A conceptual model of fracture flow in the Milk River Aquifer under transverse hydraulic gradients

The groundwater flow and geochemical processes in the Montana - Alberta transboundary Milk River Aquifer (MRA) remain poorly understood. Previous studies have described geochemical pathways (e.g., chloride trends) in the aquifer that are oriented in NW-SE direction and recent 81Kr tracer data suggest that apparent groundwater ages range from 500,000 years. The aquifer is fractured and is characterized by a NW-SE trending primary fracture system intersected by transverse shorter secondary fractures. However, hydrogeological significance of fracture flow on geochemistry and tracer ages of MRA groundwater has not been reported. The objective of this study was to combine hydraulic gradients, regional fracture characteristics and geochemical data to better explain the groundwater age distribution and the pathways of geochemical evolution.The hydraulic gradient in the aquifer is in SW-NE direction that is in part oriented in transverse direction to the primary fractures and the geochemical pathways described in the literature. Therefore, a new conceptual groundwater flow model is proposed wherein the aquifer is recharged at the outcrops by fast infiltrating water that moves quickly through the secondary fractures under regional hydraulic gradients, but disperses slowly along primary fractures due to relatively flatter gradients in the central and northern portions of the MRA. This leads to an overall slow regional groundwater flow that follows a zig-zag pattern explaining both the groundwater ages and the chloride concentration trends transverse to the hydraulic gradients in parts of the aquifer. The proposed conceptualization was successfully validated using particle-tracking modeling based on a numerical discrete fracture network groundwater flow model. We conclude that fracture flow governs groundwater movement in the MRA and likely plays a more dominant role in geochemical processes than previously recognized.