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USGS2004

Pesticide concentrations in water and in suspended and bottom sediments in the New and Alamo rivers, Salton Sea Watershed, California, April 2003

This report contains pesticide concentration data for water, and suspended and bed sediment samples collected in April 2003 from twelve sites along the New and Alamo Rivers in the Salton Sea watershed, in southeastern California. The study was done in collaboration with the California State Regional Water Quality Control Board, Colorado River Region, to assess inputs of current-use pesticides associated with water and sediment into the New and Alamo Rivers. Five sites along the New River and seven sites along the Alamo River, downstream of major agricultural drains, were selected and covered the lengths of the rivers from the international boundary to approximately 1.5 km from the river mouths. Sampling from bridges occurred at seven of the twelve sites. At these sites, streamflow measurements were taken. These same sites were also characterized for cross-stream homogeneity by measuring dissolved oxygen, pH, specific conductance, temperature, and suspended solids concentration at several vertical (depths) and horizontal (cross-stream) points across the river. Large volume water samples (200?300 L) were collected for isolation of suspended sediments by flow-through centrifugation. Water from the outflow of the flow-through centrifuge was sampled for the determination of aqueous pesticide concentrations. In addition, bottom sediments were sampled at each site. Current-use pesticides and legacy organochlorine compounds (p,p'-DDT, p,p'-DDE and p,p'-DDD) were extracted from sediments and measured via gas chromatography/mass spectrometry (GC/MS). Organic carbon and percentage of fines were also determined for suspended and bottom sediments. Cross-stream transects of dissolved constituents and suspended sediments showed that the rivers were fairly homogeneous at the sites sampled. Streamflow was higher at the outlet sites, with the Alamo River having higher flow (1,240 cfs) than the New River (798 cfs). Twelve current-use pesticides, one legacy organochlorine compound (p,p'-DDE), and the additive piperonyl butoxide were detected in water samples. Trifluralin was found in the highest concentration of all detected compounds (68.5?599 ng/L) at all sites in both rivers, except for the international boundary sites. Atrazine was also detected in high concentration (51.0?285 ng/L) at several sites. The outlet sites had among the highest numbers of pesticides detected and the international boundary sites had the lowest numbers of pesticides detected for both rivers. The numbers of pesticides detected were greater for the Alamo River than for the New River. Six current-use pesticides and two legacy organochlorines (p,p'-DDE and p,p'-DDD) were found associated with suspended and bed sediments. The DDT metabolite p,p'-DDE was detected in all suspended and bed sediments from the Alamo River, but only at two sites in the New River. Dacthal, chlorpyrifos, pendimethalin, and trifluralin were the most commonly detected current-use pesticides. Trifluralin was the compound found in the highest concentrations in suspended (14.5?120 ng/g) and bed (1.9?9.0 ng/g) sediments. The sites along the Alamo River had more frequent detections of pesticides in suspended and bed sediments when compared with the New River sites. The greatest number of pesticides that were detected in suspended sediments (seven) were in the samples from the Sinclair Road and Harris Road sites. For bottom sediments, the Alamo River outlet site had the greatest number of pesticide detections (eight).

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USGS2004

Occurrence, distribution, and transport of pesticides, trace elements, and selected inorganic constituents into the Salton Sea Basin, California, 2001-2002

A study of pesticide distribution and transport within the Salton Sea Basin, California, was conducted from September 2001 to October 2002. Sampling for the study was done along transects for the three major rivers that flow into the Salton Sea Basin: the New and Alamo Rivers at the southern end of the basin and the Whitewater River at the northern end. Three stations were established on each river: an outlet station approximately 1 mile upstream of the river discharge, a near-shore station in the river delta, and off-shore station in the Salton Sea. Water and suspended and bed sediments were collected at each station in October 2001, March-April 2002, and September 2002, coinciding with peak pesticide applications in the fall and spring. Fourteen current-use pesticides were detected in the water column. Concentrations of dissolved pesticides typically decreased from the outlets to the sea in all three rivers, consistent with the off-shore transport of pesticides from the rivers to the sea. Dissolved concentrations ranged from the limits of detection to 151 nanograms per liter (ng/L); however, diazinon, eptam (EPTC), and malathion were detected at much higher concentrations (940?3,830 ng/L) at the New and Alamo River outlet and near-shore stations. Concentrations of carbaryl, dacthal, diazinon, and eptam were higher during the two fall sampling periods, whereas concentrations of atrazine, carbofuran, and trifluralin were higher during the spring. Current-use pesticides also were detected on suspended and bed sediments in concentrations ranging from method detection limits to 106 ng/g (nanograms per gram). Chlorpyrifos, dacthal, eptam, trifluralin, and DDE were the most frequently detected pesticides on sediments from all three rivers. The number and concentrations of pesticides associated with suspended sediments frequently were similar for the river outlet and near-shore sites, consistent with the downstream transport of sediment-associated pesticides out of the rivers. Seasonal trends in pesticide concentration were similar to those for dissolved concentrations in fall 2001 and spring 2002, but not in fall 2002. Generally, the pesticides detected in the suspended sediments were the same pesticides detected in the bed sediments, and concentrations were similar, especially at the Alamo River outlet site in spring 2002 and fall 2002. Pesticides generally were not detected in sediments from the off-shore sites; however, the samples from these sites also had greater incidences of matrix interference during analysis. Sediment-associated pesticide concentrations were above equilibrium in water, suggesting a bound fraction of sediment-associated pesticides that are resistant to desorption. Concentrations of trace elements and other inorganic constituents in suspended sediments collected during the fall 2001 followed expected trends with dilution of river-derived minerals owing to highly organic autochthonous production within the Salton Sea Basin. However, calculation of enrichment ratios provided evidence for the bioconcentration of several trace elements, notably selenium in the off-shore biota.

Scientific Investigations Report↗