Abstract A conceptual model of the transboundary Milk River Aquifer (MRA), extending across the Canada–USA border, was developed based on literature, focused fieldwork and a three-dimensional geological model. The MRA corresponds to the Virgelle Member of the Milk River Formation (Eagle Formation in Montana, USA) and it is an important groundwater resource over a large area (25,000 km2). The Virgelle outcrops near the international border and along the Sweet Grass Arch in Montana. The down-gradient limit of the MRA is the unconformity separating the Virgelle from the gasbearing sandy shale of the Alderson Member. The MRA is confined above by the Pakowki/Claggett Formations aquitards and below by the Colorado Group aquitard.
The MRA contains higher transmissivity areas resulting in preferential flowpaths, confirmed by natural geochemical tracers. Tritium and 14C delineate restricted recharge areas along the outcrops on both sides of the international border. Drastic decreases in horizontal hydraulic gradients indicate that the Milk River intercepts a large proportion of groundwater flowing to the north from the recharge area. Downgradient of the Milk River, groundwater movement is slow, as shown by 36Cl residence times exceeding 1 Ma. These slow velocities imply that groundwater discharge downgradient of the Milk River is via vertical leakage through the Colorado Group and upward along buried valleys, which act as drains and correspond to artesian areas. When confined, the MRA contains a fossil groundwater resource, not significantly renewed by modern recharge. Groundwater exploitation thus far exceeds recharge, a situation requiring properly managed MRA groundwater depletion.
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