SEDIMENT DISCHARGE VARIATIONS IN MULTIPLE PENNSYLVANIA KARST SYSTEMS

Karst aquifers are unique in their ability to transmit and store sediment. Though sediments in these systems are increasingly investigated as pollutants in springs or as paleoclimate indicators in cave systems, they are less frequently used as a tool for interpreting the hydraulic settings in the inaccessible aquifer behind the spring. Storm flow input to these systems alters flow behavior and shifts sediment input. As a result, transport of sediment to the spring mouth also changes. By examining multiple shifts in sediment transport under baseflow and stormflow conditions at several Pennsylvania springs, we discover hydraulic features of the aquifer, such as aquifer flushing and storm signal attenuation, not evident in simple monitoring of stage-discharge data.

Overall, sediment concentrations are low in less mature karst systems and systems with more impervious surface in the recharge area, and concentrations are higher in more mature karst systems. Although sediment concentrations vary most after storm events, sediment concentrations fluctuate even during baseflow. During small storms, sediment concentration generally increases little, and sediment is mostly angular and poorly sorted suggesting short flow paths for transport. During rare large flow events, sediment concentration can increase substantially, but for both categories of storms, there is variation in response for similar sized storms. This points to different flow paths in the aquifer, in particular for sequential storms.

This monitoring also reveals the complicated nature of the sediment signal seen at the spring mouth. The sources of clastic sediments that travel to the spring include sinking stream input, soil wash-down from the epikarst, plug injection by sinkhole piping failures, and residual insoluble material from limestone dissolution. The conduit system acts as a mixing chamber where the injected materials, which are frequently mineralogically similar, are sorted and rearranged. Through high-resolution monitoring of multiple factors, the composite signal can sometimes be pulled apart to differentiate sediment derived from storage and from other sources. Overall, these results present another facet of the complex karst aquifer with episodic high intensity movement of sediment and water resulting from storm events.