Abstract:
The chloroacetanilide herbicide alachlor is the second most widely used herbicide
in the US, where it is used on major crops such as corn and soybean. Although alachlor is
mineralized slowly in soil, metabolites of alachlor have been found in soil indicating that
alachlor is subject to chemical or biological transformation. A water-soluble,
non-carcinogenic metabolite of alachlor, 2-[(2,6-diethlyphenyl) (methoxymethyl)
amino]-2-oxoethane-sulfonic acid (ESA), has been detected in groundwater. Because ESA
contains a sulfur atom it is likely that it was formed as a result of a glutathione-based
transformation mechanism found in many organisms. The objectives of this study were to
evaluate the use of an Enzyme-Linked ImmunoSorbent Assay (ELISA) to detect the
herbicide alachlor and its ESA metabolite in soil, and to determine if alachlor and its ESA
metabolite could be measured in a Willamette silt loam soil both before the planting,
during the growth and after harvest of a sweet corn crop. Alachlor was analyzed by gas
chromatography and ESA was analyzed by a commercial ELISA kit after separation of the
two compounds by solid phase extraction (SPE). The SPE and ELISA systems were
evaluated for efficiency of recovery and day-to-day variability. Although satisfactory
recoveries were achieved with the SPE system, there was some amount of variability in
quantifying ESA with the ELISA system. Measures taken to minimize variability included
frequent calibration of the system and the inclusion of appropriate controls in each
analysis. Alachlor levels were below the detection limit (<100 μg/kg soil) two months
after application, whereas ESA was detected throughout the soil profile (0-100 cm) at
13.7-61.2 μg/kg between July and September. In December the level of ESA dropped
substantially, and was below the detection limit in soil collected in April and June of the
following year. A laboratory study provided evidence that the transformation of alachlor
to ESA in Willamette soil was microbiologically mediated, but the question as to what
extent plants, bacteria, or fungi carry out the transformation under field conditions remains
unresolved.
