Steven W. Chapman

Review and Analysis of Chlorinated Solvent Dense Nonaqueous Phase Liquid Distributions in Five Sandy Aquifers

To select and design effective remedial measures for dense, nonaqueous phase liquid (DNAPL) source zones, better understanding of the architecture of these zones is needed. In this study, a suite of investigative techniques was applied to perform detailed vertical delineation of chlorinated-solvent source zones in sand aquifers at five contaminated industrial sites (two in Connecticut, and one each in Florida, New Hampshire, and Ontario). The DNAPL occurs in the middle of the aquifers at three of the sites and at or near the bottom at the other two. The DNAPL entered the subsurface at these sites decades ago, and therefore the DNAPL zones have aged due to groundwater dissolution. The suite of investigative techniques was used to perform profile sampling using direct-push methods, in which depth-discrete soil and groundwater samples were taken with extremely close vertical spacing. The sampling included methods to distinguish between free-product and residual DNAPL at two of the sites. At each location where DNAPL was found, the DNAPL occurred in one or a few thin layers, generally between 1 and 30 cm thick. These layers were positioned within distinct grain-size zones, or at contacts between sedimentological layers. In some cases, the DNAPL layers have no apparent textural association. For any particular sampling hole to have a high probability of finding such layers, continuous cores must be collected and sampling of these cores must be done at very close vertical spacing (5 cm or less). Free-product DNAPL occurrences in conventional wells at three of the sites indicated, misleadingly, much greater DNAPL layer thicknesses than actual, and in one case, the conventional well may have caused short-circuiting of DNAPL from the middle to the bottom of the aquifer. Although all of the DNAPL source zones are comprised of only sporadic, thin DNAPL layers representing little total mass, these source zones are the cause of high-concentration dissolved plumes down gradient.

Measurement of Oxygen Demand of Petroleum Hydrocarbon-Contaminated Groundwater

Abstract The standard biological oxygen demand (BOD) test was modified for application to petroleum hydrocarbon-contaminated groundwater. The goal was to assess the potential oxygen demand of plume constituents as part of a field trial investigating oxygen-enhanced in situ bioremediation. Modifications to standard BOD protocol included the use of an adapted microbial population developed from site groundwater and methods to minimize both the loss of volatile compounds and the exposure of samples to air. Results from this study indicated that the measured oxygen demand was significantly greater than the oxygen demand estimated solely by stoichiometric calculations from the concentrations of the analytes of typical regulatory concern, that is, benzene, toluene, ethylbenzene, and xylenes (BTEX). This is not surprising, because the petroleum hydrocarbon sources typically contain many organic contaminants other than BTEX, as well as potentially oxidizable natural dissolved organic matter and inorganic species ...

DFN-M field characterization of sandstone for a process-based site conceptual model and numerical simulations of TCE transport with degradation

Plumes of trichloroethene (TCE) with degradation products occur at a large industrial site in California where TCE as a dense non-aqueous phase liquid (DNAPL) entered the fractured sandstone bedrock at many locations beginning in the late 1940s. Groundwater flows rapidly in closely spaced fractures but plume fronts are strongly retarded relative to groundwater flow velocities owing largely to matrix diffusion in early decades and degradation processes in later decades and going forward. Multiple data types show field evidence for both biotic and abiotic dechlorination of TCE and its degradation products, resulting in non-chlorinated compounds. Analyses were conducted on groundwater samples from hundreds of monitoring wells and on thousands of rock samples from continuous core over depths ranging from 6 to 426 metres below ground surface. Nearly all of the present-day mass of TCE and degradation products resides in the water-saturated, low-permeability rock matrix blocks. Although groundwater and DNAPL flow primarily occur in the fractures, DNAPL dissolution followed by diffusion and sorption readily transfers contaminant mass into the rock matrix. The presence of non-chlorinated degradation products (ethene, ethane, acetylene) and compound specific isotope analysis (CSIA) of TCE and cis-1,2-dichloroethene (cDCE) indicate at least some complete dechlorination by both biotic and abiotic pathways, consistent with the observed mineralogy and hydrogeochemistry and with published results from crushed rock microcosms. The rock matrix contains abundant iron-bearing minerals and solid-phase organic carbon with large surface areas and long contact times, suggesting degradation processes are occurring in the rock matrix. Multiple, high-resolution datasets provide strong evidence for spatially heterogeneous distributions of TCE and degradation products with varying degrees of degradation observed only when using new methods that achieve better detection of dissolved gases (i.e., Snap Sampler™) and contaminant mass stored in the low permeability rock matrix (i.e., CORE-DFN™). Simulations using a discrete fracture-matrix (DFN-M) numerical model capable of rigorously simulating flow and transport in both the fractures and matrix, including interactions, show that even slow, first-order degradation rates (i.e., 5- to 20-year half-lives) informed by site-derived parameters can contribute strongly to natural attenuation, resulting in TCE plumes that become stationary in space and might even retreat after 50 to 100 years, if the DNAPL sources become depleted due to the combination of diffusion and degradation processes.