David W. Major

Variations in expression of carbon isotope fractionation of chlorinated ethenes during biologically enhanced PCE dissolution close to a source zone

The stable carbon isotope values of tetrachloroethene (PCE) and its degradation products were monitored during studies of biologically enhanced dissolution of PCE dense nonaqueous phase liquid (DNAPL) to determine the effect of PCE dissolution on observed isotope values. The degradation of PCE was monitored in a 2-dimensional model aquifer and in a pilot test cell (PTC) at Dover Air Force Base, both with emplaced PCE DNAPL sources. Within the plume down gradient from the source, the isotopic fractionation of dissolved PCE and its degradation products were consistent with those observed in biodegradation laboratory studies. However, close to the source zone significant shifts in the isotope values of dissolved PCE were not observed in either the model aquifer or PTC due to the constant input of newly dissolved, non fractionated PCE, and the small isotopic fractionation associated with PCE reductive dechlorination by the mixed microbial culture used. Therefore the identification of reductive dechlorination in the presence of PCE DNAPL was based upon the appearance of daughter products and the isotope values of those daughter products. An isotope model was developed to simulate isotope values of PCE during the dissolution and degradation of PCE adjacent to a DNAPL source zone. With the exception of very high degradation rate constants (>1/day) stable carbon isotope values of PCE estimated by the model remained within error of the isotope value of the PCE DNAPL, consistent with measured isotope values in the model aquifer and in the PTC.

Development of a Scaled Repeated Five-Spot Treatment Model for Examining Microbial Induced Calcite Precipitation Feasibility in Field Applications

Microbial induced calcite precipitation (MICP) has been heavily investigated in laboratory experiments with few forays into field-scale implementation. Conventionally, MICP refers to an alternative technology for improving the geotechnical properties of soils via microbially mediated urea hydrolysis inducing conditions for calcite precipitation at particle contacts. The study presented herein focuses on up-scaling the conventional treatment process to more realistic volumes through the development of a scaled repeated five-spot treatment model. Commonly used in oil recovery applications, the repeated five-spot well pattern provides for a flow symmetry condition allowing for improved laboratory model feasibility. A conventional MICP two-phase treatment technique resulted in improvement in the target treatment (0.5 m by 0.5 m by 0.15 m) zone with small spatial variation. Sensors, including bender elements and sampling wells, provided valuable insight into the evolution of biological, chemical, and mechanical changes spatially and temporally during treatment. Overall, the scaled repeated five-spot treatment model was successful at capturing a complex treatment scenario involving a bio-mediated soil improvement technology and demonstrated the potential to capture complex scenarios of soil improvement.

Smoldering Remediation of Coal-Tar-Contaminated Soil: Pilot Field Tests of STAR

Self-sustaining treatment for active remediation (STAR) is an emerging, smoldering-based technology for nonaqueous-phase liquid (NAPL) remediation. This work presents the first in situ field evaluation of STAR. Pilot field tests were performed at 3.0 m (shallow test) and 7.9 m (deep test) below ground surface within distinct lithological units contaminated with coal tar at a former industrial facility. Self-sustained smoldering (i.e., after the in-well ignition heater was terminated) was demonstrated below the water table for the first time. The outward propagation of a NAPL smoldering front was mapped, and the NAPL destruction rate was quantified in real time. A total of 3700 kg of coal tar over 12 days in the shallow test and 860 kg over 11 days in the deep test was destroyed; less than 2% of total mass removed was volatilized. Self-sustaining propagation was relatively uniform radially outward in the deep test, achieving a radius of influence of 3.7 m; strong permeability contrasts and installed barriers influenced the front propagation geometry in the shallow test. Reductions in soil hydrocarbon concentrations of 99.3% and 97.3% were achieved in the shallow and deep tests, respectively. Overall, this provides the first field evaluation of STAR and demonstrates that it is effective in situ and under a variety of conditions and provides the information necessary for designing the full-scale site treatment.

Remediation of trichloroethylene-contaminated soils by star technology using vegetable oil smoldering

Self-sustaining treatment for active remediation (STAR) is an innovative soil remediation approach based on smoldering combustion that has been demonstrated to effectively destroy complex hydrocarbon nonaqueous phase liquids (NAPLs) with minimal energy input. This is the first study to explore the smoldering remediation of sand contaminated by a volatile NAPL (trichloroethylene, TCE) and the first to consider utilizing vegetable oil as supplemental fuel for STAR. Thirty laboratory-scale experiments were conducted to evaluate the relationship between key outcomes (TCE destruction, rate of remediation) to initial conditions (vegetable oil type, oil: TCE mass ratio, neat versus emulsified oils). Several vegetable oils and emulsified vegetable oil formulations were shown to support remediation of TCE via self-sustaining smoldering. A minimum concentration of 14,000 mg/kg canola oil was found to treat sand exhibiting up to 80,000 mg/kg TCE. On average, 75% of the TCE mass was removed due to volatilization. This proof-of-concept study suggests that injection and smoldering of vegetable oil may provide a new alternative for driving volatile contaminants to traditional vapour extraction systems without supplying substantial external energy.

Liquid−Liquid Mass Transfer of Partitioning Electron Donors in Chlorinated Solvent Source Zones

A combination of batch and column experiments evaluated the mass transfer of two candidate partitioning electron donors (PEDs), n-hexanol (nHex) and n-butyl acetate (nBA), for enhanced bioremediation of trichloroethene (TCE)-dense nonaqueous phase liquid (DNAPL). Completely mixed batch reactor experiments yielded equilibrium TCE-DNAPL and water partition coefficients (KNW) for nHex and nBA of 21.7 ± 0.27 and 330.43 ± 6.7, respectively, over a range of initial PED concentrations up to the aqueous solubility limit of ca. 5000 mg/L. First-order liquid-liquid mass transfer rates determined in batch reactors with nBA or nHex concentrations near the aqueous solubility were 0.22 min(-1) and 0.11 min(-1), respectively. Liquid-liquid mass transfer under dynamic flow conditions was assessed in one-dimensional (1-D) abiotic columns packed with Federal Fine Ottawa sand containing a uniform distribution of residual TCE-DNAPL. Following pulse injection of PED solutions at pore-water velocities (vp) ranging from 1.2 to 6.0 m/day, effluent concentration measurements demonstrated that both nHex and nBA partitioned strongly into residual TCE-DNAPL with maximum effluent levels not exceeding 35% and 7%, respectively, of the applied concentrations of 4000 to 5000 mg/L. PEDs persisted at effluent concentrations above 5 mg/L for up to 16 and 80 pore volumes for nHex and nBA, respectively. Mathematical simulations yielded KNW values ranging from 44.7 to 48.2 and 247 to 291 and liquid-liquid mass transfer rates of 0.01 to 0.03 min(-1) and 0.001 to 0.006 min(-1) for nHex and nBA, respectively. The observed TCE-DNAPL and water mass transfer behavior suggests that a single PED injection can persist in a treated source zone for prolonged time periods, thereby reducing the need for, or frequency of, repeated electron donor injections to support bacteria that derive reducing equivalents for TCE reductive dechlorination from PED fermentation.

IN SITU Bioremediation Of Chlorinated Ethene Source Zones

In situ bioremediation (ISB) was not initially considered a feasible technology for treating chlorinated solvent source zones, and it still faces some skepticism. However, experience over the last decade demonstrates that it is a viable technology for treating some source zones, although realistic objectives must be set. It is reasonable to expect 90-99% reductions in groundwater concentrations and mass discharge from a source in many situations, but it is not likely that ISB can achieve complete cleanup of a source zone within a few years. ISB is best applied in stages, optimizing the design and operations over time. Some residual contamination will remain, and modeling suggests that contaminant concentrations may rebound after treatment, although no rebound has been observed so far at field sites, and it is likely that the accumulation of bacterial biomass and reduced minerals during ISB will sustain treatment for several years after active treatment ends. Practitioners considering ISB for a source zone should be aware of several potential difficulties. The electron donor demand can be so great that it is not feasible to supply enough donor and/or pH buffer, or adding large amounts of donor may cause other problems such as methane production, biofouling of wells or clogging of the subsurface. Using ISB to treat source zones requires careful design, monitoring and continuous optimization throughout treatment, and likely will require extended passive management after treatment.

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.

Enhanced Attenuation: A Reference Guide On Approaches To Increase The Natural Treatment Capacity Of A System

The objective of this document is to explore the realm of enhancements to natural attenuation processes for cVOCs and review examples that have been proposed, modeled, and implemented. We will identify lessons learned from these case studies to confirm that enhancements are technically feasible and have the potential to achieve a favorable, cost-effective contaminant mass balance. Furthermore, we hope to determine if opportunities for further improvement of the enhancements exist and suggest areas where new and innovative types of enhancements might be possible.

Research Needs for Bioaugmentation

Bioaugmentation represents a remarkable success story of applied research, and future research should lead to more successful applications. This chapter provides an introduction to the future potential of bioaugmentation (and bioremediation in general), and seeks to identify the research needs that, if addressed, will help realize this potential. First, a discussion of bioremediation at the molecular, organismal, community and ecosystem scales is presented. Cultivating a deeper understanding and/or developing new techniques at any of these levels may lead to advances and novel discoveries, and some future initiatives are suggested. Three linchpin concepts that will likely direct the future of bioremediation are examined in detail: niche specialization as a means to enhance bioremediation specificity; microcosms as valuable tools for directed research; and the enrichment paradox, which enacts a balance between research ideals, regulatory requirements and remediation activities. From these three concepts, an optimal scenario for successful bioremediation is proposed. Some practical applied research needs are outlined, and finally, future perspectives are described. While not presently feasible or in practice, these ideas give hints as to what may eventually be possible within the field of bioremediation.

Economics and Valuation Of Bioaugmentation

This chapter reviews the costs and benefits of bioaugmentation for treating chlorinated ethenes in groundwater. Detailed cost estimates are provided for a range of template site scenarios to provide practitioners a first estimate of the expected overall costs and the specific cost items involved. In most cases, bioaugmentation represents a small fraction of the total costs of an enhanced in situ bioremediation (EISB) remedy. The additional costs for bioaugmentation typically represent less than 3% of the total costs. The potential economic benefits of bioaugmentation include: (1) reduction in the time required to achieve complete dechlorination of chlorinated solvents (or complete degradation of other target compounds), thereby reducing both the monitoring costs and the overall costs for the electron donor (or capturing more of the value of the electron donor initially injected); (2) reduction in regulatory oversight by achieving treatment objectives sooner; (3) reduction in the time required to return the groundwater to beneficial use by achieving treatment goals in a shorter period of time; and (4) ability to apply EISB at sites where this approach would otherwise not be effective and where other more expensive approaches would be required.