Scientific Investigations Report 2006-5005

Scientific Investigations Report 2006-5005

Pesticides in Agricultural Irrigation-Return Flow, Columbia Basin Project, Washington, 2002-04

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The U.S. Geological Survey, in cooperation with the Bureau of Reclamation, analyzed water-quality samples from sites in four irrigation-return flow drainage basins in the Columbia Basin Project (CBP) from July 2002 to October 2004 to determine the occurrence, distribution, and transport of pesticides, in response to a recommendation by the National Marine Fisheries Service to monitor water quality in surface-water return flows in the CBP. Ten samples were collected in the Crab Creek, Sand Hollow, Red Rock Coulee, and Lind Coulee drainage basins throughout the irrigation season (generally April through October) and two samples were collected during the non-irrigation season. Samples were analyzed for water temperature, pH, specific conductance, dissolved oxygen, trace elements, major ions, and nutrients, and a suite of 107 pesticides and pesticide metabolites (pesticide transformation products). Measured pesticide concentrations were compared with drinking-water standards and aquatic-life benchmarks, and the presence of pesticides in samples from the irrigation-return flows was compared with historical data and related to current pesticide use, land use, and other environmental factors.

The four drainage basins vary in size from 19 to 710 square miles and the percentage of agricultural cropland ranges from about 35 percent in Crab Creek basin to 75 percent in Lind Coulee basin. More than 95 percent of the cropland in Red Rock Coulee, Crab Creek, and Sand Hollow basins is irrigated, whereas only 30 percent of the cropland in Lind Coulee is irrigated. Estimates of agricultural crop acreage from 2003 were used to describe agricultural land use during the current study. The largest percentage of agricultural crop acreage in Red Rock Coulee, Sand Hollow, and Crab Creek basins was alfalfa. The second largest percentage of cropland in Crab Creek and Sand Hollow basins and third largest in Red Rock Coulee basin was orchards. During 2003, onions were the second largest percentage of crop acreage in Red Rock Coulee basin. Lind Coulee basin has the smallest percentage of irrigated cropland, and about one-third of the crop acreage in dryland wheat and another one-third in fallow cropland. Nearly one-third of the remaining acreage was planted in irrigated wheat, alfalfa, potatoes, corn, and peas.

Although State of Washington water-temperature standards specify that water-temperature criteria be measured by the 7-day average of daily maximum temperatures, this was beyond the scope of this study, so temperatures measured during this study are only an indication of instantaneous temperatures at the time of sampling. Water temperature in 18 samples was greater than the State of Washington criterion of 16 degrees Celsius for salmon and trout spawning, core rearing, and migration: 7 from Red Rock Coulee, 5 from Crab Creek, 4 from Lind Coulee, and 2 from Sand Hollow. In 11 of these 18 instances, temperature also was greater than the criterion of 17.5 degrees Celsius for salmon and trout spawning, non-core rearing, and migration. State of Washington aquatic-life dissolved-oxygen criteria are listed as 1-day minimum, and the criterion of 9.5 milligrams per liter for salmon and trout spawning, core rearing, and migration was exceeded eight times from June to early October during this study at three sites: two times at Sand Hollow, three times at Red Rock Coulee, and three times at Crab Creek. State of Washington aquatic-life criterion for pH of 8.5 for fresh water was exceeded 12 times: 6 at Red Rock Coulee, 3 at Sand Hollow, 2 at Lind Coulee and 1 at Crab Creek. Concentrations of nitrate plus nitrite in two samples collected from Sand Hollow during the non-irrigation season exceeded U.S. Environmental Protection Agency Maximum Contaminant Level for drinking water.

Forty-two pesticides and five metabolites were detected in samples from the four irrigation-return flow drainage basins: 37 in Sand Hollow, 33 in Lind Coulee, 30 in Red Rock Coulee, and 28 in Crab Creek. Herbicides were the most frequently detected pesticides, followed by insecticides, metabolites, and fungicides. Atrazine, bentazon, diuron, and 2,4-D were the most frequently detected herbicides and chlorpyrifos and azinphos-methyl were the most frequently detected insecticides.

Concentrations of three insecticides and one herbicide exceeded U.S. Environmental Protection Agency or Canadian freshwater aquatic-life benchmarks. Concentrations of the insecticide azinphos-methyl exceeded the aquatic-life benchmark at least once at each of the four sites. Concentrations in samples from Sand Hollow also exceeded the aquatic-life benchmark for chlorpyrifos, lindane, and dinoseb. Water-quality aquatic-life benchmarks generally were exceeded in June and July, during the middle of the irrigation season, except the criterion for dinoseb, which was exceeded in one sample during the non-irrigation season in February 2003.

Pesticide application rates and treatment percentages from the National Agriculture Statistics Service and from a 1995 survey conducted in the study unit by the National Center for Food and Agricultural Policy and total crop acreage in the drainage basins were used to estimate the total quantity of each pesticide applied to agricultural fields in 2003. Thirty-one of the 42 herbicides, insecticides, and fungicides detected in surface-water samples were applied to the major agricultural crops in the drainage basins, and 11 of the detected pesticides are sold for residential application. Eight pesticides that were not reported as used for agricultural or residential purposes were detected in the surface-water samples. The overall pattern of pesticide use depends on which crops are grown in each basin. In drainage basins with predominantly more orchards, greater amounts of insecticides are applied, whereas in basins with larger percentages of field crops, more herbicides are applied. Similar usage patterns were in Crab Creek and Sand Hollow basins: the most heavily applied insecticides were azinphos-methyl, carbaryl, and chlorpyrifos and the most heavily applied herbicide was EPTC. DCPA was the most heavily applied herbicide in Red Rock Coulee basin, followed by the fungicide chlorothalonil, the herbicide EPTC, and the insecticides chlorpyrifos and azinphos-methyl. In Lind Coulee, which has a large percentage of dryland agricultural area, the most heavily applied pesticides were the herbicides 2,4-D and EPTC, followed by the fungicide chlorothalonil. The total amount of non-agricultural pesticides applied by residential homeowners and irrigation districts was negligible compared to total amounts applied to agricultural crops.

A statistical comparison of pesticide concentrations from this study with those in surface-water samples collected in the mid-1990s at Crab Creek and Sand Hollow showed a statistically significant increase in concentrations of diuron and a statistically significant decrease in concentrations of ethoprophos and atrazine in Crab Creek. At Sand Hollow, there were statistically significant increases in concentrations of bromacil, diuron, and pendimethalin and statistically significant decreases in concentrations of 2,6-diethylanaline, alachlor, atrazine, DCPA, and EPTC. A seasonal Kendall trend test on data from Lind Coulee showed no statistically significant trends for any pesticide for 1994 through 2004.

A comparison of pesticide concentrations detected in this study with those detected in previous U.S. Geological Survey National Water-Quality Assessment studies of the Central Columbia Plateau, the Yakima River basin, and national agricultural studies indicated that concentrations in this study generally were in the middle to lower end of the concentration spectrum for the most frequently detected herbicides and insecticides, but that the overall detection rate was at the high end.

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