A Modified Time Domain Random Walk Approach for Simulating Colloid Behavior in Fractures: Method Development and Verification

Abstract Colloids are ubiquitous in groundwater systems, and understanding their behavior is of critical importance as they can pose a threat to human and environmental health. Extensive research has been conducted to model colloid behavior in fractures leading to the development of a number of analytical solutions, albeit for single fractures. However, the application of these analytical solutions at the network scale has not yet been established; thus, fracture networks are modeled using numerical techniques that are verified in single fractures first. Time domain random walk (TDRW) is a Lagrangian‐based approach originally designed to simulate solute transport in single fractures considering advection, dispersion, and matrix diffusion. The present study develops a modified TDRW approach (MTDRW) to consider colloid‐specific transport mechanisms based on an analytical solution describing colloid behavior in single fractures. The MTDRW approach was validated through simulating the behavior of (i) monodisperse colloids in a synthetic, single fracture with and without matrix diffusion; (ii) polydisperse colloids in a synthetic, single fracture with impermeable matrix; and (iii) monodisperse colloids in a synthetic impermeable fracture network. In all three cases, the MTDRW approach replicated the results of analytical solutions in single fractures and the semianalytical solution in fracture networks. The MTDRW approach is expected to enhance the reliability of colloid transport modeling due to its capacity to simulate the physiochemical heterogeneity across the network. This is required to (i) evaluate the aquifer vulnerability to colloid migration; (ii) predict the aquifer resilience under contamination events; and (iii) develop effective planning, management, and remediation strategies. Key Points A modified time domain random walk (MTDRW) approach was developed to model colloid transport in fractures This MTDRW approach can model colloid‐specific transport and retention mechanisms at the single‐fracture and fracture‐network scales The chemical and physical heterogeneity within the aquifer can be captured using the developed approach