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The Use of Vegetation to Enhance Bioremediation of Surface Soils Contaminated with Pesticide Wastes

Principal Investigators
J.R. Coats and T.A. Anderson, Iowa State University


Goal: The primary objective of this project is to determine the role of herbicide-tolerant plants and commodity plants in facilitating microbial degradation of herbicide wastes in soils. This information can then be used in defining the potential role of vegetation, under specific types of chemical contamination (herbicides, insecticides, industrial chemicals) in the bioremediation process.

Rationale: With the increase in pesticide usage since the early 1950s has come a rapid growth in the numbers of agrochemical dealerships. Unfortunately, many of these dealerships have, through normal operating procedures, contaminated the soil and water at these sites, creating one of the most ominous issues facing the agrochemical industry. The expense of most of the current technologies for cleanup of contaminated soil and water preclude their use at agrochemical dealership sites. Biological restoration of contaminated surface soils using indigenous microbial populations is potentially an effective remediation strategy, provided that a sufficient consortium of microorganisms capable of degrading contaminants are present, and that their activity is not limited by existing environmental conditions. Environmental conditions can be altered to enhance microbial populations and/or their activity, such as through nutrient additions or aeration.

Approach: Experimental procedures will utilize pesticides from different chemical classes and with different properties as model compounds in studies to identify critical environmental and biological variables affecting rate of degradation in the root zone. Experiments will be carried out using soils and plants collected from a pesticide-contaminated site in Iowa. An initial sampling trip to the site was used to document type and percent cover of vegetation as well as to collect soil and vegetation samples from the site. Initial studies in the laboratory have developed suitable extraction and analysis techniques for quantifying pesticide wastes in soils. Experiments on influence of vegetation on microbial degradation of pesticides were conducted utilizing sterile, nonvegetated, and rhizosphere scenarios in an environmental chamber. Results of these preliminary tests indicated enhanced degradation of atrazine, metolachlor, and trifluralin in the rhizosphere of Kochia sp., an herbicide-tolerant plant compared with nonvegetated soils and sterile control soils. In addition, Kochia sp. seedlings have emerged from rhizosphere soils spiked with additional concentrations of the three test chemicals, indicating the ability of these plants to survive in soils containing high concentrations of herbicide mixtures. In addition, radiotracer experiments will be conducted to provide further evidence for biodegradation of pesticides in root zones of plants from contaminated sites, as well as allow for mass balance calculations. Finally, small-scale field trials will be conducted.

Status: An appropriate study site was located and permission was obtained. The study site is an active agrochemical dealership in central Iowa. The site was characterized with respect to (1) soil physicochemical properties, (2) vegetation, and (3) presence and distribution of pesticide contaminants. Several candidate plant species for use in the study were identified, and rhizosphere soils from these plants were collected. Nonvegetated soil samples were also collected for use in pesticide degradation experiments. Greenhouse studies have focused on the ability of candidate plant species to germinate and survive in soil contaminated with pesticides (individual compounds and mixtures). Plant germination and survival is a critical variable affecting use of plants in remediating pesticide-contaminated sites. Tests have been conducted with commercially available seeds, as well as seeds collected in the field. In addition, herbicide-resistant plant seeds have also been included in these studies because of their ability to tolerate certain classes of herbicides. In addition to plant toxicity tests, investigators are also conducting microbial toxicity tests in order to determine if high concentrations of pesticide mixtures found at these contaminated sites are inhibiting microbial growth and activity as indicated by respiration rates of incubated soil samples. Initial screening tests on mineralization of 14C-labeled atrazine identified rhizosphere soil from Kochia scoparia as having enhanced degradative capability. In subsequent tests, it was concluded that Kochia scoparia rhizospheric soil had a significantly greater rate of 14C-atrazine mineralization than nonvegetated soil and sterile control soil. This project is in its first year.

Clients/Users: This research will interest agrochemical dealerships, for heavy metal contaminated sites; to determine effects of poplar tree cultivar on survivability and growth when deep-planted at a heavy metal contaminated site; to investigate effects of soil amendments on poplar tree survival and growth; to determine heavy metal concentrations in poplar leaves, roots, and wood when grown in a heavy metal contaminated environment; to investigate optimum depth of soil cover for establishment of a perennial grass cover; and to extend a current mathematical model that simulconsulting and remediation companies, and federal agencies such as U.S. Department of Defense.

Key words: vegetation, bioremediation, pesticides, agrochemicals, rhizosphere

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