Geological Investigations and Hydrogeological Model of Fort Richardson, Alaska

• Main Authors: Hunter, Lewis E., Lawson, Daniel E., Bigl, Susan R., Astley, Beth N., Snyder, Colby F.
• Other Authors: ENGINEER RESEARCH AND DEVELOPMENT CENTER HANOVER NH COLD REGIONS RESEARCH AND ENGINEERING LAB
• Format: Text
• Language: English
• Published: 2000
• Subjects: Geology; Geochemistry and Mineralogy; Snow; Ice and Permafrost; Water Pollution and Control; *GROUND WATER; *GLACIAL GEOLOGY; ALASKA; ALLUVIUM; BOREHOLES; STRATIGRAPHY; AQUIFERS; GEOPHYSICS; SITE INVESTIGATIONS; POTENTIOMETRIC ANALYSIS; HYDROGEOLOGY; *GLACIAL STRATIGRAPHY; FORT RICHARDSON(ALASKA); ANCHORAGE(ALASKA) ELMENDORF MORAINE; QUARTERNARY GEOLOGY; MOUNTAIN VIEW FAN; Anchorage; Fan Mountain; Ice; permafrost; Alaska
• Online Access: http://www.dtic.mil/docs/citations/ADA383061; http://oai.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADA383061

The glacial stratigraphy of Fort Richardson reflects deposition in glacial and glacial-marine environments during multiple retreat phases following the last glacial maximum. A preliminary model relied heavily on the glacial history off the region, mapping by the U.S. Geological Survey, and limited borehole logs. This report expands on that model and describes new subsurface data obtained from field observations and descriptions of stratigraphic exposures and core samples from 28 new boreholes between 1997 and 1998. Geophysical techniques were applied to seven of the new boreholes and 25 existing monitoring wells, augmenting surface techniques (ground resistivity and ground penetrating radar). Beneath the cantonment is a thick unconfined aquifer, apparently deposited as a large alluvial fan (Mountain View fan), that overlies a fine-grained confining layer composed of mud and diamicton. The diamicton is a subglacial lodgement deposit bracketed by stratified debris flow deposits, being thickest to the southeast, dipping and thinning to the north and west where deposits of the Mountain View fan truncate the confining horizon, providing potential hydraulic communication between the tipper (unconfined) and lower (confined) aquifers. A second mud-diamicton horizon forms a deeper (38-66 m depth) confining layer and also appears to extend across the cantonment. Between the upper and lower confining diamicton horizons are coarse, sandy gravels that make up a confined aquifer. Ground water in the unconfined aquifer flows generally to the northwest, presumably recharged by Ship Creek. When ground water levels are low (i.e., winter), flow is locally diverted by irregularities in the surface of the upper confining layer. When recharge is high, regional flow is unaffected by these irregularities. Ground water in the confined aquifer also flows to the northwest, following the slope of the potentiometric surface.