At three industrial sites in Ontario, New Hampshire, and Florida, tetrachloroethylene (PCE) and trichloroethylene (TCE), released decades ago as dense nonaqueous phase liquids (DNAPLs), now form persistent source zones for dissolved contaminant plumes. These zones are suspended below the water table and above the bottoms of their respective, moderately homogeneous, unconfined sandy aquifers. Exceptionally detailed, depth‐discrete, ground water sampling was performed using a direct‐push sampler along cross sections of the dissolved‐phase plumes, immediately downgradient of these DNAPL source zones. The total plume PCE or TCE mass‐discharge through each cross section ranged between 15 and 31 kg/year. Vertical ground water sample spacing as small as 15 cm and lateral spacing typically between 1 and 3 m revealed small zones where maximum concentrations were between 1% and 61% of solubility. These local maxima are surrounded by much lower concentration zones. A spacing no larger than 15 to 30 cm was needed at some locations to identify high concentration zones, and aqueous VOC concentrations varied as much as four orders of magnitude across 30 cm vertical intervals. High‐resolution sampling at these sites showed that three‐quarters of the mass‐discharge occurs within 5% to 10% of the plume cross sectional areas. The extreme spatial variability of the mass‐discharge occurs even though the sand aquifers are nearly hydraulically homogeneous. Depth‐discrete field techniques such as those used in this study are essential for finding the small zones producing most of the mass‐discharge, which is important for assessing natural attenuation and designing remedial options.
zone. In recent years, several in situ technologies have been proposed for remedial restoration of chlorinated To select and design effective remedial measures for dense, nonsolvent DNAPL source zones (Interstate Technology aqueous phase liquid (DNAPL) source zones, better understanding of the architecture of these zones is needed. In this study, a suite and Regulatory Council DNAPL Team, 2002), and nuof investigative techniques was applied to perform detailed vertical merous site trials have been conducted, but no complete delineation of chlorinated-solvent source zones in sand aquifers at
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.
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