A conceptual and digital model of the aquifer, with estimates of the aquifer's capability to provide water for rice irrigation through the year 2000Open-File Report Prepared in cooperation with the ARKANSAS GEOLOGICAL COMMISSION Little Rock, Arkansas

The study area comprises about 3,200 square miles of the Mississippi Alluvial Plain in southeast Arkansas. About 90 percent of the area drains south to the Ouachita River in Louisiana.The alluvial aquifer and the streams are hydraulically connected and are studied as an aquifer-stream system. Bayou Bartholomew is a principal stream of the system. The aquifer is underlain by confining strata of the J ackson Group and Cockfield Formation. The m ean annual surface-water yield of the area that drains to the Ouachita River basin is nearly 2 million acre-feet. Floodcontrol projects have significantly reduced flooding in the area. Basin boundaries and low-flow characteristics of streams have been altered as a result of the flood-control projects and streamflow diversion for irrigation. The direction of ground-water flow generally is southward. Bayou Bartholomew functions mostly as a drain for ground-water flow from the west and as a recharge source to the aquifer east of the bayou. As a result of navigation pools, the Arkansas River is mostly a steady-recharge source to the aquifer. Pumpage from the aquifer and streams increased from about 20,000 acre-feet in 1941 to 237,000 acre-feet in 1970. Estimates of flow, derived from analog analysis but lacking field verification, indicate that recharge to the aquifer in 1970 was about 161,000 acre-feet. About 70 percent of the recharge was by capture from streams as a result of ground-water pumpage. Discharge from the aquifer was about 233,000 acre-feet. About 80 percent of the discharge was through wells. Stream diversion in 1970 from capture and open channel, excluding capture from the Arkansas and Mississippi Rivers, was about 110,000 acre-feet. Return flow to streams from rice irrigation and fishponds was about 60,000 acre-feet. The chemical quality of streamflows is excellent for irrigation. Water from the aquifer generally ranges from permissible to excellent for irrigation. The use of water from the aquifer in the flood-plain area, exclusive of irrigation, is severely limited unless it is treated to remove the iron and reduce the hardness.

No abstract

The El Dorado aquifer (lower part of the Sparta Sand) is the basal freshwater unit in Union County and the major source for municipal and industrial water supplies. Since 1960, chloride concentration in water from the aquifer has increased in the vicinity of El Dorado, the center of greatest withdrawal. The aquifer is approximately 300 feet thick and is confined by the Cane River Formation below and the middle confining unit of the Sparta Sand above. These Tertiary rocks dip gently east-southeast in Union County and are affected locally by a graben to the south and east of El Dorado. From the onset of development in the 1920's the center of greatest withdrawal from the El Dorado aquifer has remained at El Dorado. Pumpage has increased from less than 0.5 million gallons per day in 1921 to 16 million gallons per day in 1982, reaching a high of 19 million gallons per day in 1965. In response to the withdrawal, the potentiometric surface at El Dorado has been lowered more than 300 feet, increasing the hydraulic gradient toward the center of pumping. Chloride concentrations generally increase in the El Dorado aquifer to the east-southeast, conforming to the dip of the rocks and the original direction of water movement. North and west of the graben, chloride concentrations range from 25 to 150 milligrams per liter. Wells with the highest concentrations are located at or near the west end of the graben. Estimates based on interpretation of electric logs for two wells in the graben indicate chloride equivalents may be as high as 2,500 milligrams per liter in the El Dorado aquifer. Analyses of water from aquifers above the El Dorado aquifer preclude surface brines as a source of this contamination. Saltwater-bearing formations below the El Dorado aquifer are eliminated as sources of contamination on the basis of geocheraical analyses and hydrostatic pressures. Geologic, hydrologic, and chemical data indicate that the primary source of chloride is the El Dorado aquifer within the graben.

Chemical analyses of ground‐water samples taken from differing depths in a four‐county area in east Texas showed a general stratification with respect to dissolved iron, pH and hardness. On the basis of this stratification the waters of the ground‐water reservoir were divided into a shallow zone of oxidation, A; a deep zone of reduction, C; and an intermediate and unstable zone, B, in which waters from above and below are mixed. Ground water from zones A and C generally is almost free of iron, whereas water from zone B generally contains objectionable amounts. This zoning is explained by recent laboratory work and theoretical data on iron in natural waters. Wells constructed to draw only from zones A or C should yield water relatively free of iron.