Sediment transport investigation in a karst aquifer hypothesizes controls on internal versus external sediment origin and saturation impact on hysteresis

Abstract

Karst springs serve as major drinking water sources for the global population, yet there remains uncertainty regarding how contaminants like sediment are routed through karst. We find knowledge gaps pertaining to the processes that control the timing and magnitude of sediment delivery. To close these knowledge gaps, we: (1) collected high-frequency sediment and conductivity data from three surface streams and two cave sites in the inner bluegrass region of Kentucky, USA; (2) analyzed sediment hysteresis for 51 storm events over a two year period; (3) numerically modeled sediment transport through the aquifer; and (4) contextualized our results with a meta-analysis of ten karst springs in Europe and North America. Our results showed that 80% of storm events derived their sediment from external inputs through sinking streams while the remaining 20% were characterized by only internal resuspension of previously deposited cave sediment. We found that internal sediment transport was proportional to fluid shear stress in the cave and occurred exclusively during fully saturated aquifer conditions prior to the storm event. During partially-saturated aquifer conditions, externally sourced sediment pulses were efficiently routed to the spring irrespective of shear in the conduit network. Sediment hysteresis in surface streams was clockwise whereas the cave sites experienced counter-clockwise behavior. Clockwise and counter-clockwise hysteresis typically refer to proximal and distal sourcing for streams, but our findings suggest that this interpretation is not valid for subsurface karst settings. Hysteresis patterns for karst show dependence on the antecedent water conditions in the aquifer and internal versus external sourcing. meta-analysis results show that, despite high variability, counter-clockwise patterns of sediment hysteresis are very common for karst, which contrasts sediment hysteresis in surface streams. Results suggest that external sediment loading rather than internal resuspension is the most common control of sediment load at springs, which should inform management of karst groundwater resources.