Trichloroethylene Plume Research Articles

Conditioned backward probability modeling to identify sources of groundwater contaminants subject to sorption and decay

If contamination is observed in an aquifer, a backward probability model can be used to obtain information about the former position of the observed contamination. A backward location probability density function (PDF) describes the possible former positions of the observed contaminant particle at a specified time in the past. If the source release time is known or can be estimated, the backward location PDF can be used to identify possible source locations. For sorbing solutes, the location PDF depends on the phase (aqueous or sorbed) of the observed contamination and on the phase of the contamination at the source. These PDFs are related to adjoint states of aqueous and sorbed phase concentrations. The adjoint states, however, do not take into account the measured concentrations. Neupauer and Lin (2006) presented an approach for conditioning backward location PDFs on measured concentrations of non‐reactive solutes. In this paper, we present a related conditioning method to identify the location of an instantaneous point source of a solute that exhibits first‐order decay and linear equilibrium or non‐equilibrium sorption. We derive the conditioning equations and present an illustrative example to demonstrate important features of the technique. Finally, we illustrate the use of the conditioned location PDF to identify possible sources of contamination by using data from a trichloroethylene plume at the Massachusetts Military Reservation.

Effect of biotransformation on multispecies plume evolution and natural attenuation

Biological transformation of volatile organic compounds is one of the key factors that influence contaminant-plume evolution and thus natural attenuation. In this study we investigate the effect of biological transformation on the transport of contaminants in the aqueous and gaseous phases. The analysis includes the study of the effect of density-driven advection of contaminants in the gaseous phase on multiphase and multispecies flow, fate and transport modeling in the subsurface. Trichloroethylene (TCE) and its two byproducts, dichloroethylene and vinyl chloride, are analyzed as the target contaminants. Our results indicate that density-driven advection of the gaseous phase, which is initiated by evaporation of TCE as a nonaqueous phase liquid, increases the downward and also the lateral migration of TCE within the unsaturated zone. This process also influences the location of high-concentration zones of the byproducts of TCE in the unsaturated and the saturated zones. Biotransformation of TCE contributes to the reduction of dissolved TCE plume development as expected. The daughter byproducts, which are introduced into the subsurface system, show distinct transport patterns as they are affected by their independent degradation kinetics and density-driven advection. These observations, which are based on our simulation results for biotransformation and transport of TCE and its byproducts, are useful in evaluating the natural attenuation processes, its potential health hazards and also the evaluation of potential plume development at contaminated sites.

Conceptual analysis of zero‐valent iron fracture reactive barriers for remediating a trichloroethylene plume in a chalk aquifer

A novel concept, the Fe0 fracture reactive barrier (Fe0 FRB), is proposed to clean up chlorinated solvent pollution of groundwater in a chalk aquifer. Iron particles, suspended in a viscous biodegradable gel, can be injected into selected fractures to create an extended reactive zone of partly iron‐filled fractures. To evaluate the feasibility of Fe0 FRB as a remediation strategy, we conducted numerical modeling simulations to assess the treatment performance of an Fe0 FRB in a hypothetical chalk aquifer. The assessment was carried out using a numerical model for flow and solute transport in a discretely fractured porous medium coupled with an analytical expression representing degradation by iron. The hypothetical chalk aquifer was represented by a three‐dimensional discrete fracture network model that was developed using data from a number of chalk sites. Trichloroethylene reactive transport in the Fe0 FRB and mass exchange of solute between fractures and the porous matrix were fully accounted for in the model. This modeling revealed that the success of the remediation technology lies in creating a highly reactive Fe0 FRB without reducing fracture permeability, which could lead to the plume being diverted around the barrier. A parametric study of various design parameters for the Fe0 FRB suggested that high treatment efficiency could be achieved by employing highly reactive nanoscale iron or by using a high proportion of microscale iron fill and fracture enlargement. The model study also provided some preliminary conclusions on sensitive design parameters of an Fe0 FRB such as the proportion of iron fill, the size of the FRB, and the amount of fracture enlargement. A preliminary analysis suggests that an Fe0 FRB containing a small amount of highly reactive nanoscale iron could provide satisfactory treatment for up to 50 years, depending on contaminant mass flux through the barrier.

Use of geostatistics to evaluate the monitoring well network of a TCE plume

This paper summarises the use of geostatistics to evaluate an existing network of wells in a groundwater monitoring program. The primary objective was to evaluate whether selective well removal would result in a comparable characterisation of a trichloroethylene (TCE) plume. For adequate characterisation, it is important to retain enough wells to evaluate areas with higher concentrations and to show that there is high confidence or low uncertainty in the characterisation. A second objective was to utilise public or commercial software available to consultants in a workplace setting to demonstrate the ease with which this analysis can be performed. A baseline characterisation was generated with the entire well network. Then, subsequent simulations were performed with selected wells removed from the well network. Results indicate that by removing key wells from the network, the TCE plume can be adequately characterised without a significant increase in uncertainty.

Density-driven Transport of Volatile Organic Compounds and its Impact on Contaminated Groundwater Plume Evolution

Density-driven advection of gas phase due to vaporization of chlorinated volatile organic compounds (VOCs) has a significant effect on fate and transport of contaminants. In this study, we investigated the effects of density-driven advection, infiltration, and permeability on contaminant plume evolution and natural attenuation of VOCs in the subsurface system. To analyze these effects, multiphase flow and contaminant transport processes were simulated using a three-dimensional Galerkin-finite-element-based model. Trichloroethylene (TCE) is selected as a target contaminant. Density-driven advection of gas phase elevated the potential of groundwater pollution in the saturated zone by accelerating downward migration of vaporized contaminant in the unsaturated zone. The advection contributed to increased removal rates of non-aqueous phase liquid (NAPL) TCE source and reduced dissolved TCE plume development in the downstream area. Infiltration reduced the velocity of the density-driven advection and its influence zone, but raised TCE transfer from the unsaturated to the saturated zone. The variation in soil permeability showed greater impact on contaminant migration within water phase in the saturated zone than within gas phase in the unsaturated zone. Temporal variations of TCE mass within two-dimensional (2D) and three-dimensional (3D) domains under several modeling conditions were compared. These results are important in evaluation of natural attenuation processes, and should be considered to effectively design monitored natural attenuation as a remedial option.

Forward and backward location probabilities for sorbing solutes in groundwater

Location probability can be used to describe the likely position of a solute particle as it travels through an aquifer. Forward location probability describes the likely future positions of a particle, and can be used to predict the movement of a contaminant plume. Backward location probability describes the likely prior positions of a solute particle, and can be used to identify sources of contamination. For sorbing solutes, the probability distributions must also account for the phase (aqueous or sorbed) of a solute particle. We present new phase-dependent forward and backward location probabilities to describe transport of a solute undergoing linear non-equilibrium sorption. The effects of sorption are incorporated directly into the governing equations that are used to calculate the probability distributions. The shape and magnitude of the distributions depend on the phase of the contamination at both the source (or prior location) and receptor (or future location). These probabilities are related to adjoint states of concentration. Using adjoint theory, Bayes' theorem, and a clever transformation, we develop a model to efficiently calculate backward location probabilities for one or a few receptors. We illustrate important features of backward location probabilities for a sorbing solute with a hypothetical, one-dimensional confined aquifer, and we demonstrate their use in identifying sources of contamination for a trichloroethylene plume at the Massachusetts Military Reservation using a three-dimensional numerical model.

Natural Attenuation of Trichloroethylene in Rhizosphere Soils at the Savannah River Site

Extensive trichloroethylene (TCE) groundwater contamination has resulted from discharges to a former seepage basin in the A/M Area at the Department of Energy's Savannah River Site. The direction of groundwater flow has been determined and a seep line where the contaminated groundwater is estimated to emerge as surface water has been identified in a region of the Southern Sector of the A/M Area. This study was undertaken to estimate the potential of four rhizosphere soils along the seep line to naturally attenuate TCE. Microcosms were setup to evaluate both biotic and abiotic attenuation of TCE. Results demonstrated that sorption to soil was the dominant mechanism during the first week of incubation, with as much as 90% of the TCE removed from the aqueous phase. Linear partitioning coefficients (Kd) ranged from 0.83 to 7.4 mL/g, while organic carbon partition coefficients (Koc) ranged from 72 to 180 mL/gC. Diffu-sional losses from the microcosms appeared to be a dominant fate mechanism during the remainder of the experiment, as indicated by results from the water controls. A limited amount of TCE biodegradation was observed, and attempts to stimulate TCE biodegradation by either methanotrophic or methanogenic activity through amendments with methane, oxygen, and methanol were unsuccessful. The appearance of cis-1,2-dichloroethylene (c-DCE), and trans-1,2-dichloroethylene (t-DCE) confirmed the potential for anaerobic reductive dechlorination. However, these daughter products represented less than 5% of the initial TCE added. The sorption results indicate that natural attenuation may represent a viable remediation option for the TCE plume as it passes through the rhizosphere.

Complementary Investigative Techniques for Site Assessment with Low‐Level Contaminants

AbstractA remedial investigation (RI) was performed in an area downgradient from an abandoned missile silo at Vandenberg Air Force Base, California, as part of the United States Air Force Installation Restoration Program (IRP). A number of complementary investigative techniques were used to assure a reliable assessment of site contamination. These included the review of aerial photographs, the use of an organic vapor analyzer (OVA) and carbon adsorption/mass spectrometer (MS) method to conduct a soil‐gas survey; magnetic and electromagnetic geophysical surveys; bedrock permeability testing; and the chemical analysis of soil, sediment, surface water, and ground water samples. The results from this investigation revealed the presence of an undocumented landfill and a small trichloroethylene plume in ground water at concentrations ranging from 6.7 ppb to 31 ppb. The investigation also identified local ground water flow direction, provided strong evidence of the location of potential sources of contamination, and defined the downgradient extent of ground water contamination. Because the identified contaminants have not as yet reached the environmentally sensitive wetland at the base of the slope below this facility, there is still time to propose remedial alternatives that would serve to protect this environmentally sensitive area.

In situ biodegradation of TCE contaminated groundwater

Trichloroethylene (TCE) is one of the most widely used chlorinated solvents; it is also a common groundwater contaminant. TCE–contaminated groundwater is presently treated by transferring the solvent either to a solid (carbon adsorption) or to the atmosphere (air stripping). Biological treatment of the contaminated water is an alternative that causes complete destruction of the solvent. When performed within the aquifer it is termed in situ biotreatment. A specific strain of bacteria (G4) has been isolated that degrades TCE enzymatically. The process has been extensively tested in the laboratory and confirmed in a field pilot test. The pilot test involved the injection of a clean, oxygenated water stream directly into the TCE plume. Nutrients and G4 were added to the injection stream and TCE concentrations were measured up–and downgradient of the injection well. A decline in TCE levels was observed eight hours after injection and continued for the following ten days. TCE concentrations were reduced from a high of 3,000 ppb to a mean value of 78 ppb during a 20 day period.

Microbial biomass and activities associated with subsurface environments contaminated with chlorinated hydrocarbons

Abstract Soil microcosms and enrichment cultures from subsurface sediments and groundwaters contaminated with trichloroethylene (TCE) were examined. Total lipids, [I‐‘4C]acetate incorporation into lipids, and [Me‐3H]thymidine incorporation into DNA were determined in these subsurface environments. In heavily TCE‐contam‐inated zones (greater than 500 mg/L) radioisotopes were not incorporated into lipids or DNA. Radioisotope incorporation occurred in sediments both above and below the TCE plume. Phospholipid fatty acids (PLFA) were not detected, i.e., less than 0.5 pmol/L in heavily contaminated groundwater samples. In less contaminated waters, extracted PLFA concentrations were greater than 100 pmollL and microbial isolates were readily obtained. Degradation of 30–100 mg/L TCE was observed when sediments were amended with a variety of energy sources. Microorganisms in these subsurface sediments have adapted to degrade TCE at concentrations greater than 50 mg/L.

Gaseous Behavior of TCE Overlying a Contaminated Aquifer

ABSTRACTShallow soil gas (<2 meters deep) was collected and analyzed for trichloroethylene (TCE) to determine the relationship with ground‐water contamination directly below. The gaseous TCE plume was mapped with 46 probes and spanned three orders of magnitude in concentration (<0.001 to 2 μg/1). TCE concentrations in water from five monitoring wells around the study site ranged from 4 to 2800 ppb and had a correlation coefficient (r) of 0.90 with TCE concentrations in shallow soil gas. Vertical borings were completed to the water table at four locations in order to obtain profiles of soil gas contamination, air porosity, and water saturation. Gaseous diffusion and air/water partitioning are probably the dominant mechanisms involved in transporting TCE from the ground water to the shallow soil gas.