AGS / Water / Ground Water / Aquifers
Much of Arkansas’ ground-water comes from Quaternary deposits of sand and gravel in the Mississippi River Embayment. Irrigation wells, with depths ranging from 100 to 200 feet, commonly produce 1,000 to 2,000 gallons per minute (GPM). Although usable for irrigation and some domestic uses, the high iron content of Quaternary aquifers makes the water generally unsuitable for human consumption in many areas. The deeper aquifers which underlie the majority of the Gulf Coastal Plain supply most domestic and municipal needs in this part of the State.
The Cockfield Formation of Eocene age crops out in south-central Arkansas. Southeast from its outcrop belt in Chicot and Desha Counties, the Cockfielsd is the only source of serviceable ground-water for communities in this part of the state. Locally, the formation ranges from 300-400 feet in depth and well yields range from 10-300 GPM.
Below the Cockfield are the very extensive sands of the Sparta/Memphis aquifer in the middle part of the Claiborne Group (also Eocene in age). The Sparta is used in southern Arkansas and Memphis in the northeastern Arkansas. The top of this major aquifer typically occurs at depths of 200-600 feet, and in some areas as deep as 1,000 feet. Individual wells may produce up to 1,200 GPM. Most communities in southern and eastern Arkansas use this aquifer as their source of supply. Other deeper units that are also used for public supplies include the Carrizo and Wilcox (Eocene) and the Nacatoch (Cretaceous) however these deeper formations may often contain saline water and therefore the area of usefulness is limited.
In the Interior Highlands of western and northern Arkansas ground-water supplies are more limited than in the Coastal Plain. Much of the Ozark Plateaus region is underlain by carbonate rocks, which are quite soluble in the presence of water. Solution by ground water has caused many large openings through which water passes so quickly that contaminants from the surface can not be filtered out. Signs of these openings are caves, sink holes, springs and lost stream segments. As a consequence, the water in shallow wells may not be suitable for human consumption without treatment. However, there are two important aquifers at greater depth – the Roubidoux Formation and the Gunter Member of the Gasconade Formation. Both are permeable sandstone and carbonate units of Ordovician age. These aquifers serve as the principal source of high-quality water for many communities in northern Arkansas. These formations do nor outcrop anywhere in Arkansas but instead outcrop in southern Missouri.
Because of the predominance of shale in both the surface and subsurface rocks in the Arkansas Valley and Ouachita Mountains regions, and the low porosity of many of the interbedded sandstones, few rock units qualify as aquifers. Because most wells yield less than 10 GPM, most communities rely on surface-water supplies. Among the more favorable units in the Ouachita Mountains are the Hot Springs Sandstone Member of the Stanley Shale (Mississippian), the Arkansas Novaculite (Mississippian-Devonian), and the Bigfork Chert (Ordovician). Lacking appreciable intragranular porosity, most of the available water in these units is confined to fractures caused by the mountain building processes in the Ouachita Mountains these same processes help produced quartz veins that are a possible source of at least domestic supplies in some areas. The only consistent source of ground-water in this region is the alluvium along the Arkansas River. The river derived sand and gravel deposits produce mostly water of irrigation but has and still has limited use as a community water source.