Simple plant-based design strategies for volatile organic pollutants

Vegetation which enhances in-situ biodegradation of organic compounds can p lay a key role in the bioremediation of such contaminants in polluted soils and groundwater. Plants may act directly on some contaminants by degrading them, but their main effect is to enhance microbial populations in the rhi zosphere. Microbially mediated transformations are thus indirectly facilita ted by root exudates which nourish the indigenous microorganisms. Plants ma y also be viewed as a solar driven pump-and-treat system which can contain a plume and reduce the spread of contaminated water. Laboratory investigations carried out in a growth chamber with alfalfa plan ts provide evidence for the (microbially mediated) biodegradation of organi c compounds such as toluene, phenol and TCE. Alfalfa plants tolerate concen trations of these organics in contaminated water up to 100 mg/L. They facil itate transfer of the contaminants from the saturated to the vadose zone. F or volatile organic compounds such as TCE, vegetation provides a controlled release of compounds of hence assures dilution of the TCE evapotranspired into the atmosphere from contaminated soils. Using a range of calculated pl ausible scenarios, it is shown that intermedia transfer caused by volatiliz ation associated with plants is most likely to lead to exceedence of standa rds for ga phase contamination for most volatile contaminants. Possible act ion level exceedences might occur with highly toxic substances including vi nyl chloride and carbon tetrachchloride, if they are present in ground wate r at levels above kilogram amounts in a single nmp of a few hectares, and r eleased by vigorously growing plants under hot dry conditions. Information needed for the calculation and design of plant-based bioremediation systems for typical sites is discussed in this paper.