Estimates of pesticide degradation rates in subsoils are needed to improve models predicting pesticide movement to groundwater. Biodegradation rates of the herbicide alachlor [2‐chloro‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide] in surface soil, vadose zone, and aquifer samples collected from a single site near Plains, GA were determined in the laboratory under aerobic and anaerobic conditions. Degradation was described by first‐order kinetics during 126 d of incubation. Under aerobic conditions the halflife (t1/2) of alachlor in the surface soil (t1/2 = 23 d) was less than in the vadose zone (t1/2 = 73 to 285 d) and aquifer samples (t1/2 = 320 to 324 d). Alachlor in anaerobic samples degraded less rapidly in the surface (0 to 0.6 m) and the next deepest (0.6 to 2.4 m) subsoil than under aerobic conditions (t1/2 = 100 and 144 d, respectively). Degradation in anaerobic aquifer samples was very slow (t1/2 = 337 to 553 d). Addition of organic nutrients enhanced aerobic degradation in subsurface soils and one aquifer sample, indicating that nutrient availability limits biodegradation. Total aerobic microbial populations ranged from 6.6 × 103 to 2.5 × 106 cells per gram of soil in the subsoils and aquifer samples, but were not correlated with aerobic or anaerobic degradation rates. The lower degradation rates in vadose zone and aquifer materials may be due to less microbial activity or the absence of alachlor degraders.

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The fungus Cunninghamella elegans ATCC 36112 metabolized approximately 80% of the 3-14C-labeled fluoranthene (FA) added within 72 h of incubation. C. elegans metabolized FA to trans-2,3-dihydroxy-2,3dihydrofluoranthene (trans-2,3-dihydrodiol), 8-and 9-hydroxyfluoranthene trans-2,3-dihydrodiol, 3-fluoranthene-4-glucopyranoside, and 3-(8-hydroxyfluoranthene)-I-glucopyranoside. These metabolites were separated by thin-layer and reversed-phase high-performance liquid chromatography and identified by 'H nuclear magnetic resonance, UV, and mass spectral techniques. The major pathway involved hydroxylation to form a glucoside conjugate of 3-hydroxyfluoranthene and a glucoside conjugate of 3,8-dihydroxyfluoranthene which together accounted for 52% of the total ethyl acetate-soluble metabolites. C. elegans initially metabolized FA in the 2,3 position to form fluoranthene trans-2,3-dihydrodiol, which has previously been shown to be a biologically active compound in mammalian and bacterial genotoxicity tests. However, C. elegans formed predominantly glucoside conjugates of the ohenolic derivatives of FA, which suggests that this fungus has the potential to detoxify FA.

Propanil [3′,4′‐dichloropropionanilide (DCPA)] is a biologically active pollutant. Propanil is extensively used around the world for control of weeds in rice (Oryza sativa L.) production. Although propunil by itself is of low toxicity, the highly toxic impurities 3,3′,4,4′‐tetrachloroazobenzene (TCAB) and 3,3′,4,4′‐tetrachloroazoxybenzene (TCAOB) from the manufacture of 3,4‐dichloroaniline (DCA) often contaminate the 3,4‐DCA‐derived herbicide propanil. These componnds are approximately isosteric to 2,3,7,8‐tetrachlorodibenzo‐p‐dioxin (TCDD), and acting at the Ah receptor, are toxic by mechanisms similar to those of TCDD. Both TCAB and TCAOB are cytotoxic, embryolethal, and teratogenic, and hence are classified as possible human genotoxins. In soil, biodegradation of propanil liberates DCA, which is converted by microbial peroxidases to TCAB and other azo products. The TCAB and TCAOB may accumulate in rice‐growing soils, and leach into groundwater. The toxicity and fate of these compounds in the environment and the ability of microorganisms to detoxify them are discussed.