Using Plants to Remediate Petroleum-Contaminated Soil - Annual Report

Period Covered by the Report: September 1, 1999 to August 31, 2000
Date of Report: September 30, 2000
EPA Grant Number: R827015-01-0
Title: Using Plants to Remediate Petroleum-Contaminated Soil
Investigators: Greg Thoma, Duane Wolf, Craig Beyrouty
Institutions: University of Arkansas
EPA Project Officer: Bala Krishnan
Project Period: September 1, 1999 to August 31, 2000 (N/C Ext. to June 30, 2001)
Project Amount: $134,949
Research Category: Phytoremediation

Description:

Objective(s) of the Research Project:

1. To conduct greenhouse studies to screen plants for their ability to germinate and grow in weathered crude oil-contaminated soil with or without amendments.
2. To survey and collect plant species currently growing on contaminated sites and screen the plants and rhizosphere microorganisms for their ability to enhance biodegradation of petroleum contaminants.
3. To conduct an on-site field study to evaluate likely combinations of plants and management systems to enhance phytoremediation of weathered crude oil-contaminated sites.
4. To develop a model that can be used to summarize and aid in the interpretation of experimental data collected in both the laboratory and field during the first experimental season.

Progress Summary/ Accomplishments:

This report covers the period through September 30, 2000 period and summarizes our current IPEC phytoremediation studies that consist of an on-site field project in southern Arkansas, greenhouse studies, and a mathematical modeling project.

Field Study

The field site in El Dorado, AR is located in a bermed area that is the site of an intentional spill by vandals approximately three years ago. The experimental plots consist of four replicates of the following treatments: (1) nonvegetated-nonfertilized control, (2) fescue-ryegrass-alfalfa + fertilizer, and (3) fescue-bermudagrass + fertilizer. Each field plot has 12 microplots ('soil socks') that contain homogenized soil that allow monitoring of the field treatments, on a smaller scale, with less effect of field variability of the contaminant levels. On 10-12 July 2000, 6 months after establishment of vegetation at the site, soil and plant samples were collected from the plots. Biological and chemical analyses of the soil sock samples are presented in Tables 1 and 2. The bacterial and fungal numbers were typical for levels of biological populations in crude oil-contaminated soil. There appeared to be an increase in microbial numbers in response to addition of fertilizer and vegetation with fescue + ryegrass having slightly larger populations than the bermudagrass plots (Table 1). Bacteria, fungi, and petroleum degrader numbers were higher in the vegetated, fertilized plots than in the no plant, no fertilizer control plots (Table 1). Addition of fertilizer and lime resulted in the expected increase in levels of plant available P, K, Ca, and Mg (Table 2).

All plant species appeared to be exhibiting adequate plant growth at the July sampling time. Plant samples were collected for shoot biomass (50 x 50 cm area over the soil sock), root biomass, root length, and root surface area. Bermudagrass sprigging in early spring 2000 was successful and the plants were growing well. Perennial cool season grasses were over-seeded and fertilized on 30 September 2000.

Table 1. Bacterial, fungal, and petroleum degrader numbers in crude oil-contaminated soil samples collected 12 July 2000 or 6 months after initiation of the field study in El Dorado, AR.

All values are a mean of 4 samples ± 1 standard deviation except *, where values are a mean of 3 samples ± 1 standard deviation.

Table 2. Chemical properties of the crude oil contaminated-soil samples collected 12 July 2000 or 6 months after initiation of the field study in El Dorado, AR.

*Values are the mean of four samples.

The Total Petroleum Hydrocarbon (TPH) and biomarker (hopane) analyses of the soil samples collected at the time of plot establishment (t=0) are presented in Table 3. Similar analyses for the t = 6 months soil samples are currently being conducted with the next soil sampling scheduled for December 2000.

Table 3. Total Petroleum Hydrocarbon (TPH) and biomarker (hopane) levels in the crude oil contaminated-soil samples collected 6 January 1999 at the initiation (t=0) of the field study in El Dorado, AR.

*Values are the mean ± standard deviation for four samples.

Greenhouse Study

Statistical analyses of the data have been completed for the experiment evaluating the survival and growth of alfalfa, bermudagrass, crabgrass, fescue, and ryegrass in weathered crude oil-contaminated soil amended with papermill sludge, broiler litter, inorganic fertilizer, or hardwood sawdust + inorganic fertilizer. Bermudagrass grown in crude oil-contaminated soil amended with broiler litter produced higher dry shoot biomass than any other plant amendment combination at 179 mg/plant. Ryegrass and crabgrass grown in broiler litter amended soil and crabgrass grown in soil amended with inorganic fertilizer also produced high yields, at 92, 91, and 113 mg/plant, respectively. Root biomass was determined and results showed that ryegrass grown in crude oil-contaminated soil amended with inorganic fertilizer or broiler litter produced significantly higher dry root biomass than alfalfa, bermudagrass, or crabgrass with any soil amendment at 108 and 101 mg/plant, respectively. Ryegrass grown in soil amended with sawdust + inorganic fertilizer, fescue grown in broiler litter amended soil, and crabgrass grown in inorganic fertilizer amended soil also produced high root biomass yields, at 90, 73, and 62 mg/plant, respectively.

Because oil-contaminated soils have different nutrient requirements than conventional agronomic recommendations suggest, a nutrient rate study using warm season plant species was conducted. The plants were recently harvested and data are currently being analyzed. Petroleum degrader populations in the rhizosphere and bulk soil were also determined to evaluate the influence of plant species and nutrient addition. Roots from all treatment combinations will be analyzed for length, radius, and surface area using the Win-Rhizo computer software.

Mathematical Model

Our initial model of phytoremediation consisted of two soil zones, the rhizosphere and the bulk soil. The current version of the model has been extended to include a root zone, six rhizosphere zones (concentric with the root structure and with individual degradation rate constants), a decaying root zone, and a bulk soil zone. The most recent extension of the model is the inclusion of the decaying root zone. This zone is thought to be significant in many situations. Literature models of root turn over have been evaluated and the incorporation of these models as forcing functions into the phytoremediation model is ongoing. A new graduate student has been added to the project, and he is familiarizing himself with the existing model and project in general.

New, time-dependent 3-D fractal models of the root structure have also been developed so that the variability of the rhizosphere volume through time as the roots grow and senesce can be included in the model. These root structural models must still be evaluated against the structure of real grass roots. As indicated above, we are currently using the WinRhizo system to collect this data for comparison. The images of these roots are similar to those in previous summaries, and are not included here.

Publications/Presentations:

Bowen, M.L. 1999 A mathematical model of phytoremediation of hydrocarbon impacted soils including an L-system approach to rhizosphere volume estimation, Masters Thesis Univ. Arkansas, Fayetteville

Bowen, M., G. Thoma, D.C. Wolf, and C.A. Beyrouty. 1998. A mathematical model of phytoremediation for hydrocarbon-impacted soils.  In K.L. Sublette (ed.) Proc. 5th International Petroleum Environmental Conf., Albuquerque, NM, 20-23 Oct. 1998. Integrated Petroleum Environmental Consortium, Tulsa, OK.

Thoma, G.J., M.L. Bowen, C.A. Beyrouty, and D.C. Wolf. 1999. An L-systems approach to rhizosphere volume estimation. p. 125. In K.L. Sublette, G.J. Thoma, and T.J. Ward (ed.) Proc. 6th International Petroleum Environmental Conf., Houston, TX. 16-18 Nov. 1999. Integrated Petroleum Environmental Consortium, Tulsa, OK.

White, Jr., P.M., L.J. Krutz, W.D. Kirkpatrick, D.C. Wolf,  C.M. Reynolds, and G.J. Thoma. 1999. Phytoremediation of petroleum-contaminated soils. p. 392-406. In K.L. Sublette, G.J. Thoma, and T.J. Ward (ed.) 6th International Petroleum Environmental Conf., Houston, TX. 16-18 Nov. 1999. Integrated Petroleum Environmental Consortium, Tulsa, OK.

Thoma, G.J., M.L. Bowen, C.A. Beyrouty, and D.C. Wolf. 1999. An L-systems approach to rhizosphere volume estimation. p. 235. In 1999 Agronomy abstracts. ASA, Madison, WI.

Future activities:

Evaluate numerical stability of the mathematical model solution of the 17 differential equations. Analyze field samples for biomass characteristics of shoot and root growth.

Supplemental Keywords:

Arkansas (AR), petroleum, phytoremediation, EPA Region 6, rhizosphere