CONCEPTUAL MODEL OF TOPOGRAPHICAL- AND RECHARGE-DRIVEN POTENTIOMETRIC SURFACES IN THE SPRINGFIELD PLATEAU AQUIFER, NORTHERN ARKANSAS, USA

Karst aquifer systems are globally important for drinking water, irrigation, industrial use, and rare habitat. Understanding potentiometric change in these systems is critical to understanding near- and long-term groundwater availability. The Springfield Plateau aquifer (SPA) is a karst aquifer systems in northern Arkansas which comprises limestones to cherty limestones overlain by a mantled epikarst. The aquifer is characterized by both diffuse (i.e. matrix porosity and small-aperture fractures) and conduit flow regimes (i.e. large aperture fractures, dissolution conduits). Previous work has shown a significant correlation (R² = 0.98) between land-surface elevation (LSD) and groundwater levels (GWL) in the SPA. This correlation and its similarities to previous conceptual models describing potentiometric surfaces have not previously been investigated. Here we present a conceptual model based on Haitjema and Mitchell-Bruker (2001) whereby LSD and GWL correlation result from a relatively high ratio of recharge, R, to hydraulic conductivity, k. The ratio results from diffuse (i.e. low k), but steady recharge (i.e. relatively high R) from waters draining from the overlying epikarst. This observation does not preclude the importance of interbasin flow previously documented in the SPA, but better explain behavior of the potentiometric surfaces. Paradoxically, one must resolve having a potentiometric surface mimicking topography, but also have groundwater movement “crossing” topographic divides. We infer that while interbasin flow is present, its occurrence is most prominent during and shortly thereafter rain events when rapid, local recharge increases water levels causing mounding. This mounding occurs in high k feature (i.e. large fractures, sink holes) areas where the overlying epikarst is negligible and water may directly enter the aquifer. Groundwater mounding gives rise to transient, recharge driven potentiometric surfaces that do not mimic topography. These findings underscore the importance of long-term, high spatiotemporal resolution groundwater level monitoring. While identifying areas of interbasin flow are important, identifying those areas of high k features (e.g. sink holes, fractures) may be equally important where groundwater mounding is likely to occur.