Douglas M. Mackay

Transport of organic contaminants in groundwater

Distribution et devenir des polluants dans des aquiferes de sable et gravier. Processus chimiques, physiques et biologiques dans la zone saturee

Variability of aquifer sorption properties in a field experiment on groundwater transport of organic solutes: Methods and preliminary results

Abstract Methods to characterize the organic solute sorption distribution coefficient, organic carbon content, and specific surface area of aquifer solids from the site of a field experiment on solute transport in groundwater were refined for application to small subsamples of 10-cm depth increments taken from 5-cm diameter cores. Initial results indicate that the average sorption characteristics of the Borden aquifer do not vary appreciably along the trajectory of the solute plumes. However, the sorption distribution coefficient of tetrachloroethylene varied over nearly an order of magnitude among 10-cm depth increments in one core sample. Preliminary evidence suggests that the sorption distribution coefficients for four halogenated organic solutes vary proportionally among core strata. However, the distribution coefficients for sorption of tetrachloroethylene on various depth increments are not well correlated with either organic carbon content or specific surface area, suggesting that as yet unidentified mineral phases may play a significant role in sorption of such solutes by the sandy aquifer solids.

Challenges in Monitoring the Natural Attenuation of Spatially Variable Plumes

Monitored natural attenuation may be applied as a risk-based remediation strategy if it can be established that contaminants are or will be reduced to some acceptable level at or before a compliance point. Contaminant attenuation is often attributed to intrinsic biodegradation, which in some circumstances may occur only at the plume fringes where electron acceptors from the surrounding uncontaminated zones mix by dispersion and diffusion with the plume. However, due to the common spatial and temporal variability exhibited by many plumes, the centreline monitoring approaches advocated in many natural attenuation protocols may be unable to detect natural attenuation occurring primarily by fringe processes. Snapshot data from a multilevel sampling well transect across an MTBE plume at Vandenberg Air Force Base, CA, USA, illustrate the difficulty of centreline monitoring and the challenge of providing sufficient detail to detect attenuation processes that may be occurring primarily at plume fringes. In a study of a phenols plume in Wolverhampton, UK, high-resolution multilevel wells demonstrated that the key biodegradation processes were restricted spatially to the upper fringe of the plume and were rate-limited by transverse dispersion and diffusion of electron acceptors into the plume. Thus the overall extent of biodegradation was considerably less than suggested by a plume-scale analysis of total electron acceptor and contaminant budgets. These examples indicate that more robust and cost-effective MNA assessments can be obtained using monitoring strategies that focus on the location of key biodegradation processes.

Naturally Occurring Bacteria Similar to the Methyl tert-Butyl Ether (MTBE)-Degrading Strain PM1 Are Present in MTBE-Contaminated Groundwater

ABSTRACT Methyl tert-butyl ether (MTBE) is a widespread groundwater contaminant that does not respond well to conventional treatment technologies. Growing evidence indicates that microbial communities indigenous to groundwater can degrade MTBE under aerobic and anaerobic conditions. Although pure cultures of microorganisms able to degrade or cometabolize MTBE have been reported, to date the specific organisms responsible for MTBE degradation in various field studies have not be identified. We report that DNA sequences almost identical (99% homology) to those of strain PM1, originally isolated from a biofilter in southern California, are naturally occurring in an MTBE-polluted aquifer in Vandenberg Air Force Base (VAFB), Lompoc, California. Cell densities of native PM1 (measured by TaqMan quantitative PCR) in VAFB groundwater samples ranged from below the detection limit (in anaerobic sites) to 103 to 104 cells/ml (in oxygen-amended sites). In groundwater from anaerobic or aerobic sites incubated in microcosms spiked with 10 μg of MTBE/liter, densities of native PM1 increased to approximately 105 cells/ml. Native PM1 densities also increased during incubation of VAFB sediments during MTBE degradation. In controlled field plots amended with oxygen, artificially increasing the MTBE concentration was followed by an increase in the in situ native PM1 cell density. This is the first reported relationship between in situ MTBE biodegradation and densities of MTBE-degrading bacteria by quantitative molecular methods.

Direct Detection of Residual Nonaqueous Phase Liquid in the Saturated Zone Using SF6 as a Partitioning Tracer.

Sulfur hexafluoride (SF 6 ) was investigated as a potential detector of residual dense nonaqueous phase liquids (DNAPL) below the water table. In NAPL-water batch partitioning tests, SF 6 had a partitioning coefficient (K NW =concentration in NAPL÷ concentration in water) of 3211.33 to TCE, 2411.27 to DCM, and 45±1.89 to o-DCB. Tracer tests with SF 6 and bromide in packed sand columns containing residual DNAPL show that SF 6 breakthrough is retarded relative to bromide. The SF 6 retardation factor was 2.2 relative to bromide in a column containing 3.69% residual TCE and 2.6 in a column with o-DCB at 3.52% residual saturation. A relatively simple relationship based on equilibrium partitioning and retardation theory was developed that allows very accurate estimates of DNAPL residual content encountered by SF 6 during transport. Using observed retardation factors from the column tests and the experimentally determined K ow values, this simple calculation was found to predict DNAPL residual saturations to within 2.5% of that actually emplaced

A forced‐gradient experiment on solute transport in the Borden aquifer: 1. Experimental methods and moment analyses of results

A forced-gradient test of solute transport was conducted adjacent to the site of a previous natural gradient test to allow comparison of results obtained from the two types of field tests. In the forced-gradient test, one inorganic tracer and four organic solutes (carbon tetrachloride, trichloroethylene, tetrachloroethylene, and hexachloroethane) were injected for 48 hours. The migration of the dissolved chemicals toward the extraction well was monitored by multilevel samplers and a partially penetrating monitoring well. Moment analyses of the solute breakthrough curves at the multilevel points provide model-independent insight into the variation of hydraulic conductivity and organic solute retardation in the aquifer. The estimated retardation factors fall within the range estimated in the previous natural gradient test. Sorption distribution coefficients inferred from the transport data indicate spatial variability of sorption for all solutes, with mean values and ranges similar to those observed in laboratory studies of core or bulk samples. Some evidence was obtained for spatially variable in situ transformation of hexachloroethane.

A forced‐gradient experiment on solute transport in the Borden aquifer: 3. Nonequilibrium transport of the sorbing organic compounds

Transport of four organic solutes was monitored in 15 multilevel points and a partially penetrating well during a forced-gradient test in the Borden aquifer. Simulations assuming equilibrium sorption were unable to match the observed breakthrough curves, particularly for the more highly sorbing solutes. The data were better fit by first-order physical nonequilibrium simulations including mass transfer between mobile and immobile water regions. Assuming 86% of total porosity is mobile, as estimated previously, curve fits yielded estimates of Kd (sorption distribution coefficient), ƒ (fraction of sorption sites within mobile water regions), and α (first-order mass transfer coefficient), Kd and ƒ estimates compare well with those from previous laboratory studies and simulations of the natural gradient test previously conducted nearby. The estimated a values compare reasonably well with those derived from laboratory tests but are considerably higher than the value utilized in simulations of the natural gradient test.

Role of back diffusion and biodegradation reactions in sustaining an MTBE/TBA plume in alluvial media

A methyl tert-butyl ether (MTBE) / tert-butyl alcohol (TBA) plume originating from a gasoline spill in late 1994 at Vandenberg Air Force Base (VAFB) persisted for over 15 years within 200 feet of the original spill source. The plume persisted until 2010 despite excavation of the tanks and piping within months after the spill and excavations of additional contaminated sediments from the source area in 2007 and 2008. The probable history of MTBE concentrations along the plume centerline at its source was estimated using a wide variety of available information, including published details about the original spill, excavations and monitoring by VAFB consultants, and our own research data. Two-dimensional reactive transport simulations of MTBE along the plume centerline were conducted for a 20-year period following the spill. These analyses suggest that MTBE diffused from the thin anaerobic aquifer into the adjacent anaerobic silts and transformed to TBA in both aquifer and silt layers. The model reproduces the observation that after 2004 TBA was the dominant solute, diffusing back out of the silts into the aquifer and sustaining plume concentrations much longer than would have been the case in the absence of such diffusive exchange. Simulations also suggest that aerobic degradation of MTBE or TBA at the water table in the overlying silt layer significantly affected concentrations of MTBE and TBA by limiting the chemical mass available for back diffusion to the aquifer.

Analytical method for the sorption of hydrophobic organic pollutants in clay-rich materials.

A method using dialysis tubing for phase separation was developed for measuring nonionic, hydrophobic organic compound sorption in clay-rich geologic media such as aquitards. The complications caused by nonsettling particles when centrifugation is used for phase separation are eliminated. Slurried sample was sealed in the tubing and placed into a sample bottle, which was filled with synthetic groundwater amended with the sorbate of interest. Particles are retained within the tubing during equilibration, whereas the sorbate may diffuse through the tubing yielding equal aqueous-phase concentrations inside and out. Following equilibration, the aqueous phase outside the tubing was analyzed, and sorbed mass was determined by difference. Equilibration occurred within 48-72 h

The Borden Site for Groundwater Contamination Experiments: 1978–1995

The Borden experimental site for studies of groundwater contamination was established in 1978 in a small area of a large military base (Canadian Forces Base Borden) situated 60 km northwest of Toronto, Canada. The experimental site is used primarily for controlled experiments in which a variety of chemicals of industrial origin, some of which are hazardous substances, are released carefully into the subsurface environment with detailed monitoring of their movement and fate. The experiments are “controlled” in the sense that the mass and input locations of the chemicals are known precisely, and the ultimate extent of migration of the chemicals at environmentally objectionable concentrations is readily controlled by engineering means such as excavation, pump-and-treat or new technologies. In a few experiments the input chemicals are allowed to move without intervention because concentrations in the aquifer have attained or will ultimately attain environmentally acceptable levels.