Seawater intrusion modeling on Guemes Island, Washington

Thesis proposal for the Master of Science degree, Department of Geology, Western Washington University, Bellingham, Washington

Devin A. O’Brien O’Reilly March 2010

Approved by Advisory Committee Members:

______Dr. Robert Mitchell, Thesis Committee Chair

______Dr. Doug Clark

______Gary Stoyka 1 Table of Contents 1.0 Problem statement 3 2.0 Introduction 3 3.0 Background 5 3.1 Geologic setting 5 3.2 Hydrogeologic setting 7 3.3 Previous work 8 4.0 Proposed research 9 4.1 Study area 9 4.2 Primary objective 10 5.0 Methods 10 5.1 Characterize the hydrostratigraphy 10 5.2 Determine the groundwater flow regime 11 5.3 Develop a conceptual and numerical model 12 5.4 Collect and interpret water quality samples 13 5.5 Calibrate and validate the model 13 5.6 Perform simulations 14 5.7 Timeline 14 6.0 Significance of proposed research 15 7.0 References 16 Figure 1. Skagit County, Washington, with Guemes Island highlighted. 18 Figure 2. Guemes Island, Washington. 19 Figure 3. The Ghyben-Herzberg relation. 20 Figure 4. Surficial geology of Guemes Island, Washington. 21 Figure 5. Guemes Island, Washington, with study area indicated. 22

2 1.0 Problem statement The objective of this study is to develop and calibrate a groundwater flow and seawater intrusion MODFLOW/SEAWAT model of the central portion of Guemes Is- land, Skagit County, Washington. Well logs and previous work will be used to identify and map hydrostratigraphic units in the study area. Measurement of static water levels in wells will provide pontentiometric data that will be combined with hydraulic param- eters derived from aquifer test results to characterize the groundwater flow regime. Ion analysis of well water samples will be used to calibrate, test and validate seawater intrusion model results. In constructing this model, I hope to provide a predictive tool for planners and water resource managers to ensure sustainable development of fresh- water resources on Guemes Island.

2.0 Introduction Guemes Island is north of Anacortes, across Guemes Channel, in Skagit County, Washington (Figure 1). Slightly larger than 21 square kilometers, Guemes Island is a southeast member of the San Juan Islands, a major island group in coastal Wash- ington. A rural community of more than 500 people lives year-round on the island; seasonal population estimates are poorly constrained, but believed to be in excess of 2200 (Kahle and Olsen, 1995). The majority of island residents live in near-shore neigh- borhoods. Access to the island is facilitated by a county-operated ferry, running from Anacortes across narrow Guemes Channel. The central portion of Guemes Island is dominated by gently rolling topography underlain by glacial sediments, with a northeast-southwest trending lowland nearly iso- lating the northern peninsula (Figure 2). The southeastern end of the island contains a pair of higher relief hills, known locally as Guemes Mountain. These hills, at 210 meters height, comprise the only bedrock exposures on the island and represent a distinctly different hydrogeologic regime.

3 Coastal and, in particular, island aquifers present difficult challenges for resource management. In 1991, roughly 70% of Washington residents lived in one of fourteen coastal counties (Tibbott, 1992); the population of the Puget Sound Basin is expected to reach 4 million by 2020 (Swanson, 2001). As these growing nearshore populations have exceeded surface water capacity, they have turned to groundwater. For island aquifers, however, recharge is finitely limited by precipitation infiltration on the island, underscoring the importance of careful resource management. In addition to water quantity issues, island aquifers are highly susceptible to seawater intrusion. Indications of seawater intrusion have been encountered in several low-lying, nearshore neighbor- hoods on Guemes Island, including North Beach, West Beach and Potlatch Village (Kahle and Olsen, 1995). When a hydraulic connection exists between saline water and the freshwater of a coastal aquifer, the lighter fresh water floats atop the denser salt water (Kelly, 2005). The depth to the interface between the fresh and saltwater is approximated by an equation relating the densities of fresh and saltwater, known as the Ghyben-Herzberg relation (Figure 3). In the Ghyben-Herzberg relation, the depth to the interface is ap- proximated as forty times the height of the fresh water table above sea level, given a

standard seawater density of 1.025 g/cm3 and static water conditions. In most circum- stances, the interface is pushed seaward by groundwater flow. Moreover, the interface is not defined by a sharp boundary but a diffuse zone of mixing up to a hundred meters wide, in which dispersion and diffusion introduce saltwater constituents to the freshwa- ter. The location and size of this brackish water zone is influenced by seasonally wax- ing and waning groundwater levels, as well as tidal fluctuations (Tibbott, 1992). Groundwater extraction is the primary cause of seawater intrusion (Tibbott, 1992). As fresh water resources are depleted, resulting in an accompanying reduc- tion in the height of the fresh water table, the saltwater interface begins an upward and inland encroachment (Kelly, 2005). Chloride is the ion most commonly used as an

4 indicator of seawater intrusion (Tibbott, 1992). Of the approximately 35,000 mg/L of dissolved solids in seawater, 19,000 mg/L is chloride. The US Environmental Protection Agency mandated public drinking water contain less than 250 mg/L of chloride (Tib- bott, 1992), but the threshold indicating current seawater intrusion is typically set lower, at 100 mg/L (Dion and Sumioka, 1984). Seawater intrusion has already been identified on many coastal Washington islands. In 1981, nine percent of 279 wells sampled in adjacent San Juan County were suspected of experiencing seawater intrusion (Whiteman, et al., 1983); all were within a mile of the coast. Nearly half of 185 well-water samples taken from Lopez Island in a 1997 study showed signs of seawater intrusion (Orr, 1997). Statewide investigations in the 1970s and 1980s identified areas in every Washington coastal county that rep- resented either regional and minor or local and severe seawater intrusion issues (e.g., Walter, 1971; Dion and Sumioka, 1984). Seawater intrusion has been indicated on at least some wells on Guemes Island in nearly every regional assessment (e.g., Walters, 1971; Dion and Sumioka, 1984). Of the 24 wells sampled by Kahle and Olsen (1995), 8 had in excess of 100 mg/L of chlo- ride, which is indicative of seawater intrusion. All were near the shoreline; areas affect- ed include North Beach, West Beach and Potlatch Village.

3.0 Background 3.1 Geologic setting. The bedrock of Guemes Island is primarily composed of intrusive igneous rocks of the Fidalgo ophiolite sequence (Lapen, 2000). The Fidalgo ophiolite sequence is a dismembered section of Jurassic to Cretaceous-aged oceanic lithosphere prevalent in the eastern San Juan Islands. Intrusive igneous rocks common to the Fidalgo ophiolite sequence include layered gabbro, gabbroic pegmatite, horn- blende gabbro and diorite; the color ranges from light to dark olive-grey. Exposed bed- rock near the southeastern tip of the island is a member of the Lummi Formation and is

5 composed of Jurassic to Cretaceous-aged lightly metamorphosed mudstones, sand- stones and conglomerates. Bedrock only crops out on and around Guemes Mountain in the southeast portion of the island; depth to bedrock for the bulk of the island is generally well in excess of 50 meters (Kahle and Olsen, 1995). The oldest, deepest recognized Quaternary-aged unit on Guemes Island is the Double Bluff Drift (Figure 4), a broadly inclusive unit varyingly defined as consisting of till, glaciomarine drift, glaciofluvial sand and gravel, glaciolacustrine silt (Kahle and Olsen, 1995), as well as till-like stony silt and clay (Easterbrook, 1968). The Double Bluff Drift is dated at between 100,000 and 250,000 years of age (Kahle and Olsen, 1995). It is commonly exposed at or very near sea level in coastal bluffs, and usually presents on Guemes Island as a fine-to-medium, well sorted, grey to tan sand. The Whidbey Formation, an assemblage of floodplain clay and silt deposited during an interglacial, overlays the Double Bluff Drift (Kahle and Olsen, 1995). Expo- sures of the Whidbey Formation are common in Guemes Island sea cliffs, identified by Easterbrook (1969) as the best exposures of the 90,000 to 100,000 year old unit. On Guemes Island, the Whidbey Formation presents as a fairly well-sorted silt and fine sand, 12 to 40 meters thick, with well-developed sedimentary structures. A significant disconformity separates the Whidbey Formation from the next youngest unit, the 18,000 year old advance outwash of the Vashon Stade of the Fraser Glaciation (Kahle and Olsen, 1995). A regionally prevalent unit, the Vashon advance outwash is a moderately to well-sorted sandy gravel, pebbly sand, medium to coarse sand, silt and clay unit with an overall upward-coarsening sequence and variable thickness ranging up to 80 meters (Lapen, 2000). On Guemes Island, the Vashon advance outwash varies in thickness from 12 to 30 meters (Kahle and Olsen, 1995). The Vashon till, a 13,000 to 18,000 year old compact clay, silt and gravel (Kahle and Olsen, 1995), uncomformably overlies the advance outwash, varying in thickness from

6 1 to 25 meters (Lapen, 2000). The Vashon till is the modal surface unit for Guemes Island (Kahle and Olsen, 1995). The next youngest unit is the Everson drift, a glaciomarine drift deposited ~13,000 years ago near the margins of the retreating Puget Lobe (Kahle and Olsen, 1995). Everson drift consists of a poorly sorted pebbly silt and clay, with some regional exposures presenting as a till-like diamicton of cobbles and clays (Easterbrook, 1968). The thickness of the unit typically ranges from 2 to 6 meters (Easterbrook, 1968), and it mostly occurs on Guemes in low-lying, nearshore environments (Kahle and Olsen, 1995). Modern deposition on Guemes Island consists of peat and beach deposits (Kah- le and Olsen, 1995). Peat, partially decomposed organic material, is being deposited in several poorly drained low-lying island locales. Beach deposits are mostly composed of sand and gravel recycled from nearby weathered sea bluffs, as well as some mate- rial transported by longshore drift. 3.2 Hydrogeologic setting. Kahle and Olsen (1995) identified two main aquifers on the island— the Double Bluff Drift and the Vashon advance outwash. Most wells not completed in one of the two aquifers are located on Guemes Mountain and draw from fractured bedrock, with generally small well yields, ranging up to 7 gallons/minute. The Vashon advance outwash aquifer consists of partly saturated sand and gravel, with a typical thickness of 12 to 30 meters (Kahle and Olsen, 1995). It is gener- ally unconfined and above sea level; springs have been known to occur where the unit is exposed in coastal bluffs. Using specific capacity tests for wells completed in this hydrogeologic unit, Kahle and Olsen (1995) estimated a median horizontal hydraulic

conductivity of 1.5 x 10-2 cm/s. The Whidbey Formation, consisting of floodplain clay, silt, fine-grained sand and peat, generally serves as an aquitard between the two aquifer units (Kahle and Olsen, 1995). It is generally poorly permeable, with an estimated horizontal hydraulic

7 conductivity of 5.6 x 10-4 cm/s, although there is evidence for a few productive sand lenses. It ranges in thickness from 12 to 40 meters. The Double Bluff Drift aquifer is the main groundwater source for the island; half of the inventoried wells studied in Kahle and Olsen (1995) draw water from this hydro- geologic unit. The unit consists of sand and gravel, and the aquifer is confined by the overlying Whidbey Formation. Situated below sea level, the thickness of the Double Bluff Drift on Guemes Island is poorly constrained, as most wells achieve sufficient water yield with only 3 to 5 meters of penetration into the unit. Estimates for thickness range from 30 meters (Kahle and Olsen, 1995) to nearly 300 meters (Paquette, 1997). Recharge potential is limited by precipitation. Guemes Island, like neighboring Whidbey Island, is in a temperate marine climate, with warm, dry summers and cool, wet winters (Simonds, 2002). Average annual precipitation is 64 centimeters, most of which occurs between November and February (Kahle and Olsen, 1995). The quantity of freshwater recharge to Guemes Island was estimated by Kahle and Olsen (1995) using deep percolation regression equations developed for coastal King County, Wash- ington. Recharge on Guemes Island is limited by both low annual precipitation and the presence of Vashon till, a relatively impermeable dominant surficial material. Kahle and Olsen (1995) used ground cover and precipitation estimates to produce a map estimat- ing the distribution of recharge over the island. Average total annual recharge to the is- land was estimated at about 15 centimeters, a small amount when viewed in a regional context. Some areas of the island were predicted to exceed this average by as much as 50%, but these were generally coarse-grained units in nearshore areas, where most excess water would rapidly discharge to the sea (as opposed to recharging deeper units). 3.3 Previous work. In the early 1990s, Kahle and Olsen (1995) conducted a study on Guemes Island, with the aim of describing and quantifying the groundwater system. Kahle and Olsen mapped and described the island stratigraphy, using the Coastal Zone

8 Atlas of Washington, USDA soil surveys and, primarily, lithologic well driller logs. Water samples from 24 wells were used to characterize groundwater quality and an approxi- mate water budget was estimated from precipitation, demographic and soil cover data. Their study remains the most in-depth of the island to date, but poor spatial accuracy for well head locations and a reliance on proxy data sets such as the USDA soil sur- vey may have introduced significant error to what they termed a “generalized map of suficial geology.” In addition, many of the regional units mapped by Kahle and Olsen exhibit significant small-scale local variation, potentially masked by the lack of carefully surveyed well head locations. Paquette (1997) developed a steady state, single phase, sharp interface model in MODFLOW for Guemes Island. The saltwater interface was modeled as a no-flow boundary, using pressure head measurements in the Double Bluff Drift and the Ghy- ben-Herzberg relation to determine its position. Paquette (1997) found that the northern portion of the island was at risk of seawater intrusion, but concluded that substantial development on the island as a whole could occur without adversely affecting Guemes Island groundwater quality. Limitations to this study included the static saltwater in- terface keyed to the Ghyben-Herzberg relation, the sharp nature of the interface and the large grid cells used to construct the model (around 160 meters x 160 meters). The version of MODFLOW employed is also now significantly out of date; density depen- dent groundwater flow models have become more robust in the last decade (Lin, et al., 2009).

4.0 Proposed research 4.1 Study area. This study will focus on the central, more sparsely populated ele- vated core of Guemes Island (Figure 5). A northeast-southwest trending, poorly drained lowland is expected to effectively hydrogeologically isolate the north peninsular portion of Guemes Island, providing a northern bound to our study area. A similar north-south

9 trending lowland isolates the hilly, surficial bedrock dominated east half of the island and provides the eastern bound. Both the Vashon advance outwash and the Double Bluff Drift aquifers are present at depth in the study area, with the latter employed by the majority of extant wells. 4.2 Primary objective. The primary objective is to construct a predictive, func- tional model of the groundwater conditions of the study area, with special attention to modeling potential seawater intrusion conditions. I will expand on the hydrostratigraphy presented in Kahle and Olsen (1995), pay- ing special attention to identifying locally important sub-units within the broadly identi- fied hydrogeologic units. I will determine the groundwater flow regime by constructing potentiometric surfaces from water level measurements. With the hydrostratigraphy and groundwater flow regime established, I can develop a conceptual model to charac- terize the study area and use that to develop a computer-based numerical model. This numerical model will be calibrated using available data, such as water quality param- eters established through ion analysis, until it closely mimics observed conditions. Finally, I will use the numerical model to simulate a variety of potential development scenarios to explore the potential long-term response of the aquifer system to chang- ing conditions.

5.0 Methods 5.1 Characterize the hydrostratigraphy. Using the Washington State Department of Ecology’s electronic well log catalog, I have designed and developed a Microsoft Ac- cess database of characteristics for over three hundred well logs from Guemes Island. I will use the well logs from this database to identify sub-units within the broadly identi- fied named units from Kahle and Olsen (1995) in constructing a conceptual model of the hydrostratigraphy of the study area. Identifying these sub-units is critical in an envi- ronment with units as heterogeneous as those on Guemes Island. For example, Kahle

10 and Olsen (1995) calculated hydraulic conductivities for the Double Bluff Drift ranging between 4.6 x 10-4 cm/s and 0.42 cm/s, with a median of 2.4 x 10-2 cm/s— significantly higher than would be expected for a unit defined by clay-rich glaciomarine drift (Mor- gan and Jones, 1995). For quantifying hydraulic conductivities for the subunits, I will use Kahle and Ol- sen (1995) as a base, and augment it as needed with new and existing aquifer test data for the area and specific capacity tests using available data from well logs. At least one new aquifer test will be performed on a well within the study area, preferably on a well terminating within the Double Bluff Drift. Drawdown and recovery results from pump tests performed on wells will be used with established protocols to make estimates of hydraulic properties for the aquifers. For example, the Cooper-Jacob straight-line method, a simplification of the Theis analytical solution for flow to a well in a confined aquifer, allows estimation of transmissivity using a linear fit to drawdown plotted with time plotted logarithmically (Halford and Kuniansky, 2002). Transmissivity, in turn, can be used with knowledge of the potentiometric surface to estimate hydraulic conductiv- ity. Given the concern over seawater intrusion, most existing well test data for the area is comparatively robust and easily accessible through both the Washington Department of Ecology well logs and Skagit County Public Works’ archive of filed hydrogeological assessments. 5.2 Determine the groundwater flow regime. Using Western Washington Uni- versity’s Trimble 5700 survey-grade GPS unit, I will precisely locate fifteen to twenty wellheads within the study area used to construct the hydrostratigraphies, with special deference paid to improving previously poor elevation data. Static water levels will be collected from the surveyed wells in April and October, representing respectively the low and high water months. These water levels will be used to construct potentiometric surfaces for the major aquifer systems. When used in conjunction with the hydraulic

11 conductivities indicated in the hydrostratigraphy, these potentiometric surfaces will al- low characterization of the groundwater flow regime. 5.3 Develop a conceptual and numerical model. The model will be constructed in the Groundwater Modeling System (GMS), a commercial software package that provides a graphical user interface front-end for a suite of groundwater flow model- ing schemes, including MODFLOW. Point hydrostratigraphies constructed at surveyed wellheads will be interpolated by the software capabilities in GMS into three-dimen- sional aquifers for the study area. Potentiometric surfaces will allow the inclusion of the groundwater flow regime in the conceptual model. The lowland hydrologic divides that define the northern and eastern edges of the study area provide natural, no-flow model boundaries. Recharge to the system, an external input into MODFLOW, will be quanti- fied by building on estimates from Kahle and Olsen (1995), augmented by precipitation data collected by a weather station installed on Guemes Island in support of this study. In general, any artificial model boundaries that need to be selected will be chosen in such a way as to minimize the effect of boundary conditions on the study area itself. After constructing the conceptual groundwater flow model in GMS as a MOD- FLOW dataset, the program SEAWAT will be applied to model groundwater flow, in- cluding the changing state of seawater intrusion. SEAWAT, which is capable of simulat- ing three-dimensional, variable-density transient groundwater flow, is a combination of algorithms drawn from both MODFLOW and MT3DMS, a three-dimensional model that simulates solute-transport (Guo and Langevin, 2002). Building on the modulariza- tion capabilities of the original MODFLOW code (Harbaugh, et al., 2000), the SEAWAT authors modified the Ground-Water Flow Process code central to MODFLOW to con- serve fluid mass, as opposed to fluid volume, treating the equivalent freshwater head as the principal dependent variable (Guo and Langevin, 2002). Cell-by-cell flow is then calculated from freshwater head gradients, which are in turn calculated from relative density-difference equations. This flow field is passed to the MT3DMS algorithms to

12 simulate solute transport, the results of which are used to generate new relative densi- ty-difference terms and an updated density field for MODFLOW. 5.4 Collect and interpret water quality samples. For quantitative analysis of model accuracy, I plan to twice collect and send well water samples from one dozen wells to Edge Analytical in Burlington, Washington. Sample containers and a cooler are provided by Edge Analytical. I will contact residents before sampling to ensure well pumps are inactive in the period prior to sampling, preferably for at least an hour. I will sample from a spigot situated before any filtration or disinfectant system, first flush- ing the sampling tap for at least five minutes. After filling sample containers, I will note the time of sampling on the container and then immediately place it on ice in the pro- vided cooler. Edge Analytical will analyze the samples for the eight ions (bicarbonate, calcium, carbonate, chloride, magnesium, potassium, sodium, sulfate) necessary to construct a Piper diagram (Fetter, 2000). Using Piper diagrams and the techniques ex- plicated in Kelly (2005), I will identify zones of seawater intrusion and their general trend (i.e., freshening, worsening). Using these results, I can help validate computational results from SEAWAT, ensuring the modeled geometry of the saltwater interface closely resembles the actual location of the interface in the study area. 5.5 Calibrate and validate the model. In the GMS environment, I will run multiple MODFLOW/SEAWAT simulations, systematically adjusting variables such as sub-unit hydraulic conductivity until the resultant model closely mimics the observed groundwa- ter flow patterns. As a test of model robustness, I will perform a sensitivity analysis, by system- atically varying model input parameters such as recharge or unit properties such as hydraulic conductivity to examine their influence on model computed values. Doing so will reveal which inputs have the most impact on resultant simulations and quantify the potential magnitude of error introduced by any uncertain parameters.

13 5.6 Perform simulations. Following model construction and calibration, I plan to construct several hypothetical models of future island development and examine their impact on the groundwater system in the study area. These models will include sce- narios such as increased water demand from an expanding population or decreased recharge due to a climate change induced reduction in precipitation. By examining how the modeled groundwater flow and saltwater interface respond to these changing con- ditions, I hope to provide a powerful tool to future water resource managers.

5.7 Timeline • Organize an Access database of well logs from Guemes Island (Summer 2008) • Identify target wells and contact owners to obtain access (Fall 2009 — Winter 2010) • Precisely locate wellheads using survey-grade GPS (Fall 2009 — Spring 2010) • Measure static water levels in April and October (Spring 2010 — Fall 2010) • Collect water samples (Spring — Fall 2010) • Perform aquifer test (Summer 2010) • Identify subunits and hydraulic conductivities from previous pump tests (Winter — Spring 2010) • Interpolate in GMS (Spring — Summer 2010) • Run model in SEAWAT and calibrate, sensitivity analysis, etc (Spring 2010 — Fall 2010)

14 6.0 Significance of proposed research In 1997, the US Environmental Protection Agency certified the Guemes Island aquifer system as a Sole Source Aquifer for the Guemes community, a designation intended to foster conservation 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, I 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.

15 7.0 References

Dion, 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. Fetter, C. W. (2001). Applied Hydrogeology: Prentice-Hall, Upper Saddle River, New Jersey. Guo, W., & Langevin, C. D. (2002). User’s Guide to SEAWAT: A Computer Program for Simulation of Three-Dimensional Variable-Density Ground-Water Flow, U.S. Geological Survey Techniques of Water-Resources Investigations 6-A7: U.S. Geological Survey. Halford, K.J., & Kuniansky, E.L. (2002). Documentation of Spreadsheets for the Analysis of Aquifer-Test and Slug-Test Data, Open-File Report 02-197: U.S. Geological Survey. Harbaugh, A. W. (2005). MODFLOW-2005, The U.S. Geological Survey Modular Ground-Water Model— the Ground-Water Flow Process, U.S. Geological Survey Techniques and Methods 6-A16: U.S. Geological Survey. Harbaugh, A. W., Banta, E. R., Hill, M. C., & McDonald, M. G. (2000). MODFLOW-2000, The U.S. Geological Survey Modular Ground-Water Model— user guide to modularization concepts and the ground-water water flow process, Open-File Report 00-92: U.S. Geological Survey. 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. Langevin, C. D., Thorne, D. T., Dausman, A. M., Sukop, M. C., & Guo, W. (2008). SEAWAT Version 4: A Computer Program for Simulation of Multi-Species Solute and Heat Transport, U.S. Geological Survey Techniques and Methods 6A-22: U.S. Geological Survey. 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. Lin, J., Snodsmith, J. B., Zheng, C., & Wu, J. (2009). A modeling study of seawater intrusion in Alabama Gulf Coast, USA: Environmental Geology, 57, 119-130. Morgan, D.S., & Jones, J.L. (1995). Numerical Model Analysis of the Effects of Ground- Water Withdrawl on Discharge to Streams and Springs in Small Basins Typical of the Puget Sound, Open-File Report 95-470: U.S. Geological Survey.

16 Orr, L. (2000). Is Seawater Intrusion Affecting Ground Water On Lopez Island, Washington?, USGS Fact Sheet 057-00: U.S. Geological Survey. Paquette, S. M. (1997). Use of a three-dimensional flow model to simulate the position and shape of a saltwater interface: Rice University. Russell, R. H. e. (1975). Geology and water resources of the San Juan Islands, San Juan County, Washington, Water Supply Bulletin No. 46: Washington Department of Ecology. Simonds, F. Williams. (2002). Simulation of Ground-Water Flow and Potential Contaminant Transport at Area 6 Landfill, Naval Air Station Whidbey Island, Island County, Washington, Water-Resources Investigations Report 01-4252: U.S. Geological Survey. Swanson, Therese (ed). (2001). Managing Washington’s Coast: Washington’s Coastal Zone Management Program, Ecology Publication 00-06-029: Washington Department of Ecology. Tibbott, E. B. (1992). Seawater Intrusion Control in Coastal Washington: Department of Ecology Policy and Practice: U.S. Environmental Protection Agency. Walters, K. L. (1971). Reconnaissance of sea-water intrusion along coastal Washington, 1966-68, Water-Supply Bulletin No. 32: Washington Department of Ecology. Whiteman, K. J., Molenaar, D., Bortleson, G. C., & Jacoby, J. M. (1983). Occurence, quality, and use of ground water in Orcas, San Juan, Lopez, and Shaw Islands, San Juan County, Washington, Water-Resources Investigations Report 83-4019: Ground-Water Conditions in San Juan County, Washington: U.S. Geological Survey.

17 Sinclair I.

Vendovi I.

Cypress I. Samish I.

Guemes I.

Lyman Sedro-Woolley Anacortes

Burlington Skagit River

Fidalgo I. Mount Vernon

La Conner

Whidbey I.

Figure 1. Skagit County, Washington, with Guemes Island highlighted.

18 Scale and Legend

1 kilometer 100 meter segments

Elevation of contour lines

0 meters 100 meters 200 meters

North

Figure 2. Guemes Island, Washington.

19 Figure 3. The Ghyben-Herzberg relation and groundwater flow in a marine island en- vironment. After Kelly, 2005.

20 Figure 4. Suficial geology of Guemes Island, Washington, with accompanying cross- section D-D’. From Kahle and Olsen, 1995. 21 Scale and Legend

1 kilometer 100 meter segments

Elevation of contour lines

0 meters 100 meters 200 meters

Study area North

Figure 5. Guemes Island, Washington, with study area indicated.

22