Robert E. Hinchee

Bioventing soils contaminated with petroleum hydrocarbons

SummaryBioventing combines the capabilities of soil venting and enhanced bioremediation to cost-effectively remove light and middle distillate hydrocarbons from vadose zone soils and the groundwater table. Soil venting removes the more volatile fuel components from unsaturated soil and promotes aerobic biodegradation by driving large volumes of air into the subsurface. In theory, air is several thousand times more effective than water in penetrating and aerating fuel-saturated and low permeability soil horizons. Aerobic microbial degradation can mitigate both residual and vapor phase hydrocarbon concentrations. Soil venting is being evaluated at a number of U.S. military sites contaminated with middle distillate fuels to determine its potential to stimulate in situ aerobic biodegradation and to develop techniques to promote in situ vapor phase degradation. In situ respirometric evaluations and field pilot studies at sites with varying soil conditions indicate that bioventing is a cost-effective method to treat soils contaminated with jet fuels and diesel.

A Rapid In Situ Respiration Test for Measuring Aerobic Biodegradation Rates of Hydrocarbons in Soil

An in situ test method to measure the aerobic biodegradation rates of hydrocarbons in contaminated soil is presented. The test method provides an initial assessment of bioventing as a remediation technology for hydrocarbon-contaminated soil. The in situ respiration test consists of ventilating the contaminated soil of the unsaturated zone with air and periodically monitoring the depletion of oxygen (O2) and production of carbon dioxide (CO2) over time after the air is turned off. The test is simple to implement and generally takes about four to five days to complete. The test was applied at eight hydrocarbon-contaminated sites of different geological and climatic conditions. These sites were contaminated with petroleum products or petroleum fuels, except for two sites where the contaminants were primarily polycyclic aromatic hydrocarbons. Oxygen utilization rates for the eight sites ranged from 0.02 to 0.99 percent O2/hour. Estimated biodegradation rates ranged from 0.4 to 19 mg/kg of soil/day. These rates were similar to the biodegradation rates obtained from field and pilot studies using mass balance methods. Estimated biodegradation rates based on O2 utilization were generally more reliable (especially for alkaline soils) than rates based on CO2 production. CO2 produced from microbial respiration was probably converted to carbonate under alkaline conditions.

Use of hydrogen peroxide as an oxygen source for in situ biodegradation: Part I. Field studies

Abstract Hydrogen peroxide, which is commonly used as an oxygen source for in situ biodegradation, tends to decompose into water and oxygen gas. The rate of this decomposition relative to the oxygen demand of the contaminated aquifer is important to the success of an in situ process. The objective of this study, which was performed at Eglin Air Force Base in northwest Florida, was to evaluate in situ hydrogen peroxide stability and biological oxygen utilization for the biodegradation of JP-4 jet fuel. Hydrogen peroxide was injected into the subsurface, concentrations of hydrogen peroxide and oxygen were measured in monitoring wells, and in situ tests were conducted to determine hydrogen peroxide decomposition and oxygen use rates at the well locations. Because the rates of hydrogen peroxide decomposition were consistently found to be much higher than the rates of oxygen utilization, it is unlikely that any significant part of the oxygen from the hydrogen peroxide in excess of that initially required to saturate the groundwater was used to degrade jet fuel.

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).

Use of hydrogen peroxide as an oxygen source for in situ biodegradation: Part II. Laboratory studies

Abstract A review of the literature on hydrogen peroxide decomposition and stabilization identified several inorganic and organic compounds that are known to decrease the rate of decomposition of peroxide in simple systems. Phosphate, a commonly used stabilizer in groundwater applications, does not stabilize peroxide in the presence of enzymatic catalysts, primarily catalase, which are the most important catalysts of peroxide decomposition. In laboratory experiments, a variety of other peroxide stabilizers identified in the literature review also did not sufficiently stabilize peroxide. It is concluded that hydrogen peroxide may not be an efficient source of oxygen for in situ bioreclamation processes, at the Eglin Air Force Base, Florida site.

Optimizing bioventing in shallow vadose zones and cold climates

Abstract This paper describes a bioventing study design and initial activities applied to a JP-4 jet fuel spill at Eielson Air Force Base, Alaska. The primary objectives of the project were to investigate the feasibility of using bioventing technology to remediate JP-4 jet fuel contamination in a sub-arctic environment and to determine to what degree the biodegradation rate of JP-4 soil contaminants could be enhanced by increasing soil temperature, both actively by circulating heated groundwater and passively by utilizing solar energy. Biodegradation rates at the bioventing site remained relatively high during the winter months in the active-warming test plot and were consistently higher than those observed in the passive-warming and control test plots. These studies suggest that an active-warming system operated in conjunction with bioventing is a useful method for remediating fuel-contaminated areas in cold climates.