Aquifer System

Milk River

Milk River Transboundary Aquifer
The delineation of the Milk River Transboundary Aquifer covers about 24 880 square km. It covers about 160 km from east to west and about 150 km from north to south. This hydrogeological delineation corresponds to the extent of the Virgelle Member, which is the most permeable part of the Milk River Formation. It also includes the water-bearing Upper Alderson Sands, which overlaps Virgelle along its depositional limit in the northeastern part of the region. The northern, northeastern and eastern limits of the Milk River Aquifer are defined by the gas field hosted by the Alderson Member in Alberta. Another gas field located near the city of Havre (near the Bears Paw Mountains) represents the southeastern boundary of the aquifer. In northern Montana, the Eagle Formation hosts the Bears Paw gas field. The Marias River constitutes the southern limit of the aquifer. Although the Milk River / Eagle Formation extends farther south in Montana, this physiographic limit has been chosen with respect to the hydrogeological model of the aquifer. The western limit of the aquifer corresponds to the westernmost area in which water wells have been completed in the Virgelle Member. The Virgelle Member extends farther west around the longitude -113°; however, no water wells have been completed in this unit due to the significant depth of the aquifer (>400 m; Stantec, 2002, The hydrostratigraphy of the area covered by the Milk River Transboundary Aquifer is defined as follow (Pétré et al., 2015, doi: 10.4095/295754). Colorado Group: The Colorado Shale constitutes a regional aquitard in the study area. The hydraulic conductivity of the Colorado Group ranges from 1E-10 to 1E-14 m/s (Hendry and Schwartz, 1988, doi: 10.1029/WR024i010p01747). This low hydraulic conductivity limits the quantities of water that can be exploited. Milk River / Eagle Formations: The Milk River Formation is confined above by the Pakowki/Claggett Formation and below by the low-permeability shales of the Colorado. The Milk River Aquifer is within the Milk River Formation. The Milk River Formation contains, from bottom to top, the Telegraph Creek Member, the Virgelle Member, and the Deadhorse Coulee Member. The middle member (Virgelle) is the most permeable part of the Formation. The Milk River Aquifer is a confined and inclined aquifer, which shows flowing artesian conditions only locally. Telegraph Creek Member / Formation: The Telegraph Creek Member / Formation is a transition zone, interpreted as deposits having a permeability lower than the Virgelle Formation but higher than the Colorado Shale. Virgelle Member: The Virgelle Member is the most important aquifer portion of the Milk River Formation and therefore constitutes the Milk River Aquifer. The hydraulic conductivity of Virgelle Member is 1.81E-7 m/s (Robertson, 1988, Alderson Member: The upper part of the Alderson Member contains two distinct large sand bodies that form a regional aquifer in southern Alberta. According to O'Connell (2014), muddy sediments of the Alderson and Deadhorse Coulee members separate the Virgelle and Upper Alderson aquifers. The two members are locally in contact at the Virgelle erosional edge and water flow between the two aquifers is likely (O'Connell, 2014, doi: 10.4095/295603). Pakowki Formation / Claggett Shale: The Pakowki / Claggett Formation constitutes an aquitard; the hydraulic conductivity of the Pakowki Formation is 10E-11 m/s (Toth and Corbet, 1987). In Montana, the hydraulic conductivity of the Claggett Shale has an estimated value of 3.5E-11 m/s (Anna, 2011, Belly River / Judith River Formation: The Belly River Group / Judith River Formation constitutes an aquifer and the hydraulic conductivity of the Judith River Aquifer in northern Montana ranges from 9E-8 m/s to 8.8E-7 m/s (Anna, 2011, Bearpaw Formation: This marine stratum is lithologically similar to that of the Pakowki Formation; therefore, it is a regional aquitard (Tokarsky, 1974,