Bainbridge Island is located three miles (5 kilometers [km]) west of Seattle, Washington, United States. It is 28 square miles (73 km2) with a population of 25,000. Precipitation supplies over ninety percent of drinking water through recharge.
Recharge supplies seven billion gallons of water annually to aquifers. About one billion gallons of drinking water is consumed annually, obtained from two water purveyors with large production wells, and from over 1,000 private wells. Groundwater is a finite resource because:
a) Although there is much more stored groundwater than the amount used annually, an undefined amount of stored groundwater is not recoverable because of the heterogenous and discontinuous nature of the glacially-derived aquifers. Wells are sometimes drilled where the resulting production is only 20-40 gallons per minute (gpm)/75-150 liters per minute (lpm) instead of the intended 100-300 gpm.
b) To prevent seawater intrusion, some stored groundwater must be left in aquifers to act as a barrier by maintaining differential pressure between freshwater and seawater.
c) Dropping water levels over the last thirty years in the most productive Deep Aquifer, which supplies one-third of the island’s water, indicates extraction is exceeding recharge.
d) Production has doubled over the last twenty years due in part to increasing population and increased commercial uses, causing declining water levels.
e) Recharge is expected to decrease by up to ten to twenty percent due to climate change.
Policy considerations are driven by the protection of recharge areas for this federally-designated sole source aquifer island with a finite water supply, which is surrounded by salt water. The municipality has not prioritized specific recharge areas or sub-areas as to their relative importance, as specified in state regulations. Groundwater monitoring in all aquifers is essential to ensure extraction is not conducted to the point where aquifers are overpumped beyond their ability to maintain serviceable well water levels, or to allow unwanted seawater intrusion. Nevertheless, the municipality has decreased the monitoring network despite its obvious benefit. The lack of recharge area management and reductions in monitoring are indicative of a pro-growth policy and lack of emphasis in long-term management of the groundwater resource.
Recommended actions:
a) Increase monitoring in all aquifers.
b) Implement regular monitoring of perennial streams, which are all fed by groundwater, and begin watershed rehabilitation to restore stream levels.
c) Require all new construction to estimate their annual consumption and define the source aquifer.
d) Preserve critical recharge areas.