GEOLOGIC CONDITIONS AFFECTING GROUNDWATER AND ITS FLOW ON ST. GEORGE ISLAND, ALASKA

St. George Island, Pribilof Islands, Alaska, is a late Cenozoic (~2.2-1.6 Ma) volcanic center located on the southern edge of the Bering Sea shelf. The island is largely composed of layered basaltic lava flows and eroded cinder cones. Lava flows on St. George have been offset by major extensional faulting that generally steps down to the north from the ENE trending Ulakaia fault, which spans the island. The village of St. George, on the northern shore of the island, is located on an eroded and faulted cinder cone complex. The complex, which appears to have erupted from two separate vents, is primarily composed of scoria with intercalated lenses of lava and ash.

Groundwater on an oceanic island of uniform geology generally occurs as a freshwater lens above saline water, formed by the radial movement of freshwater moving toward the coast. Groundwater on St. George Island occurs as a relatively thin (~ 2.0') freshwater lens, primarily within basalts with varying lithological and hydraulic characteristics; the secondary porosity is generally high due to the layering of the flows, scoria interbeds and the presence of cooling fractures. In the northern portion of the island, groundwater is tidally-influenced; the mean hydraulic gradient was calculated using a filtering method which effectively removes all diurnal and semi-diurnal lunar and solar harmonics from consecutive hourly water level observations. Groundwater flow in the vicinity of St. George village is most likely controlled by the east-west trending normal faults which locally increase the hydraulic conductivities of the basalts and may provide preferential flow paths to the sea.