USGS Washington Water Science Center
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Aquifers of the San Juan Islands, which are the principal source of fresh water, are commonly intruded by seawater at near-shoreline locations (less than one mile from the shore). Because the demand for ground water has escalated in recent years due to population growth and is expected to continue, the progression of seawater intrusion and areas most susceptible to future seawater intrusion must be identified for proper management of the island's water resources. Additionally, moderately high chloride concentrations occur at some inland locations of Lopez Island, but it is not known if this is due to seawater intrusion. Ground-water recharge is limited due to low precipitation (20 to 30 inches per year) and poorly permeable surficial materials.