Although fresh water is piped from the Skagit River for parts of northern Whidbey, most of the island depends on rain to keep aquifers filled and household faucets flowing.

If the island had no wells, in theory this “recharge” of the underground water system would be in balance, continually filling from rainwater percolating through permeable soils, at the same time emptying at some unknown location within Puget Sound. But, two unknowns make this a difficult balance to verify: how much water escapes under the sound, and how much is returned via precipitation?

On an island with 72 separate watersheds, no alternative source of water, such as a substantial river or mountain runoff, and the threat of saltwater intruding into the aquifers, these are important questions.

A groundwater recharge study, five years in the making, now provides at least part of the answer. Scientists with the U.S. Geologic Survey explained to the Island County Board of Health Monday that Whidbey Island aquifers are “recharged” at an average rate of about 5.7 inches a year, which is on par with other regions. The number is slightly higher on Camano Island.

That information is interesting, but not necessarily useful by itself. Doug Kelly, Island County’s hydrogeologist, says it is just one critical piece of a water puzzle that will eventually be used to build a 3-D computer model of water flow and usage on the island.

“It’s a real big picture evaluation tool,” Kelly said. “So we don’t put more development on the island than the resources can hold. Are we on track or are we derailed?”

It’s not necessarily useful for evaluating individual small developments, such as whether the water supply can accommodate a cluster of six new homes, he said. Those tools already exist.

And it won’t provide a magic number for the Island County’s human carrying capacity.

“There is a constant desire to (determine) how many homes can we support,” Kelly said. “I don’t think we’re ever going to get there. We will be able to come up with some good planning tools to say ‘this area is good for more growth’ and ‘that area is not good for more growth.’ ”

With about $150 million in new construction year after year, growth in Island County is an issue that is being watched closely.

“There are locations in the county that already can’t support more homes,” Kelly said. Water supply has often been used as a limiting factor for growth, he pointed out.

Saltwater intrusion can be the end result of an overburdened aquifer. As long as the aquifer is recharged, the water pressure within will keep the saltwater out. But when the system is depleted, the saltwater will push through, spoiling the freshwater supply to a number of homes. This has already happened in Coupeville, which sits in the middle of Whidbey Island’s driest area.

Triggers can be set up to constantly test saltwater within the aquifers and give managers an early warning as intrusion is occurring, but that can already be too late.

The solution is through management, Kelly said.

“We need more predictive tools, rather than reactive tools,” he said.

On the positive side, the island is well-situated to endure the short-term droughts that have been common in recent years, he said. The time it takes water to work its way down to the island’s deep aquifers is measured in years, not months. Consequently, a draught, such as the one that parched Whidbey Island last summer, does not have an immediate effect on the water table.

“We don’t get touched by short-term drought,” Kelly said.