Novel Insights into the Hydrogeochemical Evolution of Groundwater within the Transboundary Milk River Aquifer

The Milk River Aquifer (MRA) is a transboundary fractured sandstone aquifer extending across southern Alberta, Canada, and northern Montana, USA. Although studied for more than six decades, the processes controlling groundwater flow, apparent average groundwater ages, chloride distribution, and groundwater quality evolution remain incompletely understood. Previous conceptual models have not fully reconciled the mismatch between regional hydraulic gradients, north-to-northwest trending geochemical flow paths, old average groundwater ages, and systematic changes in major ion chemistry. This thesis develops an integrated hydrogeological and hydro-geochemical conceptual model for the MRA by combining fracture data analysis, discrete fracture network groundwater flow modelling, ⁸¹Kr average groundwater age dating, major ion chemistry, stable isotope systematics, and analytical solute transport modelling. Fracture analysis identified two dominant sub-vertical fracture sets: a primary northwest-southeast set and a secondary northeast-southwest set. Although the regional hydraulic gradient trends broadly from the southwest to the northeast, the primary fracture set is partly transverse to this gradient and aligns more closely with geochemical flow paths inferred from chloride distributions. Discrete fracture network simulations show that meteoric recharge likely enters rapidly through secondary fractures near outcrop areas before dispersing slowly along longer primary fractures. This zig-zag fracture-controlled flow pattern explains both the transverse geochemical pathways and old apparent ⁸¹Kr average groundwater ages, which range from less than 50,000 years near meteoric recharge areas to more than 500,000 years in distal parts of the aquifer. Building on this fracture-flow framework, chloride transport was evaluated using coupled advective-dispersive flushing and diffusion models. Chloride concentrations increase from less than 20 mg/L near meteoric recharge areas to more than 1,350 mg/L beyond 150 km, with maximum values of 2,500 to 4,700 mg/L in distal zones. Model results indicate that differential flushing of initial marine connate groundwater through fracture networks is the dominant control on chloride distribution, accounting for approximately 70 to 80% of observed chloride concentrations. The remaining 20 to 30% can be explained by diffusive chloride input from either the underlying Colorado aquitard or low-permeability matrix zones within the Milk River Formation. A hydro-geochemical assessment of 774 groundwater samples from 549 wells identifies three sequential hydro-geochemical zones along fracture-controlled flow paths. Zone 1, within 0–50 km of meteoric recharge, is characterized by relatively young groundwater, elevated sulphate from pyrite oxidation, carbonate dissolution, cation exchange, and a Na-HCO₃-SO₄ water type. Zone 2, from 50–75 km, records sulphate depletion through bacterial sulphate reduction, organic matter oxidation, silicate dissolution, and transition to a Na-HCO₃ water type. Zone 3, beyond 75 km, contains old groundwater where methanogenesis, carbonate dissolution, cation exchange, chloride enrichment, and elevated fluoride concentrations dominate groundwater quality evolution. Overall, this thesis demonstrates that groundwater flow and water quality evolution in the MRA are controlled by fracture-network geometry, slow regional groundwater movement, flushing of connate formation water, diffusion from aquitards and/or aquifer matrix, and progressive water-rock and redox reactions.

Name thesis:degree_name

Doctor of Philosophy (PhD)

Discipline thesis:degree_discipline

Geoscience

Grantor

Science

Year dc:date.issued

2026

Author dc:creator

• Date, Avadhoot Vinayak

Advisor dc:contributor.advisor

• Mayer, Bernhard

Committee members dc:contributor.committeemember

• Cey, Edwin
• Ryan, Cathy
• Pietroniro, Alain
• Wieser, Michael E.
• Nair, Rajeev Kumar Sasidharan
• Ferguson, Grant A. G.

• Groundwater Quality
• Groundwater Age
• Numerical Modeling
• Transboundary Aquifer
• Geochemical Processes

Statement dc:rights

• Unless otherwise indicated, this material is protected by copyright and has been made available with authorization from the copyright owner. You may use this material in any way that is permitted by the Copyright Act or through licensing that has been assigned to the document. For uses that are not allowable under copyright legislation or licensing, you are required to seek permission.

Language dc:language.iso

en

DOI

https://dx.doi.org/10.11575/PRISM/51635

OAI identifier oai:identifier

oai:ucalgary.scholaris.ca:1880/125313