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A Groundwater Exposure Assessment. for Cloransulam-Methyl in the U.S. Soybean Market

Dow AgroSciences, Environmental Chemistry Labs, 9330 Zionsville Road, Indianapolis, IN 46268.

^* Corresponding author (iwesenbeeck{at}dowagro.com).

ABSTRACT

A study was conducted to examine the leaching potential for cloransulam-methyl, a new soybean herbicide, in the U.S. soybean market. Expected environmental concentrations (EECs) of cloransulam-methyl in groundwater were simulated in 60 soils in the use area using the PRZM2 model executed within the PATRIOT shell using conservative soil, weather, and chemical inputs. Modeling results suggested that cloransulam-methyl is potentially mobile in coarse-textured soils with <1.5% organic matter (OM) and not mobile in finer-textured soils. The maximum predicted EEC in groundwater for the most vulnerable soil modeled was 2.7 µg L^–1 (ppb), more than two orders of magnitude below the USEPA defined health level of 1000 µg L^–1, based on toxicological data. Comparison of EEC distributions in groundwater and EC₂₅ distributions for nontarget terrestrial plants obtained from conservative Tier 2 phytotoxicity studies suggest adequate safety, if shallow groundwater was used as a source of irrigation water on nontarget terrestrial plants. A GIS assessment of coarse textured soils with OM <1.5% and GW <1.83 m deep showed that <2, 3, and 1% of the soils in any given state in the Midwest, Eastern, or Southern soybean markets, respectively, contained soils meeting those criteria for vulnerability (based on modeling). Results of this modeling and exposure assessment are consistent with the rapid dissipation and low mobility of cloransulam-methyl observed in four terrestrial field dissipation studies conducted in the U.S. soybean market, and suggest that potential mobility of cloransulam-methyl should have no significant environmental impact.