(602c) Soil Amendments Used during Remediation of Contaminated Sites Enhance Plant Growth but Increase Mobility of Chrysotile Fibers in One Case

The ability of plants to stabilize soil and reduce erosion, a process known as phytostabilization, can be critical in remediating or restoring abandoned surface mines or other sites contaminated with pollutants such as heavy metals. Nutrient-rich soil amendments are often applied to the soil surface to improve plant growth, hasten the development of vegetation, and reduce soil erosion and mobility of contaminants.

We were interested in whether asbestos fibers, as a contaminant, move in soil and whether phytostabilization is relevant to preventing their movement. We set up a greenhouse experiment in which we grew two crop species, Brassica juncea and Sorghum bicolor, in Cone-tainers^TM containing either pure sand or sand mixed with chrysotile fibers in a 1:15 ratio by weight of chrysotile:sand, layered over pure sand. The chrysotile was obtained by grinding chrysotile ore (Glove, Arizona) in a dry condition for 15 min in a high-energy vibratory ball mill. To evaluate the utility of different soil amendments often applied in the remediation of abandoned mines or polluted sites, we added 5 cm of a soil amendment to the surface: NPK fertilizer; a commercial organic, soilless potting medium; dehydrated cow manure; topsoil; or no amendment. We examined (1) how the presence of asbestos and the different soil amendments affected plant biomass, and (2) whether leachate collected weekly from the Cone-tainers^TMafter watering contained asbestos fibers. We also collected the asbestos remaining in the containers at the end of the experiment; in the future, we will examine whether physical or chemical degradation of asbestos fibers occurred and varied among experimental treatments.

All soil amendments improved the aboveground biomass of both species compared to sand alone, but the fertilizer increased biomass the least. The effect of chrysotile on plant biomass depended on the soil amendment and species. Chrysotile increased biomass in the NPK fertilizer treatment for both plant species, perhaps because the chrysotile provided micronutrients. However, chrysotile decreased the biomass of Brassica in the presence of topsoil and decreased the biomass of Sorghumin the presence of all three amendments other than NPK. It is less likely that micronutrients are limiting in these other soil amendment treatments.

The movement of chrysotile fibers through the sandy substrate depended on the soil amendment but not the plant species. Measured by the 100.2 Method, a standard method used by EPA originally for chrysotile fibers >10 μm, chrysotile fibers >0.5 μm were found in large quantities when manure was used as a soil surface amendment but not with other soil amendments or no soil amendment. Fiber concentrations reached 100,000 MFL (millions of fibers per liter). As seen microscopically, the fibers collected in leachate were coated with organic matter particulates. The greatest concentrations were detected after 4 and 5 weeks of culture. The EPA threshold standard, applied to fibers >10 μm is 7 MFL. We found no measureable quantities of fibers >10 μm.

Data from asbestos contaminated sites corroborate our findings that asbestos can move in soil water. EPA has measured concentrations up to 0.51 MFL in test wells at a former industrial site in Pennsylvania, where chrysotile was used in manufacturing, and up to 100 MFL in surface water at an amphibole mine site in Montana. In both cases, the fiber size measured exceeded 10 μm. We found far higher concentrations in two test wells with 79 and 9 MFL at the Pennsylvania site when including fibers >0.5 μm.

Taken together, these studies provide evidence that asbestos fibers can contaminate ground water. Our experimental results suggest that adding manure as a soil amendment can increase the mobility of asbestos in soil. We, therefore, conclude that careful consideration must be given to how soil treatments affect the containment of the contaminant in question, even if the development of vegetation is facilitated.