Assessment of groundwater-sediment interplay at the Darien Basin mine complex, Peru

Thesis Abstract

Groundwater-sediment interactions in the Darien Basin mine complex present a critical hydrogeochemical control on contaminant transport, aquifer resilience, and tailings stability under tropical climatic variability. The study addresses the problem of uncertain groundwater-sediment exchange processes that influence metal mobility, particle transport, and plume evolution within geologically complex alluvial and volcanic sequences surrounding the mine. The aim is to quantify the spatial and temporal dynamics of groundwater-sediment interplay and to evaluate their implications for water quality and mine-site stability. Specific objectives are (1) to characterize lithology, hydraulic properties, and pore-water chemistry across proximal aquifers and tailings-impacted sediments; (2) to determine the role of grain-size distribution, mineralogy, and redox conditions in controlling contaminant speciation and transport; (3) to develop a predictive framework linking groundwater fluxes with sediment translocation and turbidity transport; (4) to assess seasonal and hydrological variability on groundwater-sediment interactions; and (5) to provide management-relevant recommendations for water treatment, groundwater monitoring, and tailings management. A mixed-methods approach integrates hydrogeological field measurements, laboratory analyses, and numerical modeling. The population comprises groundwater samples, pore-water extracts, and sediment cores collected from 25 strategically distributed boreholes and 8 piezometers within a 15-km2 study area surrounding the Darien Basin mine complex. Water samples (n=240) are analyzed for major cations/anions, trace metals (e.g., Fe, Mn, As, Cu, Zn), and dissolved organic carbon, using ICP-OES, IC, and ICP-MS. Sediment samples (n=120) undergo mineralogical characterization by X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS), together with grain-size distribution by laser diffraction. Pore-water extraction employs zero-tension lysimeters and high-pressure squeezing, followed by stable isotope analyses (2H, 18O) to elucidate water provenance. Hydraulic tests include single-well slug tests for transmissivity and multi-well aquifer tests to estimate hydraulic connectivity between sediments and groundwater. Groundwater-sediment interactions are modeled using a coupled HYDRUS-3D and MODFLOW-OR (organic and reactive) framework to simulate advective-dispersive transport, sorption/desorption dynamics, and redox-driven speciation. Particulate transport is addressed through a coupled advection-dispersion-attachment model calibrated with observed turbidity and suspended solids data. Temporal dynamics spanning two hydrological cycles (dry season 2024–wet season 2025) provide comparative insights into seasonal controls on exchange processes. Statistical analyses employ multivariate regression and principal components analysis to identify key lithological and geochemical drivers, while ANOVA tests evaluate differences among lithostratigraphic units. Expected findings indicate that groundwater-sediment exchange in the Darien Basin is governed by a dual mechanism preferential flow along high-permeability channels within coarse-grained tailings-impacted sediments and diffusion-dominated exchange in finer grained, reduced zones, modulated by groundwater head gradients and seasonal recharge. Redox transitions in sulfidic sulfidation-prone sediments are anticipated to control metal mobilization, with As and Fe concentrations showing significant associations with Eh-pH conditions. Sediment grain size and mineralogy will emerge as primary predictors of sorption capacity and colloidal transport, while isotope data will reveal recharge sources and groundwater residence times that shape contaminant plumes. The study will produce a robust predictive framework linking groundwater fluxes with sediment transport and contaminant mobility, enabling scenario analyses for climate variability and tailings management options. The research contributes to knowledge by integrating hydrogeology, geochemistry, and reactive transport modeling to elucidate groundwater-sediment interplay in tropical, mining-impacted systems, offering transferable insights for similar high-rainfall, alluvial-volcanic contexts. Practical implications include improved groundwater monitoring networks, targeted remediation strategies, and evidence-based guidelines for tailings isolation and water-resource protection. The main conclusion anticipates that effective management hinges on controlling preferential flow pathways and enhancing redox stability to limit metal mobilization, with recommendations emphasizing adaptive monitoring, in-situ treatment optimization, and phased tailings decommissioning plans aligned with hydrological variability.