is affected by precipitation and spring runoff, may
Subpermafrost and
produce local differences in hydraulic gradient
deep flow patterns
and therefore direction of flow north of the Chena
The hydraulic gradient of the subpermafrost
River (Fig. 22). The subregional aquifer of the
aquifer may be determined by either the subre-
Tanana River Valley, however, is probably respon-
gional aquifer of the Tanana Valley or by more
sible for the prevailing hydraulic gradient and
local aquifers, such as in Birch Hill (Fig. 23). A
overall flow pattern, particularly in the sub-
west to northwest flow will prevail if gradients
permafrost aquifer. Because the flux from both
are controlled by the Tanana system, whereas con-
aquifers varies with the season, flow direction and
trol by the Birch Hill aquifer will create a south-
velocity in the subpermafrost and suprapermafrost
erly component. In both instances, the thickness,
aquifers also vary with the seasons. In the follow-
configuration, and extent of permafrost above the
ing sections, we define hypothetical ground
aquifer will constrain flow, while it may recharge
water flow patterns, assuming that either an aqui-
or discharge into unfrozen zones. If an aquifer of
fer in Birch Hill or the Tanana Valley determines
sufficient potential exists in the bedrock, it too
hydraulic gradients in the north-central canton-
could modify or locally control flow in the
ment area.
subpermafrost aquifer. The extent of a bedrock
aquifer is limited, however, as it is restricted to
fracture zones (Cederstrom 1963).
Suprapermafrost aquifers and
near-surface flow patterns
Figure 22 shows the potential pathways for
Discussion
near-surface ground water movement and the
Data from in-situ sensors in the north-central
possible interactions with permafrost. The near-
cantonment area support the concept of an aqui-
surface ground water system is located above the
fer in Birch Hill that influences flow direction and
permafrost, where the seasonal thaw is sufficient
velocity seasonally, but they also suggest that the
to develop a mostly continuous suprapermafrost
overall pattern of flow is subregionally controlled.
aquifer. Where the depth to permafrost is less than
The subregional aquifer's gradient and flux vary
1.0 m, an aquifer probably does not develop. In
seasonally as well and, therefore, it interacts with
contrast, there is likely a significant supra-
the more local influx of ground water, particu-
permafrost aquifer beneath the landfill and in
larly from Birch Hill. For example, the lower
other deeply thawed areas beneath surface dis-
water table and hydraulic gradient of the Tanana
turbances. These aquifers communicate with
Valley aquifer in fall and winter result in a more
deeper aquifers through unfrozen and deeply
westerly flow than during early and late summer
thawed areas of former swales and channels.
peaks in water level and flux, when a northerly
Near-surface ground water may flow beneath
flow dominates (Fig. 22). The shift in flow vectors
River and Ski Hill Roads, for example, and some
from west to north through late spring is evident
of the older trails that have been in existence for
in the measurements across the area (Fig. 23, Law-
over 40 years (Fig. 5). Many of these supra-
son et al. 1996).
permafrost aquifers intercept the unfrozen zone
Whenever the flux of water through fracture
adjacent to the Chena River.
and thaw zones originating in Birch Hill is suffi-
The direction of near-surface and supra-
ciently large (this is quantitatively unknown), the
permafrost flow depends upon whether 1) sur-
distribution of permafrost dictates that the supra-
face water migrates through the landfill and
and subpermafrost aquifers will have a southerly
recharges the supra- and subpermafrost aquifers
flow. The resultant vector of flow of the sub-
near it, 2) Birch Hill runoff from snowmelt or pre-
permafrost aquifer in the area will depend largely
cipitation is high enough to create a southward-
on the subregional flux and gradient. This sce-
directed gradient, or 3) the flux of water (and
nario of interacting aquifers is consistent with the
hydraulic gradient) in the Tanana Valley alluvial
vectors for the deeper (more than 30 m) flow mea-
aquifer is higher than more local sources. In
surements (Lawson et al. 1997, in prep., App. A).
addition, water tables fluctuate as the stage of
Seasonally, there are periods of little or no flow
the Chena River fluctuates; this will locally affect
in isolated, unfrozen deposits surrounded by per-
flow patterns and movement of water through
mafrost and in some deeply thawed zones above
the suprapermafrost aquifers. The interplay and
the permafrost. Just south of Birch Hill, for
feedback of these factors result in a strong sea-
example, potential water sources for unfrozen
sonal variability to ground water movement.
zones are limited to surface infiltration and adja-
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