The importance of transects for characterizing aged organic contaminant plumes in groundwater.

A complex mixture of dissolved organic contaminants, emanating from a many decades-old, residual, dense non-aqueous phase liquid (DNAPL) source, migrates through unconfined, moderately heterogeneous, glacial-derived sediments and sedimentary rock in a residential area of Dane County, Wisconsin, USA. A portion of this contaminant plume intersects a large man-made pond, roughly 400 m downgradient of the source zone. Depth-discrete, multilevel groundwater sampling, detailed sedimentological logs, and hydraulic head profiles were used to delineate the spatial distribution of hydraulic, geologic, organic contaminant, and redox hydrochemical conditions within the established plume along two transects immediately upgradient of the pond. Twenty-one contaminants were detected and classified into four major contaminant groups: chlorinated ethenes, chlorinated ethanes, aromatics (BTEX: benzene, toluene, ethylbenzene, xylene), and aliphatic ketones. Within the glacial sediments and shallow bedrock, zones of reductive dechlorination of chlorinated ethenes and ethanes were juxtaposed with zones of BTEX and ketone degradation. Spatial heterogeneity in the concentration and distribution of contaminant groups and redox conditions was observed over lateral distances of tens of meters and vertical distances of tens of centimeters along the two transects. Although the site was situated in a complex glacial depositional environment, lithologic and hydraulic heterogeneity surprisingly only had a modest influence on the spatial distribution of plume contaminants. Depth-discrete sampling along paired, closely spaced transects (~20 m apart) was essential to assess internal plume composition/concentration evolution along flow paths with strong attenuation over short migration distances. This study shows how paired, highly resolved transects can enhance understanding of transverse and longitudinal variability in areas where contaminant-induced redox conditions control reaction zones and strong plume attenuation.

Methane gas transport in unconfined aquifers: A numerical sensitivity study of a controlled release experiment at CFB Borden.

Subsurface leakage of natural gas from petroleum wells can impact freshwater aquifers. Accurate prediction of gas migration in the subsurface will depend on knowledge of permeability, porosity, and flow system conditions. A series of two-dimensional numerical multi-phase flow simulations (CFbio) were conducted to investigate the role of multi-phase parameters (relative permeability and air entry pressure), flow system conditions (intrinsic permeability, anisotropy, and groundwater velocity), and geometric properties (layer thickness and layer lateral continuity) on the flow of gas-phase methane emanating from two variable-rate point sources in an unconfined sandy aquifer. Numerical simulations showed that for a homogeneous, weakly anisotropic aquifer, gas migrates almost exclusively vertically due to buoyancy, before venting to the vadose zone and atmosphere. As vertical migration became restricted through increased anisotropy, inclusion of lower-permeable layers, or increased horizontal groundwater velocity, an increase in the lateral component of gas migration was observed. This led to the formation of a broader lateral migration of the gas-phase plume and establishment of variably distributed vertical preferential flow paths, ultimately resulting in increased gas retention in the aquifer with relatively less methane reaching the vadose zone or atmosphere. The inclusion of a thin layer with moderately lower permeability (1-2 orders of magnitude) and increased air entry pressure was used to depict a fine-grained sand lens within a uniform aquifer. This subtle feature led to the formation of thin gas pools extending up- and down-gradient beneath the lens, allowing methane to travel much farther and faster than by groundwater advection alone, which is consistent with field observations during the experiment. In all scenarios investigated gas-phase methane was shown to migrate predominantly vertically due to buoyancy, until the aquitard permeability was <30% of the aquifer permeability. Our modelling demonstrates that even subtle permeability contrasts, together with capillary pressure changes demarcating grain-scale bedding, will lead to extensive lateral free-phase gas migration, and development of a more extensive and complex zone of impacted aquifer than presupposed.

Monitoring the evolution and migration of a methane gas plume in an unconfined sandy aquifer using time-lapse GPR and ERT.

Fugitive methane (CH4) leakage associated with conventional and unconventional petroleum development (e.g., shale gas) may pose significant risks to shallow groundwater. While the potential threat of stray (CH4) gas in aquifers has been acknowledged, few studies have examined the nature of its migration and fate in a shallow groundwater flow system. This study examines the geophysical responses observed from surface during a 72day field-scale simulated CH4 leak in an unconfined sandy aquifer at Canadian Forces Base Borden, Canada, to better understand the transient behaviour of fugitive CH4 gas in the subsurface. Time-lapse ground-penetrating radar (GPR) and electrical resistivity tomography (ERT) were used to monitor the distribution and migration of the gas-phase and assess any impacts to groundwater hydrochemistry. Geophysical measurements captured the transient formation of a CH4 gas plume emanating from the injector, which was accompanied by an increase in total dissolved gas pressure (PTDG). Subsequent reductions in PTDG were accompanied by reduced bulk resistivity around the injector along with an increase in the GPR reflectivity along horizontal bedding reflectors farther downgradient. Repeat temporal GPR reflection profiling identified three events with major peaks in reflectivity, interpreted to represent episodic lateral CH4 gas release events into the aquifer. Here, a gradual increase in PTDG near the injector caused a sudden lateral breakthrough of gas in the direction of groundwater flow, causing free-phase CH4 to migrate much farther than anticipated based on groundwater advection. CH4 accumulated along subtle permeability boundaries demarcated by grain-scale bedding within the aquifer characteristic of numerous Borden-aquifer multi-phase flow experiments. Diminishing reflectivity over a period of days to weeks suggests buoyancy-driven migration to the vadose zone and/or CH4 dissolution into groundwater. Lateral and vertical CH4 migration was primarily governed by subtle, yet measurable heterogeneity and anisotropy in the aquifer.

Multidimensional Investigation of Bedrock Heterogeneity/Unconformities at a DNAPL-Impacted Site.

Organic solvent (i.e., dense nonaqueous phase liquid, DNAPL) migration in the subsurface is known to be extremely sensitive to geologic heterogeneity. There is often a focus on heterogeneity that results from changing depositional conditions over short spatial scales. Similar or even more extreme spatial heterogeneity can result postdeposition due to erosional processes. This study applies a synergistic approach based on a combination of high-resolution lithologic logs of continuous cores, borehole geophysical logs, surface electrical resistivity, and seismic refraction tomography models to assess spatial heterogeneity in a shallow bedrock sequence subject to multiple unconformities and contaminated with a mixture of organic chemicals. The persistence of DNAPL in the source zone and an associated dissolved-phase plume led to variable impacts on formation resistivity across the study site. Seismic refraction in combination with electrical resistivity tomography improved interpretation of highly irregular erosional boundaries by delineating sharp lateral transitions in lithologic composition near the source zone and across the dissolved-phase plume. Electrical resistivity was effective at differentiating between clean and mud-rich sandstones and their unconformable contact with an underlying dolostone. Geophysical measurements revealed eroded dolostone mounds encased by a network of younger mud-rich sandstones channelized by clean semi-lithified sand, all of which was buried beneath variable glacial drift. Our synergistic multidimensional approach resulted in the development of a detailed three-dimensional shallow bedrock geospatial model, which has led to an improved understanding of DNAPL migration and contaminant plume heterogeneity.