Island County

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9722-A4H - Ground-Water Recharge Characteristics of Island County, Washington - Completed FY2003

Problem - Island County consists of two major islands, Whidbey and Camano, and lies in northern Puget Sound, north of Seattle. Whidbey Island has an area of about 165 square miles and Camano Island an area of about 45 square miles, for a total area of about 210 square miles. Ground water is the primary source of water on the islands, and the ground-water system is fairly well understood, due in part to studies of Island County conducted in the 1980s by the U.S. Geological Survey and others. However, some questions still remain about the ground-water system of Island County, especially the quantity of recharge reaching the ground-water system from precipitation.

Objectives

Estimate the total amount and the areal distribution of recharge entering the ground-water system on each of the two islands.
Identify potential areas for using runoff to artificially recharge the ground-water system.

Relevance and Benefits - The proposed work provides an opportunity to assemble data that will increase the understanding of the hydrologic system of Island County. In addition, the data and interpretations associated with the study will aid county and state agencies in making planning and management decisions. Specific benefits to the USGS include collection of additional information regarding the hydrologic system of Island County, which, in turn, may be applicable to other islands in Puget Sound.

Approach

Ground-water recharge cannot be measured directly and is difficult to estimate accurately. The estimates can be subject to large errors; consequently, multiple methods should used to verify estimates and provide some idea of the error in them. Therefore, two methods will be used to estimate recharge:
- water-budget recharge model, utilizing meteorological and various recharge-related data, and
- chloride mass-balance method.
As part of the distribution of recharge determined using the recharge model, a distribution of runoff will be estimated. This distribution, in conjunction with the appropriate geology and soils data, will be used to identify areas where artificial recharge could be considered.

WA235 - Quantitative Evaluation of the Ground-Water Resources of Island County - Completed FY1987

Problem - Island County is experiencing rapid population growth. Significant Surface Water sources do not exist and thus most of the population is served by Ground Water in the islands. GW samples collected in a 1978 study indicated that seawater intrusion could become a problem in some areas. The population increased, resulting in increased water use, is causing concern among residents and county planners about the adequacy of the GW supplies and the potential for serious Seawater intrusion. Results from a preliminary evaluation of the GW indicates that at least 80 wells have pumping levels below sea level; 31 of 329 wells sampled have chloride conentrations near or above 250 mg/L; and a correlation exists between wells withumping levels below sea level and high chloride concentrations.

Objectives

In the 1980 fiscal year a preliminary study was undertaken to determine the problems facing the county. Seawater intrusion is a major concern and the objectives of this quantitative investigation are to (1) determine the availability of ground water in the county; (2) predict the hydrologic effects of increased withdrawal of ground water on water levels and water quality; and (3) prepare a report documenting the study.

Approach The project will take 3 years to complete. Data on wells drilled since the completion of last year's field work will be collected and evaluated. Mass measurements of water levels in representative wells in all aquifer units will be made; seasonal changes will be identified and quantified by periodic measurements in an observation well network. Test wells will be drilled to determine the thickness and lateral extent of aquifers where data from existing wells are inadequate. Existing data and data to be collected during early phases of this study will be used to numerically model the system using the U.S. Geological Survey multi-layer finite difference flow model. This model will be used to predict the effects that various water-resource development alternatives would have on the ground-water flow system and the effects on seawater intrusion.