RESEARCH· ·1970·

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grain size and is greatest in fine-grained materials,
where it mu.y be as much as 6 feet.
GROUND-WATER CONDinONS NE.AR PRAI'RIE
POTHOLES
SLOAN
Some investigators of prairie-pothole hydrology
have assumed that potholes are insulated, or perched,
bodies o:f surface water separated from a regional
wn.ter table by an unsn.turn.ted zone of impervious
strata. "!"'his assumption greatly simplifies wn.ter-budget
studies becn.use it neglects the influence of groundwater
inflow or outflow. The assumption is based
hugely on the following observn.tions :
1. 'Vater levels in potholes and nearby observation
wells are usually different, .frequently as much as 1
foot or more. The difference has been interpreted as
proof that there is no hydraulic connection between the
water in the pothole ttnd the water in the surrounding
glacinJ drift. Such differences are characteristic of
glacial till and result from the low permeability of the
tillttnd the large hydraulic gradients necessary to move
ground wnter through it.
2. ~1any test holes drilled in poorly permeable glacial .
til] nppear to be dry. In such instances, water does not
fill the drill hole immediately, and till samples obtnined
during drilling seem to be dry. Saturated gravel,
sand, or silt in glacial till yields water to drill holes
readily and thoroughly wets the drilling samples.
'~Tater release from till is comparatively slow, however,
and drill holes that do not penetrate any stratified
materials require n much longer time before water in
the drill hole reaches equilibrium with the surrounding
ground water. In these dry holes the weight of the
drilling equipment can physically squeeze water out
of the samples, and the frictional heat generated by the
drilling operation can vaporize the water in the drilling
s11mples nnd drive· it off. Hot steaming samples are
commonly observed when drilling in glacial till, and
the resulting samples, although saturated in place,
appear dry and crumbly rather than wet and sticky
when they are brought to the surface.
3. Water levels in closely spaced wells frequently
differ, particularly if the wells are cased to different
depths. Cased wells in till usually act as piezometers
such that the water level represents the fluid pressure
at the bottom, or screened part, of the casing. Large
verticn.l fluid-pressure gradients are common in glacial
till becnuse of its low per1neability. If the water levels
in such wells are misinterpreted as points on the water
tnble, there would seem to b~ a series of perched water
tnbles. From this erroneous conclusion, it is logical to
assume that the potholes are also perched.
4. Closely spaced potholes are frequently separated
B229
by a large vertical distance and a steep water-table
gradient, occasionally as high as 25 percent. Such high
water-table gradients are inconsistent with normal
water-table gradients that have been described in unconfined
aquifers. They are consistent, however, with
the low permeability conditions that obtain in the
glacial till surrounding the potholes.
All of the aforementioned conditions, seemingly
anomalous, are typical of ground-water conditions surrounding
prairie potholes, primarily because of the low
permeability of glacial till. These conditions, atypical
of aquifer systems, have been construed to indicate
that ground-water flow systems are unrelated to
prairie-pothole hydrology.
RESULTS OF INVE'STIGATION
Twenty-eight observation wells were drilled during
October 1965 in the ~1t. ~1oriah area in sec. 21, T. 144
N., R. 67 vV., Stutsman County, N.Dak., to determine
the position of the water table between potholes. There
are 104 prairie potholes in sec. 21, and they have a
total area of about 58 acres. The potholes range in size
from 0.01 to 7.1 acres and have a total shoreline of
about 9.6 miles.
The ~1t. ~1oriah area is on the eastern slope or
escarpment of the Coteau du ~1issouri, a poorly drained
slope that is transitional between the undrained Coteau
surface of high-relief stagnation moraine to the west,
and well-drained, low-relief ground moraine of the
drift prairie to the east. The altitude ranges from 1,880
feet above mean sea level in the north western part of
the section to 1,705 feet in the northeastern part, a
slope of 175 feet to the mile, or 3.3 percent. Local slopes
up to 25 percent are common in the area.
The test holes that were angered in the Mt. Moriah
area ranged from 14 to 67 feet in depth, and averaged
about 24 feet. Some stratified material, including thin
stringers of sand, silt, or gravel, was found in 21 of
the 28 test holes. Glacial till was found in all the holes,
and was overlain directly by the soil zone in all but
three of the test holes. The glacial till in the Mt.
~1oriah area consists of a heterogeneous mixture of
unconsolidated gravel, sand, silt, and clay, that is well
jointed and oxidized near the surface. Both jointing
and oxidation tend to diminish with depth. A buried
oxidized zone was found in one hole at a depth of 43
feet. Only four test holes were sufficiently· deep to
penetrate the surficial oxidized zone, which in these
holes ranged in depth from 27 to 45 feet. A calcareous
marl containing many small fossil gastropods and
pelecypods was found at a depth of about 12 feet in
one hole.
Ground water attained equilibrium in all but five of