HYDROGEOLOGIC CONTROLS AFFECTING THE DIFFERENCES IN KARST SPRING DISCHARGES IN THE OZARKS OF ARKANSAS AND MISSOURI

The comparison of discharge of Ozark springs in Arkansas and Missouri yields stark differences. Although both regions have experienced intensive karstification, Missouri has ten first-magnitude springs (>2.83 m³/sec), whereas Arkansas has only one. Utilizing results from a soon-to-be-published book of Arkansas springs, this study assesses similarities and differences of hydrogeology of the two states, revealing that tectonics and lithology appear to have the greatest effect in smaller flows from Arkansas springs. Lithologically, Missouri has a thicker, more permeable sequence of karstified carbonate rocks, most which are dominantly dolomitic. These formations extend into Arkansas, but they are more deeply buried and farther removed from recharge and stream capture. Likewise, dolomitization is a process that creates a significant increase in porosity.

Tectonically, the closure of the Ouachita orogen produced far-field uplift of as much as 200 meters in northern Arkansas, resulting in fracturing and systematic jointing of carbonate/chert couplets that define smaller spring-basin boundaries in the Springfield Plateau Province of the Arkansas Ozarks. Coupled with the smaller spring-basin size, the lithologic occurrence of fine-grained sediments of the Boston Mountains Plateau Province (the southernmost part of the Arkansas Ozarks) provided a confining layer dominated by shale that sheds more of the precipitation to surface streams in Arkansas, minimizing infiltration to the groundwater that recharges springs there. So, too, do continuous, thicker chert sequences in the Springfield Plateau Province of both states. Missouri lacks these confining layers. Although the physiography of the multistate Ozarks is similar in some respects, the difference in specific hydrogeologic attributes of springs indicates a need to carefully consider the implications of lumping attributes when we assess groundwater on a larger, more regional scale.