Studies of the transport and loading of currently used pesticides in rivers draining large agricultural river basins pose numerous problems for the analytical laboratory. Most of the annual export occurs during the first two runoff events following the period of major pesticide application. Rapidly changing pesticide concentrations during these events require the use of automatic samplers for collections over the hydrograph. Each sample must be scanned for a wide variety of pesticides and samples frequently contain high concentrations of suspended sediments.

We have developed an analytical procedure which focuses on those pesticides which can be directly extracted in methylene chloride and analyzed by capillary gas chromatography. The procedure includes: extraction with methylene chloride; sonication to break up sediment-extractant-water emulsions; drying over sodium sulfate (Na₂SO₄); concentration with Kuderna-Danish apparatus; transfer to isooctane; and splitless injection into two capillary columns equipped with nitrogen-phosphorus thermionic detectors. The procedure provides quantification of the following pesticides: linuron, EPTC, butylate, ethoprop, trifluralin, phorate, simazine, atrazine, terbufos, fonofos, diazinon, cyanazine, metribuzin, alachlor, metolachlor, chlorpyrifos, penoxalin, and two atrazine metabolites. The dual column injection allows simultaneous confirmation of 14 of the compounds listed above.

The quality control (QC) program relies heavily on automated features of the analytical system. An autosampler allows analyses of a large number of QC samples including recalibration standards, blanks, replicates, spikes, and linearity checks. A data acquisition system provides chromatograms and detailed data reports for both capillary columns. Automatic transfer of the data reports to a computer system facilitates data quality review and data correction. The analytical procedure and associated quality control program provide an effective method for analyzing the large number of samples required in pesticide loading studies during runoff events in large rivers.