Numerical modeling of a regional groundwater flow system to assess groundwater storage loss, capture and sustainable exploitation of the transboundary Milk River Aquifer (Canada - USA)

Groundwater capture and storage loss play a major role in the sustainable exploitation of a regional aquifer. This study aimed to identify the impact of major and long-term groundwater exploitation on a regional aquifer system to understand the processes controlling the sustainable exploitation of the transboundary Milk River Aquifer (MRA). The MRA extends over 26,300 km(2), being a major water resource across southern Alberta (Canada) and northern Montana (USA). Concerns about the sustainability of the MRA were raised as the century-old exploitation has led to important drawdowns and the local loss of historical artesian conditions. A steady-state numerical model of the regional flow system was developed and calibrated against hydraulic heads, groundwater fluxes, and the area with flowing artesian wells. Four groundwater abstraction scenarios were simulated: 1) natural flow conditions without exploitation; 2) the mean abstraction rate over the last 108 years; 3) the historical maximum global abstraction rate of the MRA; and 4) a theoretical high abstraction rate based on the maximum rate estimated for each MRA exploitation zone. The numerical model agrees with the previously formulated conceptual model and supports its hydraulic plausibility. Results show that MRA exploitation has led to a major change in flow patterns to sustain groundwater abstraction. The MRA water balance under exploitation indicates that more recharge and reduced seepage to bedrock valleys compensate groundwater withdrawals. Based on its impact on regional discharge and the reduction in MRA storage, the mean historical level of exploitation of the MRA appears sustainable. Larger exploitation rates would significantly reduce groundwater discharge to surface seepage locations and lead to a larger reduction in groundwater storage in the MRA. Modeling also illustrates that the MRA is an internationally shared resource. This situation would justify a joint management of the aquifer system between Canada and USA; especially in the area comprised between the recharge area in Montana and the Canadian reach of the Milk River.