KP salt water intrusion distant but possible threat

By Chris Fitzgerald, KP News 

In the mid-1990s, the Pierce County Health Department (PCHD) began approving siting, construction, water quality, and quantity for new well permit applications. Since that time, “the approval of construction for new wells has become much more rigorous,” says Brad Harp, a hydrogeologist with the Environmental Health Program at PCHD.

Zoning regulations place greater restrictions on areas known to have saltwater intrusion, and the impact of new wells into existing areas undergoes additional county scrutiny. According to Harp, relative to construction activity taking place elsewhere countywide, the Key Peninsula is not experiencing a high volume of new wells being drilled. “The rural-5 and 10-acre zoning does not provide enough density of wells on Key Peninsula to get a good handle on water chemistry along the shoreline,” he said.

Yet, the Key Peninsula has an abundance of waterfront homes and land still available for new construction. (And) he cautions that the potential for saltwater intrusion exists anywhere groundwater is inadvertently directed into freshwater well systems.

The KP Community Plan’s natural environment section, dated April 27, 2005, reported minor instances of saltwater intrusion in individual wells in Longbranch, Taylor Bay, and Glen Cove. Harp acknowledged this report, and indicated neighboring wells are not necessarily affected. He says greater importance to drinking water quality lies in where a well is located relative to the shoreline, rather than to the number of wells in the immediate area.

For instance, an existing well located 5 feet from the high-tide line may experience water usage consistent with its freshwater reservoir capacity. At a neighboring location, however, a cluster of several houses whose wells are set 50 to 100 feet back from the high-tide line may experience a change in their water chemistry due to the amount of fresh water they simultaneously extract. Water follows the path of least resistance. If fresh groundwater is depleted where sea water is present nearby, it intrudes landward to fill the vacuum.

Concern for acceptable levels of chloride and conductivity (the amount of specific dissolved chemicals in water allowing it to transmit electricity) elevations in drinking water have, over the last five to eight years, tightened regulations for shoreline well construction. Unless a property owner can demonstrate to the state Department of Ecology and PCHD that no saltwater intrusion has occurred in the immediate area, according to Harp, well construction may be prohibited, and most certainly will be heavily regulated. Still, he said, the potential exists for such intrusion anywhere groundwater depletion is proximal to tidewater. It is possible to “see the impact of tides on water levels in a well and the water chemistry.”

“We charted one (well) and it matched the rise and fall of the tide. The amount of chloride and conductivity went up as the tide came in, and receded when the tide went back out,” he said.

Concern for adequate drinking water is the reason some property owners can be required to dig test wells. It’s why they must provide seasonal tests and both high- and low-water marks, along with tests during both high tide, when there is more pressure for sea water to move inward, and low tide, when that pressure is reduced. That concern is, in part, why zoning regulations mandate density limits, and why the impact of new wells into areas of pre-existing sea water intrusion problems must be thoroughly analyzed before they can be approved.

“Once you get sea water intruding landward,” Harp said, “you want to maintain equilibrium, and that’s difficult to do.”

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