Using Plants to Remediate Petroleum-Contaminated Soil (Continuation)

EPA Grant Number: R827015-01-0
Title: Using Plants to Remediate Petroleum-Contaminated Soil
Investigators: Greg Thoma, Duane Wolf, Susan Ziegler
Institutions: University of Arkansas
EPA Project Officer: Bala Krishnan
Project Period: To be announced
Project Amount: $169,712
Research Category: Phytoremediation

Introduction

Numerous techniques exist for remediating hydrocarbon-contaminated soils. Most of these are expensive and labor intensive, often requiring significant alteration of soil integrity to achieve contaminant target levels. Because of the time and capital investment, these techniques are not well suited for many businesses. Using plants, with the associated rhizosphere (root zone) microorganisms, to enhance biodegradation of petroleum contaminants in soil can provide a low-cost remediation option well suited to many sites. Phytoremediation of low solubility, relatively immobile contaminants such as spilled crude oil relies on actively growing plant roots to stimulate a diverse population of soil microorganisms, some of which are capable of metabolizing hydrocarbons. The goal is to increase the remediation rate and lower the contaminant concentration to an acceptable endpoint in a cost-efficient manner.

Third Year Objectives

Successful phytoremediation requires healthy plants, microbial communities capable of oil degradation, reduction in TPH levels, and a reduction of risks associated with the toxic components of the oil contamination. Our goal is to evaluate management strategies phytoremediation of petroleum- contaminated soil. This research is guided by two fundamental questions: Does the potential for biodegradation exist? Microorganisms are responsible for biodegradation; their presence is akin to a catalyst affecting reaction kinetics. The existence of this potential has been demonstrated in our and other works (Figure 3). The proposed experimental approach will measure the potential for biodegradation through the enumeration of alkane and PAH degraders from established field sites. What influences the rate of biodegradation? For immobile contaminants like weathered oil, the interaction of environmental and management factors affect the plant root growth, as well as the activity of the rhizosphere and major bulk soil microbial communities. The interactions of these factors ultimately define the overall biodegradation rate. The results of the proposed experiments coupled with the mathematical model will allow calculation of the degradation rate constants accounting for important environmental and management factors.

Field study: Our initial design for the field study included sufficient plots for 6 years of semiannual sampling. We will continue to monitor the field site to evaluate appropriate plant species and management systems. Samples will be collected for plant biomass and ground cover; microbial community structure including enumeration and PLFA; and monitoring of TPH and biomarker concentrations. We will continue to follow the recommendation of the Total Petroleum Hydrocarbon Criteria Working Group (TPHCWG) and quantify the 13 TPHCWG petroleum fractions in the contaminated soil. Based upon the levels of the fractions and appropriate toxicity criteria, a tiered risk-based decision protocol can be used to calculate cleanup goals for petroleum release sites. Laboratory studies: Our initial findings suggest that phytoremediation does reduce contaminant levels through the action of microbial communities associated with the rhizosphere. It is therefore important to develop successful agronomic management strategies that exploit this understanding. However, our knowledge of the microbial ecology of the rhizosphere is lacking. The laboratory experiments proposed are designed to identify the major group(s) responsible for hydrocarbon degradation, and how each contributes to this degradation under different conditions. Additionally, the identification of these actively degrading microbial group(s) in the field samples, applying the same approach used in the laboratory experiments, will be conducted to verify and apply the laboratory results to field conditions. We propose to conduct growth chamber experiments using 13C isotopic labeling of specific contaminants coupled with the analysis of the ?13C of microbial biomarker PLFA to identify the major microbial group(s) responsible for the degradation. Another study will be designed to measure the degradation rate of the substrate in both the bulk and rhizosphere soil. The information gained from this approach will enable us to focus management strategies to enhance the activity of those group(s) of microorganisms identified as major players in the degradation of hydrocarbons. The PLFA profiling of the field samples will provide a means to link the mechanistic studies and the field results.

Modeling: The current model includes seasonal variation of the root system, but relies on annual average degradation rate constants because the available TPH concentration data in the field has been collected with a frequency that only allows the estimation of annual average rate constants. As part of the proposed research we will extend the model to include both temperature and moisture level effects on the degradation rate constants in the bulk and rhizosphere soil. Realistic values for these rate constants will be ascertained through controlled experiments in the growth chamber under a range of conditions (see experiment 1-A). This information will allow the incorporation of seasonal variation into the model predictions, providing flexibility in the model necessary to validate the approach through comparison to phytoremediation data from varied climates that will be obtained from the RTDF project.