Hans F. Stroo

Chlorinated Ethene Source Remediation: Lessons Learned

Chlorinated solvents such as trichloroethene (TCE) and tetrachloroethene (PCE) are widespread groundwater contaminants often released as dense nonaqueous phase liquids (DNAPLs). These contaminants are difficult to remediate, particularly their source zones. This review summarizes the progress made in improving DNAPL source zone remediation over the past decade, and is structured to highlight the important practical lessons learned for improving DNAPL source zone remediation. Experience has shown that complete restoration is rare, and alternative metrics such as mass discharge are often useful for assessing the performance of partial restoration efforts. Experience also has shown that different technologies are needed for different times and locations, and that deliberately combining technologies may improve overall remedy performance. Several injection-based technologies are capable of removing a large fraction of the total contaminant mass, and reducing groundwater concentrations and mass discharge by 1 to 2 orders of magnitude. Thermal treatment can remove even more mass, but even these technologies generally leave some contamination in place. Research on better delivery techniques and characterization technologies will likely improve treatment, but managers should anticipate that source treatment will leave some contamination in place that will require future management.

Bioaugmentation for groundwater remediation

Bioaugmentation for Groundwater Remediation: An Overview.- Dehalococcoides and Reductive Dechlorination of Chlorinated Solvents.- Production and Handling of Dehalococcoides Bioaugmentation Cultures.- Bioaugmentation with Dehalococcoides: A Decision Guide.- Bioaugmentation Considerations.- Microbial Monitoring During Bioaugmentation with Dehalococcoides.- Bioaugmentation for Aerobic Degradation of CIS-1,2-Dichlorothene.- Bioaugmentation for the In Situ Aerobic Cometabolism of Chlorinated Solvents.- Bioaugmentation with Pseudomonas Stutzeri KC for Remediation of Carbon Tetrachloride.- Bioaugmentation for MTBE.- Economics and Valuation of Bioaugmentation.- Research Needs for Bioaugmentation.- Index.

Dermal bioavailability of benzo[a]pyrene on lampblack: Implications for risk assessment

Lampblack is the principal source of contamination in soils at manufactured gas plant (MGP) sites where oil was used as the feedstock. Risks and cleanup criteria at these sites are determined primarily by the total carcinogenic polynuclear aromatic hydrocarbon (PAH) content, particularly the concentration of benzo[a]pyrene (BaP). Dermal contact with soils at oil-gas MGP sites is a significant component of the overall risks. Seven samples were collected from oil-gas MGP sites and the steady-state dermal fluxes were measured over 96 h in vitro. The standard dermal bioassay technique (in which 3H-BaP is added to the soil matrix) was modified to allow direct measurement of the dermal absorption of the native BaP in the samples. The experimentally derived dermal absorption factors for BaP were 14 to 107 times lower than the default assumption of 15% over 24 h (55-fold lower on average). The dermal fluxes were correlated positively to the total BaP and total carbon concentrations. The measured dermal absorption factors were compared to the default risk-assessment calculations for all seven samples. The calculated excess cancer risk was reduced as a result of using the measured absorption factors by 97% on average (with reductions ranging from 93 to 99%). This work indicates the risks at oil-gas MGP sites currently are overestimated by one to two orders of magnitude, and provides a protocol for the testing and data analysis needed to generate site-specific cleanup levels.

Source Zone Remediation: The State of the Practice

Chlorinated solvents are widespread groundwater contaminants whose physical and chemical properties make them difficult to locate and treat within the subsurface. In particular, remediating chlorinated solvent source zones has proven technically challenging, costly and contentious. Nevertheless, there has been a concerted effort over the past 20-25 years to remediate these source zones, involving development of several in situ technologies. This chapter summarizes the knowledge gained from these efforts, and serves as an introduction to the entire volume. The chapter first summarizes the evolution of the conceptual model of DNAPL source zones, and then describes the current model of a 5-stage life cycle, with a long final stage of back diffusion and desorption. The chapter then summarizes the development of the important remediation technologies, followed by an overview of the current state of the practice. The chapter includes brief summaries of the remaining chapters, and concludes with a summary of the current needs, both for research and for the highly-qualified personnel required to manage groundwater restoration in the future.

Bioaugmentation with Dehalococcoides: a Decision Guide

Bioaugmentation with Dehalocccoides strains has the potential to improve the performance and/or reduce the costs of in situ bioremediation. However, deciding whether the costs of bioaugmentation are justified is often difficult. This chapter is intended to help site managers in this decision-making process. The chapter is structured around a flowchart based on a series of diagnostic questions and site conditions that are critical in determining whether to bioaugment. After introducing the key sources of uncertainty involved in the decision process, the remainder of the chapter describes these diagnostic questions and the key sources of information useful in answering them, including discussions of relevant laboratory and field test methods. This guidance is not intended to be prescriptive in nature, but to present a systematic approach to making a decision that generally requires both technical information on specific site conditions, as well as non-technical judgments regarding risk tolerance and economic assessments. The guidance begins with questions structured as “off ramps,” to allow rapid screening of sites where the decision is relatively easy. Later questions require more detailed information and testing, and the final questions require consideration of management objectives and development of comparative cost estimates.

Improving risk assessments for manufactured gas plant soils by measuring PAH availability

Abstract Remediation of soils at oil-gas manufactured gas plant (MGP) sites is driven primarily by the human health risks posed by the carcinogenic polycyclic aromatic hydrocarbons (PAHs), particularly benzo[a]pyrene (BaP), that are associated with lampblack residues. Although PAHs on lampblack are tightly sorbed, risk assessments do not account for this reduced availability. A multi-investigator study of 7 oil-gas MGP site soil samples demonstrated that the dermal and ingestion absorption factors are far lower than current default assumptions used in risk assessments. Using these sample-specific absorption factors in standard risk assessment equations increased risk-based cleanup levels by a factor of 72 on average (with a range from 23 to 142 times the default level). The rapidly released fraction of the BaP in each sample, as measured by supercritical fluid extraction, was closely correlated (r2 = 0.96) to these calculated cleanup levels. The weight of evidence developed during this research indicates that the risks posed by PAHs on lampblack are far less than assumed when using default absorption factors and that a tiered evaluation protocol employing chemical analyses, chemical release data, and in vitro bioassays can be used to establish more realistic site-specific criteria.