Millions of gallons of water per day were pumped out of an east Salt Lake water well this summer to purge it of a toxic solvent.

A dozen west-side wells were shut down during the 1980s when officials discovered high concentrations of dissolved solids and sulfates from mine tailings in the water.Much of the Salt Lake Valley's shallow aquifer - which is not a source of drinking water - is contaminated beyond repair.

The level (or pressure) of the deep aquifer - which supplies a great deal of drinking water - has been steadily dropping.

And scientists have recently detected an increasing amount of chloride in the deep aquifer.

These and other signs of deterioration in the valley's historically pristine groundwater are prompting experts to warn that the fragile resource is facing a threat that could destroy it: urbanization.

"As development increases, we must also increase our awareness that there is groundwater beneath our feet," said Florence Perez, laboratory director for Salt Lake City's Department of Public Utilities. "We have to start doing things deliberately to protect the groundwater."

That means becoming aware that whatever is put on or in the ground - solvents, discarded motor oil and antifreeze, industrial wastes, pesticides and herbicides, and dozens of other urban leftovers - may eventually reach the water.

"Urbanization is having an effect; we're seeing it more and more," said Joseph S. Gates, hydrologist and chief of the investigations section of the U.S. Geological Survey.

An Environmental Protection Agency survey released Wednesday said the groundwater of hundreds of American communities may contain unhealthy levels of pesticides and nitrates.

The agency estimated that about 10.4 percent of all community wells nationwide and 4.2 percent of private wells likely contain at least some pesticides. The survey also indicated that more than half of the nation's wells have water contaminated with nitrates from fertilizer, sewage sludge or septic tanks.

If the estimates are correct, 750 community water systems and nearly 80,000 private rural wells have unhealthy levels of contamination.

Gates and other water experts say that while most of the Salt Lake Valley's 12,000-plus wells continue to deliver water of exceptional quality, the trends are disturbing.

One development that Gates has been monitoring is the drop in the water level in the valley's principal aquifer. There have been ups and downs associated with periods of high or low precipitation, but overall, the water level has dropped by 13 percent (as much as 58 feet) since the 1950s, Gates said.

A lower water level means lower pressure, lower water quality and higher pumping costs. The decline may also explain the higher concentrations of dissolved solids that have been found in some wells.

And a continuing decline in the water pressure could eventually weaken the deep aquifer's natural defense.

"Our valley is fortunate because the principal aquifer is somewhat protected by the upward pressure," explained Don Ostler, director of the state Bureau of Water Pollution Control. "Shallow groundwater, which is where most of the contamination is, has a difficult time permeating it."

Scientists hope a four-year study currently under way may give them a clearer idea of what to expect from the decreasing water level. Ostler said the goal is to develop a model to predict potential contamination effects from overpumping.

In parts of California and Arizona, severe overpumping has actually caused the land surface to sink, Gates said.

"We've had some indication of that happening in areas of southwest Utah," he said, adding that the same thing is possible in urban areas as well. "If the surface drops uniformly, it doesn't make a lot of difference. Unfortunately, it usually doesn't happen that way."

Gates said scientists are trying to learn more about the workings of the valley's deep aquifer to find ways to protect it. For example, he said a recently completed study that identifies the aquifer's recharge area - lands where water is absorbed into the underground reservoir - could play an important role in future urban planning.

The study confirmed that the recharge to the aquifer occurs mostly in and at the base of bordering mountain ranges. In the valley itself, the groundwater is covered by a confining layer of rock that tapers to nothing as it approaches the foot-hills.

Much of the precipitation that used to seep into the recharge area along the east bench now runs into storm drains, according to a report by state and federal experts.

"Most of the water that now percolates to the water table in the recharge areas is water that has been applied to lawns and gardens," the report said. "Consequently, the water may contain fertilizers and other chemicals that could cause chemical changes in the quality of groundwater."

The higher levels of chloride in the groundwater may also be a byproduct of urbanization. Several experts have speculated that the chloride may be coming from road salt. "It's one theory," Gates said.

The chloride concentration at Salt Lake City's popular Artesian Park well, 500 E. 800 South, has more than doubled since 1970.

Perez said the chloride found there and at other wells is still considered well within acceptable limits. "But we need to find out if the increase is caused by something we're doing or whether it's a natural occurrence."

While it is possible that chloride concentrations at Artesian Park and elsewhere have increased naturally as the water level drops, the contamination of a well near Sugarhouse Park this year was anything but a natural occurrence.

The water contained perchloroethylene (PCE), a solvent used in dry cleaning. It was commonly used as a degreaser in a number of industries in the 1960s. Nothing like it had ever been found in a Salt Lake City water well.

"We are very concerned about it," Perez said. In fact, the city ordered 5 million gallons of water per day pumped out of the well - affecting water pressure and availability throughout the system - to clear it of the substance.

"We're still not using that well, and we're working with other agencies to find the source of the contamination," Perez said. "We sampled all of the culinary wells and haven't found it anywhere else."

However, PCE was found in an irrigation well at the Salt Lake Country Club. And studies revealed the presence of napthalene in groundwater from an excavation on the west side of the city; trichloroethylene in two shallow wells in West Valley City; and several toxic organic chemicals in a shallow well in South Salt Lake.

Ostler said these incidents highlight the need for additional funding to investigate cases of groundwater pollution.

"Investigations cost money because we have to dig monitoring wells that can help us identify the plume of contamination and trace it back to the source," Ostler said. "We may know where it comes from, but if the responsible party won't admit responsibility, it becomes difficult to prove without investigative resources."