A unified hydrogeological conceptual model of the Milk River transboundary aquifer, traversing Alberta (Canada) and Montana (USA)

Marie-Amélie Pétré,Alfonso Rivera,M Jim Hendry,Attila J B Folnagy,René Lefebvre

doi:10.1007/s10040-016-1433-8

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 gas-bearing 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.

Highlights

It is estimated that there are about 366 transboundary aquifers worldwide (IGRAC 2015)
Compilation of historical studies and new field data contributed to expanding our understanding of the hydrogeology of the aquifer in northern Montana and southern Alberta
There is a clear delineation of the hydrogeological extent of the Milk River Aquifer (MRA) and its three constituting sub-systems following the natural boundaries of the aquifer instead of the jurisdictional boundaries, as was done in previous studies

Summary

Introduction

It is estimated that there are about 366 transboundary aquifers worldwide (IGRAC 2015). The sound management of these shared groundwater resources is an important issue, especially in arid and semi-arid climates. Management of this resource requires a detailed understanding of the aquifer dynamics along its natural hydrogeological boundaries. Quantification of groundwater fluxes in the aquifer can be determined using groundwater numerical models. The foundation of these numerical models is the hydrogeological conceptual model (Bredehoeft 2005). Kresik and Mikszewski (2012) define the hydrogeological conceptual model as “the description of various natural and anthropogenic factors that govern and contribute to the movement of groundwater in the subsurface” The foundation of these numerical models is the hydrogeological conceptual model (Bredehoeft 2005). Kresik and Mikszewski (2012) define the hydrogeological conceptual model as “the description of various natural and anthropogenic factors that govern and contribute to the movement of groundwater in the subsurface”

Objectives

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Conclusion