Gregory B. Davis

Natural degradation rates of BTEX compounds and naphthalene in a sulphate reducing groundwater environment

Abstract Field and laboratory evidence show natural degradation of toluene, ethylbenzene, m-, p- and o-xylenes, 1,3,5-trimethylbenzene and naphthalene in sulphate reducing groundwater conditions of the Bassendean Sands in the Perth basin, Western Australia. Natural degradation rates were obtained from a groundwater tracer test with deuterated organic compounds injected into a dissolved hydrocarbon plume, down-gradient of a leaking underground storage tank at an urban service station. These were compared with similar data obtained from modelling of the whole contaminant plume itself and also with data obtained from large-scale laboratory column experiments with groundwater spiked with BTEX compounds. Toluene degradation rate was 200 to 500 times higher in the anaerobic laboratory columns than in the field. Degradation rates in the tracer test compared well with model-derived field estimates.

Estimating the retardation coefficient of trichloroethene for a sand aquifer low in sediment organic carbon - A comparison of methods.

Significant groundwater contamination by trichloroethene (TCE) has been identified in the Spearwood Sands aquifer, Perth Western Australia. The organic carbon content of the aquifer sediment is very low and reported retardation coefficients for TCE in low organic carbon subsurface environments are rare. Carbon-based correlation equations were reported to underestimate the retardation of nonionic hydrophobic organic compounds in aquifers characterised by low organic carbon contents. Estimating the retardation coefficient for TCE in the Spearwood Sands aquifer, therefore, is difficult. Various methods to estimate the retardation coefficient for TCE in the Spearwood Sands aquifer were investigated. Estimates obtained by commonly used correlation equations were compared to retardation coefficients calculated based on laboratory batch and column studies. An in situ transport experiment using deuterium labelled TCE (TCEd1) within a TCE contaminated zone was carried out to assess the reliability of the various estimation techniques. The field transport experiment showed that TCEd1 was not retarded in the Spearwood Sands aquifer. Carbon-based correlations overestimated the retardation coefficient for TCE. The equations are shown to be limited by the lack of reliable estimates of the organic carbon content for the Spearwood Sands. Laboratory batch studies resulted in more reliable estimates of TCE retardation. Retardation coefficients from the batch studies, however, were still overestimated since Teflon-coated silicone liners used in the experiments show considerably higher sorption capacity for TCE than does the aquifer sediment. Laboratory column studies were accurate in assessing the sorption behaviour of TCE in the Spearwood Sands. Column studies eliminated the additional uncertainty inherent in determining the input concentration for TCE, by assessing the retardation coefficient in relation to an inorganic tracer. Column studies also avoided the experimental difficulties experienced with the batch studies and proved to be more cost effective. It is shown, however, that also with batch experiments an accurate prediction of the field retardation for TCE can be obtained, if naphthalene is used in the batch studies to determine the sorptive properties of the sediment.

Factors controlling the concentration of methane and other volatiles in groundwater and soil-gas around a waste site

The concentration of methane in groundwater and soil-gas in the vicinity of a waste landfill on an unconfined sand aquifer has been investigated in detail. These data have been used to evaluate techniques which use volatile organic compounds in soil-gas as indicators of groundwater contamination. Simple one-dimensional models of gas advection and diffusion have been adapted for use in the study. Lateral advection of gas in the unsaturated sand was found to be seasonal and was most noticeable in winter when the profile was wet; a mean velocity of 1 m d− was measured from breakthrough of a helium tracer in an injection test. The effects of advection on trace concentrations of methane in soil-gas were limited to within 150–200m from the waste site and resulted from pressure gradients brought about by positive gas pressures in the landfill, and also as a result of ebullition (gas bubbling) from contaminated groundwater. The distribution of methane in soil-gas at shallow (2m) depth gave a general indication of the direction of movement of contaminants with groundwater in close proximity to the landfill. Outside this zone, diffusional transport of methane from groundwater to soil-gas occurred and methane in soil-gas sampled close to the water table was found to be a useful indicator of contaminated groundwater. Modelling the exchange of volatiles between aqueous and gas phases indicates that a wide range of organic compounds, particularly those with Henrys Law constants greater than 2.5 × 10t-2kPam3mol−1, would have potential for use as indicators of pollution, if these were present in groundwater and they behaved relatively conservatively. In general, the principal factors controlling the concentration of these volatiles in soil-gas were the concentration gradient at the water table and capillary fringe and the ratio of diffusion coefficients in the saturated and unsaturated zones.

Aerobic bioremediation of 1,2 dichloroethane and vinyl chloride at field scale

Aerobic bioremediation of 1,2 dichloroethane (1,2 DCA) and vinyl chloride (VC) was evaluated at field scale in a layered, silty and fine-sand anaerobic aquifer. Maximum concentrations of 1,2 DCA (2 g/L) and VC (0.75 g/L) in groundwater were within 25% and 70% of pure compound solubility, respectively. Aerobic conditions were induced by injecting air into sparging wells screened 20.5-21.5 m below ground (17-18 m below the water table). Using a cycle of 23 h of air injection followed by three days of no air injection, fifty days of air injection were accumulated over a 12 month period which included some longer periods of operational shutdown. Oxygen and volatile organic compound probes, and multilevel samplers were used to determine changes of the primary contaminants and the associated inorganic chemistry at multiple locations and depths. Air (oxygen) was distributed laterally up to 25 m from the sparge points, with oxygen partial pressures up to 0.7 atmospheres (28-35 mg/L in groundwater) near to the sparge points. The dissolved mass of 1,2 DCA and VC was reduced by greater than 99% over the 590 m(2) trial plot. Significantly, pH declined from nearly 11 to less than 9, and sulfate concentrations increased dramatically, suggesting the occurrence of mineral sulfide (e.g., pyrite) oxidation. Chloride and bicarbonate (aerobic biodegradation by-products) concentration increases were used to estimate that 300-1000 kg of chlorinated hydrocarbons were biodegraded, although the ratio of 1,2 DCA to VC that was biodegraded remained uncertain. The mass biodegraded was comparable but less than the 400-1400 kg of chlorinated compounds removed from the aqueous phase within a 10,000 m(3) volume of the aquifer. Due to the likely presence of non-aqueous phase liquid, the relative proportion of volatilisation compared to biodegradation could not be determined. The aerobic biodegradation rates were greater than those previously estimated from laboratory-based studies.

Biodegradation and retardation of PCE and BTEX compounds in aquifer material from Western Australia using large-scale columns

Abstract The behaviour of trace concentrations (∼ 1 mg L −1 ) of monoaromatic hydrocarbons (benzene, toluene, ethylbenzene and xylenes — BTEX) and a halogenated aliphatic hydrocarbon (tetrachloroethene — PCE) in groundwater were studied in large-scale columns containing uncontaminated sands and groundwater from a major Quaternary aquifer beneath an urban area on the Swan Coastal Plain of Western Australia. Columns were run in saturated mode with and without nitrate addition. Results from these experiments indicate that indigenous microorganisms in uncontaminated aquifer material are capable of degrading trace amounts of toluene and ethylbenzene under denitrifying conditions and toluene under sulfate-reducing conditions. Benzene was persistent under anoxic conditions, but degraded readily under oxygenated conditions. PCE did not degrade under either oxic or anoxic conditions in these tests. First-order degradation rates fitted from a one-dimensional model were estimated as > 1.1·10 −4 s −1 for benzene under oxic conditions, (4 ± 2)·10 −5 s −1 for toluene under anoxic conditions (denitrifying and sulfate-reducing conditions) and (4 ± 3)·10 −5 s −1 for ethylbenzene under denitrifying conditions. Degradation rates tended to increase with time (over a 150-day period), suggesting that bacteria became more efficient at degrading toluene and possibly other aromatic compounds with increasing exposure. Total bacteria counts and numbers of denitrifying bacteria were highest where toluene degradation was taking place close to the influent of the columns. The rates of loss of toluene were similar under denitrifying and sulfate-reducing conditions for different soils.

Integrating spatial and temporal oxygen data to improve the quantification of in situ petroleum biodegradation rates

Accurate estimation of biodegradation rates during remediation of petroleum impacted soil and groundwater is critical to avoid excessive costs and to ensure remedial effectiveness. Oxygen depth profiles or oxygen consumption over time are often used separately to estimate the magnitude and timeframe for biodegradation of petroleum hydrocarbons in soil and subsurface environments. Each method has limitations. Here we integrate spatial and temporal oxygen concentration data from a field experiment to develop better estimates and more reliably quantify biodegradation rates. During a nine-month bioremediation trial, 84 sets of respiration rate data (where aeration was halted and oxygen consumption was measured over time) were collected from in situ oxygen sensors at multiple locations and depths across a diesel non-aqueous phase liquid (NAPL) contaminated subsurface. Additionally, detailed vertical soil moisture (air-filled porosity) and NAPL content profiles were determined. The spatial and temporal oxygen concentration (respiration) data were modeled assuming one-dimensional diffusion of oxygen through the soil profile which was open to the atmosphere. Point and vertically averaged biodegradation rates were determined, and compared to modeled data from a previous field trial. Point estimates of biodegradation rates assuming no diffusion ranged up to 58 mg kg(-1) day(-1) while rates accounting for diffusion ranged up to 87 mg kg(-1) day(-1). Typically, accounting for diffusion increased point biodegradation rate estimates by 15-75% and vertically averaged rates by 60-80% depending on the averaging method adopted. Importantly, ignoring diffusion led to overestimation of biodegradation rates where the location of measurement was outside the zone of NAPL contamination. Over or underestimation of biodegradation rate estimates leads to cost implications for successful remediation of petroleum impacted sites.