HYDROGEOLOGY AND SEAWATER INTRUSION OF LUMMI ISLAND, WASHINGTON STATE

Lummi Island is a 28 sq-km island in northern Puget Sound west of Bellingham, Washington. The population of Lummi Island has grown steadily for decades to approximately 900 permanent and 2,000 seasonal residents. The increasing demand for groundwater resources on the island has caused some wells to experience seasonal shortages and seawater intrusion, prompting an assessment of the hydrogeology for growth-management purposes. Our study focuses on the northern half of the island where most residents live and where groundwater is the sole source of potable water. Prior to our study, the hydrogeology of Lummi Island was characterized qualitatively from maps and reports of bedrock and glacial geology which describe two units: sandstone of the Tertiary Chuckanut Formation and overlying Pleistocene glacial drift composed primarily of glaciomarine drift. Little was known about the actual vertical or lateral extent of these units, or their hydrologic properties.

To characterize the hydrogeology of northern Lummi Island, we examined data collected from over 90 wells including well logs, seasonal depth-to-water measurements, water chemistry, and precise GPS elevations and positions. Assessment of the data indicates that northern Lummi Island consists of two distinct bedrock aquifers and several glacial drift aquifers. The bedrock surface undulates dramatically beneath glacial drift that varies in thickness from 0-100 meters. Approximately 50% of wells in the study area are completed in fractured bedrock aquifers and most of these are below sea level. Wells completed in the Pleistocene glacial drift extract water from several smaller unconfined, confined, and perched aquifers defined by thin, alternating layers of clay, fine sand, and gravel that pinch-out over short lateral distances.

Only a handful of wells experience seawater intrusion because many near-shore wells are completed in confined aquifers having elevated hydraulic heads. Chloride values from well-water samples show little variance between low- and high-water sampling periods.