Project Number:
DNR-157
Other Project Number:
WR00R015
Funding Year:
2000
Contract Period:
07/01/2000 - 07/01/2001
Funding Source:
DNR
Investigator(s) and affiliations:
Kenneth R. Bradbury, Wisconsin Geological and Natural History Survey;
Herbert Wang, University of Wisconsin–Madison
Abstract:
Background/Need: The Maquoketa Formation, a dolomitic shale, forms the most important aquitard in eastern Wisconsin, USA, isolating the water-table and Silurian aquifers from the underlying Cambrian-Ordovician aquifer. The hydraulic properties of the Maquoketa aquitard are of interest for input to a regional groundwater flow model, which will be used for better groundwater resource management and well-head protection.
Analysis of earlier results showed that while the Maquoketa Formation has very low rock-matrix hydraulic conductivity, simultaneous drawdowns occurred at multiple levels within this formation when the overlying Silurian aquifer was pumped. Such observations violate equivalent porous medium assumptions but they can be explained by local vertical fracture interconnections between the observed bedding-plane fractures within the Maquoketa Formation.
Objectives: The objective of the additional fieldwork described in this report was to characterize hydraulic and transport properties of a potential fracture network extending from the Silurian dolomite aquifer into the Maquoketa Formation. If significant vertical fracture interconnections exist, they could result in much higher bulk hydraulic conductivity in the upper Maquoketa Formation. The observed vertical head distribution showed that almost half of the total vertical head loss due to regional pumping occurs across the lower contact of the formation, which is not consistent with a homogeneous, very low-conductivity aquitard at steady-state with the heads in the adjacent aquifers.