Celia Zamora

Methods, quality assurance, and data for assessing atmospheric deposition of pesticides in the Central Valley of California

1 Abstract The U.S. Geological Survey monitored atmospheric deposition of pesticides in the Central Valley of California during two studies in 2001 and 2002–04. The 2001 study sampled wet deposition (rain) and storm-drain runoff in the Modesto, California, area during the orchard dormant-spray season to examine the contribution of pesticide concentrations to storm runoff from rainfall. In the 2002–04 study, the number and extent of collection sites in the Central Valley were increased to determine the areal distribution of organophosphate insecticides and other pesticides, and also five more sample types were collected. These were dry deposition, bulk deposition, and three sample types collected from a soil box: aqueous phase in runoff, suspended sediment in runoff, and surficial-soil samples. This report provides concentration data and describes methods and quality assurance of sample collection and laboratory analysis for pesticide compounds in all samples collected from 16 sites. Each sample was analyzed for 41 currently used pesticides and 23 pesticide degradates, including oxygen analogs (oxons) of 9 organophosphate insecticides. Analytical results are presented by sample type and study period. The median concentrations of both chloryprifos and diazinon sampled at four urban (0.067 micrograms per liter [μg/L] and 0.515 μg/L, respectively) and four agricultural sites (0.079 μg/L and 0.583 μg/L, respectively) during a January 2001 storm event in and around Modesto, Calif., were nearly identical, indicating that the overall atmospheric burden in the region appeared to be fairly similar during the sampling event. Comparisons of median concentrations in the rainfall to those in the McHenry storm-drain runoff showed that, for some compounds, rainfall contributed a substantial percentage of the concentration in the runoff; for other compounds, the concentrations in rainfall were much greater than in the runoff. For example, diazinon concentrations in rainfall were about 70 percent of the diazinon concentration in the runoff, whereas the chlorpyrifos concentration in the rain was 1.8 times greater than in the runoff. The more water-soluble pesticides—carbaryl, metolachlor, napropamide, and simazine—followed the same pattern as diazinon and had lower concentrations in rain compared to runoff. Similar to chlorpyrifos,compounds with low water solubilities and higher soil-organic carbon partition coefficients, including dacthal, pendimethalin, and trifluralin, were found to have higher concentrations in rain than in runoff water and were presumed to partition to the suspended sediments and organic matter on the ground. During the 2002–04 study period, the herbicide dacthal had the highest detection frequencies for all sample types collected from the Central Valley sites (67–100 percent). The most frequently detected compounds in the wet-deposition samples were dacthal, diazinon, chlorpyrifos, and simazine (greater than 90 percent). The median wet-deposition amounts for these compounds were 0.044 micrograms per square meter per day (μg/m2/day), 0.209 μg/m2/day, 0.079 μg/m2/day, and 0.172 μg/m2/day, respectively. For the dry-deposition samples, detection frequencies were greater than 73 percent for the compounds dacthal, metolachor, and chlorpyrifos, and median deposition amounts were an order of magnitude less than for wet deposition. The differences between wet deposition and dry deposition appeared to be closely related to the Henry’s Law (H) constant of each compound, although the mass deposited by dry deposition takes place over a much longer time frame. Pesticides detected in rainfall usually were detected in the aqueous phase of the soil-box runoff water, and the runoff concentrations were generally similar to those in the rainfall. For compounds detected in the aqueous phase and suspended-sediment samples of soil-box runoff, concentrations of pesticides in the aqueous phase generally were detected in low concentrations and had few corresponding detections in the suspended-sediment samples. Dacthal, diazinon, chlorpyrifos, and simazine were the most frequently detected pesticides (greater than 83 percent) in the aqueous-phase samples, with median concentrations of 0.010 μg/L, 0.045 μg/L, 0.016 μg/L, and 0.077 μg/L, respectively. Simazine was the most frequently detected compound in the suspended-sediment samples (69 percent), with a median concentration of 0.232 μg/L. Methods, Quality Assurance, and Data for Assessing Atmospheric Deposition of Pesticides in the Central Valley of California By Celia Zamora, Michael S. Majewski, and William T. Foreman 2 Methods, Quality Assurance, and Data for Assessing Atmospheric Deposition of Pesticides Results for compounds detected in the surficial-soil samples collected throughout the study period showed that there was an increase in concentration for some compounds, indicating atmospheric deposition of these compounds onto the soil-box surface. In the San Joaquin Valley, the compounds chlorpyrifos, dacthal, and iprodione were detected at higher concentrations (between 1.4 and 2 times greater) than were found in the background samples collected from the San Joaquin Valley soil-box sites. In the Sacramento Valley, the compounds chlorpyrifos, dacthal, iprodione, parathionmethyl, and its oxygen analog, paraoxon-methyl, were detected in samples collected during the study period in low concentrations, but were not detected in the background concentration of the Sacramento Valley soil mix. Introduction California is one of the world’s leading agricultural areas, and many thousands of metric tons of pesticides are used each year on many different crops (Majewski and Baston, 2002). Pesticides have been recognized as potential air pollutants since 1946 (Daines, 1952), and a wide variety have been detected in California air (Baker and others, 1996; Majewski and Baston, 2002; Zamora and others, 2003; Majewski and others, 2005). In most cases, pesticides in agriculture are applied by spraying an aqueous suspension. As a result, a portion of the sprayed compound does not reach the target area, but is transported by wind beyond the application site as direct drift. The droplets transported by the drift are either deposited on soil or plants close to the treated area or are transported in the atmosphere over longer distances, depending on their size. A loss of pesticide after application caused by volatilization or wind erosion of soil to the atmosphere is called “indirect drift.” These direct and indirect sources to drift are the main input paths of pesticides in the atmosphere (Epple and others, 2002). Post-application volatilization from treated surfaces is often a major dissipation pathway for many pesticides (Glotfelty, 1978; Cliath and others, 1980; Glotfelty and others, 1990; Risebrough, 1990; Majewski, 1991; Majewski and others, 1993; Majewski and Capel, 1995; Seiber and Woodrow, 1995, Wania and Mackay, 1996; Majewski and Baston, 2002). Atmospheric transport and subsequent deposition of pesticides can affect the quality of streams and other surface waters adversely. Residues of pesticides in surface waters of the Central Valley have been evaluated in many previous studies (Kuivala and Foe, 1995, Domagalski, 1997a, b; Domalgalski and others, 1997; Panshin and others, 1998; Kratzer, 1998; Kratzer, 1999; Kratzer and others, 2002). Under section 303(d) of the 1972 Clean Water Act, states are required to develop lists of impaired waters that do not meet the water-quality standards set by states. The 303(d) list shows that several streams in the Central Valley are impaired because of pesticides (U.S. Environmental Protection Agency, http://www.epa.gov/region09/water/tmdl/303d-pdf/ ca-06-303d-list-final-06-28-07-combined.pdf, accessed August 2012). The most frequent impairments have been attributed to the organophosphate (OP) insecticides diazinon and chlorpyrifos. Most of the agricultural applications of diazinon and chlorpyrifos take place from December through February. They are applied to dormant orchards of several stone fruits and nuts in the San Joaquin Basin, primarily almonds (Panshin and others, 1998). These insecticides are applied during extended dry periods during the dormant-spray period, and then rainfall events after spraying cause most of the unintentional transport of pesticides from fields to streams with rainfall-induced runoff. Atmospheric transport and subsequent deposition of pesticides are most likely to affect stream water quality when rain and direct surface runoff are major sources of streamflow. Study Areas The Central Valley is a large flat valley that dominates the central portion of California and is one of California’s most productive agricultural regions. It is about 400 miles long, averages 50 miles in width, and is composed of four hydrographic subregions or drainage basins named for the major natural surface-water feature in each subregion (fig. 1). Sacramento Valley, the northernmost third of the Central Valley, has an area of about 4,400 square miles (mi2) and is drained by its namesake, the Sacramento River. San Joaquin Valley, the southern two-thirds of the Central Valley, has two subregions: the San Joaquin Basin and, at the southern end, a basin of interior drainage called the Tulare Basin after a Pleistocene lake that occupied most of the area. The fourth hydrographic subregion is the delta, a low lying area that drains directly to the Sacramento–San Joaquin Delta rather than to either river. The lower part of the delta subregion consists of wetlands interspersed with hundreds of miles of channels and numerous islands (Bertoldi and others, 1991). In this report, the study area only includes the San Joaquin Valley and Sacramento Valley, and results are summarized by the respective geographic region. Sacramento Valley The Sacramento Valley is geographically contiguous with the San Joaquin Valley to the south, but is defined by its distinct drainag