Study of the degradation of the herbicides 2,4-D and MCPA at different depths in contaminated agricultural soil

Two phenoxyacid herbicides (2,4-D and MCPA) and their six corresponding phe nols were determined in soil by using gas chomatography with electron impac t mass spectrometry (GC/MS) for confirmation/quantitation. An automatic ext raction (leaching), preconcentration, and cleanup (sorption) module was dev eloped to extract the eight compounds from soil. The average recovery of al l species, spiked to soil at mug/kg-mg/kg levels, was 95% (average standard deviation +/- 5%). A plot of agricultural clayey soil (similar to 12 m(2)) was contaminated with both herbicides (similar to 96 g/m(3), depth 10 cm, density 1.23 g/cm(3)) and irrigated with (17 mm) at variable time intervals . Both herbicides and their corresponding phenol compounds were monitored a t different soil depths over a 50 day period. The degradation of both herbi cides in the surface layer (t(1/2) similar to5 days) is a result of photode composition and microbial action; in the deeper layers, the degradation pro ducts occur in lower proportions by effect of leaching and are also the res ult of microbial action. The six phenol metabolites are only detected in th e surface layer as they form preferentially by photodecomposition. The main metabolites (viz. 2,4-DCP for 2,4-D and 4-C-2-MP for MCPA) are formed with in 24 h after the soil is contaminated; their concentration peaks are at da y 8 in the absence of irrigation.