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TitleA unified hydrogeological conceptual model of the Milk River transboundary aquifer, traversing Alberta (Canada) and Montana (USA)
AuthorPétré, M A; Rivera, AORCID logo; Lefebvre, R; Goblet, P
SourceHydrogeology Journal vol. 24, issue 7, 2016 p. 1847-1871, Open Access logo Open Access
Alt SeriesEarth Sciences Sector, Contribution Series 20150304
PublisherSpringer Nature
Mediapaper; on-line; digital
File formatpdf
AreaMontana; United States of America
Lat/Long WENS-113.0000 -109.0000 50.0000 48.5000
Subjectsgroundwater; groundwater resources; formation waters, commodity; hydraulic gradients; hydraulics; conceptual model; grounwater exploitation
Illustrationslocation maps; graphs; histograms; schematic diagrams
ProgramGroundwater Geoscience, Aquifer Assessment & support to mapping
Released2016 06 29
AbstractA conceptual model of the transboundary Canada-USA Milk River Aquifer (MRA) was developed based on literature, focused field work and a 3D geological model. The MRA corresponds to the Virgelle Member of the Milk River Formation (called Eagle Formation in Montana). The MRA is confined by aquitards of the Pakowki/Claggett Formation, and overlays Colorado Group aquitard. Confinement varies with changes in surface topography, but especially due to buried valleys eroding part of the top aquitard. The up-gradient MRA limit is the Virgelle outcrop near the international border and along the Sweetgrass Arch in Montana. The down-gradient limit is the unconformity separating the Virgelle Member from the gas-bearing sandy shale Alderson Member. The MRA shows larger transmissivity areas leading to preferential flowpaths confirmed by natural geochemical tracers. Tritium and 14C delineate restricted recharge areas along the MRA outcrops on both sides of the international border. Drastic changes in hydraulic gradient indicate that the Milk River intercepts an important proportion of the groundwater flowing through the MRA. Beyond the Milk River, groundwater flows quite slowly, as shown by 36Cl residence times exceeding a million year. There is no other direct discharge zone, but vertical leakage occurs through the underlying Colorado Aquitard and upward along buried valleys, which seem to act as drains and correspond to artesian areas. The MRA contains a mostly fossil groundwater resource not significantly renewed. Groundwater exploitation thus far exceeds renewal, a situation requiring properly managed MRA groundwater depletion.
Summary(Plain Language Summary, not published)
A conceptual model of the Milk River transboundary Aquifer (Canada-USA) is proposed. It was developed based on literature, focused field work and a 3D geological model. This model aims to link hydrogeology and geochemical evidences on both sides of the international border. This model allows a better understanding of the functioning of the aquifer and the processes that might occur. Preferential flow path have been highlighted from hydrogeological parameters and have been confirmed by geochemical data. The Milk River intercepts a large part of the groundwater flux, which in turn leads to a small and low-velocity residual flux, as indicated by residence time (obtained from isotopic analysis) over a million year. The aquifer contains mostly fossil water and is not significantly renewed. this situation requires a proper managed groundwater depletion of the aquifer.

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