Jeffrey A. Adams

TECHNICAL NOTE An experimental investigation of air flow patterns in saturated soils during air sparging

Air sparging is an emerging method used to remediate saturated soils and groundwater that have been contaminated with volatile organic compounds (VOCs). During air sparging, air is injected into the subsurface below the lowest known depth of contamination. Due to buoyancy, the injected air will rise through the zone of contamination. Through a variety of mechanisms, including volatilization and biodegradation, the air will serve to remove or help degrade the contaminants. The contaminant-laden air will continue to rise towards the ground surface, eventually reaching the vadose zone, where the vapours are collected and treated using a soil vapour extraction (SVE) system.Air sparging performance and ultimately contaminant removal efficiency is highly dependent on the pattern and type of subsurface air flow. This paper presents the results of a laboratory experimental study which investigated the injected air flow pattern development within an aquifer simulation apparatus. The test apparatus consisted of a tank measuring 61 cm long by 25.4 cm wide by 38.1 cm high. The apparatus was equipped with one air injection well and two vapour extracton wells. Three different soils were used to simulate different aquifer conditions, including a sand, a fine gravel and a medium gravel. Experiments were performed with different injected air pressures combined with different vacuum and groundwater flow conditions. Experiments were also conducted by injecting air into simulated shallow aquifers with different thicknesses. The air flow patterns observed were found to depend significantly on the soil type, groundwater flow conditions and system controls, including injected air pressure, flow rate and applied vacuum.

Air flow optimization and surfactant enhancement to remediate toluene‐contaminated saturated soils using air sparging

This paper presents the results of laboratory experiments that investigate the removal of volatile organic compounds from saturated soils through the use of air sparging. Three series of experiments were performed in a column test apparatus using two different soils to represent actual field conditions, namely, a fine gravel and a medium‐to‐fine Ottawa sand (both obtained from sources near Chicago, Illinois, USA) contaminated with toluene, a major constituent of petroleum products. The results showed that toluene was removed from gravel very efficiently using air sparging; complete removal was achieved using a variety of air flow rates. However the toluene removal rates in tests using sand were significantly less. Even at the highest air flow rate used during testing, complete toluene removal took eight times longer than in comparable tests using gravel. With low air flow rates this was not achieved even after 17 hours of testing. It was further found that the injection of foams generated with surfactants, SDS and witconol SN70, at low air flow rates during the use of air sparging was found to accelerate the bulk removal of toluene in sand, but the use of surfactants did not facilitate the removal of residual levels of contamination.

Effect of groundwater flow on remediation of dissolved-phase VOC contamination using air sparging

This paper presents two-dimensional laboratory experiments performed to study how groundwater flow may affect the injected air zone of influence and remedial performance, and how injected air may alter subsurface groundwater flow and contaminant migration during in situ air sparging. Tests were performed by subjecting uniform sand profiles contaminated with dissolved-phase benzene to a hydraulic gradient and two different air flow rates. The results of the tests were compared to a test subjected to a similar air flow rate but a static groundwater condition. The test results revealed that the size and shape of the zone of influence were negligibly affected by groundwater flow, and as a result, similar rates of contaminant removal were realized within the zone of influence with and without groundwater flow. The air flow, however, reduced the hydraulic conductivity within the zone of influence, reducing groundwater flow and subsequent downgradient contaminant migration. The use of a higher air flow rate further reduced the hydraulic conductivity and decreased groundwater flow and contaminant migration. Overall, this study demonstrated that air sparging may be effectively implemented to intercept and treat a migrating contaminant plume.

Remediation of Chlorinated Solvent Plumes Using In-Situ Air Sparging—A 2-D Laboratory Study

In-situ air sparging has evolved as an innovative technique for soil and groundwater remediation impacted with volatile organic compounds (VOCs), including chlorinated solvents. These may exist as non-aqueous phase liquid (NAPL) or dissolved in groundwater. This study assessed: (1) how air injection rate affects the mass removal of dissolved phase contamination, (2) the effect of induced groundwater flow on mass removal and air distribution during air injection, and (3) the effect of initial contaminant concentration on mass removal. Dissolved-phase chlorinated solvents can be effectively removed through the use of air sparging; however, rapid initial rates of contaminant removal are followed by a protracted period of lower removal rates, or a tailing effect. As the air flow rate increases, the rate of contaminant removal also increases, especially during the initial stages of air injection. Increased air injection rates will increase the density of air channel formation, resulting in a larger interfacial mass transfer area through which the dissolved contaminant can partition into the vapor phase. In cases of groundwater flow, increased rates of air injection lessened observed downward contaminant migration effect. The air channel network and increased air saturation reduced relative hydraulic conductivity, resulting in reduced groundwater flow and subsequent downgradient contaminant migration. Finally, when a higher initial TCE concentration was present, a slightly higher mass removal rate was observed due to higher volatilization-induced concentration gradients and subsequent diffusive flux. Once concentrations are reduced, a similar tailing effect occurs.

Social Sustainability Evaluation Matrix (SSEM) to quantify social aspects of sustainable remediation

Sustainability analysis, or triple bottom line analysis, is increasingly recognized as a holistic approach when all the three pillars of sustainability (environmental, economic and social aspects) are equally incorporated into the decision-making process of a project. Currently, the tools for assessing the environmental and economic impacts are well established. On the contrary, the development of a quantitative tool to assess the social impacts has been particularly challenging because a multitude of subjective factors may vary among social entities depending upon the type of project assessed. In this study, a new tool called Social Sustainability Evaluation Matrix (SSEM) is developed and applied to two environmental remediation project sites. In both cases, remedial options were previously identified and assessed based on environmental and economic aspects. SSEM is an Excel-based tool comprising four social dimensions: (1) socio-individual, (2) socio-institutional, (3) socio-economic, and (4) socio-environmental. Under each dimension, several key areas are identified, and a scoring system is devised to quantify the extent of resulting social impacts. Scores for the identified key areas are summed under each social dimension, and a comparative assessment is performed to allow for more informed decisions about remedy selection, design, implementation, and mitigation as necessary. Overall, SSEM was found to be quite beneficial in assessing social sustainability of the selected remedial options in this study; however, it is important to incorporate an objective basis to the highest degree practicable. Also, when negative, substantive impacts are identified, mitigation efforts should be made to minimize or avoid the impact.

State-of-the-Practice of Characterization and Remediation of Contaminated Sites

With increased manufacturing and industrial prowess following World War II, the United States entered into a period of unprecedented economic growth and advancement of the quality of life for its citizens. Nevertheless, as had occurred before in Western Europe, and is now occurring in the developing world, economic expansion and industrialization outpaced advances with respect to environmental stewardship. As a result, the environment was significantly impacted - often in the form of vivid, high-visibility events and disasters, but far more often with little notice or directly observable side effects on the subsurface environment. Regardless, in all of these cases, the cumulative effect was enormous and presented a very real threat to the gains made in quality of life in the preceding years. This state-of-the-practice paper presents a chronology of the effects of over 60 years of rapid industrialization, economic expansion, the detrimental effects to the environment, and the resulting reactions from private citizens, the corporate sector, and local, state, and federal government. An overview of the evolving regulatory framework as well as site characterization and remediation technologies used in practice is presented. Finally, the emerging holistic approaches and technologies to achieve green and sustainable remediation of contaminated sites are discussed. Many challenges and opportunities still exist for the development of efficient, reliable, simple and cost effective technologies to characterize and remediate numerous contaminated sites.