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Clark, Cleveland, and Dallas counties constitute an area of 2,151 square miles in south-central Arkansas. The area is in two physiographic provinces--the Ouachita Mountains of the Ouachita province of the Interior Highlands, and the West Gulf Coastal Plain of the Coastal Plain province. The area is drained by the Ouachita, Saline, and Little Missouri Rivers and their tributaries. Although some of the streams in the project area can furnish dependable water supplies without storage, the amount of water available for use can be increased by the construction of reservoirs. The average surface-water yield in the area is about 1.4 cubic feet per second per square mile, or a total of about 3,000 cubic feet per second. Generally, the water quality is good; but water from some of the streams, particularly from the smaller tributaries, may require treatment for excessive iron content and high color.
Ground-water yields in the project area vary considerably. The consolidated rocks in the Interior Highlands generally yield less than 10 gallons per minute to wells, precluding the development of large municipal or industrial groundwater supplies in that area.
Of the 17 geologic units present in the Coastal Plain part of the project area, 12 yield water but in varying amounts. Among the formations of Cretaceous age, the Tokio yields good-quality water in the outcrop, but the quality deteriorates downdip; the Brownstown Marl yields small amounts of water for domestic purposes, mainly in the outcrop area ; the Ozan Formation yields a highly mineralized water that is generally unsuitable for most purposes; the Nacatoch Sand yields as much as 100 gallons per minute of good-quality water in and near the outcrop, but the water becomes very salty and corrosive at distances ranging from 2 miles downdip from the outcrop in northern Clark County to 17 miles downdip in the southern part of the county.
The formations of Tertiary age offer the best possibilities for ground water, particularly in Dallas and Cleveland Counties. The Wilcox Group contains no thick widespread sands but contains thin sands locally. The quality of the water tends to deteriorate downdip, as the water becomes more mineralized and changes from a bicarbonate to a chloride type. The Carrizo Sand is undeveloped but may yield several gallons of water per minute per foot of drawdown in a large part of these two counties. High iron content may be a problem in water from the Carrizo. The Cane River Formation yields 50 gallons per minute of good-quality water to each of two wells at Sparkman. Elsewhere, high iron content of the water may be a problem. The Sparta Sand is the best aquifier in the project area, particularly east of central Dallas County. Well yields of 700 gallons per minute or more are possible. With minor treatment, the water is suitable for most purposes. The Cockfield Formation is utilized mainly for domestic supplies, but where the sands are thick, yields of as much as 300 gallons per minute are possible. The Jackson Group is utilized mainly for domestic supplies. In some areas, water from this unit contains such a high concentration of sulfate that it is unpalatable.
The deposits of Quaternary age are thin and generally suitable only for domestic supplies. However, several wells that yield more than 200 gallons per minute have been developed in the alluvium south of Arkadelphia. Transmissibility values are highly variable, and test drilling is advisable to determine if large amounts of water are available at any specific site.
Total water use in the project area in 1965 was about 6 million gallons per day, an increase of about 0.6 million gallons per day since 1960). Slightly more than one-half this amount was derived from surface-water sources. Total water use in the area in 1967 was insignificant compared with the total water available. DeGray Reservoir, now under construction on the Caddo River, will provide 250 million gallons per day for water
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Water resources of Clark, Cleveland, and Dallas Counties, Arkansas