Dicamba, leaching

The possibility for transport of Dicamba in subsurface soils, resulting in subsequent groundwater pollution, is potentially high. Both Dicamba and its initial transformation product 3,6-dichlorosalicylic acid have p2Ca values of 1.95 [392]. The high solubility of these weak acids at neutral to high pH makes it feasible for them to be mobile in lime treated or neutral pH soils. In the field, Dicamba (1) has been found to leach to a depth of 1 m over a 2-month period following application in a Missouri clay pan soil [296], (2) was discovered in approximately... 

Groundwater. According to the U.S. EPA (1986) dicamba has a high potential to leach to groundwater. 

D would also leach quickly from the soil column and will probably breakthrough immediately following dicamba. The other chemicals expected to leach from the soil column were atrazine and diazinon and both were probably of intermediate mobility. 

Leaching Experiment. Three polyethylene columns (4.8 cm ID by 50 cm height) ware employed to investigate the mobility of dicamba, 2,4-D, atrazine, diazinon, pentachlorophenol, and lindane. Each column was packed with 1,080g of fresh soil to a depth of 40 cm (sandy loam soil from Soils Incorporated, Puyallup, Washington pH 5.9 to 6.0 89 percent sand 7 percent silt 4 percent clay cation exchange capacity 7.5 meq/lOOg). 

Figure 7 Amount of dicamba (/ g) leached from columns I, II, III during the 30-day test.

Loss and detoxification of dicamba from treated plants occur by exudation through the roots and leaves and by metabolism in the plant. The main metabolite of dicamba is 5-hydroxy-2-methoxy-3,6-dichlorobenzoic acid, which forms conjugates (Broadhurst et ai, 1966 Chang and Van den Born, 1971). Dicamba is mobile in the soil, so its leaching depends on seasonal precipitation. It is readily metabolised by microorganisms in the soil. The major metabolite is 3,6-dichloro-salicylic acid (Harris, 1967). 

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