Date of Award
Master of Science in Geology
Anne I. Veeger
Quantitative description of ground-water flow in fractured-rock aquifers is difficult because flow may be non-Darcian and hydrologic parameters are scale dependent. Due to its relative efficiency, the application of a continuum, or equivalent porous medium (EPM) approach to describing flow is desired wherever it can be deemed appropriate. The suitability of imposing such conditions on a ground-water flow model of a prototype fractured-bedrock island aquifer in Narragansett Bay, Rhode Island was investigated following favorable analysis of long-term drawdown data suggestive of Darcian ground-water flow.
A borehole geophysical investigation of the island's municipal production well is corroborative, suggesting that ground-water flow into the well appears to decrease systematically with depth. Geophysical results were also used to develop transmissivity distributions from specific capacity measurements obtained throughout the study area. The distributions were useful for evaluating transmissivity values used in a finite-difference ground-water flow model.
Due to the limited borehole data, surface geophysics were employed to investigate aquifer properties at a larger scale. Very low frequency (VLF) induction electromagnetics were applied across the study area to identify electromagnetically conductive subsurface structural features, perhaps suggestive of preferential flow. Highly conductive features were identified, corresponding with observed lineaments and other geomorphological features. These are interpreted to be large water-bearing fracture zones coincident with the dominant bedrock foliation and fracture patterns observed in outcrop and acoustic borehole televiewer images. For purposes of finite-difference modeling however, explicit characterization of permeability in these areas is difficult due to the implicit nature of the geophysical method.
A simplified representation of fracture-zone permeability is incorporated into a finite-difference model of the aquifer assuming that decreases in formation factor across fracture zones, inferred from geophysical results, provide a minimum for permeability increases across these zones. The discrepancy between finite-difference model-generated head and field-measured head was minimized using a otherwise horizontally uniform distribution of layer transmissivity values. Finite-difference model transmissivity is higher, on average, than transmissivity estimated from specific capacity, however is within the range of the measured distribution. Model-head discrepancies are pronounced in fracture-zone areas identified in the VLF data. At best, a very generalized description of flow results, such that a description of solute transport is inappropriate.
Michaud, Scott Charles, "EVALUATION OF DARCIAN CONTINUUM PRECEPTS FOR DESCRIBING GROUND-WATER FLOW IN A FRACTURED-BEDROCK AQUIFER" (1998). Open Access Master's Theses. Paper 2010.