Publications—Scientific Investigation Report
By T.P. Schrader
U.S. Geological Survey Scientific Investigations Report 2006-5128
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During the spring of 2004, water levels were measured in
684 wells completed in the Mississippi River Valley alluvial
aquifer in eastern Arkansas. Ground-water levels are affected
by intense ground-water withdrawals resulting in extensive
potentiometric depressions. In 2004, the highest water-level
altitude measured was 293 feet above National Geodetic Vertical
Datum of 1929 in northeastern Clay County. The lowest
water-level altitude measured was 76 feet above National Geodetic
Vertical Datum of 1929 in the center of Arkansas County.
A large depression in the potentiometric surface was located in
Arkansas, Lonoke, and Prairie Counties during 1998 and persisted
to 2002. The area enclosed in the 100-foot contour in
Arkansas County in 2004 is about the same as in 2002, however,
the area enclosed in the 100-foot contour in Lonoke and
Prairie Counties in 2004 has receded. Two shallower cones of
depressions were located in Craighead, Cross, and Poinsett
Counties and St. Francis, Woodruff, Lee, and Monroe Counties
west of Crowleys Ridge during 1998. The 2004 potentiometricsurface
map shows that the areas enclosed by the 140-foot contour
have continued to expand.
A map of changes in water-level measurements between
2000 and 2004 was constructed using the difference between
water-level measurements from 625 wells reported in this
report and the 2000 Mississippi River Valley alluvial aquifer
report. Water-level changes between 2000 and 2004 ranged
from -31.1 feet to 16.3 feet, with a mean of -0.7 feet (negative
changes indicating water-level declines, positive changes indicating
water-level rises). The largest rise of 16.3 feet is in
Arkansas County and the largest decline of -31.1 feet is in Prairie
County.
Long-term water-level changes were calculated for 134
wells in the alluvial aquifer for the period from 1980 to 2004.
The mean annual decline in water level for the entire study area
was -0.31 feet per year with a range of -1.35 feet per year to 0.84
feet per year. The analysis of long-term water-level changes
(1980-2004) in the depression in Arkansas and Prairie Counties
shows the effects of the elongation of this depression.
Water samples were collected from 138 wells completed
in the alluvial aquifer and measured onsite for specific conductance
and temperature. Samples were collected at 71 wells for
dissolved chloride analysis at the U.S. Geological Survey
National Water Quality Laboratory. Specific conductance
ranged from 205 microsiemens per centimeter at 25 degrees
Celsius at a well in Lonoke County to 1,440 microsiemens per
centimeter at 25 degrees Celsius at a well in Monroe County.
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