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.