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The addition of group VI anions to sediment slurries resulted in the inhibition of the rate of mercury (Hg) methylation. The ranking of inhibition is as follows: tellurate (TeO) > selenate (TeO) > molybdate (MoO) > tungstate (WO). In sediment slurries treated with TeO and SeO, methylmercury (MeHg) formation was significantly inhibited (p 50 nM of TeO and >270 nM of SeO, while the significant inhibition (p < 0.05) of Hg-methylation by MoO and WO was observed in slurries spiked at final concentrations ≥100 μM and ≥700 μM, respectively. Increasing the sulfate (SO) concentration while using fixed concentrations of inhibitors led to the partial reestablishment of some MeHg production in WO-treated slurries, whereas, no such significant change was noticed in sediment slurries treated with MoO and TeO. These observations suggested that WO inhibits Hg methylation by a competitive mechanism, while MoO and TeO are noncompetitive inhibitors. Selenate and SO showed a qualitatively similar effect on Hg methylation at concentrations tested, in that each showed stimulation at low concentrations and inhibition at high concentrations. The depression of MeHg formation by group VI anions was not accompanied by an inhibition of general microbial activity, suggesting that only particular microorganisms, such as sulfate-reducing bacteria, are responsible for Hg methylation. Finally, in the concentration ranges encountered in most natural aquatic environments, the inhibition of MeHg production by group VI anions is unlikely, except in systems where those elements are found in anomalously high concentrations.

Pentachlorophenol (PCP) was applied to loamy sand and sandy clay loam soils to determine the effect of upward evaporative flux on the rates of photolytic loss. Subirrigated soil columns were irradiated by ultraviolet lamps or sunlight to measure relative rates of photolytic loss under differing soil moisture regimes. Photolysis was initially rapid in loamy sand soil under near-saturated flow conditions, with up to 55% more lost in light-exposed treatments in 14 d as compared to dark controls. Rates of transport were slower in sandy clay loam soil, reflecting a soil sorption distribution coefficient of 10 mL/g, as compared to 0.4 mL/g in the loamy sand. Despite slower upward movement, similar amounts of photodegradation of PCP had occurred in the sandy clay loam soil after 14 d. Dropping the water table to the 15 or 25 cm depth resulted in unsaturated flow and increased losses to volatilization, with a much lower relative contribution of photodegradation over the course of the experiments. Up to 77% was lost from the loamy sand soil dark treatments during the 14-day period, due to volatilization and irreversible sorption. Processes that translocate PCP into the sunlit surface zone under near-saturated soil moisture conditions can be expected to enhance rates of photolytic loss while minimizing volatilization.

On July 14, 1991, 72,000 L of the pesticide Metam (active ingredient, methyl isothiocyanate) was accidentally released into the Upper Sacramento River. We hypothesized that the spill affected streamside microbial communities and that the effects were persistent. To address this hypothesis, we sampled river soils a year later from sites above and below the spill as well as from an agricultural area and determined soil carbon dioxide efflux (as a measure of soil respiration) in response to methyl isothiocyanate in controlled-environment microcosms, resulting in estimates of the EC50 and soil degradation rates of methyl isothiocyanate. The soil respiration EC50s for the river soils ranged from 13.2 to 51.4 μg/g methyl isothiocyanate, whereas that of the agricultural soil was 72.2 μg/g. Thus, the soils from the Upper Sacramento River were more sensitive to the toxicologic effects of methyl isothiocyanate than was the agricultural soil. Soils from below the spill site also showed higher (factor two or more) EC50s than soils from reference locations above the spill site. The half-life of methyl isothiocyanate in soils ranged from 2.7 to 6.9 days and was longer by a factor of two in the river soils. These degradation rates are controlled by both biotic (microbial) and abiotic processes. The presence of a microbial community accelerated the degradation rate by a factor of two. The pattern of EC50 data demonstrates that changes in the microbial community in the river soils were persistent a full year after the spill and that the stress elicited responses that were indicative of physiologic accommodation or selection for resistance at the population, species, and/or community level.

Concentrations of mercury (Hg) were determined in surface waters and associated river bank sediment samples in a river—reservoir system contaminated by mine wastes. The distribution of total and methyl Hg in surface waters along the Carson River was similar to that measured in river bank sediments and influenced by flow regimes. High levels of Hg (up to 7,585 and 7.2 ng Hg/L for total and methyl Hg, respectively) determined on surface water samples were in large part discharged from Hg-contaminated tailings, distributed in the river bank sediments. Once introduced into the river during the spring snowmelt runoff, Hg was transported downstream and accumulated in the lacustrine part of the system. Elemental Hg (Hg0) increased from 0.02 ng/L in the noncontaminated region to about 2 ng/L in the reservoir. The vertical distribution of total methylmercury (MeHgT) in water of the reservoir differs from that observed elsewhere, in both Hg-contaminated and noncontaminated lakes. The highest levels of MeHgT ( 10 m did not enhance MeHg production. In the anoxic hypolimnion of the reservoir, the recycling of MeHgT was more influenced by the redox cycling of Mn. The addition of group VI anions (SeO2−4, MoO2−4, and WO2−4) in the range of concentrations of oxyanion-forming elements found in the Carson River system to anoxic sediment slurry spiked with SO2−4 resulted in the reduction of rates of MeHg production. Their negative effect on MeHg production was enhanced by increasing pH. Group VI anions, analogous to SO2−4 are inhibitory to sulfate-reducing bacteria, which are known to play a key role in MeHg production in anoxic sediments. Accordingly, the particular water geochemistry of the Carson River system could partly explain the observed low levels of MeHg where one would expect higher concentrations.

Silica sand contaminated with perchloroethylene (PCE) was effectively treated by Fenton's reagent. Silica sand contaminated with PCE at 1000 mg/kg was totally mineralized in 3 h by Fenton's reagent with initial concentration of about 2.1 M of H 2 O 2 and 5 mM of FeSO 4 . Results from gas chromatography-mass spectroscopy (GC-MS) revealed that dichloroacetic acid was the sole significant intermediate species generated, and was believed to be limiting species leading to miniralization

The photolysis of a series of aryl ketones on air-dried soil surfaces was examined to establish whether vapor transport has an effect on the rate and extent of photolysis. If vapor transport were significant on light-exposed soils, then differences in the observed photolysis rate profiles would be expected, with the compounds having higher vapor pressures exhibiting the greatest photolysis. The aryl ketones, including hexanophenone, octanophenone, dodecanophenone, hexadecanophenone, and octadecanophenone, had nearly identical photolysis rates in solution exposed to sunlamps (Λmax 310 nm). The vapor pressures of these five compounds (25 °C) were estimated by a capillary gas chromatographic method and varied from 8.7 × 10−3 to 8.4 × 10−8 torr. The compounds were irradiated for 15 d on an air-dried sandy loam soil surface. The photolysis rate profiles of the four aryl ketones with vapor pressures from 1.3 × 10−3 torr to 8.4 × 10−8 torr were identical within experimental error, whereas the most volatile congener, hexanophenone, showed approximately 10% greater loss. Volatilization of hexanophenone appears to have caused the compound to move into the light-exposed soil surface to a greater extent than the other compounds. These results suggest that vaporization is not important for loss or transport of the compounds with vapor pressures less than 1.3 × 10−3 torr on air-dried soils within the 15-d irradiation.

The objective of this study were to determine the relative rates of loss of various gasoline hydrocarbons from soil in a field experiment, and to evaluate conditions which maximize volatilization, thus providing an inexpensive decontamination method

— Photosensitized inhibition of mitochondrial succinoxidase by hypericin was measured in vitro and found to be drug-dose, light-dose, and wavelength dependent. Singlet oxygen generation, monitored using the singlet oxygen trap tetramethylethylene, and oxygen consumption in isolated mitochondria sensitized by hypericin were also light-dose and wavelength dependent. Unequivocal evidence for the generation of singlet oxygen was obtained using kinetic isotope ratios of products from the reaction between singlet oxygen and geminally deuterated tetramethylethylene. An action spectrum for the inhibition of succinoxidase was measured at wavelengths between 400 and 700 nm and found to parallel the recorded visible absorption spectrum of hypericin in isolated mitochondria. The greatest singlet oxygen generation, oxygen consumption, and succinoxidase inhibition occurred with white light or 600 nm irradiation. These data are consistent with a type II singlet-oxygen-mediated mechanism for hypericin induced photosensitized inhibition of mitochondrial succinoxidase.

Strychnine (C21H22N2O2) sorption on soils and uptake in alfalfa (Medicago sativa L.) were investigated. Using application rates of 52-1040 mg/L strychnine, sorption isotherms were developed on three Nevada soils. Strychnine was found to sorb strongly to the three soils studied; however, isotherms were all nonlinear and the extent of sorption varied with each soil. Desorption characteristics paralleled sorption isotherms, indicating reversibility. Strychnine extraction with ammonium chloride (NH4Cl) solutions was greater than that obtained with water extraction on all soils. An apparent relationship between specific surface area, cation exchange capacity (CEC), and total strychnine sorption was noted. Strychnine movement through soil columns under saturated flow was also investigated for the three soils. These results were similar to the sorption studies in that the soil with the lowest sorptive capacity also required the fewest pore volumes for the eluting strychnine concentration to equal that of the starting concentration. The binding mechanism of strychnine to soil is suggested to be a combination of cation exchange and surface absorption to the organic and inorganic fractions of soil. Potential uptake of strychnine in alfalfa was investigated by applying treated milo (Sorghum vulgare var. subglabrescens) bait to the base of alfalfa plants at rates that exceeded normal rodent treatments. Strychnine was not detected (detection limit 0.03 mg/kg) in whole alfalfa samples taken 4 and 11 d following treatment. The data indicate that strychnine is relatively immobile in soil systems, particularly those of moderate-to-high CEC, and is not expected to leach rapidly nor be significantly taken up by plants