title: The Hydrogeology of North Lummi Island, Washington
Creators: name: Sullivan, William M.
Publisher: name: Western Washington University
Year: 2005

description: <p>Lummi Island is a 10.8 square mile island in the northern Puget Sound Region, west of Bellingham, Washington. The population of Lummi Island has grown steadily for decades to approximately 900 permanent and 1,500 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. My study focused on characterizing the hydrogeology of the north half of the island (3.9 square miles) where most residents live and where groundwater is the sole source of potable water.</p>
<p>I examined data collected from up to 130 wells including well logs, seasonal water level measurements, water chemistry, and precise GPS well-head elevations and positions. From these data, I created a three-dimensional bedrock and unconsolidated stratigraphic model using Department of Defense Groundwater Modeling Software. A dramatically undulating bedrock surface is concealed nearly everywhere by mostly fine-grained unconsolidated Pleistocene deposits up to 300 feet thick. Bedrock in the study area is dominated by tightly-folded sandstone, shale, and conglomerate of the Tertiary Chuckanut Formation (sandstone) in the north. This is separated by a deep southeastnorthwest trending trough from metamorphosed volcanics of the pre-Tertiary Fidalgo opiolite sequence (greenstone) in the south.</p>
<p>The stratigraphic model and potentiometric data were used to identify and define the extent, volume, and thickness of at least 12 distinct aquifers. The major aquifer is the Sandstone Aquifer, one of two separate bedrock aquifers that occupy the majority of the study area. Half of 130 wells examined are in sandstone and greenstone. Hydraulic properties including horizontal hydraulic conductivity, estimated from well log data, indicate the Sandstone Aquifer is in the upper range of textbook values for fractured sandstone. The Greenstone Aquifer is much smaller and has the lowest hydraulic properties of any in the study area. Seasonal water level fluctuations are greatest in the bedrock aquifers.</p>
<p>Ten Pleistocene aquifers were identified as thin, largely discontinuous coarse-grained (mostly sand) lenses within less permeable, fine-grained silt-clay diamicton. These aquifers fill depressions in the bedrock surface. Seven Pleistocene aquifers lie below sea level and three are perched well above sea level. The Legoe Bay and Nugent aquifers are the largest and most utilized Pleistocene groundwater source, occupying most of the southern half of the study area. These aquifers have the highest hydraulic properties and mostly negligible seasonal water level fluctuations.</p>
<p>Recharge areas identified through the stratigraphic model, potentiometric surfaces, and water chemistry occupy the inland and upper regions of the study area. Infiltration of water through overlying glacial drift into bedrock aquifers is the most important recharge mechanism because of their large areal extent and because many Pleistocene aquifers receive recharge, in part, from where they are in contact with saturated bedrock. The average recharge magnitude, estimated from a site-specific water-mass balance, is 8 inches/year or 24% of average annual precipitation. A chloride-mass balance, performed as a semi-independent estimate, establishes a lower bound for recharge of 4 inches/year or 11% of average annual precipitation.</p>
<p>Water-chemistry data vary among aquifer media. Water chemistry in the Sandstone Aquifer is dominated by sodium ions while most Pleistocene aquifers are dominated by calcium ions. Despite that nearly 80% of all wells that are completed below sea level, wide-spread seawater intrusion is not evident. Only 5 wells were determined to be intruded and, 14 additional wells may be experiencing some degree of intrusion. Occurrences of seawater intrusion are localized and are most common in the Sandstone Aquifer where low storage and fracture flow combine to increase contamination susceptibility.</p>