Scientific Investigations Report 2007-5029
The U.S. Geological Survey in cooperation with the Arkansas Natural Resources Commission, the Arkansas Geological Commission, and the Louisiana Department of Transportation and Development has monitored water levels in the Sparta Sand of Claiborne Group and Memphis Sand of Claiborne Group since the 1920's. Ground-water withdrawals have increased while water levels have declined since monitoring was initiated. This report has been produced to describe ground-water levels in the aquifers in the Sparta Sand and Memphis Sand and provide information for the management of this valuable resource.
The 2005 potentiometric-surface map of the aquifers in the Sparta Sand and Memphis Sand was constructed using water-level data collected in 333 wells in Arkansas and 120 wells in Louisiana during the spring of 2005. The highest water-level altitude measured in Arkansas was 327 feet above National Geodetic Vertical Datum of 1929 located in Grant County in the outcrop at the western boundary of the study area; the lowest water-level altitude was 189 feet below National Geodetic Vertical Datum of 1929 in Union County. The highest water-level altitude measured in Louisiana was 246 feet above National Geodetic Vertical Datum of 1929 located in Bossier Parish in the outcrop area near the western boundary of the study area; the lowest water-level altitude was 226 feet below National Geodetic Vertical Datum of 1929 in central Ouachita Parish.
Three large depressions centered in Columbia, Jefferson, and Union Counties in Arkansas are the result of large withdrawals for industrial and public supplies. In Louisiana, three major pumping centers are in Ouachita, Jackson, and Lincoln Parishes. Water withdrawals from these major pumping centers primarily is used for industrial and public-supply purposes. Withdrawals from Ouachita and Lincoln Parishes and Union County, Arkansas, primarily for industrial purposes, have caused the resulting cones of depression to coalesce so that the -40 foot potentiometric contour encircles the three pumping centers.
Seven smaller depressions are evident on the 2005 Sparta-Memphis potentiometric-surface map located in Webster and Winn Parishes, Louisiana, and Calhoun, Cleveland, western Columbia, Desha, and Lafayette Counties, Arkansas. The depression in Calhoun County initially was shown in the 1996-1997 potentiometric surface. The depression in Desha County initially was shown in the 1999 potentiometric surface. The depressions in Webster and Winn Parishes were shown as early as 1975. The depressions in Cleveland, western Columbia, and Lafayette Counties initially were shown in the 2003 potentiometric surface.
A map of differences in water-level measurements between 2001 and 2005 was constructed using the difference between water-level measurements from 294 wells in Arkansas and 29 wells in Louisiana. The difference in water levels between 2001 and 2005 ranged from -30.1 to 44.6 feet. The largest rise of 44.6 feet in water level measured was in Union County in Arkansas. The largest decline of 30.1 feet in water level measured was in Columbia County in Arkansas. Areas with a general rise in water levels in Arkansas are shown in Arkansas, Columbia, Craighead, Jefferson, Prairie, and the western half of Union Counties. The area around west-central Union County had rises as much as 44.6 feet, with seven wells showing a rise of 20 feet or greater, which is an annual rise of 5 feet or greater. Areas in Arkansas with a general decline in water level are shown in western Bradley, eastern Calhoun, Cleveland, Cross, Desha, Drew, Lafayette, Lee, Lincoln, Lonoke, Poinsett, and the eastern half of Union Counties.
In Louisiana, the water-level difference map showed a general rise in water levels in northern Claiborne, northern Webster, and northwestern Union Parishes mainly because of a decrease in industrial withdrawals in southern Arkansas, particularly Union County. Another rise in water level was indicated in western Jackson Parish where industrial withdrawals have been reduced. The remainder of the study area in Louisiana showed a general decline in water level ranging from -1.5 feet in Jackson Parish to -6.1 feet in Union Parish.
Hydrographs were constructed for wells with a minimum of 25 years of water-level measurements. In Arkansas, the mean annual water-level rose only in Columbia, Lafayette, and Ouachita Counties. Mean annual declines occurred in Dallas, Grant, Phillips, Woodruff, Calhoun, Cleveland, Craighead, Cross, Desha, Drew, Jefferson, Lee, Union, Arkansas, Bradley, Lincoln, Lonoke, Poinsett, and Prairie Counties in Arkansas. In Louisiana, hydrographs were constructed using a minimum of 25 years of water-level measurements. At well Cl-149 in Claiborne Parish, a decline in water level is evident from 1981 until late 1999 at a rate of approximately -1.7 feet per year. Since 1999, a water-level increase of approximately 1.1 feet per year is evident at Cl-149, mainly because of a reduction of industrial withdrawals in Union County, Arkansas.
Water samples were collected from 61 wells in the spring of 2005 and measured onsite for specific conductance and temperature in Arkansas. Specific conductance ranged from 32.7 microsiemens per centimeter in Ouachita County to 1,356 microsiemens per centimeter in Lee County. The median specific conductance was 278 microsiemens per centimeter and the mean specific conductance was 362 microsiemens per centimeter. Along the western border of the Sparta-Memphis aquifer in Arkansas near the outcrop area, ground water has low specific conductance-generally less than 200 microsiemens per centimeter. Specific conductance increases to the east and south. The statistical distribution of specific conductance values in 2005 has changed from the distribution of specific conductance values in 2003.
Posted August 2007
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