Trouble Underground: Groundwater Contamination

What is in your tap water? Is it safe to drink? Does it contain traces of chemicals that, in larger quantities, might be poisonous or harmful? Is the water supply from which it is drawn protected, or are there plumes of hazardous substances slowly oozing towards it?

These are crucial questions for anyone, yet most people have no idea how safe their own water supply is. And others know their water is unsafe yet have no alternative but to drink it anyway. ESI-affiliated faculty members are doing research that may help people in both of these situations. Much of the research focuses on groundwater: water that lies under the surface in tiny spaces between rocks or particles of soil, and that is a source for wells and reservoirs.

Phil Gschwend, of the Department of Civil and Environmental Engineering (CEE), is concerned about what happens to groundwater when organic chemicals, such as gasoline additives and solvents, are spilled (or released in other ways). He notes that whenever we introduce a new chemical for widespread use we can expect that some of it is going to be spilled or leaked somewhere, sometime. He believes we should therefore try to determine how much of a hazard the new chemical will be to groundwater before it is introduced. He is developing tools to perform such an analysis theoretically, without having to release samples of chemicals into the environment (or even to produce such samples at all). Gschwend says it is as if he were studying the "pharmacokinetics of the Earth”, analyzing how organic chemicals move through the Earth system and examining the many “pharmacological” effects they have on it.

Charles Harvey, also of CEE, is studying a water source that is already known to be highly contaminated. He does research on groundwater in Bangladesh, where large numbers of village wells contain arsenic in concentrations that are extremely hazardous or even deadly. He wants to understand why the wells are so contaminated, and to find ways to provide safer drinking water.

For both of these investigators, it is critical to understand not only the dynamics of flowing groundwater but also the ways in which groundwater and the chemicals dissolved in it interact with the physical materials underground and the microbes that inhabit the soil. Such interdisciplinary work, cutting across the fields of physics, chemistry and biology, is critical to our understanding of the Earth system, and it is the hallmark of research done within ESL.

Gschwend notes that releasing chemicals into the environment without knowing what they will do there is like drugging a patient without knowing what effects the drugs will have. There are about 95,000 chemicals used in volume in this country. It would be impossible to do enough experiments to determine the effects each will have on groundwater. But knowing the structure of each chemical, and how it will be used, one can model computationally how it will spread and interact with subsurface minerals and solutions.

The analysis can be surprising. For example, suppose a tank that stores gasoline with added ethanol leaks. Ethanol is biodegradable, but the microbes that degrade it deplete the oxygen in the groundwater, benzene (which is also in gasoline) is best biodegraded when there is oxygen present. Thus adding ethanol to gasoline might increase the amount of benzene found in wells downhill from a gas station. Gschwend and his student, Sam Arey, are cataloguing environmental effects of organic chemicals and investigating ways to mitigate contamination.

In Bangladesh it is not a question of whether groundwater will become contaminated: millions of wells already are. There is arsenic in sail in many other places, but it does not dissolve in the groundwater in such high concentrations, In the U.S., typically less than a thousandth of the arsenic in soil is dissolved. But in parts of Bangladesh, as much as one quarter of the available arsenic is dissolved in groundwater, Why?

Perhaps microbial activity is responsible. Arsenic is often bound to the surface of mineral oxides. Could microbes be drawing away oxygen as part of their metabolic process, releasing arsenic? Harvey has established field stations, where he injects water laced with molasses to stimulate microbial activity. Then he pumps out some groundwater and tests it. He finds that stimulating microbes does increase levels of arsenic in the water.

But what stimulates the microbes when there is no molasses? The answer may have to do with agriculture. When groundwater is pumped out to irrigate crops, river water is pulled down to take its place. Many rivers are full of organic waste. That could feed underground microbes, liberating arsenic,

Irrigation pumps are often near drinking-water wells. They typically draw from roughly the same depth, about 100 feet below the surface-which is also the depth at which the concentration of dissolved arsenic peaks. Perhaps that is not a coincidence. It may be that the pumping is pulling massive amounts of organic waste down and delivering it to microbes at 100-foot depth, thus creating the arsenic maximum. Harvey Is working to determine the degree to which irrigation draws organic material into groundwater. (Irrigation may not be at fault at all. Perhaps it flushes arsenic out of the system, or brings in oxygen-rich water that enables arsenic-binding oxides to form.)

If it turns out that irrigation is to blame, people could dig drinking-water wells to 400 feet, where arsenic is at a minimum, leaving irrigation wells at 100 feet. But pumping water from 400 feet might draw organic waste down, recreating the original problem. In addition, in this approach people would still be left irrigating crops with arsenic-rich water.

Harvey's and Gschwend's projects both involve finding trade-offs among such imperfect outcomes. In Harvey's case, it may be that providing clean drinking water is worth the cost of having contaminated irrigation water. In Gschwend's, it may mean choosing among several fuel additives that each lead to different kinds of groundwater problems when they are spilled or leaked. And in both cases, these ESI researchers are finding that it is necessary to consider the system as a whole-the flow of water, the microbes in the soil, the chemical reactions that occur there-in order to make any real progress towards a solution, however imperfect.