Using natural tracers to evaluate flow and transport in saprolite and fractured sedimentary rocks.

摘要

Natural tracers (dissolved ions and gases, and delta18O) were sampled on spatial scales of 1 m to over a km and temporal scales of a few hours up to a year and used to develop a conceptual model of the shallow (20 m depth) flow system on the Oak Ridge Reservation, Tennessee. Groundwater flow is controlled by preferential pathways (fractures or other macropores). Advection is the dominant transport process in the unsaturated zone while advection and diffusion control transport below the water table. Periodic fluctuations in groundwater chemistry driven by storm events are hypothesized to result in episodic transport of solutes and colloidal material. Conservative tracers (Na+ and delta18O) are used in mass balance calculations to quantify flow.; A two dimensional (2-D) numerical model of the unsaturated zone is developed using an existing code capable of simulating variably saturated conditions in fractured, porous media. Simulations were performed to explore fracture/matrix interaction during repeated wetting/drying events. These simulations demonstrate that advection is the dominant transport process in the unsaturated zone, and that more solute is transported into the matrix during wetting than is returned to the fractures during drying. The advective pumping of the matrix during wetting/drying cycles is discussed in terms of the impact on contaminant transport, mineral weathering, and application of agricultural amendments. The numerical model is also used to examine assumptions used in quantitative hydrograph separation. These simulations demonstrate that the assumption of constant and known compositions for the soil water and groundwater reservoirs may not be valid in many hydrogeologic settings.; Hydrogeologic systems that are characterized by preferential pathways for flow are difficult to remediate once contaminated. Contaminants initially enter the system through the preferential pathways, and are transported into the surrounding lower permeability matrix by advection and/or diffusion. In these settings, the traditional pump and treat remedial scenarios are not efficient since pumping draws water primarily from the preferential pathways while contaminants in the matrix slowly diffuse back into the preferential pathways. This investigation takes advantage of the chemical disequilibrium that may exist between the preferential pathways and surrounding matrix to study flow and transport in these settings.