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Scientific Investigations Report 2007–5180

Scientific Investigations Report 2007–5180

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Pesticide Occurrence and Distribution

An estimated 146 organic pesticides2 were applied to crops in the Yakima River Basin during the 2000 growing season (table 3). Estimates were based on county-level agricultural statistics from the National Agricultural Statistics Service (NASS) and were verified and corrected in interviews with private crop chemical consultants and agriculture-extension agents in Kittitas, Yakima, and Benton Counties. Data on right-of-way applications were obtained from State and local transportation departments and irrigation districts. Details of the pesticide compilation are provided by Ebbert and Embrey (2002). Seventy-five of the 146 applied pesticides (51 percent) were analyzed for this study, and of these 75 pesticides, 47 were detected (63 percent). Only glyphosate (Roundup®, Rodeo®) was applied in large amounts, but not analyzed in this study. Pesticides that were applied but not often detected were applied in small quantities or have chemical properties that inhibit their transport to waterways, such as rapid degradation or a large soil organic carbon-water partitioning coefficient (Koc). Summaries of detected pesticides and their concentrations in water samples are provided in table 4 and figure 5, respectively. Screened and unscreened summary statistics are presented for the reader (see Sidebar 1: Pesticide Reporting Levels).

2 Inorganic, petroleum, and biological controls also are used as pesticides in the Yakima River Basin, but are outside the scope of this report.

Sidebar 1: Pesticide Reporting Levels

The minimum concentration reported by the NWQL varies from analyte to analyte. Pesticides routinely are detected below the established laboratory RL because the analytical methods are considered “information rich” and use multiple lines of evidence to identify and quantify an analyte (Childress and others, 1999). Analytes detected and reported below the RL are noted as such by the NWQL.

Comparisons among pesticides with different RLs can misrepresent the frequency and distribution of occurrence. Pesticides with lower RLs may seem to be distributed more widely or to be detected more frequently in the water than pesticides with a higher RL. For these types of comparisons, pesticide concentrations were screened at a concentration of 0.020 µg/L, which represents the lowest common RL for all pesticides that were detected. Unless specifically noted, data presented in this report are unscreened.

Samples were analyzed for 45 pesticides that had no record of application (table 5). Seven of these pesticides were detected. Their presence is a minor footnote to the larger picture of pesticides in the Yakima River Basin. These seven pesticides were rarely detected, and when they were, concentrations were near the laboratory RL. Application of some pesticides might be unrecorded because they were secondary ingredients in pesticide formulations, they were from supplies left over from previous seasons, or they were applied by a farmer who did not participate in the statistical survey. Two pesticides (dieldrin and dinoseb) are no longer registered for use in the United States, and the detection of these compounds is most likely of residuals from past use.

The most frequently detected pesticide during the July 2000 sampling was 2,4-D. Eighty-two percent of all samples collected contained 2,4-D at concentrations exceeding 0.02 µg/L. Three detections exceeded 1 µg/L, including two from canal-water samples in the Kittitas Valley. Although these concentrations are high in the context of this study, they are well below the U.S. Environmental Protection Agency (EPA) drinking water maximum contaminant level of 70 µg/L (the only available regulatory benchmark; U.S. Environmental Protection Agency, 2004a). Between 15 and 20 percent of all the 2,4-D used in catchments sampled for this study was applied to control weeds along road, canal, and drain rights-of-way. Because most of these rights-of-way were along flowing waterways, drift and overspray might result in the direct application to waterways.

The second most frequently detected pesticide in July 2000 (based on screened values) was the insecticide azinphos-methyl, which was found in 37 percent of the samples. Every occurrence of azinphos-methyl exceeded the chronic guidelines for the protection of aquatic organisms (0.01 µg/L) established by the EPA (U.S. Environmental Protection Agency, 2004b). In the Yakima River Basin, azinphos-methyl was used almost exclusively in tree-fruit orchards, where it was applied with airblast sprayers. Drift and overspray probably contributed to its widespread occurrence. In an environment similar to the Yakima River Basin, on a relative calm day late in the growing season, Schultz and others (2001) reported deposition rates of azinphos-methyl 15 m downwind of an orchard equal to 1.2 percent of the deposition rate in the orchard. They concluded that this was a best-case scenario. Spray drift deposited directly on water or on an adjacent field that is rill irrigated can facilitate its transport from the field of use. In addition, this author has observed runoff from a sprinkler-irrigated orchard flowing into roadside ditches or regional drains on two separate occasions at different sites. Anecdotal reports from farmers in the basin confirm that these observations were not isolated instances.

Pesticide Mixtures

More than 90 percent of the July samples and 79 percent of the October samples contained at least two pesticides or degradates (fig. 6). In July, the median number of chemicals in a mixture was 9, and the maximum was 26. In October, the median number of chemicals in a mixture was 5, and the maximum was 13. The most frequently occurring herbicides in mixtures were atrazine, 2,4-D, and the degradate deethylatrazine. The most frequently occurring insecticides in mixtures were azinphos-methyl, carbaryl, and p,p’-DDE (a degradate of DDT).

Research to understand the impact of mixtures of pesticides on human health and aquatic life is in its early stages (Mileson 1999/2000; Richardson and others, 2001). For some pesticide mixtures, test organisms in laboratory studies are affected by the compounds in the mix as if they were exposed to each compound individually (additive effect)—the total toxicity to the test organisms is represented as the toxicity due to compound 1 plus the toxicity due to compound 2 plus the toxicity due to compound 3, and so on. For other mixtures, test organisms respond as if they were exposed to lower concentrations of both compounds; that is, the mixture is less toxic than the summation of the individual compounds (antagonistic or protective effect). The opposite effect also has been observed—mixtures of some pesticides are more toxic than their individual components (synergistic effect) (Danish Veterinary and Food Administration, 2003).

The EPA has taken the initial steps to regulate mixtures of pesticides by conducting exposure and risk assessments for groups of chemicals having a common mode of toxicity. The first of these assessments has been completed for the organophosphate insecticides, which include azinphos-methyl and diazinon (U.S. Environmental Protection Agency, 2002b). Additional cumulative risk assessments are in progress for N-methyl carbamate, triazine, and chloroacetanilide pesticides. Changes in pesticide handling and application required by these risk assessments are designed to protect human health, but also could help reduce environmental contamination. Guidelines to protect aquatic life, however, remain based on single chemical exposures and, for reasons already noted, probably do not reflect the actual toxicity when multiple pesticides are present. In addition, existing guidelines are based on mortality from direct toxicity and do not consider behavioral (Sholtz and others, 2000) and physiological changes (Hayes and others, 2006) that are detrimental to the reproduction and survival of an organism.

Pesticide Degradates

In addition to the analyses for parent pesticides, samples were analyzed for 54 pesticide degradates (table 6). Degradates are important because many maintain pesticidal action. Some pesticides, such as diazinon, produce a degradate that is more toxic than the parent (Agency for Toxic Substances and Disease Registry, 1996). Understanding and monitoring degradates can provide insight into the pathways through which pesticides are transported into waterways. During this study, 14 degradates were detected among the July and October samples. Four triazine herbicide degradates (deethylatrazine, deisopropylatrazine, deethyldeisopropylatrazine, and 2-hydroxy-atrazine) were among the most commonly detected. Degradates of the insecticides carbaryl (1-naphthol and 1,4-naphthoquinone) and DDT (p,p’-DDE and 4,4’-dichlorobenzophenone) also were detected regularly in water samples. DDT was used widely in the Yakima River Basin prior to its cancellation in 1972.

Spatial Distribution

The Yakima River Basin has three large, distinct agricultural areas with about 450,000 ha in cultivation: the Kittitas Valley, the Mid Valley, and the Lower Valley (fig. 7). The Kittitas Valley is the most northern of the agricultural areas. Most of the farmland in this region is devoted to raising timothy hay. Other hays, small grains, sweet corn, potatoes, and apples also are grown in this area. The Mid Valley agricultural area surrounds the city of Yakima. Hops and fruit orchards (primarily apples and pears) are the major crops produced in this part of the Yakima River Basin. The Lower Valley begins south of the city of Yakima and extends to Richland, and is the largest of the three agricultural areas. A large variety of crops are grown in this region, including tree-fruit orchards (apples, pears, cherries, nectarines, apricots), juice grapes, wine grapes, feed corn, alfalfa hay, hops, asparagus, mint, sweet corn, potatoes, and onions, along with a variety of other minor crops. In addition, more than 250,000 cattle are raised for milk and beef in this area and are an important part of the agricultural landscape.

The spatial distribution of pesticide detections in the Yakima River Basin is summarized in table 7. Sites in the Kittitas Valley were dominated by detections of herbicide and herbicide degradates. Sites in the Mid Valley and Lower Valley contained complex mixtures of herbicides, fungicides, insecticides, and degradates, which reflect differences in crop patterns noted above.

Temporal Distribution

With few exceptions, samples collected in July contained more pesticides than samples collected in October (table 7). In addition, the number of pesticides detected in July was larger (63 pesticides and degradates) compared with October (27 pesticides and degradates; table 4). Concentrations of pesticides generally were greater in July than in October or they were comparable between the two samples. Rarely were concentrations greater in October. These observations are consistent with greater pesticide use during the growing season and the availability of water to transport them off the plants and fields to which they were applied.

It is notable however, that most of the samples collected in October did contain at least one pesticide (82 percent contained deethylatrazine and 70 percent contained atrazine). Few insecticides or insecticide degradates were detected in October. In mid-October 2000, the irrigation canals were drained, and at most sites, the water in the streams and drains at the time of sampling was entirely ground-water discharge, which occurs either as seepage directly into open-channel drains and streams or as seepage into onfield and regional tile drains. It is possible, however, that a few sites might have been receiving water from onfarm storage ponds that were being drained at the end of the season. The incidence of pesticide detections after mid-October suggests there is widespread, low-level contamination of the shallow ground water in agricultural areas of the Yakima River Basin. Subsequent investigations in the Lower Valley have provided additional information about the role of ground water in pesticide transport (Capel and others, 2004; Steele and others, in press, 2007).

Based on the limited data available, it seems unlikely that insecticides are distributed widely in the ground water. However, chronic, low-level exposure to a mixture of herbicides and their degradation products is likely for residents drinking from shallow wells in some parts of the Yakima River Basin. The effects of long-term exposure to low-level herbicide concentrations have not been well studied, in part because widespread herbicide use dates only to the 1960s, and chronic health effects often take decades to manifest themselves. The few studies available in the literature suggest a potential link between long-term, drinking water exposure to herbicide exposure and increased incidences of cancer (Kettles and others, 1997; Van Leeuwen and others, 1999) and retarded fetal development (Munger and others, 1997). All three studies acknowledge their limitations and urge additional studies to verify the preliminary findings.

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