UNSATURATED FLOW AND MULTICOMPONENT REACTIVE TRANSPORT IN PHYSICALLY AND CHEMICALLY HETEROGENEOUS SEDIMENTS

Coupled processes of unsaturated flow and multicomponent strontium ion exchange are investigated in a subsurface system with multiscale heterogeneities in sediment properties. High-resolution geophysical techniques and image analysis are used to generate sub-millimeter resolution maps of physical and chemical heterogeneities in an exposed 4 meter by 1 meter cut face of subsurface sediments. Permeability distribution parallel and perpendicular to laminae is measured by mini-permeameter. Water content, particle size distribution, particle density, bulk density, and cation exchange capacity measured on selected samples are used to develop pedotransfer functions for chemical and unsaturated hydraulic properties. Spatial distributions of temperature from high-resolution infrared images correlated to particle size statistics are used to interpolate hydraulic and chemical properties across the entire cut face. A geochemical model of multicomponent strontium ion exchange developed in a previous laboratory study and tested under unsaturated conditions with sediments from the field study, is the basis for simulating the aqueous, mineral, and sorption reactions in this system. The migration of a conservative tracer and a strontium solution is simulated under a range of recharge rates to explore the impact of the negative correlation between permeability and reactive surfaces. Results show the development of saturation-dependent flow paths that can limit exposure to zones of highest cation exchange capacity.