Results The conceptual model we developed in GMS largely conforms to the stratigraphy laid out in Kahle and Olsen (1995) and described in the Geologic Setting section (Figure 4). The majority of island residents pull water from the Double Bluff Drift, a confined aquifer at or below sea level and contiguous beneath Guemes Island. Fewer wells draw from the Vashon advance outwash, a generally unconfined, shallower aquifer, particularly within the study area. One notable deviation from Kahle and Olsen (1995) is that more wells than expected draw from productive zones within the Whidbey Formation, especially within the GPS-surveyed subset.

HYDROGEOLOGY AND SEAWATER INTRUSION CHARACTERIZATION OF SOUTHWEST GUEMES ISLAND, WASHINGTON

Devin A. O’Reilly and Robert Mitchell, Geology Department, Western Washington University, Bellingham, WA 98225

Figure 4. Comparison between a cross-section from Kahle and Olsen (1995) and a comparable cross-section from our conceptual model. The cross-section is indicated in green on the conceptual model oblique view at bottom.

Figure 5. Results of ion analysis for Guemes Island wellheads. Individual wells are differentiated by unique sym-bols; symbol color indicates sampling month and source aquifers are indicated in key. Analyses performed by Edge Analytical of Burlington, WA. Piper diagram zoning after Kelly (2005).

Ion analyses of water samples collected in April and October 2010 were plotted on a Piper diagram to evaluate potential seawater intrusion (Figure 5). The highest chlo-ride concentration measured was 51 mg/L at a nearshore well tapping the Whidbey Formation; none contained over 100 mg/L of chloride, a common benchmark for indi-cating active seawater intrusion (Dion and Sumioka, 1984). The majority of samples are classified as “fresh” water with the remainder plotting in one of the “freshening” categories. Sampled sites showed little seasonal variation; the most pronounced varia-tions were towards “freshening” (improving) conditions, although none varied enough to move into a new classification on the Piper diagram.

Future work The development of a numerical, MODFLOW-based model from the conceptual model is underway. Once construction and calibration of the MODFLOW model is com-plete in GMS, the model will be passed to SEAWAT, a density-dependent extension program of the core MODFLOW code. We will use SEAWAT with historical chloride data and ion analyses completed as a part of this study to model the location and changing geometry of the seawater interface over time, including under a variety of climatologi-cal or development stressed hypothetical scenarios.

Introduction Guemes Island is north of Anacortes in Skagit County, Washington (Figure 1). Slightly larger than 21 square kilometers, Guemes Island is a southeast member of the San Juan Islands. A ru-ral community of over 500 lives year-round on the island (Kahle and Olsen, 1995). The majority of is-land residents live in predominantly coastal neigh-borhoods. Coastal and, in particular, island aquifers pres-ent unique and difficult challenges for resource management. As growing nearshore populations have exceeded surface water capacity, they have turned to groundwater. For island aquifers, howev-er, recharge is finitely limited by precipitation infil-tration on the island, underscoring the importance of diligent resource management. In addition to water quantity issues, island aquifers are high-ly susceptible to seawater intrusion. Indications of seawater intrusion have been encountered in several low-lying, nearshore neighborhoods on Guemes Island, including North Beach, West Beach and Potlatch Village (Kahle and Olsen, 1995). In 1997, the US Environmental Protection Agency certified the Guemes Island aquifer sys-tem as a Sole Source Aquifer for the Guemes community, a designation intended to foster con-servation and careful management of the water resources by the applicable local agencies. From that, Skagit County has assumed a mandate to sustainably manage the groundwater resource for the use of current and future island residents. In constructing this model, we hope to both better characterize the Guemes Island aquifer system and provide a powerful tool for county officials and citizens to use in planning island development while protecting their drinking water.

Research objectives The primary objective is to construct a predic-tive, functional model of the groundwater condi-tions of the study area, with special attention to modeling potential seawater intrusion conditions. This study is focused on on the central, more sparsely populated elevated core of Guemes Island (Figure 2). A northeast-southwest trending, poor-ly drained lowland is expected to effectively hy-drogeologically isolate the north peninsular portion of Guemes Island, providing a northern bound to our study area. A similar north-south trending low-land isolates the hilly, surficial bedrock dominated east half of the island and provides the eastern bound. Expanding on the hydrostratigraphy presented in Kahle and Olsen (1995), we developed a con-ceptual model to characterize the study area and construct a computer-based numerical model. This numerical model will be calibrated using available data, such as water quality parameters estab-lished through ion analysis, until it closely mimics observed conditions. Ultimately, we will use the numerical model to simulate a variety of poten-tial future scenarios to explore the potential long-term response of the aquifer system to changing conditions.

Figure 1. Location of Guemes Island in Skagit County, Washington.

Figure 2. Locations of wells used on Guemes Island for this study. Orange dots are the locations of well log-derived point stratigraphies used in the conceptual model. Blue dots are wells that were additionally GPS surveyed and measured for static water level. Green dots are wells with GPS surveying, static water levels and ion analyses.

Figure 3. A Guemes Island wellhead being surveyed with the Trimble 5700.

Acknowledgements Deep appreciation is owed the residents of Guemes Island for allowing me access time and again to their homes, spigots and well casings— with particular thanks to Marianne Kooiman, whose tireless efforts provided valuable data to this project. Thanks also to my field assistants Niki Thane, Flip O’Reilly and Sam O’Reilly. Funding for this project has been graciously provided by Skagit County and the Western Washington University Geology De-partment Advance for Field Research.

ReferencesDion, N. P., & Sumioka, S. S. (1984). Seawater intrusion into coastal aquifers in Washington, 1978, Water-

Supply Bulletin 56: Washington Department of Ecology.Easterbrook, D. J., & Anderson, H. W. (1968). Pleistocene stratigraphy of Island County and ground-water

resources of Island County, Water Supply Bulletin No. 25: Department of Water Resources, State of Washington.

Easterbrook, D. J. (1969). Pleistocene chronology of the Puget Lowland and San Juan Islands, Washington. Geological Society of America Bulletin, 80(11), 2273-2286.

Kahle, S. C., & Olsen, T. D. (1995). Hydrogeology and Quality of Ground Water on Guemes Island, Skagit County, Washington, Water-Resources Investigations Report 94-4236: U.S. Geological Survey.

Kelly, D. (2005). Seawater Intrusion Topic Paper: Island County Health Department.Lapen, T. J. (2000). Geologic Map of the Bellingham 1:100,000 Quadrangle, Washington, Open File Report

2000-5: Washington Division of Geology and Earth Resources.

Methods •Characterized the hydrostratigraphy using 56 Department

of Ecology well logs selected from a custom-developed Microsoft Access database for Guemes Island

•Determined the groundwater flow regime -Surveyed a subset of a dozen wells using a Trimble

5700 survey-grade GPS unit (Figure 3) -Measured subset static water levels in April and October•Collected eleven water samples and analyzed for seawater

intrusion indications using a Piper diagram•Developed a conceptual model using the Groundwater

Modeling System (GMS) to interpolate the subsurface between point stratigraphies (Figure 4)

•Develop a numerical model using MODFLOW and SEAWAT•Calibrate and validate the model•Perform simulations to examine potential scenarios for the

evolution of the groundwater system

Conceptual Model UnitsExample (oUnit code)• Unit description/ lithology• Hydrogeologic function• Distribution

Everson Drift (oQe)• Pebbly silt and clay diamicton; 13000 years old• Aquitard/ confining unit• Filled-in nearshore environments; 2 to 40 meters thick

Vashon till (oQvt)• Compact clay, silt and gravel; 13000-18000 years old• Aquitard/ confining unit• Modal surface unit; 1 to 25 meters thick

Vashon advance outwash (oQva)• Moderately to well-sorted sandy gravel, pebbly sand,

medium to coarse sand, silt and clay unit with an overall upward-coarsening sequence; 18000 years old

• Aquifer• Above sea level; 12 to 30 meters thick

Whidbey Formation (oQw)• Floodplain clay, silt, fine-grained sand and peat with well-

developed sedimentary structures; 90000-100000 years old

• Aquitard (with productive sand lenses)• Abundant in sea cliffs; 12 to 50 meters thick

Double Bluff Drift (oQdb)• Till, glaciomarine drift, glaciofluvial sand and gravel,

glaciolacustrine silt, till-like stony silt and clay; usually presents on Guemes Island as a fine-to-medium, well sorted, grey to tan sand; 100000-250000 years old

• Aquifer• At or below sea level

Bedrock (oBr)• Fidalgo ophiolite sequence– layered gabbro, gabbroic

pegmatite, hornblende gabbro, diorite; Jurassic-aged• No flow boundary/ aquiclude• Surface exposures on island only around Guemes Mountain

highland