Formatted Title

Impacts of Matrix Diffusion on Solute Transport and Groundwater Remediation in Fractured Crystalline Bedrock: Empirical and Modeling Demonstrations 

Background/Objectives

Addressing groundwater contamination in fractured crystalline (igneous and metamorphic) bedrock is particularly challenging due to the high cost of drilling monitoring and remediation wells, complexity of fracture networks, potentially deep extent of contaminant plumes, and other factors. Moreover, there does not seem to be broad consensus among groundwater and environmental professionals regarding the role that matrix diffusion may play in contaminant transport in crystalline bedrock or the implications that diffusion may have on the feasibility of groundwater remediation. Decades of scientific research indicates that crystalline bedrock has significant primary porosity that can store groundwater and solutes. Matrix diffusion in crystalline bedrock can control contaminant transport and timeframes of remediation because the majority of the solute mass is likely to be in the matrix and not in fractures where flow occurs. Consequently, the time required to clean up contaminant plumes may be governed by the rate at which contaminants can diffuse back out of the rock matrix. Porous media (i.e., single-porosity) solute transport models are not appropriate for simulating contaminant transport in fractured crystalline bedrock where matrix diffusion occurs. We present data obtained from projects, the literature, and modeling results which demonstrate that matrix diffusion occurs in crystalline bedrock and can dominate plume transport and significantly impact the feasibility of remediation.

Approach/Activities

This study uses instructive examples from the literature that document the presence, shapes, and scale of pore spaces in crystalline bedrock. Fate and transport modeling simulation results are presented that quantify the importance of matrix diffusion in plume transport and remediation. One example from the literature involves a large granite block placed into the sea as part of a coastal fortification project. After decades immersed in seawater, the block was cored, and concentrations of chloride and bromide were measured at multiple positions within the block. Results showed that those ions diffused into the granite block and equilibrated even in the center of the block. Literature examples like this provide clear, empirical evidence of matrix diffusion in crystalline bedrock. Fate and transport model simulations were performed using parameters measured by the authors at crystalline bedrock sites to illustrate the impact of matrix diffusion on solute transport and remediation. The simulations include scenarios such as initial plume advance, natural plume flushing after source removal, pump and treat, and pulsed pumping.

Results/Lessons Learned

Published and unpublished data from sites with crystalline bedrock indicate matrix porosity values are log-normally distributed, range from approximately 0.075% to 9.4%, and have a geometric mean of approximately 1.5%. Similarly, the geometric mean matrix porosity value from 13 crystalline bedrock project sites where we have worked is 1.4%. Although this is considerably less than the geometric mean matrix porosity of clastic sedimentary rocks (6.4% from four sites where we have worked), it is sufficient to significantly influence plume migration and remediation. Similar to contaminant transport in fractured sedimentary bedrock, the degree of plume attenuation in fractured crystalline bedrock–for conservative and nonconservative solutes–increases with time and travel distance, eventually reaching an asymptotic level. We have measured matrix-diffusion based plume retardation values of 100 or more at crystalline bedrock sites, even for non-reactive solutes, which has significant implications for groundwater remediation.  Due to diffusion into the matrix as a plume migrates, plume velocities may be 100 to 600 times less than groundwater velocity in fractures. Our model simulations confirm that solute plumes advance much more slowly, but also flush more slowly, due to the influence of matrix diffusion. Due to reverse diffusion from the matrix back into the fractures during natural flushing or remediation: natural flushing may take 100+ years to reach remedial goals; pump-and-treat may not expedite groundwater cleanup even with pulsed-pumping schemes; and concentrations in fractures should be expected to rebound after pumping stops. These findings call for setting realistic expectations regarding the prospects for groundwater remediation in fractured crystalline bedrock.