John L. Rayner

The variability and intrinsic remediation of a BTEX plume in anaerobic sulphate-rich groundwater

Data from long-term groundwater sampling, limited coring, and associated studies are synthesised to assess the variability and intrinsic remediation/natural attenuation of a dissolved hydrocarbon plume in sulphate-rich anaerobic groundwater. Fine vertical scale (0.25- and 0.5-m depth intervals) and horizontal plume-scale (>400 m) characteristics of the plume were mapped over a 5-year period from 1991 to 1996. The plume of dissolved BTEX (benzene, toluene, ethylbenzene, xylene) and other organic compounds originated from leakage of gasoline from a subsurface fuel storage tank. The plume was up to 420 m long, less than 50 m wide and 3 m thick. In the first few years of monitoring, BTEX concentrations near the point of leakage were in approximate equilibrium with non-aqueous phase liquid (NAPL) gasoline. NAPL composition of core material and long-term trends in ratios of BTEX concentrations in groundwater indicated significant depletion (water washing, volatilisation and possibly biodegradation) of benzene from residual NAPL after 1992. Large fluctuations in BTEX concentrations in individual boreholes were shown to be largely attributable to seasonal groundwater flow variations. A combination of temporal and spatial groundwater quality data was required to adequately assess the stationarity of plumes, so as to allow inference of intrinsic remediation. Contoured concentration data for the period 1991 to 1996 indicated that plumes of toluene and o-xylene were, at best, only partially steady state (pseudo-steady state) due to seasonal groundwater flow changes. From this analysis, it was inferred that significant remediation by natural biodegradation was occurring for BTEX component plumes such as toluene and o-xylene, but provided no conclusive evidence of benzene biodegradation. Issues associated with field quantification of intrinsic remediation from groundwater sampling are highlighted. Preferential intrinsic biodegradation of selected organic compounds within the BTEX plume was shown to be occurring, in parallel with sulphate reduction and bicarbonate production. Ratios of average hydrocarbon concentrations to benzene for the period 1991 to 1992 were used to estimate degradation rates (half-lives) at various distances along the plume. The estimates varied with distance, the narrowest range being, for toluene, 110 to 260 days. These estimates were comparable to rates determined previously from an in situ tracer test and from plume-scale modelling.

The effects of nitrogen and water on mineralisation of hydrocarbons in diesel-contaminated terrestrial Antarctic soils

Bioremediation of petroleum-contaminated soil in the Antarctic will be logistically and technically difficult and will cost more than similar treatment in temperate regions or the Arctic because of the remote location and unfavourable environmental conditions. To optimise nutrient amendments for the remediation of a long-term hydrocarbon-contaminated site at Old Casey Station in Antarctica, we investigated the effects of nitrogen (and phosphorus) amendments on microbial mineralisation using radiometric microcosm experiments and gas chromatography. Hydrocarbon mineralisation at nine different inorganic nitrogen concentrations (ranging from 85 to over 27,000 mg N kg-soil-H2O−1) was monitored over 95-day incubation at 10 °C. Total 14C-octadecane mineralisation increased with increasing nutrient concentration peaking in the range 1000–1600 mg N kg-soil-H2O−1. The microcosms with the lowest and highest concentrations of N had extended lag phases of over 12.5 days prior to significant mineralisation. Gas chromatographic analysis of the aliphatic components of Special Antarctic Blend (SAB) diesel in the contaminated soil showed good agreement with the 14C-octadecane mineralisation outcomes. Ratios of n-C17/pristane and n-C18/phytane indicated that low nutrient concentrations rather than water were the main limiting factor for biodegradation of hydrocarbons in the soil collected from Old Casey Station when incubated at 10 °C. However, because the soils from this site are characterised by low water holding capacities, it would be difficult to maintain optimal nutrient concentrations during full-scale treatment, and thus the use of a controlled release nutrient is being considered as a nutrient source in the bioremediation of SAB-contaminated Antarctic soils.

Volatilisation and biodegradation during air sparging of dissolved BTEX-contaminated groundwater

Abstract The relative contributions of volatilisation and biodegradation are quantified for a field trial of air sparging for the remediation of groundwater contaminated with dissolved petroleum hydrocarbons. Groundwater in the unconfined sand aquifer at Kwinana in Western Australia was grossly contaminated by benzene, toluene, ethylbenzene, xylene (BTEX) and other dissolved organics from spills of gasoline. Multi-depth sampling bores, in situ oxygen sensors and neutron access tubes were used to determine changes in groundwater chemistry, oxygen utilisation and the fate of injected air in the aquifer. Oxygen utilisation was used to infer rates of biodegradation. A vadose zone soil vapour extraction system was used to quantify the volatile organic compounds (VOCs) that partitioned from the aqueous phase into the gas phase. Volatilisation was found to be the dominant mechanism for the removal of dissolved VOCs. This was indicated by the close correspondence between calculated masses and the timing of losses. The rate of removal was very rapid, with most organics removed within 3 days of the start of sparging. The rate of loss was also observed to follow the Henrys Law constant for the particular compounds. Estimating biodegradation of dissolved petroleum hydrocarbons was complicated by other sinks for dissolved O2, the presence of residual entrapped air in the aquifer and bulk movement of groundwater. However, biodegradation rates were at least an order of magnitude less than volatilisation rates over the period of greatest losses. It was also notable that dissolved VOCs were reduced over a larger volume of the aquifer than directly contacted by injected air. This may have been due to groundwater movement enhanced by stopping and starting sparging during the trial.

Effectiveness of in situ air sparging for removing NAPL gasoline from a sandy aquifer near Perth, Western Australia

In situ air sparging has the potential to augment the removal of nonaqueous phase liquid (NAPL) contaminants in soil vapour extraction (SVE) systems when the NAPL is present in the capillary fringe or below the water table. NAPL removal can also be enhanced from above the water table by improving air access. Results are presented from a pilot-scale field trial aimed at evaluating the performance of such a remediation strategy where in situ air sparging was used in conjunction with a soil vapour extraction system to remove weathered gasoline NAPL from an unconfined sandy aquifer. A simple analysis that partitions extracted soil vapour between air injected through the sparge well and air drawn from the atmosphere across the soil surface, together with petroleum hydrocarbon concentrations in the extracted soil vapour, was used to interpret the effectiveness of air sparging. The composition and mass of the NAPL in the aquifer were also monitored along with observations on the distribution of air in the aquifer. Results showed that sparged air constituted 42% of the extracted soil vapour but contributed the majority of the petroleum hydrocarbons removed. For the first 5 days of sparging, hydrocarbon concentrations in the sparged air were in equilibrium with the NAPL in the aquifer leading to total petroleum hydrocarbon concentrations in the combined system being three to four times greater than for soil vapour extraction alone. Petroleum hydrocarbon concentrations in the extracted soil vapour decreased over time as a result of the depletion of the more volatile constituents from the NAPL, possible development of mass transfer limitations and increased fraction of clean air from depleted zones. Overall, 65% of the 673 kg of petroleum hydrocarbons extracted in soil vapour over a period of 30 days was carried in sparged air from the single sparge well. Percentages of the mass carried in the sparged air were even higher (median 70%) for individual aromatic hydrocarbons. Inclusion of air sparging increased the mass extracted by a factor of 1.9 (more for individual petroleum hydrocarbons) over and above that for soil vapour extraction alone for the 30 days of sparging. Air sparging was also effective in removing residual NAPL from below the water table. The mass of petroleum hydrocarbons removed from the site was not reflected in changes to the mass of NAPL in the aquifer. This result is apparently due to lateral inflow of NAPL to the site although evidence of a depleted source of volatilised hydrocarbons suggests the possible development of mass transfer limitations.

Hydraulic performance of a permeable reactive barrier at Casey Station, Antarctica.

A permeable bio-reactive barrier (PRB) was installed at Casey Station, Antarctica in 2005/06 to intercept, capture and degrade petroleum hydrocarbons from a decade old fuel spill. A funnel and gate configuration was selected and implemented. The reactive gate was split into five separate cells to enable the testing of five different treatment combinations. Although different treatment materials were used in each cell, each treatment combination contained the following reactive zones: a zone for the controlled release of nutrients to enhance degradation, a zone for hydrocarbon capture and enhanced degradation, and a zone to capture excess nutrients. The materials selected for each of these zones had other requirements, these included; not having any adverse impact on the environment, being permeable enough to capture the entire catchment flow, and having sufficient residence time to fully capture migrating hydrocarbons. Over a five year period the performance of the PRB was extensively monitored and evaluated for nutrient concentration, fuel retention and permeability. At the end of the five year test period the material located within the reactive gate was excavated, total petroleum hydrocarbon concentrations present on the material determined and particle size analysis conducted. This work found that although maintaining media reactivity is obviously important, the most critical aspect of PRB performance is preserving the permeability of the barrier itself, in this case by maintaining appropriate particle size distribution. This is particularly important when PRBs are installed in regions that are subject to freeze thaw processes that may result in particle disintegration over time.

Combining coring and suction cup data to improve the monitoring of pesticides in sandy vadose zones: a field-release experiment

Soil coring and vertically and horizontally installed suction cup monitoring techniques were compared during a field release experiment conducted in an urban area of the Swan Coastal Plain of Western Australia. Sodium bromide and low concentrations of diazinon, chlorpyrifos, atrazine and fenamiphos were released into the vadose zone and rates of migration and mass loss with respect to a bromide tracer investigated. Only bromide and atrazine showed significant migration through the vadose zone. The relative half-life mass losses from the vadose zone of the pesticides ranged from 3 to >40 days. The use of soil coring complemented the use of vertically and horizontally installed suction cups for investigating relatively mobile non-volatile compounds, such as atrazine. Data from horizontally installed suction cups accounted for mass losses due to dilution and transport that could not be accounted for by coring, and enabled a better estimate of degradation and migration rates through the vadose zone. From core data alone, atrazine migration rates for the first 0.25 m were underestimated by more than 50% (0.0039 m day−1 compared to 0.013 m day−1), and removal rates (and inferred degradation rates) were overestimated by more than 100% (half-life of 14 days compared to a half-life of 40 days), compared with rates determined by using core data and horizontal suction cup data in combination. Migration rates may have been even further underestimated at greater depths.

Evaluation of a permeable reactive barrier to capture and degrade hydrocarbon contaminants

A permeable reactive barrier (PRB) was installed during 2005/2006 to intercept, capture and degrade a fuel spill at the Main Power House, Casey Station, Antarctica. Here, evaluation of the performance of the PRB is conducted via interpretation of total petroleum hydrocarbon (TPH) concentrations, degradation indices and most probable number (MPN) counts of total heterotroph and fuel degrading microbial populations. Results indicate that locations which contained the lowest TPH concentrations also exhibited the highest levels of degradation and numbers of fuel degrading microbes, based on the degradation indices and MPN methods selected. This provides insights to the most appropriate reactive materials for use in PRB’s in cold and nutrient-limited environments.

Fate and transport of petroleum hydrocarbons in engineered biopiles in polar regions.

A dynamic multi-media model that includes temperature-dependency for partitioning and degradation was developed to predict the behaviour of petroleum hydrocarbons during biopiling at low temperature. The activation energy (Ea) for degradation was derived by fitting the Arrhenius equation to hydrocarbon concentrations from temperature-controlled soil mesocosms contaminated with crude oil and diesel. The model was then applied to field-scale biopiles containing soil contaminated with diesel and kerosene at Casey Station, Antarctica. Temporal changes of total petroleum hydrocarbons (TPH) concentrations were very well described and predictions for individual hydrocarbon fractions were generally acceptable (disparity between measured and predicted concentrations was less than a factor two for most fractions). Biodegradation was predicted to be the dominant loss mechanism for all but the lightest aliphatic fractions, for which volatilisation was most important. Summertime losses were significant, resulting in TPH concentrations which were about 25% of initial concentrations just 1 year after the start of treatment. This contrasts with the slow rates often reported for hydrocarbons in situ and suggests that relatively simple remediation techniques can be effective even in Antarctica.

Evaluating the reliability of equilibrium dissolution assumption from residual gasoline in contact with water saturated sands

Understanding dissolution dynamics of hazardous compounds from complex gasoline mixtures is a key to long-term predictions of groundwater risks. The aim of this study was to investigate if the local equilibrium assumption for BTEX and TMBs (trimethylbenzenes) dissolution was valid under variable saturation in two dimensional flow conditions and evaluate the impact of local heterogeneities when equilibrium is verified at the scale of investigation. An initial residual gasoline saturation was established over the upper two-thirds of a water saturated sand pack. A constant horizontal pore velocity was maintained and water samples were recovered across 38 sampling ports over 141days. Inside the residual NAPL zone, BTEX and TMBs dissolution curves were in agreement with the TMVOC model based on the local equilibrium assumption. Results compared to previous numerical studies suggest the presence of small scale dissolution fingering created perpendicular to the horizontal dissolution front, mainly triggered by heterogeneities in the medium structure and the local NAPL residual saturation. In the transition zone, TMVOC was able to represent a range of behaviours exhibited by the data, confirming equilibrium or near-equilibrium dissolution at the scale of investigation. The model locally showed discrepancies with the most soluble compounds, i.e. benzene and toluene, due to local heterogeneities exhibiting that at lower scale flow bypassing and channelling may have occurred. In these conditions mass transfer rates were still high enough to fall under the equilibrium assumption in TMVOC at the scale of investigation. Comparisons with other models involving upscaled mass transfer rates demonstrated that such approximations with TMVOC could lead to overestimate BTEX dissolution rates and underestimate the total remediation time.

Effect of compositional heterogeneity on dissolution of non-ideal LNAPL mixtures.

The extent of dissolution of petroleum hydrocarbon fuels into groundwater depends greatly on fuel composition. Petroleum fuels can consist of thousands of compounds creating different interactions within the non-aqueous phase liquid (NAPL), thereby affecting the relative dissolution of the components and hence a groundwater plumes composition over long periods. Laboratory experiments were conducted to study the variability in the effective solubilities and activity coefficients for common constituents of gasoline fuels (benzene, toluene, p-xylene and 1,2,4-trimethylbenzene) (BTX) in matrices with an extreme range of molar volumes and chemical affinities. Four synthetic mixtures were investigated comprising BTX with the bulk of the NAPL mixtures made up of either, ethylbenzene (an aromatic like BTX with similar molar volume); 1,3,5-trimethylbenzene (an aromatic with a greater molar volume); n-hexane (an aliphatic with a low molar volume); and n-decane (an aliphatic with a high molar volume). Equilibrium solubility values for the constituents were under-predicted by Raoults law by up to 30% (higher experimental concentrations) for the mixture with n-hexane as a filler and over-predicted by up to 12% (lower experimental concentrations) for the aromatic mixtures with ethylbenzene and 1,3,5-trimethylbenzene as fillers. Application of PP-LFER (poly-parameter linear free energy relationship) model for non-ideal mixtures also resulted in poor correlation between experimentally measured and predicted concentrations, indicating that differences in chemical affinities can be the major cause of deviation from ideal behavior. Synthetic mixtures were compared with the dissolution behavior of fresh and naturally weathered unleaded gasoline. The presence of lighter aliphatic components in the gasoline had a profound effect on estimating effective solubility due to chemical affinity differences (estimated at 0.0055 per percentage increase in the molar proportion of aliphatic) as well as reduced molar volumes (estimated at -0.0091 in the activity coefficient per unit increase in molar volume, mL/mol). Previously measured changes in activity coefficients due to natural weathering of 0.25 compares well to 0.27 calculated here based on changes in the chemical affinity and molar volumes. The study suggests that the initial estimation of the composition of a fuel is crucial in evaluating dissolution processes due to ideal and non-ideal dissolution, and in predicting long term dissolution trends and the longevity of NAPL petroleum plume risks.