Experimental and numerical study of solute transport through saturated fractured porous aquifer.

摘要

The evaluation of groundwater pollution in different subsurface media bas always been a challenging task. The knowledge required for the conceptual basis for analysis is often insufficient. Site specific data are usually not available. In fractured formations further difficulties arise since contaminant migration patterns are influncedd in a complex wsy by the variability of fractute characteristics. In addition many numerical models have been found to be inefficient in handling the equations governing subsurface contaminant transport. This means that the investigator is confronted with several sources of uncertainties arising from both the input data and the numerical model itself.; This thesis presents a field experiment and numerical study of solute transport through a saturated fractured aquifer located in St. John's, Newfoundland, Canada. The aquifer at the test site consists of a thin glacial till overlying a fractured bedrock. The investigation comprised two parts. The first part involved several months of groundwater level monitoring, in situ tests for hydraulic conductivity and study of the groundwater chemistry. High spatial variability of hydraulic conductivity in the bedrock aquifer was observed. The second part involved two natural gradient tracer tests. Experimental results indicate that there is limited hydraulic communication between the overlying till and the fractured bedrock Tracer migration patterns in the bedrock suggest a dense network of highly interconnected fractures which cannot be represented on individual scale. Tracer migration in the bedrock showed so evidence of flow channelling. The concentration distribution has been represented by breakthrough curves.; Knowledge from the field study has been used to develop an efficient numerical model for solute transport based on the advection-dispersion equation. The numerical model was based on the dual reciprocity boundary element method (DRBEM). A new approach in the application of this technique to a class of non-linear, nonhomogeneous equations has been proposed. Several numerical experiments to test the accuracy and efficiency of the new model have been performed. Numerical results showed that the new approach improves the accuracy of the DRBEM. Good agreement was obtained between model results and the analytical solutions for several theoretical test problems. The ability of the DRBEM to handle the dual nature of the advection-dispersion equation was also recognized by achieving accurate results at high Peclet numbers. The efficiency of the DRBEM compared with other numerical solution techniques was also observed.; The DRBEM model was applied to a practical field situation by using the results from a tracer test. Numerically simulated breakthrough curves were compared with the experimental results obtained from the tracer test. Aquifer parameters optimized for a reasonable match between the numerically simulated and the experimental results have been estimated. Simulation results suggest that the advection-dispersion model is highly suitable for solute transport analysis in this aquifer. Sensitivity analysis has been performed by computing sensitivity coefficients for the various aquifer parameters. The field experiment and the numerical study have provided considerable insights into the processes controlling solute transport in this fractured aquifer.