The evolution of microbial communities with increasing carbon tetrachloride concentrations was studied in two anaerobic columns containing sand and two different clay soils, one of which contained high levels of iron. Microbial communities were characterized through analysis of column effluents with denaturing gradient gel electrophoresis and quantitative polymerase chain reaction for archaea and eubacteria as inlet carbon tetrachloride concentrations were increased from 0.8 to 29 μM. Inhibition of microbial activity was observed in both columns, and was associated with the accumulation of chloroform at concentrations of 0.2 to 0.4 μM as inlet CT concentrations were increased to 2.4-3.0 μM in the low-iron clay column and approximately 16 μM in the iron rich clay column. Inhibition was indicated by decreasing rates of methanol and carbon tetrachloride degradation, decreases in effluent levels of DNA, and shifts in microbial communities of the columns. Even with the inhibition observed, in the iron-rich clay column CT degradation continued to the end of the study with inlet CT concentrations of 29 μM, in contrast to the low-iron clay column in which minimal CT degradation occurred once CT inlet concentrations exceeded 3 μM. The greater capacity for CT degradation in the column containing the iron-rich clay was hypothesized to be the result of reaction with biogenic ferrous iron produced by biological dissimilatory iron reduction.

Little is known about the transport of microorganisms through freeze-fractured clay soils. Normally consolidated clay (NCC) and compacted clay (CC) columns (representing a natural clay barrier and a compacted barrier, respectively) were exposed to six consecutive freeze-thaw cycles and permeated for 21 days with an Escherichia coli cell suspension (approximately 1 × 10⁷ colony forming units (CFU)/mL) containing a 2.1-mM bromide tracer. An unfractured sand column was also examined for comparison with the clay columns. While no E. coli was detected in the effluent of both untreated NCC and CC control clay columns, a relatively low density of E. coli (between 228 and 270 CFU/mL compared to 1 × 10⁷ CFU/mL in the influent) was first detected in the effluent of the freeze-fractured NCC and CC columns at 0.29 and 0.31 pore volumes (or at 5.4 and 4.1 h), respectively. It took 11 min for a full breakthrough of E. coli through the sand column, but only about 0.1% of the influent E. coli density was detected in the effluents of the freeze-fractured NCC and CC columns at day 21. These observations show that despite the high bacterial retention capacity of the freeze-fractured clay columns, the fractures were large enough for the E. coli to flow through. Based on batch sorption tests and the permeation data, it is estimated that 18%, 7%, and 84% of the freeze-fractured NCC, CC, and sand columns would be exposed to the influent, respectively, under a full E. coli breakthrough condition. Our data show that the high bacterial retention capacity of clay barriers can be compromised by freeze-thaw conditions.

Water retention and transport in soils is dependent upon the surface tension of the aqueous phase. Surfactants present in aqueous solution reduce the surface tension of aqueous phase. In soil-water systems, this can result in water drainage and reductions in field capacity and hydraulic conductivity. In this investigation, the surface tension of surfactant solutions mixed with soil—in a constant fixed ratio—was measured as a function of surfactant concentration. Two anionic surfactants were used: sodium dodecyl sulphate and sodium bis (2-ethylhexyl) sulfosuccinate. Two soils were also used—a clay soil and a sandy soil. The key observation made by this investigation was that the addition of soil to the surfactant solution provided a further component of surface tension reduction. Neither soil sample reduced the surface tension of water when surfactant was absent from the aqueous phase, though both soils released soil organic matter at low surfactant concentrations as shown by measurement of the chemical oxygen demand of the supernatant solutions. Furthermore, both surfactants were shown to be weakly adsorbed by soil as shown by the use of a methylene blue assay. It is therefore proposed that the additional reduction in surface tension arises from synergistic interactions between the surfactants and dissolved soil organic matter.

Background, aim and scope Chlormequat (Cq) is a plant growth regulator used throughout the world. Despite indications of possible effects of Cq on mammalian health and fertility, little is known about its fate and transport in subsurface environments. The aim of this study was to determine the fate of Cq in three Danish subsurface environments, in particular with respect to retardation of Cq in the A and B horizons and the risk of leaching to the aquatic environment. The study combines laboratory fate studies of Cq sorption and dissipation with field scale monitoring of the concentration of Cq in the subsurface environment, including artificial drains. Materials and methods For the laboratory studies, soil was sampled from the A and B horizons at three Danish field research stations—two clayey till sites and one coarse sandy site. Adsorption and desorption were described by means of the distribution coefficient (K d) and the Freundlich adsorption coefficient (K F,ads). The dissipation rate was estimated using soil sampled from the A horizon at the three sites. Half life (DT₅₀) was calculated by approximation to first-order kinetics. A total of 282 water samples were collected at the sites under the field monitoring study— groundwater from shallow monitoring screens located 1.5-4.5 m b.g.s. at all three sites as well as drainage water from the two clayey sites and porewater from suction cups at the sandy site, in both cases from 1 m b.g.s. The samples were analysed using LC-MS/MS. The field monitoring study was supported by hydrological modelling, which provided an overall water balance and a description of soil water dynamics in the vadose zone. Results The DT₅₀ of Cq from the A horizon ranged from 21 to 61 days. The Cq concentration-dependant distribution coefficient (K d) ranged from 2 to 566 cm³/g (median 18 cm³/g), and was lowest in the sandy soil (both the A and B horizons). The K F,ads ranged from 3 to 23 (µg¹ ⁻ ¹/n (cm³)¹/n g⁻¹) with the exponent (1/n) ranging from 0.44 to 0.87, and was lowest in the soil from the sandy site. Desorption of Cq was very low for the soil types investigated (<10%w). Cq in concentrations exceeding the detection limit (0.01 µg/L) was only found in two of the 282 water samples, the highest concentration being 0.017 µg/L. Discussion That sorption was highest in the clayey till soils is attributable to the composition of the soil, the soil clay and iron content being the main determinant of Cq sorption in both the A and B horizons of the subsurface environment. Cq was not detected in concentrations exceeding the detection limit in either the groundwater or the porewater at the sandy site. The only two samples in which Cq was detected were drainage water samples from the two clayey till sites. The presence of Cq here was probably attributable to the hydrogeological setting as water flow at the two clayey till sites is dominated by macropore flow and less by the flow in the low permeability matrix. In contrast, water flow at the sandy site is dominated by matrix flow in the high permeability matrix, with negligible macropore flow. Given the characteristics of these field sites, Cq adsorption and desorption can be expected to be controlled by the clay composition and content and the iron content. Combining these observations with the findings of the sorption and dissipation studies indicates that the key determinant of Cq retardation and fate in the soil is sorption characteristics and bioavailability. Conclusions The leaching risk of Cq was negligible at the clayey till and sandy sites investigated. The adsorption and desorption experiments indicated that absorption of Cq was high at all three sites, in particular at the clayey till sites, and that desorption was generally very limited. The study indicates that leaching of Cq to the groundwater is hindered by sorption and dissipation. The detection of Cq in drainage water at the clayey till sites and the evidence for rapid transport through macropores indicate that heavy precipitation events may cause pulses of Cq. Recommendations and perspectives The present study is the first to indicate that the risk of Cq leaching to the groundwater and surface water is low. Prior to any generalisation of the present results, the fate of Cq needs to be studied in other soil types, application regimes and climatic conditions to determine the Cq retardation capacity of the soils. The study identifies bioavailability and heavy precipitation events as important factors when assessing the risk of Cq contamination of the aquatic environment. The possible effects of future climate change need to be considered when assessing whether or not Cq poses an environmental risk.