Graduation Year

2010

Document Type

Thesis

Degree

M.S.

Degree Granting Department

Geology

Major Professor

Sarah Kruse, Ph.D.

Keywords

Collapse conduits, Hydrology, SP, Groundwater, Self-potential, Sinkholes

Abstract

For the protection of wetland and water resources it would be beneficial to understand when collapse conduits function as recharge points to the underlying aquifer. Inexpensive, noninvasive methods to detect recharge are desirable. Previous studies show negative self-potential (SP) anomalies over sinkholes that correspond to the expected electrokinetic effects of groundwater flowing downward through a conduit. SP surveys are less labor-intensive than high-resolution 3D GPR and resistivity, and continuous long-term monitoring is possible. However, before SP surveys can be reliable indicators of flow, SP contributions from ET, conductivity changes, redox reactions, thermoelectric effects, cultural noise, and lateral flow must be understood. A year of continuous SP monitoring was combined with high-resolution 3-D GPR surveys, and intermittent water table monitoring over two small covered-karst sinkholes in Tampa, Florida.

Positive and negative SP anomalies episodically manifested over conduits, suggesting that conduit flow is dynamic, not static. Three distinct SP flow regimes in the conduits are postulated: fast flow open to the aquifer, slow flow open to the confining layer through the collapse conduit walls, and a conduit, plugged high enough to behave like the rest of the confining layer. SP responses after rain events also appear to measure the effects of two moving Gaussian wetting front curves, one striking the monitoring electrode, one the reference. viii The wetting front volumes are differently dispersed by traveling different distances. By comparing curve shapes for all possible pairs of electrodes, it may be possible to establish surficial infiltration and flow patterns. Temporal SP response clearly shows SP is also affected by soil conductivity, rainfall history, and cultural noise. Ultimately, SP changes too frequently to rely on measurements many hours or days apart.

Over the course of the year, the electrodes became less responsive to rainfall and more erratic. Extremely wet and dry conditions seemed to affect responses. The porous faces of the electrodes or the bentonite clay gel used to enhance contact may decline. It appears a better design for electrodes and electrode contact needs to be developed. To test the intermittent behavior hypothesis, more conduits need to be studied, and moisture and SP must be studied concurrently. Several reference electrodes placed in various topographic, vegetative, geologic, and climatic settings could help distinguish groundwater flow from other SP sources. SP is a valuable research tool; however external complexities such as cultural noise, sinkhole lithology, and the state of the unsaturated zone make SP data difficult to interpret without ancillary information.

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