Quantitative Assessment of Groundwater-Rock Coupling in Fractured Aquifers

Thesis Abstract

In fractured aquifers, groundwater-rock interactions govern hydraulic conductivity, storage, and chemical evolution, yet quantitative characterization of this coupling remains challenging due to heterogeneity, scale effects, and limited field observables. This study seeks to quantify groundwater–rock coupling by integrating hydrogeophysical, hydrochemical, and statistical approaches to elucidate how fracture networks mediate exchange processes, storage dynamics, and tracer transport under variable stress conditions. The aim is to determine how fracture aperture distributions, mineralogy, and poroelastic responses control effective hydraulic properties and solute mobility, thereby improving predictive models of groundwater flow and contaminant fate in fractured media. Specific objectives are (i) to characterize fracture-scale hydraulic conductivities and aperture distributions using borehole data, outcrop analogs, and pumped-storage tests; (ii) to quantify groundwater–rock exchange by analyzing stable isotopes (18O/16O, 2H/1H) and major ions to infer rock- water interaction rates; (iii) to evaluate the influence of stress-induced aperture changes on flow and transport using in situ hydraulic testing and time-lapse electrical resistivity tomography (ERT); (iv) to develop and validate a coupled hydrogeochemical-porous-media model that integrates fracture network statistics with reactive transport; and (v) to identify thresholds in fracture connectivity that lead to regime shifts inaquifer response under pumping and recharge scenarios. The methodology adopts an explanatory sequential design initial field characterization in a basement-fractured aquifer system selected for its well-documented hydrogeology and existing data (n = 6 boreholes, 12 packer tests, and 3 tracer tests). Data collection employs (a) high-resolution borehole logging and side-view MRI-achtige fracture imaging to derive fracture apertures and connectivity; (b) slug and step-drawdown tests to estimate hydraulic conductivities and storativity; (c) groundwater sampling for isotope and major ion analyses using isotope ratio mass spectrometry (IRMS) and Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES); (d) time-lapse ERT during pumping and recharge to monitor dynamic saturation and fracture aperture changes; (e) rock sample analysis for mineralogical composition and reactive surface area via X-ray diffraction (XRD) and Brunauer–Emmett–Teller (BET) surface area measurements. The analytical framework combines multivariate regression and Bayesian hierarchical modeling to link hydraulic properties with fracture geometry and mineralogy, complemented by reactive transport modeling using a dual-porosity, dual-permeability approach parameterized with field-derived fracture statistics. Model validation uses observed drawdown, tracer breakthrough curves, and time-lapse resistivity data, with sensitivity analysis to quantify uncertainties in fracture aperture distributions and reaction rate constants. Expected findings include (i) robust quantification of the coupling between fracture aperture variability and effective hydraulic conductivity, (ii) empirical relationships between mineralogical composition and geochemical alteration rates driving groundwater chemistry along fracture pathways, (iii) evidence of stress-induced mobilization or clogging effects on fracture networks quantified through ERT and pumping tests, and (iv) a validated coupled model capable of reproducing observed hydraulic and chemical evolution under transient boundary conditions. The study's contribution to knowledge lies in providing a replicable, data-driven framework for measuring groundwater–rock coupling in fractured aquifers, integrating physical, chemical, and geophysical indicators within a probabilistic modeling regime, and delivering transferable insights for groundwater management under pumping stress and climate-driven recharge variability. The anticipated conclusion emphasizes that fracture-scale processes control macroscopic aquifer response more than homogeneous porosity models predict, with practical implications for well-field design, contaminant risk assessment, and management strategies that account for coupled hydrogeochemical-poroelastic dynamics. Recommendations include routine acquisition of fracture-characterization data in fractured aquifers used for water supply, incorporation of time-lapse geophysical monitoring into management plans, and refinement of reactive transport parameters through site-specific laboratory experiments to reduce predictive uncertainty in couched aquifer models.