Soil analyses for 1,3-dichloropropene (1,3-DCP), sodium n-methyldithiocarbamate (metam-sodium), and their degradation products near Fort Hall Idaho, September 1999 through March 2000

Between September 1999 and March 2000, soil samples from the Fort Hall, Idaho, area were analyzed for two soil fumigants, 1,3-dichloropropene (1,3-DCP) and sodium n-methyldithiocarbamate (metam-sodium), and their degradation products. Ground water is the only source of drinking water at Fort Hall, and the purpose of the investigation was to determine potential risk of ground-water contamination from persistence and movement of these pesticides in cropland soils. 1,3-DCP, metam-sodium, or their degradation products were detected in 42 of 104 soil samples. The samples were collected from 1-, 2-, and 3-foot depths in multiple backhoe trenches during four sampling events—before pesticide application in September; after application in October; before soil freeze in December; and after soil thaw in March. In most cases, concentrations of the pesticide compounds were at or near their laboratory minimum reporting limits. U.S. Environmental Protection Agency Method 5035 was used as the guideline for soil sample preparation and analyses, and either sodium bisulfate (NaHSO4), an acidic preservative, or pesticide-free water was added to samples prior to analyses. Addition of NaHSO4 to the samples resulted in a greater number of compound detections, but pesticide-free water was added to most samples to avoid the strong reactions of soil carbonate minerals with the NaHSO4. As a result, nondetection of compounds in samples containing pesticide-free water did not necessarily indicate that the compounds were absent. Detections of these compounds were inconsistent among trenches with similar soil characteristics and histories of soil fumigant use. Compounds were detected at different depths and different trench locations during each sampling event. Overall results of this study showed that the original compounds or their degradation products can persist in soil 6 months or more after their application and are present to at least 3 feet below land surface in some areas. A few of the soil analyses results were unexpected. Degradation products of metam-sodium were detected in samples from croplands with a history of 1,3-DCP applications only, and were not detected in samples from croplands with a history of metam-sodium applications. Although 1,2-dibromoethane (EDB) has not been used in the area for many years, EDB was detected in a few soil samples. The presence of EDB in soil could be caused by irrigation of croplands with EDBcontaminated ground water. Analyses of these soil samples resulted in many unanswered questions, and further studies are needed. One potential study to determine vertical extent of pesticide compound migration in sediments, for example, would include analysis of one or more columns of soil and sediments (land surface to ground water, about 35 to 50 feet below land surface) in areas with known soil contamination. Another study would expand the scope of soil contamination to include broader types of cropland conditions and compound analyses.