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Missoula Technology & Development Center

Treatment of Petroleum-Contaminated Soils

Introduction

The number of sites contaminated with hazardous compounds in recent years has prompted environ-mental engineers to develop new technologies and to adapt existing technologies to treat contaminated groundwater and soil. Environmental engineers already knew how to treat domestic wastewater and industrial wastewater. Because they had some control over these processes, engineers had a fairly good understanding of the characteristics of the wastes and the technologies that would work best to treat them. Environmental engineers are now faced with treating soil and ground-water contaminated with petroleum products. Engineers usually have less knowledge of the characteristics of this waste. Adding to the new challenges, contaminants in soil and groundwater are dispersed throughout the media; in other words, the contaminants are not contained as an industrial or domestic waste would be.

Environmental engineers have adapted proven technologies and have developed new technologies to reduce contaminants in soil and groundwater ex situ (removed from the original location) and in situ (in place). Ex situ treatment methods incorporate both chemical and biological processes. Over the years, experience has shown that ex situ treatment processes can be costly in capital, operations, and maintenance. In situ methods attempt to stabilize or reduce the mass of contaminants in soil and groundwater using physical, chemical, and biological processes. In most cases, these treatment methods are more difficult to control than ex situ methods. The complexity of these treatment methods can be attributed to heterogeneities in soil properties.

Soil and groundwater contamination is not limited to industrial or urban locations. Contamination can be found in rural and even in remote locations. The U. S. Department of Agriculture, Forest Service is now addressing small-volume (less than 400 cubic meters) soil contamination by petroleum products in remote national forest locations where climate, utility access, and accessibility present challenges to economical restoration. This report addresses the Tongass and Chugach National Forests in Alaska (figures 1 and 2). These forests are in cold regions with temperatures dropping as low as about –2 °C during winter (Valdez weather station, data from Alaska Climate Research Center, University of Alaska Fairbanks). They are also wet, receiving more than 500 centimeters of rainfall annually (Whittier weather station, data from Alaska Climate Research Center, University of Alaska Fairbanks). Access and utility accessibility are key issues for engineers in these remote national forests.

Figure 1—Approximate boundaries of the Tongass National Forest.
Boundaries are shown as dotted lines.

Figure 2—Approximate boundaries of the Chugach National Forest.
Boundaries are shown as dotted lines.

This report will provide Forest Service engineers some guidance to help them choose a treatment technology that will reduce the level of petroleum contamination in contaminated soils. Selecting the appropriate treatment technology (or technologies) is a challenge that requires knowledge of the science involved and some professional judgment. This report is not meant to provide a "cookbook" method of selecting a technology for treating contaminated soils.

Because of uncertainties in soil properties, the total mass of contaminant released, the lateral and vertical extent of contamination, seasonal fluctuations in the water table, and weather conditions, it is very difficult to predict the outcome of a soil treatment technology (Massmann, J.; Schock, S.; Johannesen, L. 2000; Barnes and McWhorter 2000). The goal of the design engineer is to choose and design the most economical treatment system (a treatment system can be made up of more than one technology) that has a high probability of attaining the intended goal. Because of the uncertainties listed above, the design engineer may have difficulty achieving the stated goal. For example, if the engineer specifies the use of landfarming to treat contaminated soil, an unseasonably cold summer could inhibit the effectiveness of this technique, delaying the project's completion. Designing systems under conditions of uncertain variables is not a new problem. Terzaghi (1961) addressed this issue as it applied to geotechnical engineering. Others have also investigated this topic (Peck 1969; Peck 1980; Dean and Barvenik 1992). The conclusion from decades of working on this issue is that "cookbook" design protocols do not work. A method that has been successfully applied in geotechnical engineering is one that relies on good engineering judgment and a 'learn as you go' approach to the design. While this manuscript is not a primer on this design approach, the manuscript was written with this approach in mind.

This report discusses several technologies that are known to be successful for treating petroleum-contaminated soils in cold, wet, and remote regions. The documented performance of each technology is a compilation of the experience of different commercial companies and Alaskan engineers specializing in restoration of contaminated soil. Appendix C lists the commercial and engineering companies that provided information for this report.

Actual costs for the different technologies have not been included in this document. Instead, items that need to be included in a cost estimate are listed. Including actual costs for different items would date the report, making the information less useful with time. The costs for services, such as incineration of contaminated soil, will vary depending on the vendor. Finally, the authors assume that Forest Service engineers have the best information on costs for such items as labor and equipment rental.

Before engineers address any waste treatment issue, they must have an under-standing of the waste to be treated and, in the case of contaminated soil, the medium in which the waste exists. This report will provide a brief description of petroleum products and a simple conceptual model of the movement of petroleum through unsaturated soil. It includes brief descriptions of the climatic conditions in the Tongass and Chugach National Forests. The report also describes each viable treatment technology and discusses its applicability for treating contaminated soil in the Tongass and Chugach National Forests. Treatment technologies addressed in this report are shown in table 1.

Before contaminated soil can be treated, the lateral and vertical extent of the contamination needs to be determined. While site investigation is an important topic, a detailed discussion of site investigation is beyond the scope of this report.

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