Multilayer Sediment Dynamics in Fluviokarst Systems: Insights from Stable Isotopes and Sensor Measurements

Karst aquifers, supplying nearly 25% of the global population with drinking water, are critical yet vulnerable resources. While sediment transport modeling in karst systems has progressed, fluviokarst systems remain underexplored, particularly regarding multilayer sediment dynamics during diverse hydrologic events. This study introduces a novel framework integrating stable isotope analyses (δ¹³C, δ¹⁵N) with high-resolution sensor data to investigate sediment transport mechanisms in Kentucky’s mature fluviokarst systems, focusing on the Cane Run Watershed and Royal Spring Basin in central Kentucky, USA.

 

The multilayer framework hypothesizes that sediment stored at different depths within fluviokarst caves originates from distinct sources and undergoes unique transport processes. During large hydrologic events, a single active sediment layer forms, mobilizing material from sources in the fluvial system to the cave outlet via suspension. Smaller hydrologic events activate only the top sediment layer within the cave, with transport characterized by intermittent deposition and resuspension of surface sediments, fluctuating between suspension and saltation. Surface processes across the fluvial system exhibit a similar pattern: smaller hydrologic events mobilize loose surface material, while larger hydrologic events erode deeper soil layers through gully formation, transporting material into the karst cave systems.

 

Tracer-aided modeling, combining sediment fingerprinting and sediment continuity equations, revealed two primary transport types: (1) larger hydrologic events characterized by high sediment trap weights, lower soil organic carbon (SOC) and total nitrogen (TN) values, less negative δ¹³C, and more positive δ¹⁵N values; and (2) smaller hydrologic events dominated by surface-derived material with higher SOC and TN values, more negative δ¹³C, and less positive δ¹⁵N values. Isotopic trends highlighted interactions between cave-stored sediment and external sources, with high-discharge hydrologic events mixing bottom layers and low-discharge hydrologic events remobilizing deposited sediment.

 

Three temporal periods were identified, reflecting the stream’s response to in-stream reconstruction: (1) a pre-reconstruction period with low sediment loads, (2) a reconstruction period with elevated sediment loads from disturbances, and (3) a post-reconstruction recovery period with reduced sediment loads as the stream stabilized.

 

This research provides critical insights into sediment transport dynamics in fluviokarst systems, emphasizing the interplay between hydrologic variability, sediment sources, and anthropogenic impacts. It advances predictive capabilities for sediment behavior under future hydrologic and climatic conditions.