CLASTIC SEDIMENT FLUXES AS AN INTRINSIC PART OF THE FLOW SYSTEM IN KARSTIC AQUIFERS

Unique among aquifers, carbonate aquifers with well-developed conduit permeability transmit a flux of clastic sediment superimposed on the flux of water. Sediment is injected into the aquifer from sinking streams, from infiltration of surface soils, and from piping failures in sinkholes. The clastic sediment load, as observed in temporary storage in active stream caves, consists of sediment ranging from clay size particles to boulders. Sediment sizes and particle arrangements provide information on the transport mechanisms and flow regimes operating in the aquifer.

Coarse sediments move primarily as bedload except during extraordinary storm events, which may take entire sediment piles into suspension. Suspended loads contain very fine grained - colloid to clay size - particles, which remain in suspension even under base flow conditions. Sediment may be easily re-mobilized by storm flows when unconsolidated, but fine-grained compacted sediments require high boundary shears to take the material into suspension.

Because much of the conduit system acts as closed pipes, flow velocity and thus boundary shear varies widely, from minimal to substantial, with storm recharge. Sediment transport is episodic related to storm flow with a sequence of thresholds for suspended load, bedload of various particle sizes, and, at the most extreme, complete entrainment of sediment piles held in storage. To move the largest sediment observed in cave deposits, boundary shear stresses in excess of 400 Pa are required. Among groundwater systems, only karstic aquifers manifest adequate shear stress to move sediments of this size. In specific cave systems, sediment deposits can be used to back-calculate the minimum flows required to produce them. Calculations based on sediment transport theory and sediment examples from caves in West Virginia, Virginia, and Pennsylvania determine the minimum storm flow shear stresses required to mobilize sediment.