From Ground Water Currents, September 1995, Issue No. 13

A Solution to Bioremediation's Soil Plugging

By Peter R. Jaffe, Ph.D., Princeton University

The results of research sponsored by EPA's Northeast Hazardous Substances Research Center (HSRC) have direct application for the design of in situ bio-remediation of contaminated ground water. The research results clearly show under what conditions soil clogging occurs and how to mitigate this problem.

Manipulating the ground water by adding constituents that enhance the growth of microorganisms can cause the soil to become plugged or clogged -- that is, gases and water are no longer able to flow through it freely. Several factors can lead to clogging of soils during in situ bioremediation, including the accumulation of biomass in soil pores; the precipitation of chemicals such as iron, which can occur as the oxygen in ground water increases; and the entrapment in soil of small gas bubbles formed by bacterial metabolism. Biomass, precipitates and gas bubbles all reduce the area through which the water can flow in the soil. If such clogging occurs, pumping and controlling the ground water flow in the subsurface will be difficult; the bio-remediation design may even fail.

The research focused on identifying which types of in situ bioremediation designs are more likely to have clogging; in what types of soils clogging is more likely to occur; and how to design and operate a bioremediation scheme in order to minimize clogging of soils. In order to address these issues, a computer model was developed that simulates the accumulation of biomass in the soils. This computer model was based on results of laboratory studies on the physics of soil clogging by biomass. With the aid of this model, researchers at the Northeast HSRC investigated clogging during in situ bioremediation of contaminated ground water sites having different soil and pollutant properties.

The results of this research project have shown that heterogeneous soils are more likely than homogeneous soils to exhibit clogging during in situ bioremediation. This potential for clogging has been expressed in terms of soil-type parameters used in soil physics, such as a pore-size distribution index, which gives engineers and geo-hydrologists a tool to assess the potential for clogging in a specific soil condition. In terms of the pollutant itself, it was shown that clogging will be most severe in the vicinity of injection wells when the bioremediation design requires that a specific substrate for the microorganisms and oxygen both be injected simultaneously. The model developed in this project was used to illustrate how to operate such injection wells in order to minimize soil clogging. By alternating the injection of the food source and the oxygen, the researchers estimated that the injection pressure at the injection well could be reduced tenfold.

For more information, call Peter Jaffe at Princeton University at 609-258-4653. See also: Taylor, S.W. and P.R. Jaffe, "Enhanced In Situ Biore-mediation and Aquifer Permeability Reduction," JOURNAL OF ENVIRONMENTAL ENGINEERING, ASCE, Vol. 117, No. 1, Jan. 1991, pp. 25-46.