Kayti Didricksen

Abstract

Increasing development and demand for water resources has led to concern about the impact of additional stresses on the ground water and surface water resources. This report describes the ground water system in the unconsolidated sediments of the Swinomish Indian Reservation in northwestern Washington.

The 10.5 square-mile study area is underlain by sediments as much as 900 feet thick, deposited during multiple glacial and interglacial periods. Drillers' well logs for about 200 wells were the primary source of information used to define the hydrogeologic framework. The preparation of cross-sections helped define the hydrogeologic units (Vashon till/Everson glaciomarine semi-confining layer, Vashon advance outwash aquifer, Olympia confining bed, a "sea level" aquifer, and undifferentiated glacial and non-glacial aquifers and confining beds). Maps depicting the extent and the thickness of the units also were prepared.

A field inventory visited 36 wells to measure water levels and collect samples for water analysis. Instrumentation was installed in six wells to provide long-term water level monitoring of the major aquifers. Overall, recent water levels are similar to historic water levels, suggesting that the ground water storage has not changed significantly and that quantities of recharge and discharge are at equilibrium. Water level and potentiometric surface maps were based on measurements during the field inventory and historic water levels for selected wells. Horizontal hydraulic conductivity was determined from specific capacity data for about 100 wells in the study area.

Long-term average annual recharge was estimated using regression equations developed by the U.S. Geological Survey in similar hydrogeologic conditions in the southern Puget Lowland. A range of recharge values was determined, depicting various percentages of coniferous and deciduous forest cover. The calculated average annual recharge is 9.9 to 12.1 inches per year. Discharge from the ground water system occurs as withdrawals from wells, flow to streams, seeps and springs and diffuse subsurface flow to the saltwater bodies surrounding the study area. Total ground water use is about 106,000 gallons per day, while more than 172,000 gpd of surface water is imported into the study area. Baseflow in three perennial streams was derived from hydrograph separation and totals about 195 gallons per minute. These calculations indicate that about 80 to 90 percent of measured stream flow is derived from ground water system discharge.

Twenty-four water samples were collected from wells during the field inventory and analyzed for common inorganic constituents and the presence of coliform bacteria. Water in the study area generally meets water quality standards. As is common in the Puget Sound area, several of the wells had elevated concentrations of iron and manganese. Twenty-five percent of the sample tested positive for the presence of coliform bacteria. Sampled chloride concentrations were low, indicating that seawater intrusion is not a problem on the Reservation.