Reconnaissance of the Bedrock Aquifers and Groundwater Chemistry of Crawford, Franklin, and Sebastian Counties, Arkansas

A ground water reconnaissance of Crawford, Franklin, and Sebastian counties was performed utilizing 122 wells having drillers\u27 logs from bedrock aquifers. North of the Arkansas River, essentially all bedrock wells produce from the Atoka Formation. There are many low producing aquifers within the Atoka with a range in yield of 0.1 to 55 gpm, but having a median yield of only 2 gpm. Well depths range from 18 to 248 feet with a median of 122 feet. More water is generally obtained from the shale/siltstone aquifers than the sandstones due to more bedding-plane partings and more closely spaced fractures. Greater yields are also found in valleys. South of the Arkansas River, three additional bedrock aquifers are utilized. The aquifers and median yield are as follows: (1)Savannah Sandstone (11.7 gpm), (2) Hartshorne Sandstone (10 gpm), and (3) McAlester Shale (5.2 gpm). Well depths range from 40 to 300 feet. Seventeen wells in the Atoka were sampled and analyzed. The median iron concentration was 0.15, but four wells had over the 0.3 ppm health limit. Sulfate values ranged from31 to 125 ppm with a median of 45 ppm. Chloride concentrations ranged from 16 to 58 ppm with a median of 33 ppm. These relatively high values commonly give the water a bitter and strigent taste with some H₃S odor. The source of these ions may be from pyrite weathering or to contamination from the many gas fields in the area