Cristin L. Bruce

Bioaugmentation for MTBE Remediation

Bioaugmentation to treat methyl tert-butyl ether (MTBE) in groundwater was considered a promising technique for several years, with several researchers involved in developing and testing MTBE-degrading bacteria. Although the effort yielded important insights into the biodegradation of MTBE and its daughter product tert-butyl alcohol (TBA), bioaugmentation for these contaminants is not used currently, and it is likely to remain a minor field of endeavor. The costs and time required to generate enough biomass to seed a barrier system has proven to be prohibitive for most applications, and generally not necessary for effective aerobic treatment. Although MTBE and TBA were originally considered to be highly recalcitrant, indigenous microorganisms have proven capable of effective aerobic treatment, given sufficient oxygen, and it also has proven possible to create stable and robust zones of oxygenation even in relatively complex lithologic systems. Lessons learned in the course of the research on MTBE/TBA bioremediation include: (1) sufficient delineation is critical, (2) sufficient oxygen delivery is a common limitation, (3) biostimulation is typically sufficient, though a time lag may be experienced, (4) bioaugmentation activity can persist for years in situ, (5) typical cocontaminants must be considered, (6) effective treatment typically requires a minimum of 6–12 months, and (7) large numbers of bacteria may be needed for effective treatment.

Advances in In Situ Air Sparging/Biosparging

In situ air sparging (IAS) is a technology commonly used for treatment of submerged source zones and dissolved groundwater plumes. The acceptance of IAS by regulatory agencies, environmental consultants, and industry is remarkable considering the degree of skepticism initially surrounding the technology in the early 1990s. Much has been learned and reported in the literature since that time, but it appears that practice has changed little. In particular, conventional pilot testing, design, and operation practices reflect a lack of appreciation of the complex phenomena governing IAS performance and the unforgiving nature of this technology. Many systems are poorly monitored and likely to be inefficient or ineffective. Key lessons-learned since the early 1990s are reviewed and their implications for practice are discussed here. Of particular importance are issues related to: (a) the understanding of air flow distributions and the effects of geology and injection flowrate, (b) the need to characterize air flow distributions at the pilot- and field-scale, (c) how changes in operating conditions (e.g., pulsing) can affect performance improvements and reduce equipment costs, and (d) how conventional monitoring approaches are incapable of assessing if systems are performing as designed.

A Practical Approach for the Selection, Pilot Testing, Design, and Monitoring of In Situ Air Sparging/Biosparging Systems

The use of in situ air sparging (IAS) has increased rapidly since the early 1990s, and it is now likely to be the most practiced engineered in situ remediation option when targeting the treatment of hydrocarbon-impacted aquifers. To date, IAS system design has remained largely empirical, with significant variability in approaches and results. Here, the valuable knowledge gained from IAS studies and applications over the past decade has been integrated into a new paradigm for feasibility assessment, pilot testing, design, and operation. The basis for this Design Paradigm, the initial feasibility assessment, monitoring, and the overall design approach are discussed in detail here; other referenced documents contain the details of specific recommended activities. The proposed design approach is unique in that it contains two design routes; the first is a non-site-specific approach requiring minimal site characterization and testing (Standard Design Approach), while the second is a more site-specific approach (Site-Specific Design Approach).

Diagnostic Tools for Integrated In Situ Air Sparging Pilot Tests

In situ air sparging (IAS) pilot test procedures have been developed that provide rapid, on-site information about IAS performance. The standard pilot test consists of six activities conducted to look for indicators of infeasibility and to characterize the air distribution to the extent necessary to make design decisions about IAS well placement. In addition, safety hazards that need to be addressed prior to full-scale design are identified. Two additional pilot test activities are described in those cases where air distribution must be more precisely defined. The test activities include both chemical tests (tracking contaminant concentrations, dissolved oxygen and tracers) and physical tests (air flow rate and injection pressure, groundwater pressure response). Pilot test data from Eielson Air Force Base, Alaska illustrates implementation of the pilot test and interpretation of the data.

Use of an SF6-Based Diagnostic Tool for Assessing Air Distributions and Oxygen Transfer Rates during IAS Operation

A diagnostic test designed to assess air distribution and oxygen delivery rate to the aquifer during in situ air sparging (IAS) is described. The conservative tracer gas, sulfur hexafluoride (SF6), is added upstream of the air injection manifold during steady IAS operation and groundwater samples are collected from the target treatment zone after some time period (usually 4 to 24 h). The appearance of SF6 in groundwater is used to characterize the air distribution in the target treatment zone, while the SF6 concentration increase with time is used to assess oxygen transfer rates to the target treatment zone. Conversion from SF6 concentration to oxygen mass transfer rate involves correcting the SF6 concentration increase over time for differences in the relevant chemical properties and injection air concentration. Data presented from a field demonstration site illustrate the utility of this test for identifying air distribution details not readily identified by deep vadose zone helium and groundwater pressure transducer response tests. Oxygen transfer rates at this site ranged from 0 to 20 mg-O2/L-H2O/d. Finally, a comparison of short-term SF6 test data with longer-term dissolved oxygen data illustrated this tests utility for anticipating long-term dissolved oxygen distributions.

A multi-tracer push-pull diagnostic test for in situ air sparging systems

This document describes the development and initial application of a multi-tracer push-pull test designed to provide near real-time point-specific measures of contaminant volatilization and aerobic biodegrada-tion rates during in situ air sparging (IAS) operation. Measured biodegradation and volatilization rates are specific to the tracers used, so the results provide relative measures useful for identifying spatial differences in treatment performance and changes in performance with changes in system operation and design. The diagnostic test involves injecting a solution containing multiple tracer compounds into the target treatment zone through a monitoring well, piezometer, or drive point. The injected solution is initially deoxygenated and can contain: (a) a nondegradable, non-volatile conservative tracer, (b) one or more nondegradable, volatile chemicals, (c) an aerobically biodegradable, nonvolatile compound, and (d) a visible dye. After some predetermined hold time, an excess quantity of groundwater is extracted from the same injection point and the change in the concentrations of the tracer compounds is measured. Volatilization and oxygen utilization rates are then estimated from mass balances on the tracer components. The development of this diagnostic tool was conducted in a controlled physical model study and then initial field tests were conducted at the U.S. Navy Hydrocarbon National Test Site (HNTS) in Port Hueneme, California. Spatial variations in oxygenation and volatilization rates were observed, with oxygenation rates varying from 0 to 51 mg-O2/L-water/d, and tracer volatilization rates varying from 0 to 47%/d. Acetate and sulfur hexafluoride (SF6) were used as tracers in the initial testing, and it was discovered that these are not ideal choices due to the potential for anaerobic acetate biodegradation and SF6 partitioning into trapped gas in the aquifer.

Air Sparging for the Treatment of Chlorinated Solvent Plumes

In its simplest form, in situ air sparging (IAS) is a source zone and dissolved groundwater plume remediation technology that involves injection of air into an aquifer through a collection of vertical wells screened below the water table. Modifications to this basic design may include the use of horizontal wells placed below the water table, vertical wells placed in an engineered trench, the delivery of gaseous reactants (hydrogen, propane, oxygen, etc.), the use of vapor recovery and treatment systems, pulsing of the gas injection and heating of the injection gas. The basic process components of IAS systems are shown in Figure 14.1.

Large-Scale Mixed MTBE-BTEX Plume Containment at Port Hueneme, CA, Using a Combination of Biostimulation and Bioaugmentation

Karen D. Miller—Naval Facilities Engineering Service Center, Restoration Development Branch, 1100 23rd Ave., Port Hueneme, CA 93043, Tel: 805-982-1010, Fax: 805-982-4304; Paul C. Johnson—Arizona State University, Dept of Civil and Environmental Engineering, Box 875306, Tempe, AZ 85287-5306, Tel: 480-965-9115, Fax: 480-965-0557; Cristin L. Bruce —Arizona State University, Dept of Civil and Environmental Engineering, Box 875306, Tempe, AZ 85287-5306, Tel: 480-965-0055, Fax: 480-965-0557