San Juan County Completed

9722-CZN - Estimates of average monthly ground-water recharge, precipitation, and runoff on Lopez, San Juan, Orcas, and Shaw Islands, San Juan County, Washington - Completed FY2007

Problem - Due to water shortages on Lopez, San Juan, Orcas, and Shaw Islands, San Juan County has allowed a number of residences to use rainwater as an alternative water source. As an outgrowth of the Washington State watershed planning process (HB2514 for short), local stakeholders, San Juan County, and the Washington State Department of Ecology (Ecology) are considering issuing one comprehensive surface-water right to the County under which the County could re-issue individual rainwater permits. As a result, Ecology is in the process of evaluating what restrictions should be applied to such a water right to prevent impairment of existing water rights if the new water right were fully exercised. Specifically, Ecology is evaluating possible spatial and temporal impacts on ground-water recharge and surface-water runoff due to rainwater use on an island-by-island basis. To conduct its analysis, Ecology needs spatially distributed, average monthly water-budget information for each island.

Objectives - The objective of the proposed study is to estimate spatially distributed, average monthly recharge, runoff, and precipitation for Lopez, San Juan, Orcas, and Shaw Islands.

Relevance and Benefits - This study is consistent with the national USGS mission and goals and water-resources issues identified in the USGS Washington Water Science Center Science Plan. Specifically, the study addresses the following issues in "Strategic Directions of the Water Resources Division 1999-2008": effects of urbanization and suburbanization on water resources (issue 1); effects of land use and population increases on water resources in the coastal zone (issue 2); drinking water availability and quality (issue 3); and surface-water and ground-water interactions as related to water-resources management (issue 8). The study will estimate spatially distributed, selected average monthly water-budget components on Lopez, San Juan, Orcas, and Shaw Islands that can be used to help evaluate the impact on water resources of rainwater use as an alternate water source.

Approach - The objective will be met by rerunning the Deep Percolation Model (DPM), a near-surface water-balance model used to compute spatially distributed average annual recharge for Lopez, San Juan, Orcas, and Shaw Islands by Orr and others (2002), for water years 1997-98 and computing average monthly recharge, runoff, and precipitation for the same period and spatial resolution. Modeling results will be published in GIS coverages that will be documented with metadata and published on the USGS Washington Water Science Center Web site and at Geospatial One Stop (GOS), a National GIS clearing house.

WA399 - Assessment of Seawater Intrusion into the Aquifers of the San Juan Islands, San Juan County, Washington - Completed FY2002

Problem - Aquifers of the San Juan Islands, which are the principal source of fresh water, are commonly intruded by seawater at near-shoreline locations (less than one mile from the shore). Because the demand for ground water has escalated in recent years due to population growth and is expected to continue, the progression of seawater intrusion and areas most susceptible to future seawater intrusion must be identified for proper management of the island's water resources. Additionally, moderately high chloride concentrations occur at some inland locations of Lopez Island, but it is not known if this is due to seawater intrusion. Ground-water recharge is limited due to low precipitation (20 to 30 inches per year) and poorly permeable surficial materials.

Objectives - The goals of this study are to describe and interpret occurrences of chloride in selected wells on Lopez Island as related to possible seawater intrusion, and to make preliminary estimates of ground-water recharge on the four islands Lopez, San Juan, Shaw, and Orcas.

Relevance and Benefits - Ground water is the sole source of drinking water for all the San Juan Islands. It is, therefore, important to understand the amount of available fresh water and the balance between ground-water withdrawal and possible induced seawater intrusion. This study will provide local governments with information they can use to manage their limited ground-water resource. The chloride data from this study will be used to assess and monitor seawater intrusion. Estimates of the magnitude and seasonal distribution of ground-water recharge from this study will be valuable in managing water resources. The data and interpretive information from this study will advance the understanding of island ground-water flow systems and will improve strategies for protecting drinking-water sources in coastal zones.

Approach - The two main approaches being used are: 1) Collect water samples and analyze for chloride concentrations from previously sampled and newly inventoried wells on Lopez Island to assess the changes in ground-water reservoir intrusion since previous investigations conducted 15 and 18 years ago, and 2) estimate recharge to ground water from precipitation on the four islands Lopez, San Juan, Shaw, and Orcas by analysis of the near-surface water balance.