Sediment Discharges from Proximal Urban and Rural Karst Basins during Storm Flow

During the past decade, studies of sediment sources and transport within fluviokarst basins have proliferated. Most of these studies have focused on timing of transport to springs during storm flow or on the composition and provenance of sediment emerging from springs. Fewer have addressed flow thresholds at which sediment is mobilized or basin-scale sediment yields. We examined each of these topics for a pair of spring basins (one mainly urban [Blue Hole], the other mainly rural [SP-2]) in the Inner Bluegrass region of central Kentucky. Suspended sediment from both springs was mainly quartz silt, with lesser amounts of calcite and organic matter. Bed sediment included these phases as well as dolomite, Ca phosphate, and Fe (oxyhydr)oxide. Traces of mica and feldspar in Blue Hole bed sediments may be artifacts of 19th-century grain milling. Initial specific conductance (SC) and turbidity peaks following storms reflected displacement of water and resuspension of sediment within karst conduits. Subsequent SC declines and secondary turbidity peaks were consistent with throughflow of runoff and allochthonous sediment. By aggregating results from four storms over 2 years, we found that total suspended sediment tended to vary with discharge (Q) and capacity approached 1 g/L at Blue Hole, but curve shapes varied among storms. For both springs, competence appeared to increase as Q became > ~ 0.5 m³/s for grain diameters > ~ 10 µm. Prior delineation of the Blue Hole basin boundary by qualitative dye tracing permitted estimates of area-normalized sediment fluxes for individual storms. These fluxes were ~ 2 orders of magnitude greater for storms in March (2002 and 2004) than in September (2002 and 2003). The overall range of fluxes (0.0092 to 5.93 kg/[ha-hr]) generally bracketed longer-term average fluxes reported for the Bluegrass region and other karst terrains in the USA and western Europe.