Without drawing conclusions about its significance, U.S. Geological Survey hydrologists said this week in a preliminary report that Fletcher Bay aquifer’s water level, which is more than 1,000 feet deep, has drawn down about 25 feet between pre-development (before people started extracting groundwater) and 2008.

According to the federal agency’s Bainbridge Island study, however, the expected decrease during the next 25 years will be between four and 10 feet in the aquifer, which is the source for about 38 percent of all groundwater used on the island.

While the sea-level aquifers (representing 53 percent of all completed island wells) are also expected to see a decrease through 2035, increases are expected in the perched and semi-perched aquifers (mostly above mean sea level).

With the doubling of the island’s population during the last 20 years or so depleting the main water source, city officials asked USGS scientists to create a model that the city could use to help manage the island water resource in respect to population growth.

USGS hydrologists Lonna Frans and Matt Bachmann presented preliminary results of a conceptual groundwater model Tuesday at City Hall. The modeling effort, which began in 2006 and will be published and placed online in a couple of months, was jointly funded by the city and USGS.

After two years of assessing historical studies and data on the island, the hydrologists researched current well levels, springs and seep flows, stream flow and weather patterns. After establishing a groundwater flow model, the USGS, city staff and residents came up with six scenarios to run through the models.

The major objectives of the study – using the most extreme ranges of potential population, land use and climate change – was to characterize the island’s water system and its interaction with associated surface-water features by monitoring it monthly. Then putting this information into a numerical flow model to assist water-resource managers in development of a long-term watershed management plan.

The city and Kitsap Public Utility District also pump water from wells reaching the sea-level and perched aquifers, which are the primary water sources for homeowners, said Frans.

USGS hydrologists visited more than 400 wells on the island, gathering specifics on the lateral extent and thickness of all layers. Overall, they identified five aquifers on the island. They also did more superficial modeleling of Kitsap Peninsula aquifers to identify underground flows into the Fletcher Bay aquifer, Frans said.

“Because of the eastward direction of the flow,” she said, “there’s no doubt there is a strong subsurface connection between the Kitsap Peninsula and the deepest aquifer on the west side of the island.”

She said the study looked at Kitsap Peninsula because “it helped us set up boundary conditions – going as far away as possible from the area of interest. But we didn’t project any drawdown nor impact scenarios over there.”

She said that it was estimated that the total flow from Kitsap Peninsula to the Fletcher Bay aquifer through 2008 was only 5 to 6 percent of the total volume of its recharged amount. She said that much of the aquifer’s recharged water comes from precipitation; about 20 inches of total precipitation annually is recharged, though much is short-lived and goes right into streams.

Frans said the model doesn’t project any saltwater intrusion in wells currently or through 2035, though she is aware there have been documented cases of intrusion on the northeast part of the island.

“We know it happens but the amounts are so small they are not detectable in our model,” she said.

The model revealed that drawdown levels were influenced mostly by population growth and weather patterns. The population model used minimum, moderate and maximum projections through 2035.

Bachmann said the study is seen primarily as a tool and “we try not to make value judgments. What the city staff and managers do with the information is up to them. We’re just providing a model for planning purposes.”

More information is available at http://wa.water.usgs.gov/projects/bainbridge.