somewhere below Fort Richardson. Confining
Fort Richardson hydrogeology
The hydrogeology of Fort Richardson has been
layers appear to exist at the north and southeast-
briefly summarized in numerous engineering
ern sections of the cantonment, but well AP-3591
reports, but detailed studies are restricted largely
found no confining layer in the center of the can-
to Freethy (1976), Anderson (1977), and USACE
tonment, where the confined and unconfined
(1996b). Despite these studies, the ground water
aquifers appear to converge (USACE 1996b).
conditions below Fort Richardson remain poorly
Sand and gravel of the unconfined and con-
known and require further detailed investiga-
fined aquifers are exceptionally permeable and
tions. Detailed subsurface information is generally
hydraulically conductive. Recharge studies were
lacking for the glacial sediments that range from
conducted by temporarily diverting the flow of
70 to 97 m thick below the cantonment (Ceder-
Ship Creek into storage basins on Fort Richard-
son. The recharge rate was measured at 1.5 104
strom et al. 1964).
m3/day, while a second test in coarser gravel
The shallow, near-surface aquifer occurs in the
achieved a recharge rate of 5.3 104 m3/day
Mountain View fan sediments (Fig. 10 and 12).
The water table lies at a depth of 3 to 13 m in sedi-
(Anderson 1977, Updike et al. 1984). Anderson
ments that are approximately 18.3 m thick in the
(1977) calculated a permeability of 68.6 m/day,
main cantonment (Cederstrom et al. 1964, USACE
while Freethy (1976) proposed a much higher
value of 720 m2/day for transmissivity. The
1996b). The water table dips to almost 37 m bgs
ground water mounded during the second test,
(below ground surface) at AP-3462, beyond the
equivalent to 9.4 and 4.9 m in the unconfined and
apparent limit of the confining layer where the
confined aquifers, respectively (Anderson 1977);
confined and unconfined aquifers converge (Fig.
however, water levels dropped rapidly once the
14 and 15a). The confining layer may be the Dish-
artificial recharge was shut off.
no Pond or Fort Richardson diamicton or possi-
a. Top of unconfined aquifer in May 1995. (After USACE 1996b.)
Figure 15. Ground water maps. (Contours in feet, as originally measured. To convert to
meters, multiply by 0.3048.)
21
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