The City of Bainbridge Island Groundwater Management Program consists of monitoring groundwater levels in 46 private and public wells and testing 28 wells for saltwater intrusion. The goal of the monitoring is to ensure the City can make an appropriate response if levels in our aquifers drop and/or saltwater begins to infiltrate them. COBI just released the results of the 2013 monitoring.
Samples were taken since 2007 for some but not all of the wells, and so there is not a complete data set for all the wells in all sample years. But there is sufficient data to give the city a sense of the state of our water, and, over time, a more complete picture will emerge.
The City has determined Early Warning Levels for both water levels and seawater intrusion. When EWLs are exceeded, the City (a) conducts an additional investigation to determine whether a problem is developing and/or (b) takes remedial or protective action such as water conservation, putting limits on the development of new wells, or pursuing additional water sources.
Water Level
The state of Island aquifers, the sources of our drinking water, is assessed by water level. Safe yield is defined as the amount of water that can be taken from the aquifer without adverse effects. When water levels decline by one-half foot per year or more over a 10-year period and the decline can’t be attributed to precipitation variations, the aquifer is said to be at the EWL.
Our Island aquifers are all rain fed, so the City also measures Cumulative Rainfall Departure to determine whether rainfall exceeded or fell short of the average. That way, if aquifer levels change, the City can determine whether the change is due to CRD or to another factor such as overuse.
The 2013 data show that none of the monitored wells dropped below the EWLs and that water levels in the monitored aquifers stayed the same or varied with changes in precipitation.
Read the report here.
Seawater Intrusion
Seawater intrusion is measured by the presence of chloride (chlorine ion). The EWL for chloride concentration is at or above 100 mg/L. The 28 wells monitored for seawater intrusion were chosen for their proximity (less than 1/2 mile) to shoreline or for being part of vulnerable aquifers.
Samples were taken from five aquifers: Perched, Semi-perched, Glaciomarine, Sea Level, and Fletcher Bay aquifers. No samples exceeded the EWL. And in four of the aquifers there was no upward trend. But in the Sea Level aquifer, one private well near Fletcher Bay showed a slight increase in chloride. Confirmation that there is an upward trend will require four consecutively increasing samples. In a different KPUD well, chloride increased slightly between 2003 and 2006 and also between 2008 and 2012. Despite the increase, the concentration is still low, below 14 mg/L, but the City will continue to monitor it.
Read the report here.
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Photo by Saaby.
