Summary and Conclusions

Recent population growth along the Interstate 5 corridor near Mount Vernon, Washington, has led to increased domestic water use, with many new wells serving residents in the lower part of the Skagit River basin. Planning for future development in the lower basin, including the reservation of water for new domestic wells requires identification of areas where withdrawals from existing and new wells could adversely impact streamflow in the Skagit River or its tributaries. A study to characterize shallow groundwater movement in an area between the lower Skagit River and Puget Sound was conducted by the U.S. Geological Survey to assist Skagit County and the Washington Department of Ecology with the identification of areas where withdrawals from existing and new wells could have adverse impact on streamflow in the Skagit River.

Land-surface altitude in the study area ranges from near sea level adjacent to the Swinomish Channel to about 130 ft in upland areas. The shallow groundwater system consists of alluvial, lahar runout, and recessional outwash deposits composed of sand, gravel, and cobbles, with minor lenses of silt and clay. The aquifer is unconfined where it is not fully saturated or exposed at land surface, however, confined conditions are likely where it is fully saturated and overlain by confining layers of clay. Upland areas are underlain by glacial till and outwash deposits that show evidence of terrestrial and shallow marine depositional environments. Bedrock exposures are limited to a few upland outcrops in the southwestern part of the study area, and consist of metamorphic, sedimentary, and igneous rocks.

Water levels were measured in 47 wells on a quarterly basis (August 2007, November 2007, February 2008, and May 2008), and measurements from 34 wells completed in the shallow groundwater system were used in the construction of groundwater-level maps and linear-regression analysis. Estimates of Skagit River stage were used to constrain groundwater-level contours along the eastern margin of the study area. Groundwater flow in the shallow groundwater system generally moves in a southwestward direction away from the Skagit River and towards the Swinomish Channel and Skagit Bay. Local groundwater flow towards the river was inferred during February 2008 in areas west and southwest of Mount Vernon. Water levels varied seasonally, however, generally ranged from less than 3 ft (August 2007) in the west to about 15 ft (May 2008) in the east. Concave and convex bends in the shape of water-level altitude contours indicate the presence of convergent and divergent groundwater flow patterns on a local scale, and likely reflect variations in hydraulic conditions within the aquifer resulting from spatial variations in aquifer material properties (clay, silt, and sand). The time-averaged shallow groundwater-flow direction derived from regression analysis (8.5° south of west) was similar to flow directions depicted on the quarterly water-level maps, with a gradient of 2.67 ft/mi,

Seasonal changes in groundwater levels in most of the wells in the Skagit River Delta follow a typical pattern for shallow wells in western Washington. Water levels rise in the fall and winter, when precipitation is high, and decline during the spring and summer, when precipitation is lower. Groundwater levels in wells along the eastern margin of the study area also are likely influenced by stage on the Skagit River. Water levels in these wells remained elevated through April, and did not begin to decline until the end of May, in response to declining river stage. Groundwater levels in a well equipped with a continuous water-level recorder exhibited periodic fluctuations that are characteristic of ocean tides (for example, component wave lengths of about 24 hours, and 12 hours and 25 minutes). The well (34N/03E-19F01) is less than a mile east of the Swinomish Channel (tidally influenced), and exhibited water-level fluctuations that correspond closely to predicted tidal extremes obtained from a tide gage near La Conner.