[1] At an industrial site on a sand aquifer overlying a clayey silt aquitard in Connecticut, a zone of trichloroethylene dense nonaqueous phase liquid (DNAPL) at the aquifer bottom was isolated in late 1994 by installation of a steel sheet piling enclosure. In response to this DNAPL isolation, three aquifer monitoring wells located approximately 330 m downgradient exhibited strong TCE declines over the next 2-3 years, from trichloroethylene (TCE) concentrations between 5000 and 30,000 mg/L to values leveling off between 200 and 2000 mg/L. TCE concentrations from analysis of vertical cores from the aquitard below the plume and also from depth-discrete multilevel systems in the aquifer sampled in 2000 were represented in a numerical model. This shows that vertical back diffusion from the aquitard combined with horizontal advection and vertical transverse dispersion account for the TCE distribution in the aquifer and that the aquifer TCE will remain much above the MCL for centuries.
In a new conceptual model for immiscible‐phase organic liquids in fractured porous media that specifically includes the effect of molecular diffusion on the persistence of organic liquid in fractures, dissolved contaminant mass from the liquid in fractures is lost by diffusion from the fractures into the porous matrix between the fractures. Theoretical calculations for one‐dimensional diffusive fluxes from single, parallel‐plate fractures using parameter values typical of fractured porous geologic media establishes the concept of immiscible‐phase disappearance time, which is the time required for a given volume of immiscible liquid in a specified aperture to disappear following its arrival in the fracture. Nonlithified surficial clayey deposits with matrix porosities ranging from 25 to 70% are extensive across many regions of North America and Europe, and at shallow depth, typically have fractures with apertures in the range of 1 to 100 microns. The common chlorinated solvents such as dichloromethane (DCM), trichloroethene (TCE), and tetrachloroethene (PCE) are expected to completely disappear in these deposits within a few days to weeks. For fractured sedimentary rocks with much lower matrix porosities (5–15%), disappearance times for these solvents are generally less than several years for fracture apertures ranging from 10 to 200 microns typical for shales, siltstones, sandstones, and carbonate rocks. This is sufficient time for the immiscible phase of chlorinated solvent contamination to have disappeared at many industrial sites. This conceptual model has important implications with respect to ground‐water monitoring, diagnosis of the nature and degree of contamination, and expectations for ground‐water remediation at many contaminated sites. Proposed methods for enhancing immiscible‐phase mass removal using hydraulic manipulation, surfactants, or alcohols will be futile where the immiscible phase has disappeared into the clay or rock matrix, and reverse diffusion and desorption will control clean‐up time frames. Therefore, prospects for permanent restoration of many DNAPL and LNAPL sites in fractured porous media are more limited than previously thought.
Expansion of shale gas extraction has fueled global concern about fugitive methane impacts on groundwater and climate. Although methane leakage from wells is common, information regarding impacts to groundwater remains sparse, and is believed by many to be minor. We injected methane gas into a shallow, flat-lying sand aquifer for 72 days. While a significant fraction of methane vented to the atmosphere, an equal portion remained in the groundwater.Methane migration in the aquifer was governed by subtle grain-scale bedding that impeded buoyant free-phase gas flow, leading to episodic releases of free-phase gas, and fostering lateral gas migration farther than anticipated based on groundwater advection. Methane persisted in the groundwater zone despite active growth of methanotrophic bacteria, while much of the methane venting into the vadose zone was degraded. Our results show even small-volume releases of methane gas cause extensive free-phase and solute plumes emanating from leaks only detectable using well-established contaminant hydrogeology monitoring methods.
Chlorinated ethenes often migrate over extended distances in aquifers and may originate from different sources. The aim of this study was to determine whether stable carbon isotope ratios remain constant during dissolution and transport of chlorinated ethenes and whether the ratios can be used to link plumes to their sources. Detailed depth-discrete delineation of the carbon isotope ratio in a tetrachloroethene (PCE) plume and in a trichloroethene (TCE) plume was done along cross-sections orthogonal to groundwater flow in two sandy aquifers in the Province of Ontario, Canada. At the TCE site, TCE concentrations up to solubility were measured in one high concentration zone close to the bottom of the aquifer from where dense non-aqueous phase liquid (DNAPL) was collected. A laboratory experiment using the DNAPL indicated that only very small carbon isotope fractionation occurs during dissolution of TCE (0.26x), which is consistent with field observations. At most sampling points, the y 13 C of dissolved TCE was similar to that of the DNAPL except for a few sampling points at the bottom of the aquifer close to the underlying aquitard. At these points, a 13 C enrichment of up to 2.4x was observed, which was likely due to biodegradation and possibly preferential diffusion of TCE with 12 C into the aquitard. In contrast to the TCE site, several distinct zones of high concentration were observed at the PCE site and from zones to zone, the y 13 C values varied substantially from À 24.3x to À 33.6x. Comparison of the y 13 C values in the high concentration zones made it possible to divide the plume in the three different domains, each probably representing a different episode and location of DNAPL release. The three different zones could still be distinguished 220 m from the DNAPL sources. This demonstrates that carbon isotope ratios can be used to differentiate between different zones in chlorinated ethene plumes and to link plume zones to their sources. In addition, subtle variations in y 13 C at plume fringes provided insight into mechanisms of plume spreading in transverse vertical direction. These variations were identified because of the high-resolution provided by the monitoring network.
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