The City of Bainbridge Island, in conjunction with the United States Geological Survey, has released the initial models and scenarios that will be used in its groundwater flow study.
The six models employed by the study use data compiled to give estimates of future demographics.
“To complete the initial scenarios (model runs), the city had to provide population, population distribution, and land use data for each of the model runs,” said City Water Resources Specialist Cami Apfelbeck.
The study, which cost the city $500,000 and the USGS $250,000, was commissioned in 2006 with a goal of developing a model describing the interrelation between water sources on the island and how changes in population growth and density could affect those water systems.
The initial models used include the predevelopment steady state, which is what the state of the water system like before human inhabitation. Current condition is the health of the water systems with no additional population, land use or climate change. The other four scenarios, expected impact, highest and least potential impact, use growth rates to predict future population and that group’s effect on the island’s water supply.
The final scenario, future land use modification, describes how resources will benefit from protection of critical recharge areas and will be affected by change.
Apfelbeck said this run will be helpful with policy-making decisions. Once the model is complete, if a policy affects land use or population, staff can run the changes through the model and it will show the council the effect of the policy on the water supply.
“You can test proposed land use changes or other kinds of changes that are being considered and they (the council) can make a more educated decision,” she said.
The scenarios dictate that Bainbridge will see growth rates between .45 percent for the low end and 3.72 percent at the top, resulting in a population somewhere between 26,176 and 60,200 residents by 2035. The expected scenario, which is the city’s educated estimate of what will happen in the future, concludes that the city will grow by 1.31 percent and have 32,647 inhabitants in 2035.
When the city and USGS joined together to create this model, the goal was to answer questions about the source of Bainbridge’s water supply. Since then, residents have filed a petition with the Environmental Protection Agency to have Bainbridge declared a sole-source aquifer, which means the aquifer is the “sole or principal” source of drinking water.
One of the questions the model aims to answer is whether or not there is exchange of water between aquifers on Bainbridge and those throughout the rest of Kitsap County, said Matt Bachmann of USGS.
The city expects to have the study by late summer 2010, but Bachmann said the report should be finished by the spring.
Bachmann said the USGS has a good idea what the results of the study will be, but until all phases are carried out, the variables can still change.
“I expect that local citizens will want to know right away what kind of situation they’re in,” Bachmann said. “The model may say one thing, but as we go through and measure the aquifer we could be off by a factor of two or three and that could change everything,” he said.
The report recommends re-calibration of the model every 10 years to insure that it remains viable. The city intends to re-calibrate the model with each update to the Comprehensive Plan, while city staff continues to collect monthly water data.
