Estimates of ground-water recharge from precipitation to glacial-deposit and bedrock aquifers on Lopez, San Juan, Orcas, and Shaw islands, San Juan County, Washington

Abstract

An important source of fresh water on Lopez, San Juan, Orcas, and Shaw Islands in San Juan County off the northwestern coast of Washington is glacial-deposit and bedrock aquifers. Two methods were used to estimate recharge from precipitation to the water tables on the islands. A daily near-surface water-balance method, the Deep Percolation Model (DPM), was used to simulate water budgets for the period October 1, 1996, through September 30, 1998 (water years 1997-98) for six small drainage basins?three on Lopez Island and one each on San Juan, Orcas, and Shaw Islands. The calibrated soil and subsoil parameters from the DPM for each small basin were then used in island-wide applications of the DPM where the direct runoff component (which is not available on an island-wide basis) was simulated, rather than input, and calibration was not required. A spatial distribution of annual recharge was simulated for each island, with island averages of: Lopez Island, 2.49 inches per year; San Juan Island, 1.99 inches per year; Orcas Island, 1.46 inches per year; and Shaw Island, 1.44 inches per year.A chloride mass-balance method that requires measurements of atmospheric chloride deposition, precipitation, streamflow, and chloride concentrations in ground water was used to estimate recharge to the glacial-deposit aquifers of Lopez Island. Only average recharge could be estimated using this method rather than area-specific recharge. Average recharge for Lopez Island estimated by this method was only 0.63 inch per year. The range of chloride concentrations in ground-water samples from selected wells indicates that the average recharge in areas of glacial deposits is between 0.29 and 1.95 inches per year. Recharge simulated using the DPM for two drainage basins on Lopez Island overlain by glacial deposits are 2.76 and 2.64 inches per year. Sources of chloride in ground water other than from the atmosphere would cause the recharge estimated by the chloride mass-balance method to be less than the actual recharge, therefore these estimates may represent lower limits which are, at least, consistent with the higher simulated recharge from the DPM. The average island-wide recharge is most closely related to the amount of area overlain by glacial deposits. Thus, even though Lopez Island receives the least precipitation, it has the most recharge per square mile because it proportionally has the largest area overlain by glacial deposits. Recharge simulated by the DPM for areas of shallow to outcropping bedrock generally were less than 1.5 inches per year, but recharge simulated in areas of glacial deposits ranged from less than 0.5 to 3 inches per year, with recharge as high as 9 inches per year in some small areas.