Area-differentiated modelling of excess nutrient and contaminant leaching risks at the river-basin scale using a hydrological approach in India

Application of excess nutrients, such as fertilizers, is a significant and sometimes even major component of groundwater pollution. Diffuse inputs of nutrients and contaminants to the groundwater are related to runoff generated from precipitation on a catchment. This implies that the analysis of diffuse fluxes from the land surface to the groundwater requires an analysis of water fluxes for a catchment. This requires the simulation and modelling of total runoff, groundwater recharge, and plant-available water as a function of the regional interaction of the climate, soil, hydrogeology, topography and land-use conditions in the river basin. A model has been developed for large river basins in India, and has been applied to the Upper Yamuna basin, to quantify the exchange probability of plant-available soil water, which can be taken as a measure to determine the nutrient and contaminant leaching risk of a site. It was found that, with the available large-scale databases and methods, regional patterns of the total runoff could be simulated successfully. In this way, about 75001km² of the total 121000 km² of the Upper Yamuna basin was classified as an area sensitive to nutrient and contaminant leaching.     

A direct passive method for measuring water and contaminant fluxes in porous media

This paper introduces a new direct method for measuring water and contaminant fluxes in porous media. The method uses a passive flux meter (PFM), which is essentially a self-contained permeable unit properly sized to fit tightly in a screened well or boring. The meter is designed to accommodate a mixed medium of hydrophobic and/or hydrophilic permeable sorbents, which retain dissolved organic/inorganic contaminants present in the groundwater flowing passively through the meter. The contaminant mass intercepted and retained on the sorbent is used to quantify cumulative contaminant mass flux. The sorptive matrix is also impregnated with known amounts of one or more water soluble 'resident tracers'. These tracers are displaced from the sorbent at rates proportional to the groundwater flux; hence, in the current meter design, the resident tracers are used to quantify cumulative groundwater flux. Theory is presented and quantitative tools are developed to interpret the water flux from tracers possessing linear and nonlinear elution profiles. The same theory is extended to derive functional relationships useful for quantifying cumulative contaminant mass flux. To validate theory and demonstrate the passive flux meter, results of multiple box-aquifer experiments are presented and discussed. From these experiments, it is seen that accurate water flux measurements are obtained when the tracer used in calculations resides in the meter at levels representing 20 to 70 percent of the initial condition. 2,4-Dimethyl-3-pentanol (DMP) is used as a surrogate groundwater contaminant in the box aquifer experiments. Cumulative DMP fluxes are measured within 5% of known fluxes. The accuracy of these estimates generally increases with the total volume of water intercepted.     

Effects of reduced contaminant loading on downgradient water quality in an idealized two-layer granular porous media

The following explores the issue of how reductions in contaminant loading to plumes will effect downgradient water quality. An idealized scenario of two adjacent layers of uniform geologic media, one transmissive and the other low permeability, is considered. A high concentration source, similar to a thin DNAPL pool, is introduced in the transmissive layer immediately above the low permeability layer. While the source is active, dissolved constituents are driven along the contact by advection and into the low permeability layer by transverse diffusion. Removing the source reverses the concentration gradient between the layers, driving back diffusion of contaminants from the low permeability layer. Laboratory studies involving four contaminants demonstrate that 15 to 44% of the introduced contaminant moves into the low permeability zone (along a distance of 87 cm in a sand tank) over a period of 25 days. The greatest movement of contaminants into the low permeability zone is seen with the contaminants with the greatest sorption coefficients. A unique two-dimensional analytical solution is developed for the two-layer scenario. Processes addressed include advection; transverse dispersion; adsorption and degradation in the transmissive zones; and diffusion, adsorption, and degradation in the low permeability layer. Laboratory data agree favorably with the analytical solutions. Collectively, the laboratory results and analytical solutions provide a basis for testing other modeling approaches that can be applied to more complex problems. A set of field-scale scenarios are considered using the analytical solutions. Results indicate that improvement in water quality associated with source removal diminish with distance downgradient of the source. Furthermore, contaminant degradation and contaminant adsorption in the stagnant zone are shown to be critical factors governing the timing and magnitude of downgradient improvements in water quality. For five of six scenarios considered, observed improvements in water quality 100 m downgradient of the source fall in the range of 1 to 2 orders of magnitude 15 years after complete source removal. The sixth scenario, involving a contaminant half-life of three years and no adsorption, shows greater than three order of magnitude improvements in downgradient water quality within one year of source removal.     

Pulsed pumping process optimization using a potential flow model

A computational model is applied to the optimization of pulsed pumping systems for efficient in situ remediation of groundwater contaminants. In the pulsed pumping mode of operation, periodic rather than continuous pumping is used. During the pump-off or trapping phase, natural gradient flow transports contaminated groundwater into a treatment zone surrounding a line of injection and extraction wells that transect the contaminant plume. Prior to breakthrough of the contaminated water from the treatment zone, the wells are activated and the pump-on or treatment phase ensues, wherein extracted water is augmented to stimulate pollutant degradation and recirculated for a sufficient period of time to achieve mandated levels of contaminant removal. An important design consideration in pulsed pumping groundwater remediation systems is the pumping schedule adopted to best minimize operational costs for the well grid while still satisfying treatment requirements. Using an analytic two-dimensional potential flow model, optimal pumping frequencies and pumping event durations have been investigated for a set of model aquifer-well systems with different well spacings and well-line lengths, and varying aquifer physical properties. The results for homogeneous systems with greater than five wells and moderate to high pumping rates are reduced to a single, dimensionless correlation. Results for heterogeneous systems are presented graphically in terms of dimensionless parameters to serve as an efficient tool for initial design and selection of the pumping regimen best suited for pulsed pumping operation for a particular well configuration and extraction rate. In the absence of significant retardation or degradation during the pump-off phase, average pumping rates for pulsed operation were found to be greater than the continuous pumping rate required to prevent contaminant breakthrough.     

Fugacity modelling to predict the distribution of organic contaminants in the soil:oil matrix of constructed biopiles

Level I and II fugacity approaches were used to model the environmental distribution of benzene, anthracene, phenanthrene, 1-methylphenanthrene and benzo[a]pyrene in a four phase biopile system, accounting for air, water, mineral soil and non-aqueous phase liquid (oil) phase. The non-aqueous phase liquid (NAPL) and soil phases were the dominant partition media for the contaminants in each biopile and the contaminants differed markedly in their individual fugacities. Comparison of three soils with different percentage of organic carbon (% org C) showed that the % org C influenced contaminant partitioning behaviour. While benzene showed an aqueous concentration worthy of note for leachate control during biopiling, other organic chemicals showed that insignificant amount of chemicals leached into the water, greatly reducing the potential extent of groundwater contamination. Level II fugacity model showed that degradation was the dominant removal process except for benzene. In all three biopile systems, the rate of degradation of benzo(a)pyrene was low, requiring more than 12 years for soil concentrations from a spill of about 25 kg (100 mol) to be reduced to a concentration of 0.001 microgg(-1). The removal time of 1-methylphenanthrene and either anthracene or phenanthrene was about 1 and 3 years, respectively. In contrast, benzene showed the highest degradation rate and was removed after 136 days in all biopile systems. Overall, this study confirms the association of risk critical contaminants with the residual saturation in treated soils and reinforces the importance of accounting for the partitioning behaviour of both NAPL and soil phases during the risk assessment of oil-contaminated sites.     

Risk assessment and prioritisation of contaminated sites on the catchment scale

Contaminated sites pose a significant threat to groundwater resources worldwide. Due to limited available resources a risk-based prioritisation of the remediation efforts is essential. Existing risk assessment tools are unsuitable for this purpose, because they consider each contaminated site separately and on a local scale, which makes it difficult to compare the impact from different sites. Hence a modelling tool for risk assessment of contaminated sites on the catchment scale has been developed. The CatchRisk screening tool evaluates the risk associated with each site in terms of its ability to contaminate abstracted groundwater in the catchment. The tool considers both the local scale and the catchment scale. At the local scale, a flexible, site specific leaching model that can be adjusted to the actual data availability is used to estimate the mass flux over time from identified sites. At the catchment scale, a transport model that utilises the source flux and a groundwater model covering the catchment is used to estimate the transient impact on the supply well. The CatchRisk model was tested on a groundwater catchment for a waterworks north of Copenhagen, Denmark. Even though data scarcity limited the application of the model, the sites that most likely caused the observed contamination at the waterworks were identified. The method was found to be valuable as a basis for prioritising point sources according to their impact on groundwater quality. The tool can also be used as a framework for testing hypotheses on the origin of contamination in the catchment and for identification of unknown contaminant sources.     

Processes controlling the distribution and natural attenuation of dissolved phenolic compounds in a deep sandstone aquifer

Processes controlling the distribution and natural attenuation (NA) of phenol, cresols and xylenols released from a former coal-tar distillation plant in a deep Triassic sandstone aquifer are evaluated from vertical profiles along the plume centerline at 130 and 350 m from the site. Up to four groups of contaminants (phenols, mineral acids, NaOH, NaCl) form discrete and overlapping plumes in the aquifer. Their distribution reflects changing source history with releases of contaminants from different locations. Organic contaminant distribution in the aquifer is determined more by site source history than degradation. Contaminant degradation at total organic carbon (TOC) concentrations up to 6500 mg l(-1) (7500 mg l(-1) total phenolics) is occurring by aerobic respiration NO3-reduction, Mn(IV)-/Fe(III)-reduction, SO4-reduction, methanogenesis and fermentation, with the accumulation of inorganic carbon, organic metabolites (4-hydroxybenzaldehyde, 4-hydroxybenzoic acid), acetate, Mn(II), Fe(II), S(-II), CH4 and H2 in the plume. Aerobic and NO3-reducing processes are restricted to a 2-m-thick plume fringe but Mn(IV)-/Fe(II)-reduction, SO4-reduction, methanogenesis and fermentation occur concomitantly in the plume. Dissolved H2 concentrations in the plume vary from 0.7 to 110 nM and acetate concentrations reach 200 mg l(-1). The occurrence of a mixed redox system and concomitant terminal electron accepting processes (TEAPs) could be explained with a partial equilibrium model based on the potential in situ free energy (deltaGr) yield for oxidation of H2 by specific TEAPs. Respiratory processes rather than fermentation are rate limiting in determining the distribution of H2 and TEAPs and H2 dynamics in this system. Most (min. 90%) contaminant degradation has occurred by aerobic and NO3-reducing processes at the plume fringe. This potential is determined by the supply of aqueous O2 and NO3 from uncontaminated groundwater, as controlled by transverse mixing, which is limited in this aquifer by low dispersion. Consumption to date of mineral oxides and SO4 is, respectively, <0.15% and 0.4% of the available aquifer capacity, and degradation using these oxidants is <10%. Fermentation is a significant process in contaminant turnover, accounting for 21% of degradation products present in the plume, and indicating that microbial respiration rates are slow in comparison with fermentation. Under present conditions, the potential for degradation in the plume is very low due to inhibitory effects of the contaminant matrix. Degradation products correspond to <22% mass loss over the life of the plume, providing a first-order plume scale half-life >140 years. The phenolic compounds are biodegradable under the range of redox conditions in the aquifer and the aquifer is not oxidant limited, but the plume is likely to be long-lived and to expand. Degradation is likely to increase only after contaminant concentrations are reduced and aqueous oxidant inputs are increased by dispersion of the plume. The results imply that transport processes may exert a greater control on the natural attenuation of this plume than aquifer oxidant availability.     

Surface catalyzed Fenton treatment of bis(2-chloroethyl) ether and bis(2-chloroethoxy) methane

This study examined the feasibility of using surface catalyzed Fenton treatment to remediate soil and groundwater contaminated by the chlorinated ethers, bis(2-chloroethyl) ether (BCEE) and bis(2-chloroethoxy) methane (BCEM). Parameters that affect the contaminant loss rate such as porewater pH, hydrogen peroxide concentration, and solid/water ratio were systematically evaluated. Batch reactors were set-up utilizing either contaminated or uncontaminated soil, obtained from an industrial site in Moss Point, MS, that was mixed with synthetic groundwater containing the contaminants of interest. The results show an increase in contaminant reduction with a decrease in pH, an increase in hydrogen peroxide concentration, or an increase in the solid/water ratio. For a similar set of conditions, contaminant reduction was greater for systems utilizing contaminated soil as compared to the systems containing uncontaminated soil. In addition, specific oxygen uptake rates (SOURs) were measured for biomass, collected from an activated sludge plant, exposed to different dilutions of untreated and surface catalyzed Fenton treated water to evaluate whether residual BCEE, BCEM, and their co-contaminants as well as their oxidation by-products were potentially inhibitory or can potentially serve as a substrate for the biomass. The measured SOURs show that the surface catalyzed Fenton treatment enhanced the biodegradability of the contaminated groundwater and served as a substrate for the biomass.     

Laboratory studies to characterize the efficacy of sand capping a coal tar-contaminated sediment

Placement of a microbial active sand cap on a coal tar-contaminated river sediment has been suggested as a cost effective remediation strategy. This approach assumes that the flux of contaminants from the sediment is sufficiently balanced by oxygen and nutrient fluxes into the sand layer such that microbial activity will reduce contaminant concentrations within the new benthic zone and reduce the contaminant flux to the water column. The dynamics of such a system were evaluated using batch and column studies with microbial communities from tar-contaminated sediment under different aeration and nutrient inputs. In a 30-d batch degradation study on aqueous extracts of coal tar sediment, oxygen and nutrient concentrations were found to be key parameters controlling the degradation rates of polycyclic aromatic hydrocarbons (PAHs). For the five PAHs monitored (naphthalene, fluorene, phenanthrene, anthracene, and pyrene), degradation rates were inversely proportional to molecular size. For the column studies, where three columns were packed with a 20-cm sand layer on the top of a 5 cm of sediment layer, flow was established to sand layers with (1) aerated water, (2) N(2) sparged water, or (3) HgCl(2)-sterilized N(2) sparged water. After steady-state conditions, PAH concentrations in effluents were the lowest in the aerated column, except for pyrene, whose concentration was invariant with all effluents. These laboratory scale studies support that if sufficient aeration can be achieved in the field through either active and passive means, the resulting microbially active sand layer can improve the water quality of the benthic zone and reduce the flux of many, but not all, PAHs to the water column.     

Establishing and Evaluating the Risk Implications of Uniform Soil Remediation Goals

Abstract

There is growing interest in the establishment of uniform, concentration-based soil remediation goals to accelerate the cleanup of contaminated sites and to provide consistency in the extent of cleanup required for similar sites. If uniform goals are used in a region, post-remediation risk will vary across sites due to regional variability in site properties. Thus far, of the national and state agencies that have established or proposed uniform concentration guidelines, none have quantified the regional variability in risk that is likely to result with their use as remediation goals.

This paper describes a methodology for evaluating regional cancer risk variability resulting from use of uniform, concentration-based soil remediation goals, presents an example application of the methodology, and examines its utility for establishing uniform goals for organic contaminants in soil. The methodology is based on the integrated transport, fate, exposure, and risk model SoilRisk. Also addressed is the issue of contaminants for which regional risk variability is expected to be large with use of uniform soil remediation goals. An approach is described for reducing such variability by development of alternative uniform contaminant concentration levels for categories of sites differentiated by site geographic location or physical-chemical properties.     

Allocating Remedial Costs at Superfund Sites with Commingled Groundwater Contaminant Plumes

Remedial efforts at Superfund sites across the country focus on groundwater contaminant plumes that have been produced by contributions from multiple parties. Allocating cleanup costs between the parties in a fair and equitable manner can be a problem of substantial complexity. Considerable time and money may be spent determining the amount of contamination attributable to each party in order to apportion liability. Contaminant plumes that have evolved over long periods of time may affect large volumes of groundwater and require extensive remediation. Pump and treat remedial costs are driven by both the volume of water extracted and the mass of contaminants removed. Allocation methods based solely on the mass of contaminants contributed by each party are inadequate in this setting since they do not account for both components of the remedial costs. This paper presents an approach for equitably allocating remedial costs when addressing overlapping or commingled groundwater plumes. The method accounts for the major elements driving the costs of remediating dispersed contaminant plumes.     

Allocating Remedial Costs at Superfund Sites with Commingled Groundwater Contaminant Plumes

Remedial efforts at Superfund sites across the country focus on groundwater contaminant plumes that have been produced by contributions from multiple parties. Allocating cleanup costs between the parties in a fair and equitable manner can be a problem of substantial complexity. Considerable time and money may be spent determining the amount of contamination attributable to each party in order to apportion liability. Contaminant plumes that have evolved over long periods of time may affect large volumes of groundwater and require extensive remediation. Pump and treat remedial costs are driven by both the volume of water extracted and the mass of contaminants removed. Allocation methods based solely on the mass of contaminants contributed by each party are inadequate in this setting since they do not account for both components of the remedial costs. This paper presents an approach for equitably allocating remedial costs when addressing overlapping or commingled groundwater plumes. The method accounts for the major elements driving the costs of remediating dispersed contaminant plumes.     

Factors controlling groundwater quality in the Yeonjegu District of Busan City,Korea, using the hydrogeochemical processes and fuzzy GIS

The hydrogeochemical processes and fuzzy GIS techniques were used to evaluate the groundwater quality in the Yeonjegu district of Busan Metropolitan City, Korea. The highest concentrations of major ions were mainly related to the local geology. The seawater intrusion into the river water and municipal contaminants were secondary contamination sources of groundwater in the study area. Factor analysis represented the contamination sources of the mineral dissolution of the host rocks and domestic influences. The Gibbs plot exhibited that the major ions were derived from the rock weathering condition. Piper’s trilinear diagram showed that the groundwater quality was classified into five types of CaHCO₃, NaHCO₃, NaCl, CaCl₂, and CaSO₄ types in that order. The ionic relationship and the saturation mineral index of the ions indicated that the evaporation, dissolution, and precipitation processes controlled the groundwater chemistry. The fuzzy GIS map showed that highly contaminated groundwater occurred in the northeastern and the central parts and that the groundwater of medium quality appeared in most parts of the study area. It suggested that the groundwater quality of the study area was influenced by local geology, seawater intrusion, and municipal contaminants. This research clearly demonstrated that the geochemical analyses and fuzzy GIS method were very useful to identify the contaminant sources and the location of good groundwater quality.

     

Evaluating the risks of methyl tertiary butyl ether (MTBE) pollution of urban groundwater

MTBE, a fuel oxygenate added to gasoline in parts of the USA, appears to have imposed significant adverse impacts on groundwater. In the UK, the impacts of MTBE are not well known in part because insufficient data has been presented to allow an accurate assessment. With the recognition of urban groundwater as a potentially valuable resource due to the growing pressure on rural groundwater, there is need for pollution risks to urban groundwater to be evaluated for contaminants such as MTBE. This paper presents the application of a risk-based water management tool called Borehole Optimisation System (BOS) in the evaluation of the risk of MTBE to urban groundwater at city scale using Nottingham city as our case study. The risk model was validated by comparison of its predictions with observations of MTBE detections for about 1100 boreholes in England and Wales. The output of the risk analysis was the production of a map showing the predicted MTBE concentration at all locations in the city. The results indicate that MTBE does not currently pose a major risk to urban groundwater although there may be a potential risk to groundwater in the southern part of the city where most industries are concentrated.     

Integrated methodology for assessing the HCH groundwater pollution at the multi-source contaminated mega-site Bitterfeld/Wolfen

A large-scale groundwater contamination characterises the Pleistocene groundwater system of the former industrial and abandoned mining region Bitterfeld/Wolfen, Eastern Germany. For more than a century, local chemical production and extensive lignite mining caused a complex contaminant release from local production areas and related dump sites. Today, organic pollutants (mainly organochlorines) are present in all compartments of the environment at high concentration levels. An integrated methodology for characterising the current situation of pollution as well as the future fate development of hazardous substances is highly required to decide on further management and remediation strategies. Data analyses have been performed on regional groundwater monitoring data from about 10 years, containing approximately 3,500 samples, and up to 180 individual organic parameters from almost 250 observation wells. Run-off measurements as well as water samples were taken biweekly from local creeks during a period of 18 months. A kriging interpolation procedure was applied on groundwater analytics to generate continuous distribution patterns of the nodal contaminant samples. High-resolution geological 3-D modelling serves as a database for a regional 3-D groundwater flow model. Simulation results support the future fate assessment of contaminants. A first conceptual model of the contamination has been developed to characterise the contamination in regional surface waters and groundwater. A reliable explanation of the variant hexachlorocyclohexane (HCH) occurrence within the two local aquifer systems has been derived from the regionalised distribution patterns. Simulation results from groundwater flow modelling provide a better understanding of the future pollutant migration paths and support the overall site characterisation. The presented case study indicates that an integrated assessment of large-scale groundwater contaminations often needs more data than only from local groundwater monitoring. The developed methodology is appropriate to assess POP-contaminated mega-sites including, e.g. HCH deposits. Although HCH isomers are relevant groundwater pollutants at this site, further organochlorine pollutants are present at considerably higher levels. The study demonstrates that an effective evaluation of the current situation of contamination as well as of the related future fate development requires detailed information of the entire observed system.     

Factors affecting the distribution of hydrocarbon contaminants and hydrogeochemical parameters in a shallow sand aquifer

The distributions of hydrocarbon contaminants and hydrogeochemical parameters were investigated in a shallow sand aquifer highly contaminated with petroleum hydrocarbons leaked from solvent storage tanks. For these purposes, a variety of field investigations and studies were performed, which included installation of over 100 groundwater monitoring wells and piezometers at various depths, soil logging and analyses during well and piezometer installation, chemical analysis of groundwater, pump tests, and slug tests. Continuous water level monitoring at three selected wells using automatic data-logger and manual measuring at other wells were also conducted. Based on analyses of the various investigations and tests, a number of factors were identified to explain the distribution of the hydrocarbon contaminants and hydrogeochemical parameters. These factors include indigenous biodegradation, hydrostratigraphy, preliminary pump-and-treat remedy, recharge by rainfall, and subsequent water level fluctuation. The permeable sandy layer, in which the mean water table elevation is maintained, provided a dominant pathway for contaminant transport. The preliminary pump-and-treat action accelerated the movement of the hydrocarbon contaminants and affected the redox evolution pattern. Seasonal recharge by rain, together with indigenous biodegradation, played an important role in the natural attenuation of the petroleum hydrocarbons via mixing/dilution and biodegradation. The water level fluctuations redistributed the hydrocarbon contaminants by partitioning them into the soil and groundwater. The identified factors are not independent but closely inter-correlated.     

Redistribution of contaminants by a fluctuating water table in a micro-porous, double-porosity aquifer: field observations and model simulations

Large seasonal fluctuations of the water table are characteristic of aquifers with a low specific yield, including those fractured, double-porosity aquifers that have significant matrix porosity containing virtually immobile porewater, such as the Chalk of northern Europe. Where these aquifers are contaminated, a strong relationship between water table elevation and contaminant concentration in groundwater is commonly observed, of significance to the assessment, monitoring, and remediation of contaminated groundwater. To examine the processes governing contaminant redistribution by a fluctuating water table within the 'seasonally unsaturated zone', or SUZ, profiles of porewater solute concentrations have been established at a contaminated site in southern England. These profiles document the contaminant distribution in porewater of the Chalk matrix over the SUZ at a greater level of detail than recorded previously. A novel double-porosity solute transport code has been developed to simulate the evolution of the SUZ matrix porewater contaminant profiles, given a fluctuating water table, when the groundwater is initially contaminated and the SUZ is initially free of contamination. The model is simply characterised by: the matrix-fracture porosity ratio, the matrix block geometry, and a characteristic diffusion time. De-saturation and re-saturation of fractures is handled by a new approximation method. Contaminant accumulates in the upper levels of the SUZ, where it is less accessible to mobile groundwater, and acts as a persistent secondary source of contamination once the original source of contamination has been removed or has become depleted. The 'SUZ process' first attenuates the progress of contaminants in groundwater, and subsequently controls the slow release of contamination back to the mobile groundwater, thus prolonging the duration of groundwater contamination by many years. The SUZ process should operate in any fractured, micro-porous lithology e.g. fractured clays and mudstones, making this approach widely applicable.     

Assessing the natural attenuation of organic contaminants in aquifers using plume-scale electron and carbon balances: model development with analysis of uncertainty and parameter sensitivity

A quantitative methodology is described for the field-scale performance assessment of natural attenuation using plume-scale electron and carbon balances. This provides a practical framework for the calculation of global mass balances for contaminant plumes, using mass inputs from the plume source, background groundwater and plume residuals in a simplified box model. Biodegradation processes and reactions included in the analysis are identified from electron acceptors, electron donors and degradation products present in these inputs. Parameter values used in the model are obtained from data acquired during typical site investigation and groundwater monitoring studies for natural attenuation schemes. The approach is evaluated for a UK Permo-Triassic Sandstone aquifer contaminated with a plume of phenolic compounds. Uncertainty in the model predictions and sensitivity to parameter values was assessed by probabilistic modelling using Monte Carlo methods. Sensitivity analyses were compared for different input parameter probability distributions and a base case using fixed parameter values, using an identical conceptual model and data set. Results show that consumption of oxidants by biodegradation is approximately balanced by the production of CH4 and total dissolved inorganic carbon (TDIC) which is conserved in the plume. Under this condition, either the plume electron or carbon balance can be used to determine contaminant mass loss, which is equivalent to only 4% of the estimated source term. This corresponds to a first order, plume-averaged, half-life of > 800 years. The electron balance is particularly sensitive to uncertainty in the source term and dispersive inputs. Reliable historical information on contaminant spillages and detailed site investigation are necessary to accurately characterise the source term. The dispersive influx is sensitive to variability in the plume mixing zone width. Consumption of aqueous oxidants greatly exceeds that of mineral oxidants in the plume, but electron acceptor supply is insufficient to meet the electron donor demand and the plume will grow. The aquifer potential for degradation of these contaminants is limited by high contaminant concentrations and the supply of bioavailable electron acceptors. Natural attenuation will increase only after increased transport and dilution.