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12/17/91

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You CAN drink the water: hydrogeologists at work

STANFORD -- Most people think of fresh water as residing in lakes and rivers, but more than 95 percent of the world's usable fresh water lies underground, in soil pores and cracks in rocks.

That is the realm of hydrogeology.

"Hydrogeology is just what it sounds like," said Irwin Remson, a professor in the Stanford department of applied earth sciences. "It's the study of water in the ground."

Stanford's hydrogeologists are working on sustainable management of this resource, so there will be enough clean water for the future. Among their efforts:

  • Remson developed the first finite differential simulations of groundwater flow. Using that, he and colleague Steven Gorelick have devised models to define the best pumping strategies for cleaning up contaminated aquifers. Among their successes: a strategy to protect drinking water from contamination leached from a landfill near the Canadian capital, Ottawa.
  • Gorelick and graduate student Claire Tiedeman are using similar techniques to predict the amount of pumping necessary to contain a contaminant plume of vinyl chloride in an aquifer in St. Joseph, Mich.
  • Gorelick and postdoctoral fellow Haim Gvirtzman recently patented a technology to remove pollutants from groundwater by pumping air into wells. The pollutants move into the air, which is then removed and cleaned.
  • Graduate student Chris Koltermann has devised a new way to map water-bearing strata without drilling samples. She uses data on ancient rainfall and temperature patterns to predict the location of soil types with different water-carrying characteristics.
  • Graduate student Russ Philbrick is comparing above- ground reservoirs with underground water reserves in the San Francisco Bay Area. He has found that in some circumstances, underground storage is a better choice: The water is less likely to be contaminated by pollutants from the air or from surface run-off, it stays at an even temperature and it does not evaporate.

On the downside, however, underground storage raises questions of groundwater rights, the energy cost of pumping and potential contamination from toxic plumes, Philbrick said.

Sinking Santa Clara Valley

From Stanford's earliest days, faculty have been interested in finding and using underground water to make arid land bloom, but they also recognized the limits of groundwater resources. During the Great Depression, Prof. C.F. Tolman, with graduate student Joseph Poland, documented that land can sink as a result of pumping too much groundwater to the surface. Tolman's studies, showing that parts of the Santa Clara Valley were subsiding at a rate of four inches annually, were among the research that led to public policies aimed at limiting pumping to sustainable rates.

Sustainability requires knowledge of how groundwater moves. Remson published the first finite differential numerical simulation of groundwater flow almost 30 years ago. More recently, he and Gorelick and their students have used computer models of flows to devise strategies to locate, store and clean up groundwater.

The first step in removing contaminants from an underground aquifer is to define the boundaries of the contaminanted volume or "plume," as well as the rates at which it is moving in different directions.

Next, a series of wells is drilled into the plume so water can be pumped to the surface for treatment. As pumping continues, purer water flows into the plume, diluting contaminant levels and flushing additional contaminant to a place where it can be diluted or treated. Eventually, contaminant levels in the target plume fall to within acceptable ranges.

Well-drilling, pumping and water treatment are expensive, however. That is why Gorelick and Tiedeman are using models to determine the minimal amount of pumping necessary to contain a plume of vinyl chloride in an aquifer in St. Joseph, Mich. They consider the uncertainty involved with predicting underground flows by expressing their results in probabilities of success: If workers pump this much water out, they have a 75 percent chance of success; if they pump more, they have a 95 percent chance.

"We are appropriately overdesigning the clean-up operation," Gorelick said.

Gorelick is taking another approach to groundwater clean-up with Gvirtzman. They have patented a technology for removing some types of contaminants - volatile organic chemicals - without pumping the contaminated water to the surface.

When air is pumped into the wells, this type of chemical moves freely from the water into the air. The tainted air is brought back to the surface where the chemicals are extracted at less cost than by pumping and treating the wtaer directly.

Koltermann has developed a powerful new tool that uses ancient climate information to predict the location of water-bearing strata.

Normally, hydrogeologists use soil samples to predict the location or size of an underground aquifer. Water moves more freely through material with large pore spaces, like gravel or sand, than it does through material made of small, closely packed particles, like clay. So, large quantities of groundwater often lie in layers of sand or gravel deposited by ancient stream flows.

Researchers take samples of soil materials with a hollow-core drill, then apply statistical techniques to interpolate the underlying geology between sampling points.

Koltermann, however, predicts the location of ancient streams without sampling. Instead, she uses climate data for the last 600,000 years, compiled by other scientists, to predict where water-carrying layers of sand and gravel lie. Working from the hypothesis that heavier rains produced greater flows capable of moving coarser material, she has demonstrated a correlation between the amount of rainfall in a given era and the size of the material deposited at different depths.

The days are gone when clean water could be taken for granted, the hydrogeologists say.

"As we move toward a more sustainable human culture, thrifty management of water resources and the protection of their quality will become ever more important," Philbrick said.

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