Quantitative studies of suspended sediment in karst aquifers

Open Access
Author:
Herman, Ellen K
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
September 29, 2006
Committee Members:
  • Rudy L Slingerland, Committee Chair
  • William Blaine White, Committee Chair
  • Derek Elsworth, Committee Member
  • Kamini Singha, Committee Member
Keywords:
  • sediment
  • springs
  • karst
  • time series
Abstract:
Though sediment transport is an intrinsic part of the functioning of fluviokarst aquifers, this flux has not received the attention devoted to spring chemistry and discharge. The fluviokarst conduit system acts as a mixing chamber where sediments from multiple sources are sorted and rearranged. The storm recharge in the basin largely controls the sediment transport through the drainage to the spring. Continuous and event-based monitoring at five karst springs in Pennsylvania over the past four years offered insight into the basic functioning of storms and sediment transport in fluviokarst aquifers. The episodic nature of storm flows can make prediction of sediment transport and flow quite difficult. To this end, researchers have employed conceptual models based on both surface water and groundwater flow to describe flow response in karst. Frequently, karst is envisioned as a mid-point between surface water and groundwater resources or some mix of the two types. Analysis of the flow at multiple springs points to complications with employing this approach. Different karst springs fall at multiple locations on the surface to groundwater continuum, and the relationship between location on the continuum and specific physical properties is difficult to deduce. Understanding the functioning of storm flow in karst systems has direct impact on comprehending sediment transport. The composition of the fine-grained portion transported through the aquifer even during minor storms is dependent on source area, mixing within the aquifer, and transport mechanisms. Composition can vary substantially in composition from spring to spring. This portion can also reveal aspects of the flow behind the spring mouth that aren’t apparent from detailed chemistry and flow monitoring. Nolte Spring in Lancaster County, Pennsylvania, discharged a variety of calcite sediments in spite of being undersaturated with respect to calcite. This unique sediment discharge enabled deduction of processes not visible in the karst system, and pointed to the idea that sediment discharge at the spring mouth can reveal a complex signal of washed through, conduit-sourced, and stored sediment at any particular time. Additional observations at Arch Spring in Blair County, Pennsylvania confirmed this complex signal in sediment transport. Exceptional runoff events can lead to sediment transport where concentrations increase an order of magnitude, and thresholds required to mobilize stored sediment in the conduits are exceeded. Though such thresholds in flow and transport have been discussed in the literature for decades, very few have been captured through direct monitoring and no similar sediment transport event has been captured. This monitoring during large storm events also quantified a threshold in the flow system where any excess water beyond a certain level flows on the surface rather than through the conduit system. Though much additional work remains in the field of karst sediment transport, long-term continuous and event-based monitoring revealed these complicated aspects of the five karst aquifers. The nature of these behaviors would not have been uncovered through periodic site visits, and results presented here reinforce the necessity of long-term monitoring protocols in karst.