Abstract
A hydrogeologic conceptual model that improves understanding of variability in aquitard integrity is presented for a fractured sedimentary bedrock unit in the Cambrian-Ordovician aquifer system of midcontinent North America. The model is derived from multiple studies on the siliciclastic St. Lawrence Formation and adjacent strata across a range of scales and geologic conditions. These studies employed multidisciplinary techniques including borehole flowmeter logging, high-resolution depth-discrete multilevel well monitoring, fracture stratigraphy, fluorescent dye tracing, and three-dimensional (3D) distribution of anthropogenic tracers regionally. The paper documents a bulk aquitard that is highly anisotropic because of poor connectivity of vertical fractures across matrix with low permeability, but with ubiquitous bed parallel partings. The partings provide high bulk horizontal hydraulic conductivity, analogous to aquifers in the system, while multiple preferential termination horizons of vertical fractures serve as discrete low vertical hydraulic conductivity intervals inhibiting vertical flow. The aquitard has substantial variability in its ability to protect underlying groundwater from contamination. Across widespread areas where the aquitard is deeply buried by younger bedrock, preferential termination horizons provide for high aquitard integrity (i.e. protection). Protection is diminished close to incised valleys where stress release and weathering has enhanced secondary pore development, including better connection of fractures across these horizons. These conditions, along with higher hydraulic head gradients in the same areas and more complex 3D flow where the aquitard is variably incised, allow for more substantial transport to deeper aquifers. The conceptual model likely applies to other fractured sedimentary bedrock aquitards within and outside of this region.
| Translated title of the contribution | A multidisciplinary-based conceptual model of a fractured sedimentary bedrock aquitard: improved prediction of aquitard integrity |
|---|---|
| Original language | Spanish |
| Pages (from-to) | 2133-2159 |
| Number of pages | 27 |
| Journal | Hydrogeology Journal |
| Volume | 26 |
| Issue number | 7 |
| DOIs | |
| State | Published - Nov 1 2018 |
Bibliographical note
Funding Information:Acknowledgements We thank the Minnesota Department of Natural Resources (DNR) for access to the site of the Afton MLS well, and the Minnesota Department of Health (MDH) for assistance with video logging and well permitting. The staff at Afton State Park was particularly helpful in many respects during construction and continued monitoring of the MLS hole. We would also like to thank the scientists from the US Geological Survey in Storrs, Connecticut for their work on the borehole geophysical surveys conducted in the Afton borehole, and scientists from the US Geological Survey office in Middleton, Wisconsin for their work on the packer tests conducted in the Afton borehole. Tom Al, University of New Brunswick, provided insights that helped us interpret the water chemistry data from the Afton MLS hole. The University of Guelph’s G360 Institute for Groundwater Research provided the Westbay tools and Westbay transducers for hydraulic head and groundwater sample collection and research staff time, financially supported by the University Consortium for Field Focused Groundwater Contamination Research. Peeter Pehme of the University of Guelph provided assistance with the MLS design and borehole geophysics. Westbay Instruments Inc. (a division of Nova Metrix Ground Monitoring (Canada) Ltd.), provided support via a discount in cost of the MLS for the Afton borehole, as well as technical support, as part of their research relationship with the Guelph G360 Institute for Groundwater Research. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government.
Funding Information:
We thank the Minnesota Department of Natural Resources (DNR) for access to the site of the Afton MLS well, and the Minnesota Department of Health (MDH) for assistance with video logging and well permitting. The staff at Afton State Park was particularly helpful in many respects during construction and continued monitoring of the MLS hole. We would also like to thank the scientists from the US Geological Survey in Storrs, Connecticut for their work on the borehole geophysical surveys conducted in the Afton borehole, and scientists from the US Geological Survey office in Middleton, Wisconsin for their work on the packer tests conducted in the Afton borehole. Tom Al, University of New Brunswick, provided insights that helped us interpret the water chemistry data from the Afton MLS hole. The University of Guelph?s G360 Institute for Groundwater Research provided the Westbay tools and Westbay transducers for hydraulic head and groundwater sample collection and research staff time, financially supported by the University Consortium for Field Focused Groundwater Contamination Research. Peeter Pehme of the University of Guelph provided assistance with the MLS design and borehole geophysics. Westbay Instruments Inc. (a division of Nova Metrix Ground Monitoring (Canada) Ltd.), provided support via a discount in cost of the MLS for the Afton borehole, as well as technical support, as part of their research relationship with the Guelph G360 Institute for Groundwater Research. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the US government.
Publisher Copyright:
© 2018, Springer-Verlag GmbH Germany, part of Springer Nature.
Keywords
- Aquitard
- Conceptual models
- Fractured rocks
- Groundwater protection
- USA
