Aquifer Recharge Study and Water-Budget Components of San Juan County, Washington
The Issue: San Juan County encompasses an archipelago within the Salish Sea between the southeastern end of Vancouver Island, British Columbia and Skagit and Whatcom Counties on mainland western Washington. Although the year-round population is generally small (approximately 16,000 in 2010), there is an ever-growing influx of visitors to the scenic islands especially during the summer months. An ongoing need in the management of the islands’ relatively scarce fresh water resources is an accounting of water-budget components and water-use. Current water-use estimates could aid in planning and management of water resources at current levels and inform management decisions related to potential changes to the natural systems due to increased use, drought, climate warming, and sea-level rise.
How USGS will help: An analysis of water budget components is an important next step in characterizing the availability of good-quality groundwater for island residents. The USGS will review and compile available hydrologic data, estimate groundwater recharge, compile or estimate water use information, and provide an updated water balance for San Juan County.
Problem: Most of San Juan County’s relatively limited fresh water resources are provided by local rainfall that feeds lakes and streams, recharges the groundwater system, and refills rainwater catchment systems. Smaller amounts of fresh water are provided by desalination systems. The water resources of San Juan County are constrained by the rain shadow created by the Olympic Mountains to the south and Vancouver Island to the west, by mostly steep terrain and bedrock geology, by small watershed catchment areas, and by extensive shoreline (San Juan County, 2004). These conditions result in low rainfall, limited groundwater recharge and storage, susceptibility to seawater intrusion of nearshore wells, and extensive runoff and discharge to the sea.
Objectives: This USGS study will build on work completed in the early 2000s (Orr and others, 2002) and help identify aquifer recharge areas, provide better estimates of rates of recharge to the groundwater system, and provide an updated water balance for the County.
Relevance and Benefits: This study will provide information about the aquifer recharge areas and water budget components of San Juan County and lay the groundwork for potential future studies to describe the groundwater flow system of the islands. The results of this study will be summarized in maps, tables, and text published online in a USGS Scientific Investigation Report (SIR). Additionally, a Data Release (DR) will be published in ScienceBase including selected hydrologic datasets.
Approach: To meet the objectives of this study, the following tasks will be completed: (1) evaluate available groundwater, surface water, and meteorological data; (2) complete recharge estimation using the USGS Soil-Water-Balance (SWB) model; and (3) update the water balance for San Juan County.
