Paul Dahlen

Identification of Alternative Vapor Intrusion Pathways Using Controlled Pressure Testing, Soil Gas Monitoring, and Screening Model Calculations

Vapor intrusion (VI) pathway assessment and data interpretation have been guided by an historical conceptual model in which vapors originating from contaminated soil or groundwater diffuse upward through soil and are swept into a building by soil gas flow induced by building underpressurization. Recent studies reveal that alternative VI pathways involving neighborhood sewers, land drains, and other major underground piping can also be significant VI contributors, even to buildings beyond the delineated footprint of soil and groundwater contamination. This work illustrates how controlled-pressure-method testing (CPM), soil gas sampling, and screening-level emissions calculations can be used to identify significant alternative VI pathways that might go undetected by conventional sampling under natural conditions at some sites. The combined utility of these tools is shown through data collected at a long-term study house, where a significant alternative VI pathway was discovered and altered so that it could be manipulated to be on or off. Data collected during periods of natural and CPM conditions show that the alternative pathway was significant, but its presence was not identifiable under natural conditions; it was identified under CPM conditions when measured emission rates were 2 orders of magnitude greater than screening-model estimates and subfoundation vertical soil gas profiles changed and were no longer consistent with the conventional VI conceptual model.

Treatment of Heavy, Long-Chain Petroleum-Hydrocarbon Impacted Soils Using Chemical Oxidation

AbstractChemical oxidation is a promising approach for in situ or ex situ treatment of heavy, long-chain (C12−C40) petroleum-hydrocarbon impacted soils. Aqueous chemical oxidation treatments (sodium percarbonate, hydrogen peroxide, sodium persulfate, chlorine dioxide, sodium permanganate, and ozone) using two oxidant concentrations were tested in batch tests on soils containing C12–C40 total petroleum hydrocarbon (TPH) concentrations of 1.6 and 2.0% weight/weight (w/w) resulting in TPH reductions from 20 to 90%. Gas chromatography with flame ionization detector (GC-FID) chromatograms for hydrocarbons were obtained and presented as chain-length fractions. Sodium percarbonate and hydrogen peroxide achieved the highest TPH reduction. There was little difference between 1 and 10% weight/volume (w/v) for all oxidant doses on TPH removal. Soluble organics in the liquid supernatants after oxidation of the TPH-containing soils were characterized by TPH analysis and excitation-emission matrix fluorescence spectros...

Long-Term Evaluation of the Controlled Pressure Method for Assessment of the Vapor Intrusion Pathway

Vapor intrusion (VI) investigations often require sampling of indoor air for evaluating occupant risks, but can be confounded by temporal variability and the presence of indoor sources. Controlled pressure methods (CPM) have been proposed as an alternative, but temporal variability of CPM results and whether they are indicative of impacts under natural conditions have not been rigorously investigated. This study is the first involving a long-term CPM test at a house having a multiyear high temporal resolution indoor air data set under natural conditions. Key observations include (a) CPM results exhibited low temporal variability, (b) false-negative results were not obtained, (c) the indoor air concentrations were similar to the maximum concentrations under natural conditions, and (d) results exceeded long-term average concentrations and emission rates under natural conditions by 1-2 orders of magnitude. Thus, the CPM results were a reliable indicator of VI occurrence and worst-case exposure regardless of day or time of year of the CPM test.

Proof-of-concept study of an aerobic vapor migration barrier beneath a building at a petroleum hydrocarbon-impacted site.

A proof-of-concept study was conducted to evaluate an alternative to traditional extraction-based subslab vapor mitigation systems at sites with petroleum hydrocarbon and/or methane vapor impact concerns. The system utilizes the slow delivery of air beneath a foundation to attenuate vapor migration to the building via aerobic biodegradation. The study was conducted at a site having elevated hydrocarbon plus methane and depleted O(2) vapor concentrations (160 mg/L and <1% v/v, respectively) beneath a building having a 195 m(2) footprint and a basement extending 1.5 m below ground surface (BGS). Nonaqueous phase liquid (NAPL)-impacted soils, first encountered at about 7.6 to 9.1 m BGS, were the source of hydrocarbon and methane vapors, with the latter being generated by anaerobic methanagenesis of the former. O(2) concentrations beneath and around the building were monitored prior to and during air injection through a horizontal well installed about 1.5 m beneath the foundation. The air injection rate was increased from 1 to 5 to 10 L/min, with each held steady until the O(2) distribution stabilized (46-60 d). The 10 L/min flow rate achieved >5% v/v soil gas O(2) concentrations beneath the foundation and spanning a 1.5 m vertical interval. It was within 3× of the pretest stoichiometric requirement estimate of 3.8 L/min. This resulted in reductions in subslab hydrocarbon plus methane concentrations from 80 to <0.01 mg/L and benzene, toluene, ethylbenzene, and xylenes (BTEX) reductions to below detection limits (0.5-0.74 ppb(v)). This air injection rate is <1% of flows for typical extraction-based mitigation systems.

Effect of dissolved oxygen manipulation on diffusive emissions from NAPL-impacted low permeability soil layers.

Aquifer physical model experiments were performed to investigate if diffusive emissions from nonaqueous phase liquid (NAPL)-impacted low-permeability layers into groundwater moving through adjacent NAPL-free high-permeability layers can be reduced by creating an aerobic biotreatment zone at the interface between the two, and if over time that leads to reduced emissions after treatment ceases. Experiments were performed in two 1.2-m long × 1.2-m high × 5.4 cm wide stainless steel tanks; each with a high-permeability sand layer overlying a low-permeability crushed granite layer containing a NAPL mixture of indane and benzene. Each tank was water-saturated with horizontal flow primarily through the sand layer. The influent water was initially deoxygenated and the emissions and concentration distributions were allowed to reach near-steady conditions. The influent dissolved oxygen (DO) level was increased stepwise to 6.5-8.5 mg/L and 17-20 mg/L, and then decreased back to deoxygenated conditions. Each condition was maintained for at least 45 days. Relative to the near-steady benzene emission at the initial deoxygenated condition, the emission was reduced by about 70% when the DO was 6.5-8.5 mg/L, 90% when the DO was 17-20 mg/L, and ultimately 60% when returning to low DO conditions. While the reductions were substantial during treatment, longer-term reductions after 120 d of elevated DO treatment, relative to an untreated condition predicted by theory, were low: 29% and 6% in Tank 1 and Tank 2, respectively. Results show a 1-2 month lag between the end of DO delivery and rebound to the final near-steady emissions level. This observation has implications for post-treatment performance monitoring sampling at field sites.

Carbonaceous nano-additives augment microwave-enabled thermal remediation of soils containing petroleum hydrocarbons

Remediating soils contaminated with heavy hydrocarbons (C12–C40) from petrochemical exploration activities is a major environmental challenge across the globe. This study evaluated microwave irradiation in the presence of nano- and macro-scale graphitic additives as a rapid remediation technology for removing heavy hydrocarbons from soil. Adding inert materials (i.e., glass wool fibers or washed silica sand) as controls had no effect on total petroleum hydrocarbons (TPH) removal upon microwave irradiation. Carbonaceous nanomaterials (i.e., carbon nanotubes, graphene nanosheets, and carbon nanofibers) because of their favorable dielectric properties showed extraordinary heating performances when mixed with soil and microwave irradiated. As a result, adding these carbonaceous nanomaterials to contaminated soils removed more TPH compared with macro-scale carbonaceous additives. TPH concentrations decreased from 11 000 to between 2000 and 6000 mg TPH kg−1 soil within one minute using carbon nanomaterial additives and a 2.45 GHz, 1000 W conventional microwave oven. In separate experiments, this technology decreased TPH from 2500 to 650 mg TPH kg−1 soil from soils containing recalcitrant, non-biodegradable fractions of TPH. Large scale microwave systems are available and hold promise for remediating soils when used in conjunction with carbon nanomaterials.

Creation of a Sub-Slab Soil Gas Cloud by an Indoor Air Source and Its Dissipation Following Source Removal

It is accepted that indoor sources of volatile organic compounds can confound vapor intrusion (VI) pathway assessment. When they are discovered during pre-sampling inspection, indoor sources are removed and air sampling is delayed, with the assumption that a few hours to a few days are sufficient for indoor source impacts to dissipate. This assumption was tested through the controlled release of SF6 and its monitoring in indoor air and soil gas at a study house over 2 years. Results show that indoor sources generate subsurface soil gas clouds as a result of fluctuating direction in the exchange between soil gas and indoor air and that it may take days to weeks under natural conditions for a soil gas cloud beneath a building to dissipate following indoor source removal. The data also reveal temporal variability in indoor air and soil gas concentrations, long-term seasonal patterns, and dissipation of soil gas clouds over days to weeks following source removal. Preliminary modeling results for similar conditions are consistent field observations. If representative of other sites, these results suggest that a typical 1-3 day waiting period following indoor source removal may not be sufficient to avoid confounding data and erroneous conclusions regarding VI occurrence.

Optical fiber-mediated photosynthesis for enhanced subsurface oxygen delivery

Remediation of polluted groundwater often requires oxygen delivery into subsurface to sustain aerobic bacteria. Air sparging or injection of oxygen containing solutions (e.g., hydrogen peroxide) into the subsurface are common. In this study visible light was delivered into the subsurface using radially emitting optical fibers. Phototrophic organisms grew near the optical fiber in a saturated sand column. When applying light in on-off cycles, dissolved oxygen (DO) varied from super saturation levels of >15 mg DO/L in presence of light to under-saturation (<5 mg DO/L) in absence of light. Non-photosynthetic bacteria dominated at longer radial distances from the fiber, presumably supported by soluble microbial products produced by the photosynthetic microorganisms. The dissolved oxygen variations alter redox condition changes in response to light demonstrate the potential to biologically deliver oxygen into the subsurface and support a diverse microbial community. The ability to deliver oxygen and modulate redox conditions on diurnal cycles using solar light may provide a sustainable, long term strategy for increasing dissolved oxygen levels in subsurface environments and maintaining diverse biological communities.