Department of Geology
Eastern Washington University

Recent Masters theses in Geology

ASSESSMENT OF AQUIFER VULNERABILITY INFLUENCED BY CLIMATIC DIFFERENCES IN
SOUTHEAST, SOUTHCENTRAL, AND NORTHERN ALASKA

Michael G. Alcorn

Abstract

Climate and soil characteristics critically influence aquifer contamination
potential at Federal Aviation Administration facilities at Juneau
International Airport in southeast Alaska, Anchorage Air Route Traffic
Control Center in southcentral Alaska, and Deadhorse Airport in northern
Alaska.

Because of high soil infiltration rates (3 cm/min) the Anchorage aquifer is
considered the most vulnerable to surface spill contamination.  Soil pits
dug revealed about 80 cm of silty to very rocky or sandy soil overlying
highly permeable (5.7 cm/min) unconsolidated sediments.  Wells showed the
water table is 9 m below ground surface.  Large surface spills would
infiltrate through the soil and unconsolidated sediments to reach the water
table of the 19 m thick, unconfined sand and gravel aquifer in about 2
hours, 50 minutes.  Ground water flows due west at about 90 m/year towards
the Knik Arm of the Cook Inlet.  Water wells located down gradient would be
contaminated in an estimated 3 to 5 years after a large surface spill.  No
known contamination problems exist, however, immediately north of the site
on Elmendorf Air Force Base, monitoring wells have detected low levels of
fuels, solvents, and metals coming from a closed landfill.  Future
contamination potential at the site is moderate due to the existence of
several above ground fuel storage tanks.

The Juneau aquifer is the least vulnerable because low soil infiltration
rates (0 - 0.48 cm/min) impedes infiltration and will cause spills to flow
mostly as surface runoff to streams and the ocean.  The coastal location
and surface- and ground-water flow directions limit down-gradient
contamination potential to undeveloped intertidal zones and coastal waters.
Soil pits revealed very silty soils of varying depths over sandy gravel
outwash.  Although the land beneath the airport runway and its associated
buildings has been raised with undifferentiated fill material, aquifer
contamination potential remains low because the low permeability of the
buried soil.  Monitoring wells showed the water table about 1 to 5 m below
ground surface.  Ground water flows southwest towards the Gastineau Channel
in the 60 m thick unconfined aquifer.  High precipitation rates of 1,400
mm/year will help to dilute and disperse contaminant spills.  The dominant
maritime climate here produces mild temperatures that promote natural
attenuation of contaminants by microbial biodegradation.  Petroleum
hydrocarbons and low concentrations of polychlorinated biphenyls have been
detected in soil samples, but water samples have been uncontaminated.
Spill damage could be minimized by isolating the spill by damming surface
flow and promptly recovering the product.  Future site spill potential is
high due to aircraft fuel storage tanks and the operation, maintenance, and
storage of aircraft.

The Deadhorse aquifer is considered a special case because of complete
seasonal freezing due to the arctic climate.  Although soil infiltration
rates (0.1 to 6.0 cm/min) fall between those of Anchorage and Juneau, the
unique permafrost environment increases aquifer vulnerability because any
remediation efforts would be destructive.  Soil pits revealed a thick
vegetative mat, 10 to 20/ cm thick, overlying silty soil which extends
below the permafrost at about 5 m below ground surface.  Permafrost acts as
a near-surface confining layer that will restrict contaminant migration.
Natural attenuation of contaminants by microorganisms is impeded by cold
temperatures.  During the short summer when the upper 0.5 m of soil is
thawed, a surface spill in unsaturated areas may contaminate the soil down
to the frost table, while spills in saturated areas may float on the water
surface.  Disturbance or removal of the vegetative mat during cleanup
operations may result in new freeze-thaw features.  Buildings and roads are
built on raised gravel pads (1.5 to 2.5 m thick), designed to insulate the
soiland keep it perpetually frozen.  Liquids spilled on the gravel pads
can be expected to infiltrate through the gravel and accumulate on the
frost table.  The site has no known contamination problems.  Future spill
potential is moderate due to the limited operation of aircraft and fuel
storage.