Water levels and water quality in the Sparta-Memphis aquifer (middle Claiborne aquifer) in Arkansas, spring-summer 2011

Abstract

The U.S. Geological Survey, in cooperation with the Arkansas Natural Resources Commission and the Arkansas Geological Survey, has monitored water levels in the Sparta Sand of Claiborne Group and Memphis Sand of Claiborne Group (herein referred to as “the Sparta Sand” and “the Memphis Sand,” respectively) since the 1920s. Groundwater withdrawals have increased while water levels have declined since monitoring was initiated. Herein, aquifers in the Sparta Sand and Memphis Sand will be referred to as “the Sparta-Memphis aquifer” throughout Arkansas. During the spring of 2011, 291 water levels were measured in wells completed in the Sparta-Memphis aquifer and used to produce a regional potentiometric-surface map. During the summer of 2011, groundwater-quality samples were collected and measured from 61 wells for specific conductance, pH, and temperature. In the northern half of Arkansas, the regional direction of groundwater flow in the Sparta-Memphis aquifer is generally to the south-southeast and flows east and south in the southern half of Arkansas. The groundwater in the southern half of Arkansas flows away from the outcrop area except where affected by large depressions in the potentiometric surface. The highest and lowest water-level altitudes measured in the Sparta-Memphis aquifer were 326 feet above and 120 feet below National Geodetic Vertical Datum of 1929 (NGVD 29), respectively. Five depressions are located in the following counties: Arkansas, Cleveland, Jefferson, Lincoln, and Prairie; Union; Cross, Poinsett, St. Francis, and Woodruff; Columbia; and Bradley. Two large depressions, centered in Jefferson and Union Counties, are the result of large withdrawals for industrial, irrigation, or public supply. The depression centered in Jefferson County has expanded in recent years into Arkansas and Prairie Counties as a result of large withdrawals for irrigation and public supply. The lowest water-level altitude measured in this depression is approximately 20 feet (ft) higher in 2011 than in 2009. The area enclosed within the 40-ft contour on the 2011 potentiometric-surface map has decreased in area, shifting north in Lincoln County and west in Arkansas County when compared with the 2009 potentiometric-surface map. The depression in Union County is roughly circular within the -60-ft contour. The lowest water-level altitude measurement was 157 ft below NGVD 29 in 2009, with a 37-ft rise to 120 ft below NGVD 29 in 2011. The depression in Union County has diminished and encloses a smaller area than in recent years. In 1993, the -60-ft contour enclosed 632 square miles (mi 2 ). In 2011, the -60-ft contour enclosed 375 mi 2 , a decrease of 41 percent from 1993. The lowest water-level altitude measurement during 2011 in the center of the depression in Union County represents a rise of 79 ft since 2003. The area enclosed by the lowest altitude contour, 120 ft below NGVD 29, on the 2011 potentiometric-surface map is less than 10 percent of the area enclosed by that same contour on the 2009 potentiometric-surface map. A broad depression in western Poinsett and Cross Counties was first shown in the 1995 potentiometric-surface map. In 2011, the lowest water-level altitude measurement in this depression, 129 ft above NGVD 29, is 2 ft lower than in 2009. The 140-ft contour has extended southwest into northwestern St. Francis and east-central Woodruff Counties in 2011. In Columbia County in 2011, the area of the depression has decreased, with water levels rising about 1 ft since 2005 in the well with the lowest water-level altitude measurement. The depression in Bradley County in 2011 has decreased in area compared to 2007. A water-level difference map was constructed using the difference between water-level measurements made during 2007 and 2011 at 247 wells. The differences in water level between 2007 and 2011 ranged from -17.3 to 45.4 ft, with a mean of 4.1 ft. Water levels generally declined in the northern half of the study area and generally increased in the southern half of the study area. Areas with a general decline in water levels include Lonoke and western Prairie Counties; northern Arkansas County; Miller County; and Craighead, Poinsett, Cross, and Woodruff Counties. Areas with a general rise in water levels include Lafayette, Columbia, Union, Calhoun, and Bradley Counties; Grant, Jefferson, southern Arkansas, Lincoln, Drew, and Desha Counties; and Phillips County. Hydrographs from 183 wells with a minimum of 25 years of water-level measurements were constructed. During the period 1987–2011, county mean annual water levels generally declined. Mean annual declines were between 0.5 foot per year (ft/yr) and 0.0 ft/yr in Ashley, Chicot, Crittenden, Drew, Grant, Jefferson, Lafayette, Mississippi, Monroe, Ouachita, Phillips, Pulaski, St. Francis, and Woodruff Counties. Mean annual declines were between 1.0 ft/yr and 0.5 ft/yr in Bradley, Calhoun, Cleveland, Craighead, Cross, Desha, Lonoke, Miller, Poinsett, and Prairie Counties. Mean annual declines were between 1.5 ft/yr and 1.0 ft/yr in Arkansas, Lee, and Lincoln Counties. The county mean annual water level rose in Columbia, Dallas, and Union Counties about 0.3 ft/yr, 0.1 ft/yr, and 1.2 ft/yr, respectively. Water samples were collected in the summer of 2011 from 61 wells completed in the Sparta-Memphis aquifer and measured onsite for specific conductance, temperature, and pH. Although there is a regional increase in specific conductance to the east and south, anomalous increases occur in some parts of the study area. Specific conductance ranged from 35 microsiemens per centimeter (μS/cm) in Ouachita County to 1,380 μS/cm in Monroe County. Relatively large specific conductance values (greater than 700 mS/cm) occur in samples from wells in Arkansas, Ashley, Clay, Monroe, Phillips, and Union Counties.