A new method is presented for calculating a building-specific subslab to indoor air attenuation factor for use in assessing subsurface vapor intrusion to indoor air. The technique includes (1) subslab gas extraction with flow and vacuum measurements and mathematical modeling to characterize the bulk average vertical gas conductivity of the floor slab, (2) monitoring of the ambient pressure gradient across the floor slab with a micromanometer, (3) calculating the volumetric flow of soil gas into the building ( Q), and (4) dividing Q by the building ventilation rate ( Q) to calculate a building-specific attenuation factor. Sample calculations using order statistics from 121 individual tests are comparable to the U.S. Environmental Protection Agency empirical attenuation factors for residential buildings and the U.S. Navy empirical attenuation factors for commercial/industrial buildings. A case study of a commercial building shows encouraging agreement between the attenuation factors calculated via this method and via conventional subslab and indoor air sampling.

Despite the installation in the 1980s and 1990s of hydraulic containment systems around known source zones (four slurry walls and ten pump-and-treat systems), trichloroethene (TCE) plumes persist in the three uppermost groundwater-bearing units at the Middlefield-Ellis-Whisman (MEW) Superfund Study Area in Mountain View, California. In analyzing TCE data from 15 recovery wells, the observed TCE mass discharge decreased less than an order of magnitude over a 10-year period despite the removal of an average of 11 pore volumes of affected groundwater. Two groundwater models were applied to long-term groundwater pump-and-treat data from 15 recovery wells to determine if matrix diffusion could explain the long-term persistence of a TCE plume. The first model assumed that TCE concentrations in the plume are controlled only by advection, dispersion, and retardation (ADR model). The second model used a one-dimensional diffusion equation in contact with two low-permeability zones (i.e., upper and lower aquitard) to estimate the potential effects of matrix diffusion of TCE into and out of low-permeability media in the plume. In all 15 wells, the matrix diffusion model fit the data much better than the ADR model (normalized root mean square error of 0.17 vs. 0.29; r 2 of 0.99 vs. 0.19), indicating that matrix diffusion is a likely contributing factor to the persistence of the TCE plume in the non-source-capture zones of the MEW Study Area's groundwater-extraction wells. O Virgilio (Vic) Cocianni is the remediation manager for Schlumberger, a leading oilfield services company. Cocianni provides leadership and guidance on remediation activities in the various countries where Schlumberger operates. Du'Bois (Joe) Ferguson is the remediation technical manager for Schlumberger Technology Corporation,where he provides the technical leadership for the company's remediation projects on a global basis.

No abstract

The data mining/groundwater modeling methodology developed in McDade et al. (2013) was performed to determine if matrix diffusion is a plausible explanation for the lower‐concentration but persistent chlorinated solvent plumes in the groundwater‐bearing units at three different pump‐and‐treat systems. Capture‐zone maps were evaluated, and eight wells were identified that did not draw water from any of the historical source areas but captured water from the sides of the plume. Two groundwater models were applied to study the persistence of the plumes in the absence of contributions from the historical source zones. In the wells modeled, the observed mass discharge generally decreased by about one order of magnitude or less over 4 to 10 years of pumping, and 1.8 to 17 pore volumes were extracted. In five of the eight wells, the matrix diffusion model fit the data much better than the advection dispersion retardation model, indicating that matrix diffusion better explains the persistent plume. In the three other wells, confounding factors, such as a changing capture zone over time (caused by changes in pumping rates in adjacent extraction wells); potential interference from a high‐concentration unremediated source zone; and limited number of pore volumes removed made it difficult to confirm that matrix diffusion processes were active in these areas. Overall, the results from the five wells indicate that mass discharge rates from the pumping wells will continue to show a characteristic “long tail'' of mass removal from zones affected by active matrix diffusion processes. Future site management activities should include matrix diffusion processes in the conceptual site models for these three sites. © 2013 Wiley Periodicals, Inc.

Cone penetromcler testing (CPT). combined with discrete‐depth ground water sampling methods, can significantly reduce the time and expense required to characterize large sites that have multiple aquifers. Results from the screening site characterization can then be used to design and install a cost‐effective monitoring well network. At a site in northern California, it was necessary to characterize the stratigraphy and the distribution of volatile organic compounds (VC)Cs) to a depth of 80 feet within a ½‐mile‐by‐½‐mile residential and commercial area in a complex alluvial fan setting. To expedite characterization, a five‐week field screening program was implemented that consisted of a shallow ground water survey, CPT soundings and pore‐pressure measurements, and discrete‐depth ground water sampling. Based on continuous lithologic information provided by the CPT soundings, four predominantly coarse‐grained, water yielding stratigraphic packages were identified. Individual coarse‐grained units within each package are discontinuous, as they coalesce and pinch out in longitudinal and transverse directions. Seventy‐nine discrete‐depth ground water samples were collected using either shallow ground water survey techniques, the BAT Enviroprobe, or the QED HydroPunch I, depending on subsurface conditions. Using results from these efforts, a 20‐well monitoring network was designed and installed to monitor critical points within each stratigraphic package. Good correlation was found for hydraulic head and chemical results between discrete‐depth screening data and monitoring well data. Understanding the vertical VOC distribution and concentrations produced substantial time and cost savings by minimizing the number of permanent monitoring wells and reducing the number of costly conductor casings that had to be installed. Additionally, significant long‐term cost savings will result from reduced sampling costs, because fewer wells comprise the monitoring network. We estimate these savings to be 50 percent for site characterization costs, 65 percent for site characterization time, and 60 percent for long term monitoring costs.