Abstract

Eocene-age sand beds near the base of the Cockfield Formation of Claiborne Group constitute the aquifer known locally as the Cockfield aquifer. Upper-Paleocene age sand beds within the lower parts of the Wilcox Group constitute the aquifer known locally as the Wilcox aquifer. In 2005, reported water withdrawals from the Cockfield aquifer in Arkansas totaled 16.1 million gallons per day, while reported water withdrawals from the Wilcox aquifer in Arkansas totaled 27.0 million gallons per day. Major withdrawals from these units were for industrial and public water supplies with lesser but locally important withdrawals for commercial, domestic, and agricultural uses.

During February 2009, 56 water-level measurements were made in wells completed in the Cockfield aquifer and 57 water-level measurements were made in wells completed in the Wilcox aquifer. The results from the 2009 water-level measurements are presented in potentiometric-surface maps and in combination with previous water-level measurements.

Trends in water-level change over time within the two aquifers are investigated using water-level difference maps and well hydrographs. Water-level difference maps were constructed for each aquifer using the difference between depth to water measurements made in 2003 to 2009. Well hydrographs for each aquifer were constructed for wells with 20 or more years of historical water-level data. The hydrographs were evaluated individually using linear regression to calculate the annual rise or decline in water levels, and by aggregating the regression results by county and statistically summarizing for the range, mean, and median water-level change in each county.

The 2009 potentiometric surface of the Cockfield aquifer map indicates the regional direction of groundwater flow generally towards the east and southeast, except in two areas of intense groundwater withdrawals that have developed into cones of depression. The lowest water-level altitude measured was 43 feet and the highest water-level altitude measured was 351 feet.

A water-level difference map was constructed from 54 wells completed in the Cockfield aquifer within Arkansas. The largest rise in water level was 14.9 feet and the largest decline was 27.4 feet. Seven wells had a rise in water level, and the remaining 47 wells had a decline in water level.

Hydrographs for 33 wells completed in the Cockfield aquifer were developed. Hydrographs indicate water-level changes in individual wells ranged from rises of 0.33 feet per year to declines of 1.21 feet per year over the 20-year period (1990-2009). County summaries of the linear regression analysis indicate Cleveland and Columbia Counties have mean annual rises. Arkansas, Ashley, Bradley, Calhoun, Chicot, Desha, Drew, Lincoln, and Union Counties have mean annual declines.

The potentiometric surface for the Wilcox aquifer is presented using two maps, one for a southern area and another for a northeastern area, because of the absence of water-level data in the central part of the State. The direction of groundwater flow in the southern area is generally the east, except around two cones of depression and around two mounds of elevated water levels. Water-level altitudes in the southern area range from 147 feet to 400 feet. The direction of groundwater flow in the northeastern area is generally to the south and southeast except in an area of intense groundwater withdrawals that has altered the flow to a westerly direction.

Two water-level difference maps were constructed using water-level altitudes measured in 2003 to 2009 from 53 wells completed in the Wilcox aquifer within southern and northeastern Arkansas. In the southern area the largest rise in water level was 16.0 feet and the largest decline was 17.7 feet. Eight wells in the southern area had rising water levels and the remaining five wells had declining water levels. In the northeastern area, the largest rise in water level was 1.3 feet and the largest decline was 21.7 feet. Four wells in the northeastern area had rising water levels, 35 wells had declining water levels, and the remaining well had no change in water level.

Hydrographs for 42 wells completed in the Wilcox aquifer were developed to show water-level altitude variation over the past 20 years. Hydrographs for the southern area indicate water-level changes in individual wells ranged from rises of 0.44 feet per year to declines of 1.68 feet per year over the 20-year period. County summaries of the linear regression analysis for the southern area indicate that Hot Spring County had a mean annual rise; all other counties had mean annual declines. Hydrographs for the northeastern area indicate water-level changes in individual wells ranged from rises of 0.51 feet per year to declines of 2.16 feet per year over the 20-year period. County summaries of the linear regression analysis for the northeastern area indicate that Greene County had a mean annual rise; all other counties had mean annual declines.

First posted April 19, 2010