Hydrogeology

A simplified surficial geologic map (fig. 4) for the Skagit River Delta area was constructed based on previous mapping by Schuster (2000). Geologic units were grouped into one of five new unit designations (table 1) based on similarities in lithologic and hydrogeologic characteristics. The hydrogeologic characteristics attributed to these units are similar to those defined for similar units by other investigations in areas adjacent to this study (Thomas and others, 1997; Dragovich and Grisamer, 1998; GeoEngineers, 2003). Descriptions of the units used in this study are given below.

The alluvial and recessional outwash aquifer (Qago) is present throughout the low lying areas of the Skagit River Delta and represents: (1) active or abandoned channel and overbank deposits associated with the present and ancient Skagit River, (2) lahar-runout deposits, originating from Glacier Peak, and (3) terrestrial and marine recessional and deltaic glacial-outwash deposits. This unit comprises the shallow groundwater system in the study area, and consists of sand, gravel, and cobbles, with minor lenses of silt and clay. Groundwater in this 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.

The till confining unit (Qgt) is present within local upland areas and shows evidence of terrestrial and shallow marine depositional environments. This low-permeability unit is composed of glacial diamicton and consists of various proportions of clay, silt, sand, gravel, cobbles, and boulders, with locally occurring sand and gravel lenses capable of providing water for domestic use.

Exposures of advance outwash aquifer (Qga) deposits are limited to a few areas along the base of the glacial upland, and the presence of this unit is inferred, at least in places, beneath the till confining unit within upland areas. The unit consists primarily of sand and gravel with minor amounts of silt and scattered layers of pebble-cobble gravel and local silt and clay interbeds. Groundwater in this aquifer is potentially confined by the overlying till confining unit, however, unconfined conditions may occur locally where it is not fully saturated or exposed at land surface.

Generally, glacial deposits are believed to be largely absent beneath low lying areas of the Skagit River Delta to a depth of approximately 300 ft below sea level, likely due to removal by southward flowing subglacial meltwater, prior to subaerial exposure of the glacier bed during ice recession (Booth, 1994; Dragovich and others, 1994). This interpretation is supported by the absence of glacial deposits in wells from low lying areas of the delta, and similar interpretations from previous investigations (Dragovich and Grisamer, 1998; GeoEngineers, 2003). Glacial deposits in upland areas within the delta likely represent erosion remnants of a previously more continuous distribution of glacial units.

The sedimentary bedrock aquifer (OEc) crops out within and adjacent to uplands in the southern part of the study area; a lack of outcrop and well-log information makes it difficult to determine the extent of this unit in the subsurface. The unit consists primarily of pebble and cobble conglomerate and medium- to coarse-grained sandstone, with fine-grained intervals of mudstone, siltstone, coal, and shale. Groundwater in the bedrock aquifer is unconfined where it crops out; however, confined conditions are likely where it is fully saturated and overlain by glacial till and glaciolacustrine units. Fine-grained intervals within the sedimentary bedrock aquifer also may produce locally confined conditions.

The igneous and metamorphic confining unit (KJ) crops out within and adjacent to uplands in the southwestern part of the study area; a lack of outcrop and well-log information makes it difficult to determine the extent of this unit in the subsurface. This low-permeability unit is composed of igneous and metamorphic rocks and consists of a complex assemblage of volcaniclastic deposits, and low-grade metasediments, and metavolcanics. This unit is considered to be nonwater bearing except in localized areas of fracturing.