Remediation techniques for mitigating vapor intrusion from sewer systems to indoor air

Investigations at former dry cleaning sites in Denmark show that sewer systems often are a major vapor intrusion pathway for chlorinated solvents to indoor air. In more than 20 percent of the contaminated drycleaner sites in Central Denmark Region, sewer systems were determined to be a major vapor intrusion pathway. Sewer systems can be a major intrusion pathway if contaminated groundwater intrudes into the sewer and contamination is transported within the sewer pipe by water flow in either free phase or dissolved states. Additionally, the contamination can volatilize from the water phase or soil gas can intrude the sewer system directly. In the sewer, the gas phase can migrate in any direction by convective transport or diffusion. Indications of the sewer as a major intrusion pathway are:

• higher concentrations in the upper floors in buildings,
• higher concentrations in indoor air than expected from soil gas measurements,
• higher concentrations in bathrooms/kitchen than in living rooms,
• chlorinated solvents in the sewer system, and
• a pressure gradient from the sewer system to indoor air.

Measurements to detect whether or not the sewer system is an intrusion pathway are simple. In Central Denmark Region, the concentrations of contaminants are routinely measured in the indoor air at all floors, the outdoor air, behind the water traps in the building, and in the manholes close to the building. The indoor and outdoor air concentration, as well as concentrations in manholes, are measured by passive sampling on sorbent samplers over a 14‐day period, and the measurements inside the sewer system are carried out by active sampling using carbon tubes (sorbent samplers). Furthermore, the pressure gradient over the building slab and between the indoor air and the sewer system are also measured. A simple test is depressurization of the sewer system. Using this technique, the pressure gradient between the sewer system and the indoor air is altered toward the sewer system—the contamination cannot enter the indoor air through the sewer system. If the sewer system is a major intrusion pathway, the effect of the test can be observed immediately in the indoor air. Remediation of a sewer transported contamination can be:

• prevention of the contaminants from intruding into the sewer system or
• prevention of the contaminated gas in the sewer system from intruding into the indoor air.

Remediation techniques include the following:

• lining of the sewer piping to prevent the contamination from intruding into the sewer;
• sealing the sewer system in the building to prevent the contamination from the sewer system to intrude the indoor air;
• venting of manholes; and
• depressurizing the sewer system.

     

Vapor intrusion mitigation challenges posed by a classic urban setting

AECOM's Environmental Remediation Services Group, under a Pennsylvania Department of Environmental Protection (PADEP) remediation contract, was tasked with assessing potential vapor intrusion risks and mitigating identified risks at an active commercial property affected by a former adjacent laundry and dry cleaning operation. The former dry cleaning operation was positioned in the center of row‐buildings containing various commercial businesses. The neighboring active commercial property is an antiquated building that operates as a coffee shop. The coffee shop building contains a service area, a kitchen, and a rear conference room on the first floor, a storage area on the second floor, and a partial basement. The basement has been observed to periodically flood. Adjacent to the coffee shop building is an operating hair and nail salon that formerly functioned as one of the dry cleaning operations buildings with known soil, groundwater, and soil gas impacts. Due to the disposition of the coffee shop building, a methodical approach was implemented to characterize the potential vapor intrusion risk and included a diagnostic building inspection that identified required management of basement apertures and abatement of periodic flooding prior to vapor intrusion mitigation system design and installation.     

Automated continuous monitoring and response to toxic subsurface vapors entering overlying buildings—Selected observations,implications and considerations

Vapor intrusion characterization and response efforts must consider four key interactive factors: background indoor air constituents, preferential vapor migration pathways, complex patterns of vapor distribution within buildings, and temporal concentration variability caused by pressure differentials within and exterior to structures. An additional challenge is found at sites contaminated by trichloroethylene (TCE), which in the United States has very low indoor air screening levels due to acute risk over short exposure durations for sensitive populations. Timely and accurate characterization of vapor intrusion has been constrained by traditional passive time‐averaging sampling methods. This article presents three case studies of a robust new methodology for vapor intrusion characterization particularly suited for sites where there is a critical need for rapid response to exposure exceedances to minimize health risks and liabilities. The new methodology comprises low‐detection‐level field analytical instrumentation with grab sample and continuous monitoring capabilities for key volatile constituents integrated with pressure differential measurements and web‐based reporting. The system also provides automated triggered alerts to project teams and capability for integration with engineered systems for vapor intrusion control. The three case studies illustrate key findings and lessons learned during system deployment at two sites undergoing characterization studies and one site undergoing thermal remediation of volatile contaminants.     

Building pressure control test to examine if VOC contamination in indoor air is caused by an interior source or by a subslab source

When determining whether or not remediation of subslab contamination is warranted, it is often important to know if the presence of volatile organic compounds (VOCs) in indoor air is caused primarily by interior sources, or primarily by subslab contamination. With a Building Pressure Control test, the pressure differential across the building envelope and, thereby, also the pressure differential across the slab, can be controlled by a blower. By measuring the indoor air concentrations of VOCs under depressurization (upward pressure gradient over the slab) and under pressurization (downward pressure gradient over the slab), it can be determined if the subslab contamination contributes significantly to the VOC concentrations in indoor air. This technique has been applied at several sites in the Central Denmark Region, and the results show that a Building Pressure Control test is a quick and effective method to examine if VOC contamination in indoor air is caused primarily by interior sources or primarily by subslab sources. With numerous measurement points in the indoor air, at some sites this test method has identified the area where the subslab contamination is located. A pressure differential at ?5/+5 Pa across the building envelope was sufficient at all the test sites to control the pressure gradient across the slab to be uniquely upward/downward.     

High‐Frequency Continuous Monitoring to Track Vapor Intrusion Resulting From Naturally Occurring Pressure Dynamics

Vapor intrusion characterization efforts can be challenging due to complexities associated with background indoor air constituents, preferential subsurface migration pathways, and response time and representativeness limitations associated with conventional low‐frequency monitoring methods. For sites experiencing trichloroethylene (TCE) vapor intrusion, the potential for acute risks poses additional challenges, as the need for rapid response to exposure exceedances becomes critical in order to minimize health risks and associated liabilities. Continuous monitoring platforms have been deployed to monitor indoor and subsurface concentrations of key volatile constituents, atmospheric pressure, and pressure differential conditions that can result in advective transport. These systems can be comprised of multiplexed laboratory‐grade analytical components integrated with telemetry and geographical information systems for automatically generating time‐stamped renderings of observations and time‐weighted averages through a cloud‐based data management platform. Integrated automatic alerting and responses can also be engaged within one minute of risk exceedance detection. The objectives at a site selected for testing included continuous monitoring of vapor concentrations and related surface and subsurface physical parameters to understand exposure risks over space and time and to evaluate potential mechanisms controlling risk dynamics which could then be used to design a long‐term risk reduction strategy. High‐frequency data collection, processing, and automated visualization efforts have resulted in greater understanding of natural processes such as dynamic contaminant vapor intrusion risk conditions potentially influenced by localized barometric pumping induced by temperature changes. For the selected site, temporal correlation was observed between dynamic indoor TCE vapor concentration, barometric pressure, and pressure differential. This correlation was observed with a predictable daily frequency even for very slight diurnal changes in barometric pressure and associated pressure differentials measured between subslab and indoor regimes and suggests that advective vapor transport and intrusion can result in elevated indoor TCE concentrations well above risk levels even with low‐to‐modest pressure differentials. This indicates that vapor intrusion can occur in response to diurnal pressure dynamics in coastal regions and suggests that similar natural phenomenon may control vapor intrusion dynamics in other regions, exhibiting similar pressure, geochemical, hydrogeologic, and climatic conditions. While dynamic indoor TCE concentrations have been observed in this coastal environment, questions remain regarding whether this hydrogeologic and climatic setting represent a special case, and how best to determine when continuous monitoring should be required to most appropriately minimize exposure durations as early as possible. ©2017 Wiley Periodicals, Inc.     

Addressing soil gas vapor intrusion using sustainable building solutions

Soil gas vapor intrusion (VI) emerged in the 1990s as one of the most important problems in the investigation and cleanup of thousands of sites across the United States. A common practice for sites where VI has been determined to be a significant pathway is to implement interim building engineering controls to mitigate exposure of building occupants to VI while the source of contamination in underlying soil and groundwater is assessed and remediated. Engineering controls may include passive barriers, passive or active venting, subslab depressurization, building pressurization, and sealing the building envelope. Another recent trend is the emphasis on “green” building practices, which coincidentally incorporate some of these same engineering controls, as well as other measures such as increased ventilation and building commissioning for energy conservation and indoor air quality. These green building practices can also be used as components of VI solutions. This article evaluates the sustainability of engineering controls in solving VI problems, both in terms of long‐term effectiveness and “green” attributes. Long‐term effectiveness is inferred from extensive experience using similar engineering controls to mitigate intrusion of radon, moisture, mold, and methane into structures. Studies are needed to confirm that engineering controls to prevent VI can have similar long‐term effectiveness. This article demonstrates that using engineering controls to prevent VI is “green” in accelerating redevelopment of contaminated sites, improving indoor air quality, and minimizing material use, energy consumption, greenhouse gas emissions, and waste generation. It is anticipated that engineering controls can be used successfully as sustainable solutions to VI problems at some sites, such as those deemed technically impracticable to clean up, where remediation of underlying soil or groundwater contamination will not be completed in the foreseeable future. Furthermore, green buildings to be developed in areas of potential soil or groundwater contamination may be designed to incorporate engineering controls to prevent VI. © 2009 Wiley Periodicals, Inc.     

Successful unsaturated zone treatment of PCE with sodium permanganate

In situ chemical oxidation (ISCO) with permanganate has been widely used for soil and groundwater treatment in the saturated zone. Due to the challenges associated with achieving effective distribution and retention in the unsaturated zone, there is a great interest in developing alternative injection technologies that increase the success of vadose‐zone treatment. The subject site is an active dry cleaner located in Topeka, Kansas. A relatively small area of residual contamination adjacent to the active facility building has been identified as the source of a large sitewide groundwater contamination plume with off‐site receptors. The Kansas Department of Health and Environment (KDHE) currently manages site remedial efforts and chose to pilot‐test ISCO with permanganate for the reduction of perchloroethene (PCE) soil concentrations within the source area. KDHE subsequently contracted Burns & McDonnell to design and implement an ISCO pilot test. A treatability study was performed by Carus Corporation to determine permanganate‐soil‐oxidant‐demand (PSOD) and the required oxidant dosing for the site. The pilot‐test design included an ISCO injection approach that consisted of injecting aqueous sodium permanganate using direct‐push technology with a sealed borehole. During the pilot test, approximately 12,500 pounds of sodium permanganate were injected at a concentration of approximately 3 percent (by weight) using the methods described above. Confirmation soil sampling conducted after the injection event indicated PCE reductions ranging from approximately 79 to more than 99 percent. A follow‐up treatment, consisting of the injection of an additional 6,200 pounds of sodium permanganate, was implemented to address residual soil impacts remaining in the soil source zone. Confirmation soil sampling conducted after the treatment indicated a PCE reduction of greater than 90 percent at the most heavily impacted sample location and additional reductions in four of the six samples collected. © 2009 Wiley Periodicals, Inc.     

Dynamic subsurface explosive vapor concentrations: Observations and implications

Conventional vapor intrusion characterization efforts can be challenging due to background indoor air constituents, preferential subsurface migration pathways, sampling access, and collection method limitations. While it has been recognized that indoor air concentrations are dynamic, until recently it was assumed by many practitioners that subsurface concentrations did not vary widely over time. Newly developed continuous monitoring platforms have been deployed to monitor subsurface concentrations of methane, carbon dioxide, oxygen, hydrogen sulfide, total volatile organic constituents, and atmospheric pressure. These systems have been integrated with telemetry, geographical information systems, and geostatistical algorithms for automatically generating two‐ and three‐dimensional contour images and time‐stamped renderings and playback loops of sensor attributes, and multivariate analyses through a cloud‐based project management platform. The objectives at several selected sites included continuous monitoring of vapor concentrations and related physical parameters to understand explosion risks over space and time and to then design a long‐term risk reduction strategy. High‐frequency data collection, processing, and automated visualization have resulted in greater understanding of natural processes, such as dynamic contaminant vapor intrusion risk conditions potentially influenced by localized barometric pumping. For instance, contemporaneous changes in methane, oxygen, and atmospheric pressure values suggest there is interplay and that vapor intrusion risk may not be constant. As a result, conventional single‐event and composite assessment technologies may not be capable of determining worst‐case risk scenarios in all cases, possibly leading to misrepresentation of receptor and explosion risks. While dynamic risk levels have been observed in several initial continuous monitoring applications, questions remain regarding whether these situations represent special cases and how best to determine when continuous monitoring should be required. Results from a selected case study are presented and implications derived. © 2011 Wiley Periodicals, Inc.     

Remediation of a Former Dry Cleaner Using Nanoscale Zero Valent Iron

Nanoscale zero valent iron (nZVI) was evaluated in a laboratory treatability study and subsequently injected as an interim measure to treat source area groundwater impacts beneath a former dry cleaner located in Chapel Hill, North Carolina (the site). Dry cleaning operations resulted in releases of tetrachloroethene (PCE) that impacted site soil at concentrations up to 2,700 mg/kg and shallow groundwater at concentrations up to 41 mg/L. To achieve a design loading rate of 0.001 kg of iron per kilogram of aquifer material, approximately 725 kg of NanoFe? (PARS Environmental) was injected over a two‐week period into a saprolite and partially weather rock aquifer. Strong reducing conditions were established with oxidation–reduction potential (ORP) values below –728 mV. pH levels remained greater than 8 standard units for a period of 12 months. Injections resulted in near elimination of PCE within one month. cis‐1,2‐Dichloroethene accumulated at high concentrations (greater than 65 mg/L) for 12 months. MAROS software (Version 2.2; AFCEE, 2006 ) was used to calculate mass reduction of PCE and total ethenes at 96 percent and 58 percent, respectively, compared to baseline conditions. Detections of acetylene confirmed the presence of the beta‐elimination pathway. Detections of ethene confirmed complete dechlorination of PCE. Based on hydrogen gas generation, iron reactivity lasted 15 months. © 2013 Wiley Periodicals, Inc.     

Introduction to Therm Net Technology

The Introduction to Therm Net technology is an overview of how radio frequency (RF) beating can be implemented to enhance conventional remediation technologies. Included in the article is a case study of a project conducted in March 1996 at a gasoline release site. The project consisted of a “hot spot” application to reduce BETX concentrations to achieve site closure. The application consisted of RF heating, soil vapor extraction, and groundwater ventilation. A bench scale study was also conducted to measure the effects that RF heating and vapor extraction had on the removal of PAH constitutents from a coal tar sludge. Up to 100 percent reductions were observed for some of the constituents in the study. Also included is a graphical representation of temperature versus vapor pressure for contaminants typically encountered at remediation sites, illustrating that as temperature increases, the removal rate increases.     

Treatment Optimization Through Refinement of a Conceptual Site Model Using Compound Specific Isotope Analysis

Two adjacent automotive component manufacturers in Japan had concentrations of trichloroethene (TCE) and perchloroethene (PCE) in soils and groundwater beneath their plants. One of the manufacturers extensively used these solvents in its processes, while the adjacent manufacturer had no documentation of solvent use. The conceptual site model (CSM) initially involved a single source that migrated from one building to under the adjacent building. Further, because low concentrations of daughter products (e.g., cis‐1,2‐dichloroethene; 3.6 to 840 micrograms per liter [μg/L]) were detected in groundwater, the CSM did not consider intrinsic degradation to be a significant fate mechanism. With this interpretation, the initial remedial design involved both source treatment and perimeter groundwater control to prevent offsite migration of the solvents in groundwater. Identifying whether intrinsic degradation was occurring and could be quantified represented a means of eliminating this costly and potentially redundant component. Further, incorporating intrinsic degradation into the remediation design would also allow for a more focused source treatment, resulting in further cost savings. Three rounds of sampling and data interpretation applying compound specific isotope analysis (CSIA) were used to refine the CSM. The first sampling round involved three‐dimensional CSIA (¹³C, ³⁷Cl, and ²H), while the second two rounds involved ¹³C only, focusing on degradation over time. For the May 2012 sampling, δ¹³C for PCE ranged from –31‰ to –29.6 ‰ and for TCE ranged from –30.4‰ to –28.3‰; showing similar values. δ²H for TCE ranged from 581‰ to 629‰, indicating a manufactured TCE rather than that resulting from dehalogenation processes from PCE. However, mixing of manufactured TCE with that resulting from degraded PCE cannot be ruled out. Because of the similar δ¹³C ratios for PCE and TCE, and ³⁷Cl data for PCE and TCE, fractionation and enrichment factors could not be relied upon. Fractionation patterns were evaluated using graphical methods to trace TCE to the source location to better focus the locations for steam injection. Graphical methods were also used to define the degradation mechanism and from this, incorporate intrinsic degradation processes into the remedial design, eliminating the need for a costly perimeter pump and treat system. ©2015 Wiley Periodicals, Inc.     

Overview of state approaches to vapor intrusion

A large number of states have issued guidance addressing the vapor intrusion pathway making it difficult to keep up with various policies and requirements. We have compiled and reviewed guidance from 35 states, half of which have issued documents within the last three years. A comparison of policies among states shows reasonable consistency in some areas—for example, 20 of 23 states that provide an exclusion distance for subsurface sources of chlorinated volatile organic compounds (VOCs) use a distance of 100 feet. However, more commonly, the policy decisions vary widely. Among states, indoor air screening concentrations for the same VOC vary by more than 2,000 times and subsurface screening concentrations vary by more than 2,000,000 times. These wide discrepancies suggest a need for communication and consensus building in order to increase consistency in the management of the vapor intrusion pathway. © 2012 Wiley Periodicals, Inc.     

Enhanced anaerobic bioremediation of a TCE source at the Tarheel Army Missile Plant using EOS

EOS, or emulsified oil substrate, was used to stimulate anaerobic biodegradation of trichloroethene (TCE) and tetrachloroethene (PCE) at a former Army‐owned manufacturing facility located in the Piedmont area of North Carolina. Previous use of chlorinated solvents at the facility resulted in soil and groundwater impacts. Ten years of active remediation utilizing soil vacuum extraction and air sparging (SVE/AS) were largely ineffective in reducing the TCE/PCE plume. In 2002, the Army authorized preparation of an amended Remedial Action Plan (RAP) to evaluate in situ bioremediation methods to remediate TCE in groundwater. The RAP evaluated eight groundwater remediation technologies and recommended EOS as the preferred bioremediation alternative for the site. Eight wells were drilled within the 100 × 100 feet area believed to be the primary source area for the TCE plume. In a first injection phase, dilute EOS emulsion was injected into half of the wells. Distribution of the carbon substrate through the treatment zone was enhanced by pumping the four wells that were not injected and recirculating the extracted water through the injection wells. The process was repeated in a second phase that reversed the injection/extraction well pairs. Overall, 18,480 pounds of EOS were injected and 163,000 gallons of water were recirculated through the source area. Anaerobic groundwater conditions were observed shortly after injection with a corresponding decrease in both PCE and TCE concentrations. Dissolved oxygen, oxidation‐reduction potential, and sulfate concentrations also decreased after injection, while TCE‐degradation products, ferrous iron, and methane concentrations increased. The reduction in TCE allowed the Army to meet the groundwater remediation goals for the site. Approximately 18 months after injection, eight wells were innoculated with a commercially prepared dechlorinating culture (KB‐1) in an attempt to address lingering cis‐1,2‐dichloroethene (cis‐DCE) and vinyl chloride (VC) that continued to be observed in some wells. Dehalococcoides populations increased slightly post‐bioaugmentation. Both cis‐DCE and VC continue to slowly decrease. © 2007 Wiley Periodicals, Inc.     

Vapor intrusion risk evaluation using automated continuous chemical and physical parameter monitoring

Vapor intrusion risk characterization efforts are challenging due to complexities associated with background indoor air constituents, preferential subsurface migration pathways, and representativeness limitations associated with traditional randomly timed time‐integrated sampling methods that do not sufficiently account for factors controlling concentration dynamics. The U.S. Environmental Protection Agency recommends basing risk related decisions on the reasonable maximum exposure (RME). However, with very few exceptions, practitioners have not been applying this criterion. The RME will most likely occur during upward advective flux conditions. As such, for RME determinations, it is important to sample when upward advective flux conditions are occurring. The most common vapor intrusion assessment efforts include randomly timed sample collection events, and therefore do not accurately yield RME estimates. More specifically, researchers have demonstrated that randomly timed sampling schemes can result in false negative determinations of potential risk corresponding to RMEs. For sites experiencing trichloroethylene (TCE) vapor intrusion, the potential for acute risks poses additional challenges, as there is a critical need for rapid response to exposure exceedances to minimize health risks and liabilities. To address these challenges, continuous monitoring platforms have been deployed to monitor indoor concentrations of key volatile constituents, atmospheric pressure, and pressure differential conditions that can result in upward toxic vapor transport and entry into overlying buildings. This article demonstrates how vapor intrusion RME‐based risks can be successfully and efficiently determined using continuous monitoring of concentration and parameters indicating upward advective chemical flux. Time series analyses from multiple selected 8‐ and 24‐hr time increments during upward advective TCE flux conditions were performed to simulate results expected from the most commonly employed sampling methods. These analyses indicate that, although most of the selected time increments overlap within the same 24‐hr window, results and conclusions vary. As such, these findings demonstrate that continuous monitoring of concentration and parameters such as differential pressure and determination of a time‐weighted concentration average over a selected duration when upward advective flux is occurring can allow for a realistic RME‐based risk estimate.     

Overview of state approaches to vapor intrusion: 2018

Regulatory requirements for the evaluation of vapor intrusion vary significantly among states. For site owners and responsible parties that have sites in different regulatory jurisdictions, one challenge is to know and understand how the requirements or expectations for vapor intrusion differ from one jurisdiction to the next. Differences in requirements can make it difficult to manage sites in a consistent manner across jurisdictions. Eklund, Folkes, et al. (2007, February, Environmental Manager, 10–14) published an overview of state guidance for vapor intrusion in 2007 that provided a detailed summary of pathway screening values and other key vapor intrusion policies. An update by Eklund, Beckley, et al. (2012, Remediation, 22, 7–20) was published in 2012, which expanded the evaluation to additional states. Since that time, numerous states have substantially revised their guidance and some states that did not have vapor intrusion‐specific guidance have issued new guidance. This article provides an update to the 2012 study. For each state, the review includes tabulations of the types of screening values included (e.g., groundwater, soil, soil gas, indoor air) and the screening values for selected chemicals that commonly drive vapor intrusion investigations (i.e., trichloroethylene [TCE], tetrachloroethylene, and benzene) along with other compounds of potential interest. In addition, for each state, the article summarizes a number of key policy decisions that are important for the investigation of vapor intrusion including: distance screening criteria, default subsurface to indoor air attenuation factors, mitigation criteria, and policies for evaluation of short‐term TCE exposure.     

Optimizing remediation strategies for a former dry‐cleaner site

Residual tetrachloroethene (PCE) contamination at the former Springvilla Dry Cleaners site in Springfield, Oregon, posed a potential risk through the vapor intrusion, direct contact, and off‐site beneficial groundwater uses. The Oregon Department of Environmental Quality utilized the State Dry Cleaner Program funds to help mitigate the risks posed by residual contamination. After delineation activities were complete, the source‐area soils were excavated and treated on‐site with ex situ vapor extraction to reduce disposal costs. Residual source‐area contamination was then chemically oxidized using sodium permanganate. Dissolved‐phase contamination was subsequently addressed with in situ enhanced reductive dechlorination (ERD). ERD achieved treatment goals across more than 4 million gallons of aquifer impacted with PCE concentrations up to 7,800 micrograms per liter prior to remedial activities. The ERD remedy introduced electron donors and nutrient amendments through groundwater recirculation and slug injection across two aquifers over the course of 24 months. Adaptive and mass‐targeted strategies reduced total remedy costs to approximately $18 per ton within the treatment areas. © 2010 Wiley Periodicals, Inc.     

Insufficient source area remediation results in the rebound of TCE breakdown products in groundwater

In the early 1990s, a soil removal action was completed at a former disposal pit site located in southern Michigan. This action removed waste oil, cutting oil, and chlorinated solvents from the unsaturated zone. To contain groundwater contaminant migration at the site, a groundwater pump‐and‐treat system comprised of two extraction wells operating at a combined flow of 50 gallons per minute, carbon treatment, and a permitted effluent discharge was designed, installed, and operated for over 10 years. Groundwater monitoring for natural attenuation parameters and contaminant attenuation modeling demonstrated natural attenuation of the contaminant plume was adequate to attain site closure. As a result of incomplete contaminant source removal, a rebound of contaminants above the levels established in the remedial action plan (RAP) has occurred in the years following system shutdown and site closure. Groundwater concentrations have raised concerns regarding potential indoor air quality at adjacent residential properties constructed in the past 9 to 10 years. The only remedial option available in the original RAP is to resume groundwater pump‐and‐treat. To remediate the source area, an alternate remediation strategy using an ozone sparge system was developed. The ozone sparge remediation strategy addresses the residual saturated zone contaminants beneath the former disposal pit and reestablishes site closure requirements without resumption of the pump‐and‐treat system. A pilot study was completed successfully; and the final system design was subsequently approved by the Michigan Department of Environmental Quality. The system was installed and began operations in July 2010. As of the January 2011 monitoring event, the system has shown dramatic improvement in site contaminant concentrations. The system will continue to operate until monitoring results indicate that complete treatment has been obtained. The site will have achieved the RAP objectives when the system has been shut down and meets groundwater residential criteria for four consecutive quarters. © 2011 Wiley Periodicals, Inc.     

Cost‐Effective Rapid and Long‐Term Screening of Chemical Vapor Intrusion (CVI) Potential: Across Both Space and Time

Reviews including the latest “data‐rich” chemical vapor intrusion‐radon (CVI‐Rn) studies indicate buildings/times can be “screened‐in” as having Rn‐evident‐susceptibility/priority for soil gas intrusion, and elevated‐potential for CVI concerns, or not. These screening methods can supplement conventional indoor‐air chemical sampling, under naturally varying conditions, by prioritizing buildings and times based on indoor Rn levels. Rn is a widespread, naturally occurring component of soil gas and a tracer of soil gas intrusion into the indoor air of overlying buildings. Rn is also an indicator for generally similar behavior of other components of near‐building soil gas, possibly including chemical contaminant vapors. Indoor Rn is easily measured at a low cost, allowing continuous observations from essentially all buildings with the potential for CVI across time. This presents cost savings and other benefits for all CVI stakeholders.     

Residential vapor‐intrusion evaluation: Long‐duration passive sampling vs. short‐duration active sampling

Sampling indoor air for potential vapor‐intrusion impacts using current standard 24‐hour sample collection methods may not adequately account for temporal variability and detect contamination best represented by long‐term sampling periods. Henry Schuver of the U.S. Environmental Protection Agency Office of Solid Waste stated at the September 2007 Air & Waste Management Association vapor‐intrusion conference that the US EPA may consider recommending longer‐term vapor sampling to achieve more accurate time‐weighted‐average detections. In November 2007, indoor air at four residences was sampled to measure trichloroethene (TCE) concentrations over short‐ and long‐duration intervals. A carefully designed investigation was conducted consisting of triplicate samplers for three different investigatory methods: dedicated 6‐liter Summa canisters (US EPA Method TO‐15), pump/sorbent tubes (US EPA Method TO‐17), and passive diffusion samplers (MDHS 80). The first two methods collected samples simultaneously for a 24‐hour period, and the third method collected samples for two weeks. Data collected using Methods TO‐15 (canisters) and TO‐17 (tubes) provided reliable short‐duration TCE concentrations that agree with prior 24‐hour sampling events in each of the residences; however, the passive diffusion samplers may provide a more representative time‐weighted measurement. The ratio of measured TCE concentrations between the canisters and tubes are consistent with previous results and as much as 28.0 μg/m³ were measured. A comparison of the sampling procedures, and findings of the three methods used in this study will be presented. © 2008 Wiley Periodicals, Inc.     

ART in‐well air stripping—An innovative technology succeeds where other technologies have failed

Accelerated Remediation Technologies LLC (ART) developed a proprietary (patent‐pending) effective remediation technology that is based on verified and established concepts. The ART technology combines in‐situ air stripping, air sparging, soil vapor extraction, enhanced bioremediation/oxidation, and Dynamic Subsurface Circulation^TM in an innovative wellhead system. The system is designed to accommodate a 4‐inch well and is cost‐effective when compared with other remediation technologies. The air‐sparging component results in lifting the water table. This lifting of the water in the well causes a net reduction in head at the well location. Vacuum pressure (the vapor‐extraction component) is applied on top of the well point to extract vapor from the subsurface. The negative pressure from the vacuum extraction results in water suction that creates additional water lifting (mounding). A submersible pump is placed at the bottom of the well to recirculate water to the top for downward discharge through a spray head. The water cascades down the interior of the well similar to what occurs in an air‐stripping tower. Enhanced stripping via air sparging near the bottom of the well occurs simultaneously. In essence, the well acts as a subsurface air‐stripping tower. The pumped‐and‐stripped, highly oxygenated water flows down well annulus and over the “mounded” water back in to the aquifer, which creates a circulation zone around the well to further enhance cleanup. The ART technology has been implemented at several sites nationwide, including industrial laundry facilities, manufacturing plants, and service stations, and has achieved significant reductions in contaminant concentrations. Specifically, a concentration of tetrachloroethene (PCE) decreased from 2,700 to 240 μg/l, in 13 days. In less than three months, the concentrations dropped further to 79 μg/l, which is within the range of background levels. Other sites utilizing the technology have exhibited similar reduction trends in complex subsurface environments. © 2002 Wiley Periodicals, Inc.