High-resolution delineation of chlorinated volatile organic compounds in a dipping, fractured mudstone: Depth- and strata-dependent spatial variability from rock-core sampling
Graphical abstract
Introduction
Remediation of groundwater at fractured-rock sites contaminated with Dense Non-Aqueous Phase Liquids (DNAPL) and dissolved chlorinated volatile organic compounds (CVOC) remains a major challenge. A recent strategic study by the U.S.'s National Research Council (NRC) (2013) noted “The most problematic [hazardous waste] sites are those with potentially persistent contaminants including chlorinated solvents recalcitrant to biodegradation, and with hydrogeologic conditions characterized by large spatial heterogeneity or the presence of fractures.” In fractured-rock settings, especially sedimentary rocks, the rock matrix is a reservoir for contaminants that can act as a long-term secondary source by DNAPL dissolution, back diffusion, and de-sorption (Dearden et al., 2013, Mutch et al., 1993, Parker et al., 2010, Révész et al.,2014).
Characterization of the CVOC mass distribution in fractured sedimentary rocks has recently advanced using rock core sampling techniques, providing a fine scale vertical profile of CVOC mass diffused into or sorbed onto rock strata (Sterling et al., 2005, Parker et al., 2010). Analysis of rock core for CVOCs provides fine-scale delineation of contaminants in and on the rock matrix, and is not affected by possible vertical contaminant migration in the borehole after coring (cross contamination). Dearden et al. (2013) used rock core sampling to identify the CVOC profile into about 4 m of Triassic-age mudstone underlying a surficial aquifer. Chapman et al. (2013) recently presented rock core sampling results from 3 coreholes in fractured sedimentary rocks of the Newark Basin, with depths of about 70, 75, and 120 m, respectively, and horizontal spacings of more than 200 m apart along a flow path.
Our study extends previous interpretation of the rock core sampling results by synthesis with borehole geophysical methods and multi-level water-quality monitoring within a site conceptual model based on a detailed hydrogeologic framework, using closely spaced coreholes of depths of up to 60 m. This approach allows a site-scale characterization of the control of weathering and strata-bound permeability features on CVOC migration and persistence in sedimentary fractured rocks. Our study also provides additional direct confirmation of some of the fine scale features included in current conceptual models of CVOC migration in fractured sedimentary rocks, such as limited diffusion into deep unweathered rock. These results are used to refine the conceptual model for CVOC fate and transport in fractured sedimentary rock aquifers.
In this investigation, we synthesize closely spaced rock core sampling results with multi-level water-quality monitoring, borehole geophysical logging, and hydraulic testing at the former Naval Air Warfare Center (NAWC), West Trenton, New Jersey (Fig. 1). Lacombe, 2000, Lacombe, 2002, Lacombe, 2011 describes monitoring well configurations at the site and interpretation of water levels and contaminant concentrations measured in monitoring wells installed by the U.S. Navy. Industrial use of TCE at the NAWC resulted in DNAPL TCE, and dissolved TCE, cis-1,2-dichloroethene (DCE), and vinyl chloride (VC) in groundwater, and historic contaminant discharges to Gold Run, a culverted stream along the site's southern boundary (Fig. 1). The distribution of peak CVOC concentration reflects source areas where DNAPL TCE was released, and is still present, and migration of dissolved CVOCs towards Gold Run. Since the mid-1990s a pump and treat (P&T) system has minimized discharge of CVOCs beyond the site boundary and to Gold Run (Lewis-Brown and Rice, 2002).
The evolving conceptual site model for contaminant migration at NAWC is based on a detailed hydrogeologic framework (Lacombe, 2000, Lacombe and Burton, 2010). The study area is underlain by mudstones of the Lockatong Formation of Triassic age, of the Newark Basin, one of a series of early Mesozoic Basins in the eastern U.S. in which mudstone, shale, and sandstone aquifers are used for water supply (Trapp and Horn, 1997). At NAWC the mudstone strata are gently dipping to the NW, and a fault along the southeastern boundary of the site separates the Lockatong from the older sandstone of the Stockton Formation (Fig. 1). The detailed lithology of the local mudstones is divided into distinctly colored strata that occur in cycles of deposition: thin (less than 0.2-m thick) black strata containing up to 7% organic carbon by weight (Marjorie S. Schulz, USGS, written comm., 2007; Lebron et al., 2013) and generally highly fractured (fissile), gray mildly fractured thinly layered (laminated) strata, and light-gray weakly fractured massive strata. Several of the high-carbon fissile black beds are herein designated by their respective depths in corehole 43BR at the site: for example the top of a black fissile stratum designated “BlkFis-233” occurs at a depth of 233 ft (71 m) in 43BR.
Below relatively high-permeability weathered and highly fractured strata, groundwater flow is primarily in bedding-plane fractures and thin fissile high-carbon strata, or in fractured, laminated strata in the dipping mudstones, as identified by borehole (Williams et al., 2007) and aquifer testing (Lacombe, 2000, Tiedeman et al., 2010). Large-scale horizontal anisotropy in transmissivity is a well-known characteristic of Newark Basin aquifers, with preferential water-level responses to pumping in the direction of strike of the dipping beds (Herpers and Barksdale, 1951, Longwill and Wood, 1965, Vecchioli, 1965). This apparent horizontal anisotropy is caused by the dipping hydro-stratigraphy, together with the vertical anisotropy of hydraulic conductivity (HK): HK is highest parallel to the dipping beds and lowest perpendicular to beds (Michalski, 1990, Michalski and Britton, 1997, Goode and Senior, 1998, Goode and Senior, 2000, Senior and Goode, 1999, Lacombe, 2000).
Although the precise timing, magnitude, and location of spills of DNAPL TCE are unknown, it is presumed that spills occurred at multiple locations over several decades (Lacombe, 2000). These presumed localized source areas and highly heterogeneous groundwater flowpaths have resulted in highly variable spatial distributions of CVOCs (Fig. 1b) as characterized by water samples from wells with open intervals of generally between 4 and 8 m, at depths of up to about 125 m (Lewis-Brown and Rice, 2002). Despite more than 18 years of P&T remediation, and presumably longer natural attenuation, CVOC concentrations remain elevated at many locations (Fig. 2; Lacombe, 2011; also see Supplementary information figs. SI2 and SI3). TCE concentrations are usually much higher than that of degradation byproducts DCE or VC in wells open to deeper strata (24BR, 36BR, 56BR), whereas degradation byproduct DCE is often the predominant CVOC in wells open to shallower strata (BRP1, 7BR, 15BR). Chapelle et al. (2012) estimated much higher rates of TCE degradation in shallower strata than in deeper strata at the site. DeFlaun et al. (2006) and Révész et al. (2014) observed that the relation of TCE > DCE in highly contaminated deep wells was reversed to DCE > TCE after bioaugmentation.
Section snippets
Methods
New core holes were installed using standard rock core diamond-bit drilling methods. Shallow alluvium was removed by auger, and in a limited number of shallow intervals, tri-cone rotary drilling (not coring) was used for short intervals of highly fractured, weathered rock that could not be recovered in the core barrel. Tap water was used to circulate cuttings, and small amounts of synthetic mud was added to the recirculating fluids when needed to prevent binding of the bit. The coreholes were
Results and synthesis
The site-scale synthesis and stratigraphic correlation of rock core CVOC concentrations are illustrated using closely spaced coreholes in two areas of the site. The results and synthesis are presented in detail for the “West Area” (Fig. 1c). CVOC concentrations in water samples pumped from monitoring wells (or sampled by diffusion bags) as part of the U.S. Navy's site monitoring program and from other USGS research studies are briefly summarized to compare the spatial variability shown in those
Refined conceptual model of CVOC transport
Consistent rock-core sampling results from five coreholes, in synthesis with hydraulic and water-quality monitoring and hydrogeologic characterization, suggest refinements to the conceptual site model for the migration of CVOCs and depth-dependent rock matrix concentrations. Active remediation included P&T from wells starting in 1995 (Lacombe, 2000) and excavation in 1998 of overburden between existing buildings to a depth of about 2 m. Prior to P&T, groundwater flow was primarily subhorizontal
Conclusions
High-resolution sampling and analysis of bulk CVOC concentrations in rock core delineated TCE and DCE in more detail than possible from water-quality data provided by water samples from typical fractured-rock monitoring wells, leading to refinement of a conceptual site model for CVOC transport in fractured mudstones of the Newark Basin. These data indicated depth- and strata-dependent spatial variability and confirmed the importance of discrete high-permeability bedding-oriented fractures in
Acknowledgments
This study was part of the USGS Toxic Substances Hydrology Program project “Chlorinated Solvents in Fractured Sedimentary Rock” and the Strategic Environmental Research and Development Program project ER-1555. We are grateful for the support of the U.S. Navy and Jeffery M. Dale, Remedial Project Manager. We thank the U.S. Navy's contractors (EA Engineering, Science and Technology Inc.; International Technology Corp.; ECOR Solutions Inc.; Geosyntec Consultants Inc.; Watermark Environmental Inc.)
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