Seawater Intrusion Modeling on Guemes Island, Washington

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 freshwater 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 approximated 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 freshwater. 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 chloride, 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 bedrock 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).

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