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Distribution of Dissolved Pesticides and Other Water Quality Constituents in Small Streams, and their Relation to Land Use, in the Willamette River Basin, Oregon, 1996

By Chauncey W. Anderson, Tamara M. Wood, and Jennifer L. Morace

USGS Water-Resources Investigations Report 97-4268, 87 pages, 12 figures, 19 tables, 3 appendices, 1 CD-ROM

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Water quality samples were collected at sites in 16 randomly selected agricultural and 4 urban subbasins as part of Phase III of the Willamette River Basin Water Quality Study in Oregon during 1996. Ninety-five samples were collected and analyzed for suspended sediment, conventional constituents (temperture, dissolved oxygen, pH, specific conductance, nutrients, biochemical oxygen demand, and bacteria) and a suite of 86 dissolved pesticides. The data were collected to characterize the distribution of dissolved pesticide concentrations in small streams (drainage areas 2.6-13 square miles) throughout the basin, to document exceedances of water quality standards and guidelines, and to identify the relative importance of several upstream land use categories (urban, agricultural, percent agricultural land, percent of land in grass seed crops, crop diversity) and seasonality in affecting these distributions.

A total of 36 pesticides (29 herbicides and 7 insecticides) was detected basinwide. The five most frequently detected compounds were the herbicides atrazine (99% of samples), desethylatrazine (93%), simazine (85%), metolachlor (85%), and diuron (73%). Fifteen compounds were detected in 12-35% of samples, and 16 compounds were detected in 1-9% of samples.

Water quality standards or criteria were exceeded more frequently for conventional constituents than for pesticides. State of Oregon water quality standards were exceeded at all but one site for the indicator bacteria E. coli, 3 sites for nitrate, 10 sites for water temperature, 4 sites for dissolved oxygen, and 1 site for pH. Pesticide concentrations, which were usually less than 1 part per billion, exceeded State of Oregon or U.S. Environmental Protection Agency aquatic life toxicity criteria only for chlorpyrifos, in three samples from one site; such criteria have been established for only two other detected pesticides. However, a large number of unusually high concentrations (1-90 parts per billion) were detected, indicating that pesticides in the runoff sampled in these small streams were more highly concentrated than in the larger streams sampled in previous studies. These pulses could have had short term toxicological impli cations for the affected streams; however, additional toxicological assessment of the detected pesticides was limited because of a lack of available information on the response of aquatic life to the observed pesticide concentrations.

Six pesticides, including atrazine, diuron, and metolachlor, had significantly higher (p<0.08 for metolachlor, p<0.05 for the other five) median concentrations at agricutural sites than at urban sites. Five other compounds--carbaryl, diazinon, dichlobenil, prometon, and tebuthiuron--had significantly higher (p<0.05) concentrations at the urban sites than at the agricultural sites. Atrazine, metolachlor, and diuron also had significantly higher median concentrations at south ern agricultural sites (dominated by grass seed crops) than northern agricultural sites. Other compounds that had higher median concentrations in the south included 2,4-D and metribuzin, which are both used on grass seed crops, and triclopyr, bromacil, and pronamide.

A cluster analysis of the data grouped sites according to their pesticide detections in a manner that was almost identical to a grouping made solely on the basis of their upstream land use patterns (urban, agricultural, crop diversity, percentage of basin in agricultural production). In this way inferences about pesticide associations with different land uses could be drawn, illustrating the strength of these broad land use categories in determining the types of pesticides that can be expected to occur. Among the associations observed were pesticides that occurred at a group of agricultural sites, but which have primarily noncropland uses such as vegetation control along rights-of-way. Also, the amount of forested land in a basin was negatively associated with pesticide occurrence, suggesting that riparian growth or runoff from forested lands helped reduce pesticide concentrations.

Estimates of pesticide application also were made for the 16 agricultural study basins. Concentrations of pesticides in streams were significantly (albeit weakly) correlated (p<0.05) with estimated use for only a few compounds that are applied to a wide variety of crop types. Because of the large acreages involved, several compounds that are applied to grass seed were better cor related with the fraction of upstream land use in agricultural production or in grass seed crops than with their respective estimated applications. Application estimates for some compounds, including atrazine and metolachlor, were probably low because of uses that are not indicated in current literature.

Significant correlations were also found among certain individual compound concentrations, and between these and concentrations of suspended sediment. Included in both groups were atrazine and metolachlor, suggesting that environmental factors that mobilize atrazine and metolachlor can mobilize other compounds, and that hydrologic conditions are as important as the specific amount and timing of application in determining the transport of many compounds to the streams. The suspended sediment concentration was not, however, significantly correlated with discharge, and concentrations of only one pesticide were correlated with discharge. Even though correlations between discharge and pesticide concentration were poor, the similar seasonal pattern in both variables is evidence that transport to the streams is related to discharge and consequently to the amount of runoff.

Median concentrations of atrazine, metolachlor, diuron, metribuzin, pronamide, and suspended sediment were significantly higher in the late fall than in the summer. Additionally, winter "baseline" sampling for both atrazine and metolachlor confirmed that median concentrations as high as those in the fall or spring were maintained well past any periods of initial flushing, suggesting that a steady supply of atrazine and metolachlor is retained in soils in the study basins. Two intensive immunoassay studies illustrated variations in pesticide concentration over storm hydrographs. During a large storm with localized flooding, atrazine concentration increased on the rising limb of the hydrograph, started to decrease just prior to peak stage (indicating dilution), and continued to decrease as the water level decreased. Metolachlor concentrations decreased throughout the storm by a factor of two from their concentrations prior to the storm.

The future prospects for successfully correlating the stream loads of certain pesticides with estimates of application rates may be good if current and locally specific rates of application to various crop types can be obtained. Alternatively, atrazine concentration appears to be at least a rough indicator for conditions that move several other compounds, and it was shown that it can be measured relatively cheaply and with good accuracy and precision, with enzyme immunoassays. However, the prevalence of atrazine in stream water throughout the basin precludes its use for prediction of occurrence or concentrations of specific compounds in the absence of other information.

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