Kitsap groundwater model points to promising future

Overall, the Kitsap Peninsula is expected to have enough water for people and fish for many years into the future, as long as the water is managed well, according to a groundwater model developed by the U.S. Geological Survey.

The model offers reassuring findings for residents of the Kitsap Peninsula. It is also encouraging to see local water, sewer and public works officials working together to plan for infiltrating stormwater along with recycling wastewater for irrigation. Those efforts will not only protect the peninsula’s water resources but will save money for water customers.

Drilling for water on the Kitsap Peninsula Kitsap Sun file photo
Drilling for water on the Kitsap Peninsula
Kitsap Sun file photo

Lonna Frans of the U.S. Geological Survey met this week with members of WaterPAK — the Water Purveyors of Association of Kitsap — to discuss the conclusions of a five-year, $1.4 million study of water resources across the Kitsap Peninsula. Lonna said a final written report should be available in about a month. (See website Kitsap GW model.)

The most impressive part of the groundwater model is the mapping of geology across the entire peninsula, based on more than 2,100 well-driller logs that describe the type of soil at various depths. Putting that information together provides a three-dimensional picture of the underground structure, including sand and gravel deposits, which contain water, along with layers of clay and compressed soils, which slow down the water movement.

By monitoring water levels in 66 wells over time and accounting for rainfall and groundwater withdrawals, the computer model provides a dynamic picture of what happens under various conditions. The model can be used to predict what will happen to Kitsap’s aquifers under various rainfall scenarios, including long periods of drought.

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The model also can predict what will happen to streamflows under various rainfall scenarios. The Kitsap Peninsula has no mountain snowpack to supply the streams with water during dry summer months, so the water must come from slow-moving underground supplies.

Now that the model is complete, it can be run for almost any pattern of rainfall or drought that one wishes to dream up. For example, running the model with average rainfall and no pumping at all (close to a predevelopment condition) would bring the average groundwater level up about 25 feet — although groundwater levels in some places would be raised more than in other places.

Streamsflows under the no-pumping scenario would be an average of about 2 percent higher — although this would be difficult to measure with current instruments. Nobody would really notice the difference.

If pumping across the peninsula were increased by 15 percent, there would not be much difference in aquifers near the surface and only a two- or three-foot drop in aquifers around sea level. Streamflows would go down by a fraction of a percent but not enough to notice.

Decreasing groundwater recharge by 15 percent, such as paving over the landscape with new roads, houses and parking lots, would have a greater effect on streamflows.

Again, not all areas on the peninsula will see the same effects. The model can be used to zero in on specific streams and their watersheds — although the smaller the area of study, the less accurate the prediction is likely to be.

Bob Hunter, manager of Kitsap Public Utility District, said the model can be used to predict the effects that new wells would have on streamflows as the population grows. The model could advise managers whether it would be advisable to pump certain wells at certain times of the year and hold back at other times.

Kathleen Cahall, water resources manager for the city of Bremerton, said the model can also be used to make sure aquifer-recharge areas are protected and that industrial facilities that store large quantities of chemicals are not located where a spill could contaminate a major underground water supply.

Morgan Johnson, general manager of Silverdale Water District, said he would like to use the model to predict what will happen when highly treated effluent from the Central Kitsap Wastewater Treatment Plant is used to irrigate ball fields and other areas in Central Kitsap. Efforts between the water districts and Kitsap County might lead to greater infiltration of water and greater groundwater supplies to be pumped from existing wells throughout Central Kitsap.

The model was built on background information, which can be found in the report “Hydrogeologic Framework, Groundwater Movement, and Water Budget of the Kitsap Peninsula” (PDF 49.8 mb).

The USGS provided half the costs for the study. The other half was shared among Kitsap PUD; Silverdale Water District; West Sound Utility District; North Perry Water District; Manchester Water District; the cities of Bremerton, Port Orchard, Poulsbo and Gig Harbor; Washington Water, a private utility; and the Suquamish and Port Gamble S’Klallam tribes.

In September of 2014, I wrote about water resources for the series we called “Taking the Pulse of Puget Sound.” The story was called “Making sure there is enough water to go around.”

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