticides--usually greater than 15--were detected (Panshin and others, in press). Pesticides detected more frequently in Orestimba Creek than at the other sites include DDE, dieldrin, fonofos, napropamide, and propargite. The presence of these pesticides is attributed to past or present application primarily on dry beans and truck crops.
Some pesticides were detected frequently in samples from all three subbasins, whereas other pesticides were detected in samples from only one or two subbasins.
Salt Slough drains a low-lying part of the San Joaquin Valley, which includes large areas of wetlands and cotton; the slough does not have a significant upland area within its basin, and its streamflow is dominated by agricultural drainage much of the year. Twenty-five herbicides and eight insecticides were detected at this site. Pesticides detected more frequently at Salt Slough than at other sites were atrazine, cyanazine, diuron, EPTC, malathion, and molinate. The presence of these pesticides is attributed to application primarily on cotton, rice, alfalfa, and truck crops.
The Merced River is one of three tributaries that carry runoff from the Sierra Nevada year round, often as reservoir release, and runoff from agricultural areas during the summer. Although 26 pesticides were detected in this river, the frequency of detection and concentrations were usually much lower than corresponding levels in the other two basins. This relatively low occurrence is due to a combination of factors: the generally coarse-grained soils of the eastern San Joaquin Valley result in little surface runoff during rainfall or irrigation; and pesticides that do reach the Merced River are diluted by the release of the relatively pesticide-free water from a reservoir in the Sierra Nevada foothills.