Numerous anthropogenic chemicals of environmental concern--including some phenoxy acid herbicides, organophosphorus insecticides, polychlorinated biphenyls, phthalates, freon substitutes and some DDT derivatives--are chiral. Their potential biological effects, such as toxicity, mutagenicity, carcinogenicity, and endocrine disrupter activity, are generally enantiomer-selective, and different enantiomers are preferentially degraded (transformed) by micro-organisms in various environments. Here we use field and laboratory experiments to demonstrate that environmental changes in soils can alter these preferences, and to suggest that the preferences shift owing to different groups of related microbial genotypes being activated by different environmental changes. In Brazilian soils, almost all pasture samples preferentially transformed the non-herbicidal enantiomer of dichlorprop ((RS)-2-(2,4-dichlorophenoxy)propionic acid), while most forest samples either transformed the herbicidal enantiomer more readily or as rapidly as the non-herbicidal enantiomer. Organic nutrient enrichments shifted enantioselectivity for methyl dichlorprop ((RS)-methyl 2-(2,4-dichlorophenoxy)propionic acid) strongly towards preferentially removing the non-herbicidal enantiomer in soils from Brazil and North America, potentially increasing phytotoxicity of its residues relative to that of the racemate. Assessments of the risks chemical pollutants pose to public health and the environment need to take into account the chiral selectivity of microbial transformation processes and their alteration by environmental changes, especially for pesticides as up to 25 per cent are chiral.

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Enantiomeric ratios (ERs) for eight polychlorinated biphenyl (PCB) atropisomers were measured in aquatic sediment from selected sites throughout the United States by using chiral gas chromatography/mass spectrometry. Nonracemic ERs for PCBs 91, 95, 132, 136, 149, 174, and 176 were found in sediment cores from Lake Hartwell, SC, which confirmed previous inconclusive reports of reductive dechlorination of PCBs at these sites on the basis of achiral measurements. Nonracemic ERs for many of the atropisomers were also found in bed-sediment samples from the Hudson and Housatonic Rivers, thus indicating that some of the PCB biotransformation processes identified at these sites are enantioselective. Patterns in ERs among congeners were consistent with known reductive dechlorination patterns at both river sediment basins. The enantioselectivity of PCB 91 is reversed between the Hudson and Housatonic River sites, which implies that the two sites have different PCB biotransformation processes with different enantiomer preferences.

To assess the fate of current-use pesticides, it is important to understand their bioaccumulation and biotransformation by aquatic biota. We examined the dietary accumulation and enantioselective biotransformation of the chiral current-use pesticide fipronil, along with a mixture of selected chiral [alpha-hexachlorocyclohexane (alpha-HCH), heptachlor epoxide (HEPX), polychlorinated biphenyls (PCBs) 84, 132, 174, o,p'-DDT, and o,p'-DDD] and nonchiral (p,p'-DDT, p,p'-DDD) organochlorine compounds in juvenile rainbow trout (Oncorhynchus mykiss). Fish rapidly accumulated all compounds, as measured in the carcass (whole body minus liver and GI tract) during the 32 d uptake phase, which was followed by varying elimination rates of the chemicals (half-lives (t1/2s) ranging from 0.6 d for fipronil to 77.0 d for PCB 174) during the 96 d depuration period. No biotransformation was observed for alpha-HCH, HEPX, PCB 174, o,p'-DDT, or o,p'-DDD based on consistent enantiomeric fractions (EFs) in the fish and their t1/2s falling on a log K(ow)--log t1/2 relationship established for recalcitrant contaminants in fish. p,p'-DDT and PCBs 84 and 132 were biotransformed based on the former's t1/2 position below the log K(ow)--log t1/2 relationship, and the PCBs change in EF. Fipronil was rapidly biotransformed, based on a change in EF, a t1/2 that fell below the log K(ow)--log t1//2 relationship, which accounted for 88% of its elimination, and the rapid formation of fipronil sulfone, a known metabolite. Fipronil sulfone was found to persist longer (t1/2 approximately 2 d) than its parent compound fipronil (t1/2 approximately 0.6 d) and needs to be considered in fate studies of fipronil. This research demonstrates the utilities of the log K(ow)--log t1/2 relationship as a mechanistic tool for quantifying biotransformation and of chiral analysis to measure biotransformation in fish.

The chiral herbicide dichlorprop (2,4-dichlorophenoxy-2-propionic acid), which is sold and applied as the racemic mixture, was observed to degrade completely in soil within 31 days, with a half-life of 6.6 d. Degradation occurred with enantiomeric selectivity, indicating biologically mediated reactivity as opposed to strictly abiotic degradation. The S-(−)-isomer degraded significantly faster (t 1/2 = 4.4 d) than the R-(+)-isomer (t 1/2 = 8.7 d); this is contrary to other published results that show selective degradation of the R-(+)-enantiomer, although in other media. Soil samples taken from a field plot at increasing time intervals after application of Foxtril, a commercial herbicide formulation, were solvent-extracted and analyzed for total dichlorprop by capillary zone electrophoresis (CZE), using an acetate buffer at pH 4.7. Heptakis (2,3,6-tri-O-methyl)-β-cyclodextrin, a chiral reagent, was then added to the buffer to effect separation of the (+)- and (−)-isomers of dichlorprop. Baseline resolution allowed calculation of relative concentrations (enantiomer ratios) of the two isomers. CZE is a fast and efficient technique for the analysis of ionic organic species (such as the anion of dichlorprop), including their enantiomers, in pesticide formulations as well as in environmental samples. It thus was possible to analyze Foxtril directly after dilution with water for ioxynil (2,6-diiodo-4-cyanophenol) as well as for dichlorprop. Ioxynil also was detected in the soil extract on the day of application. The hydrolysis product [methyl 2-nitro-5-(2,4-dichlorophenoxy) benzoic acid] of bifenox methyl ester, another herbicide component of Foxtril, was detected in the soil samples taken at 17 and 31 d.

The enantiomeric composition of polychlorinated biphenyl (PCB) atropisomers was measured in river and riparian biota (fish, bivalves, crayfish, water snakes, barn swallows) from selected sites throughout the United States by using chiral gas chromatography/mass spectrometry. Nonracemic enantiomeric fractions (EFs) were observed for PCBs 91, 95, 136, and 149 for aquatic and riparian biota from Lake Hartwell, SC, a reservoir heavily contaminated with PCBs, and for these congeners and PCBs 132, 174, 176, and 183 in river fish and bivalves nationwide. Fish and bivalves showed marked differences in EFs as compared to sedimentfound atthe same sampling sites, thus suggesting that PCBs are bioprocessed in biota in a different manner from those found in sediment (e.g., reductive dechlorination). Species-dependent patterns in PCB EFs were observed, which suggest differences in the ability of different species to bioprocess PCBs enantioselectively, most likely by metabolism. The presence of nonracemic PCBs in fish and bivalves suggests greater metabolic degradation of PCBs in these organisms than indicated from previous achiral studies and underscores the powerful potential of chiral analysis as a tracer of environmental bioprocesses.

Changes in soil water retention of the surface soil brought about by tillage can significantly alter the amount of rain water that infiltrates into the root zone and is available for plant growth. Soil tillage generally increases porosity and changes the pore‐size distribution, leading to changes in the soil water retention curve and hydraulic conductivities. The objective of this study was to investigate some simple ways of estimating the soil water retention curve of a tilled soil from that of an untilled soil, knowing the change in soil porosity or bulk density due to tillage. The study of literature and empirical analysis of the available data indicated: (i) under field conditions the tillage did not significantly change the air‐entry value of the soil; (ii) tillage increased the absolute value of the slope of the log‐log relationship below the air‐entry value; and (iii) the changes due to tillage in the retention curve occurred only in the larger pore‐size range, approximately between the air‐entry pressure head value and 10 times the air‐entry value. Assuming these observations hold in general, two simple methods of estimating the water retention curve of a tilled soil from that of its untilled condition are proposed. The first method is a simple imposition of the Brooks and Corey function between the air‐entry value and 10 times this value. The second method assumes that the change in soil water content at a given pressure head in the above range of pressure heads was inversely proportional to the value of the pressure head. The tests on four pairs of measured water retention curves on three different soils showed that these methods provided good approximations.

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