Introduction
Crude oil-contamination of soil often occurs in areas adjacent to wellheads and storage facilities. Phytoremediation is a promising tool that can be used to remediate such sites and uses plants and agronomic techniques to enhance biodegradation of the hydrocarbon compounds. The research objective of this study was to evaluate fertilizer addition and vegetation establishment on phytoremediation of crude oil-contaminated soil. Our current IPEC phytoremediation studies consist of an on-site field project in southern Arkansas, greenhouse studies, and a mathematical modeling.
Field Study
The field site in El Dorado, AR is located in a bermed area that is the site of an intentional spill in 1997 by vandals. 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. Plant shoot biomass and root biomass, length, surface area, and volume for each of the treatments were determined. All plant species appeared to be exhibiting adequate plant growth. The Total Petroleum Hydrocarbon (TPH) and biomarker (hopane) analyses of the soil samples collected 6 months after plot establishment (t=6) are currently being conducted.
A survey was conducted to identify plant species currently growing on petroleum-contaminated sites near the El Dorado, AR field site. Plants that were observed to be growing at the various sites included bermudagrass (Cynodon dactylon L.), yellow nutsedge (Cyperus esculentus L.), fall panicum (Panicum dichotomiflorum Michx.), brome grass (Bromus spp.), and witchgrass (Panicum capillare L.).
Greenhouse Study
Selection of plant species and soil amendments is essential to phytoremediation. The research goal is to identify plants and management techniques to effectively remediate oil-contaminated soil. Germination and survival of 22 plant species in oil-contaminated soil has been determined in a growth chamber study. A greenhouse study evaluated biomass production of 5 plant species in oil-contaminated unamended soil or soil amended with broiler litter, hardwood sawdust, papermill biosolids, or inorganic fertilizer. Oil-contaminated field plots were sampled at 0 to 15-cm and 15 to 45-cm depths and analyzed for TPH and plant nutrients. Treatments were: non-fertilized vegetation-free control; fertilized fescue-ryegrass mixture; or fertilized bermudagrass-fescue mixture. Plant growth, TPH levels, soil nutrients, and oil-degrading microbial levels will be evaluated.
Mathematical Model
We have developed a simple mathematical model of the phytoremediation process that accounts for the ‘exploration’ of contaminated soil by the plant roots. Unlike pesticides, metals, and explosives that are readily transported to the plant via water movement, and thus exposed to the root-zone-enhanced remediation rates, spilled crude oil is not mobile, so root growth through the soil must be accounted for in the model to incorporate the effect of enhanced degradation in the rhizosphere zone. Both the rhizosphere volume and the rate of root turnover are important parameters in this system. However, accurate and precise experimental measurements of the rhizosphere volume are difficult to make, and we are developing tools to estimate these parameters. In addition, we have proposed a simple model for root growth that accounts for annual root turnover, maximum standing biomass, and spatial and temporal distribution.