Simulations of Advective Transport and Importance of High Permeability Facies in an Outwash Deposit
Peter E. Riemersma, Jean M. Bahr, and Mary P. Anderson
Predictions of clean-up times for groundwater pump-and-treat systems are difficult to make for heterogeneous aquifers due to uncertainty in our knowledge of the hydraulic conductivity distribution. Our approach has been to specify the spatial distribution of hydraulic conductivity in terms of different facies and to examine the importance of highly permeable facies on transport. A 50 by 60 meter section of proximal glacial outwash deposits were mapped by a previous investigator in a quarry in Wisconsin and separated into seven lithofacies. Hydraulic conductivity values were estimated for facies based on grain size characteristics of sediment samples. These values range over 4 orders of magnitude, with the highest and lowest hydraulic conductivity facies each representing ess than 5 percent of the deposit.
We simulated contaminant transport using a particle tracking code coupled to a three-dimensional model for advective flow to trace the path of particles in a steady-state flow field. The model grid is composed of 7,500 nodes in 10 layers. Simulations assuming homogeneous facies and interfacies heterogeneity were conducted. Breakthrough curves were generated for particle arrival at the model exit face and for particle capture by a pumping well, and exhibit such features as early particle arrival and tailing that are characteristic of heterogeneous aquifers. Slow and fast particle pathways were examined and related to the distribution of hydraulic conductivity.
Simulations were performed to examine the sensitivity of particle transport to the orientation of the flow field, pumping well location and absence of high conductivity facies. Particle recovery is sensitive to both well location and high conductivity facies. Less converging of particle pathways and later arrival occurred without the high conductivity facies. Simulation results suggest that site characterization should be focused on identification and delineation of high conductivity units, even when they compose a small percentage of the aquifer.
AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California