A down to earth solution

Professors D. Andrew Barry and Christof holliger are experts in soil remediation, or cleaning up underground pollution. Toxic substances creep slowly downward over the years through the pores in the soil, ultimately ending up in the groundwater supply and causing health problems for humans and animals, including cancer. We’ve made progress encapsulating underground storage tanks so they don’t leak, and many countries have passed regulations prohibiting the dumping of toxic substances such as cleaning fluids and motor oil. But huge swaths of land are still contaminated from past spills, uninhabitable and dangerous to those living nearby. Instead of taking the usual approaches – “dig and dump”, in which the contaminated soils are just dumped in abandoned sites that don’t pose groundwater issues, “cap and forget” in which the stuff is encapsulated so that it cannot enter the groundwater table, or “dilute and displace” in which the pollution is spread out over a larger area, reducing concentrations to acceptable levels – Barry and holliger are confronting the problem, armed with the latest in twenty-first century technology: computers and bacteria.

Bacteria are masters of dismantling things. It’s thanks to bacteria that garbage eventually enriches the soil. Their clever chemical machinations net them some carbon and result in various harmless by-products like hydrogen, carbon dioxide and water. They can transform the toxic chemicals in contaminated soil into other substances which, although not too palatable, are at least no longer toxic.

Holliger is perfecting a veritable bacterial armada that can tackle the nastiest and most carcinogenic substances of the lot, chlorinated solvents. Used in dry cleaning and as cleaning solvents, these chemicals are denser than water, so they tend to sink down to the bottom of the aquifer. It’s hard to predict how they will travel through the soil. Because garden-variety bacteria need air to operate, they can’t degrade these compounds. Other species that respire anaerobically can break up the chlorine-carbon bonds deep underwater, but they need a steady supply of hydrogen in order to function. This need can be met by adding organic matter to the water; other kinds of bacteria ferment it, producing hydrogen in the process. Unfortunately, though, one of the by-products of the chlorine-reducing reaction is hydrochloric acid, and the anaerobic bacteria have a limited acidity tolerance. Things get too acidic, and they go on strike. That can be handled, says holliger, by adding a buffering agent. The soup is getting thick here. Are the solvents degrading?

Well, yes and no. Things get a little complicated because the ground is not like a test tube. There are other species that compete with the anaerobic bacteria for the extra hydrogen. Would adding more biomass speed things up? What kind of buffer should be added to handle increasing acidity levels? Where and how should it be placed? A trial and error approach – tweaking the recipe to get the soup right – is not an efficient way to handle a real groundwater contamination problem. Enter Barry, who is an expert in modeling the physics and hydrology of soil and groundwater. In a project funded by the swiss national science Foundation, the researchers have put together a model that can handle all these interwoven factors in a simple way – the amount of biomass added, the fermentation situation, the species of bacteria involved and their rates of reaction, including the various competing species, and the interaction with various buffering agents. Together, holliger and Barry are taking the test tube situation and turning it into a real aquifer model that may one day provide cleanup consultants with a real design-based tool for decontaminating soil and groundwater.