Carbamate groundwater contamination

The potential use of substitute crop protectants in areas where contamination has been discovered and the general lack of Information with respect to the leaching properties and groundwater persistence of many carbamate and organophosphorus pesticides make It Imperative to Investigate the degradation behavior of a broad range of these compounds. 

An example of an extremely toxic carbamate is aldicarb [2-methyl-2-(methylthio) propionaldehyde]. Both oral and dermal routes are the primary portals of entry, and it has an oral LD50 of 1.0 mg/kg (rat)and a dermal LD50 of 20 mg/kg (rabbit). For this reason it is recommended for application to soils on crops such as cotton, citrus, and sweet potatoes. This compound moves readily through soil profiles and has contaminated groundwater supplies. 

A number of carbamates have been used as insecticides and a few as herbicides. Kinetic parameters for several of these compounds are summarized in Table 8.6. Note that the base-catalyzed reaction is the more important, however, some reports indicate that for some compounds, the direct reaction with water can be significant with values of 6-7. Unfortunately, there is only limited information in this area. Note that these data also illustrate how the substituents on the N influence hydrolysis. The feb values for the dimethyl compounds are orders of magnitude lower than the methyl counterparts. Aldicarb represents a variation in that the ester component of the molecule is derived from an oxime (RCH=NOH) rather than an alcohol. It also illustrates the significance of hydrolysis in understanding its environmental behavior. Aldicarb has been very effective in controUing insect pests in potatoes, but because of its high water solubility (6000 ppm) has been detected as a contaminant in groundwater. This effect has been observed only in acid (pH 5.5) but not in basic (pH 8.0) soils. 

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