Temporal patterns and sources of atmospherically deposited pesticides in Alpine Lakes of the Sierra Nevada, California, U.S.A.

Agricultural pesticides are being transported by air large distances to remote mountain areas and have been implicated as a cause for recent population declines of several amphibian species in such locations. Largely unmeasured, however, are the magnitude and temporal variation of pesticide concentrations in these areas, and the relationship between pesticide use and pesticide appearance in the montane environment. We addressed these topics in the southern Sierra Nevada mountains, California, by sampling water weekly or monthly from four alpine lakes from mid-June to mid-October 2003. The lakes were 46-83 km from the nearest pesticide sources in the intensively cultivated San Joaquin Valley. Four of 41 target pesticide analytes were evaluated for temporal patterns: endosulfan, propargite, dacthal, and simazine. Concentrations were very low, approximately 1 ng/L or less, at all times. The temporal patterns in concentrations differed among the four pesticides, whereas the temporal pattern for each pesticide was similar among the four lakes. For the two pesticides applied abundantly in the San Joaquin Valley during the sampling period, endosulfan and propargite, temporal variation in concentrations corresponded strikingly with application rates in the Valley with lag times of 1-2 weeks. A finer-scale analysis suggests that a large fraction of these two pesticides reaching the lakes originated in localized upwind areas within the Valley.

Development of a solid phase extraction method for agricultural pesticides in large-volume water samples.

An analytical method using solid phase extraction (SPE) and analysis by gas chromatography/mass spectrometry (GC-MS) was developed to determine trace levels of a variety of 41 agricultural pesticides and selected transformation products in high-elevation surface waters. Large-volume water sampling (up to 100L) was employed because it was anticipated that pesticide contamination, if present, would be at very low levels. The target compounds comprise pesticides (and selected oxygen transformation products) known to have been extensively used in agriculture in the San Joaquin Valley, CA, USA. Solid phase extraction using the polymeric resin Abselut Nexus was optimized to extract the pesticide analytes from water samples. A single determinative method using GC-MS with electron ionization was used for all the analytes. Recoveries from 100L of reagent water at 100pg/L and 1ng/L concentrations were generally greater than 75%, although dimethoate, disulfoton, and phorate were not recovered. Analysis of the extracts without cleanup yielded detection limits for the remaining 38 analytes between 0.1 and 30ng/L. A silica cleanup with separate analysis of 3 eluant fractions improved detection limits for 37 of the compounds to between 6 and 600pg/L in high-elevation surface waters.

Analysis of volatiles and semivolatiles in drinking water by microextraction and thermal desorption.

A new technique to analyze aqueous samples for nanograms per liter levels of volatile and semivolatile compounds using microextraction and thermal desorption into a gas chromatograph/ion trap mass spectrometer (GC/MS) is described. This method is inherently sensitive (50 mL of aqueous sample is extracted prior to each desorption), uses no solvents, and detects volatiles and semivolatiles in the same analysis. Aqueous standards and environmental samples are pumped through a length of porous-layer open-tubular capillary column, which is then thermally desorbed onto a 30 m x 0.25 mm i.d. analytical column interfaced to an ion trap mass spectrometer for subsequent separation and detection. Sharp chromatographic peaks and reproducible retention times (RT) were observed. Replicate injections of surrogates (n=6) averaged 32.6% R.S.D. Analysis of domestic tap water detected 55 analytes, some at the low-nanograms per liter level, and detected 3 halogenated ethenes, not previously reported in drinking water. Analysis of an aqueous sample from a municipal ground water source detected the presence of numerous semivolatile compounds at trace-levels.