A Particle-Based Conditional Sampling Scheme for the Simulation of Transport in Fractured Rock With Diffusion Into Stagnant Water and Rock Matrix

Journal Article · 01 April 2020 · Water Resources Research

DOI: https://doi.org/10.1029/2019wr026958 · OSTI ID:1649437

Trinchero, Paolo  ^[1]; Painter, Scott L.  ^[2]; Poteri, Antti ^[3]; Sanglas, Jordi ^[1]; Cvetkovic, Vladimir  ^[4]; Selroos, Jan‐Olof  ^[5]

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1. AMPHOS 21 Consulting S.L., Barcelona (Spain)
2. Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States). Environmental Sciences Division
3. Posiva Oy, Eurajoki (Finland)
4. KTH Royal Inst. of Technology, Stockholm (Sweden). Land and Water Resources Engineering
5. Swedish Nuclear Fuel and Waste Management Company, Solna (Sweden)

In situ experiments and field-scale characterization studies have pointed out that, in fractured crystalline media, groundwater flow is highly channelized. This implies that, at the scale of a single fracture, only part of the fracture surface area is in contact with flowing water, while the rest of in-plane water is essentially stagnant and can be accessed by solutes via molecular diffusion. Despite their importance for contaminant retention, to date, there are no numerical or analytical approaches that could be used to assess the implication of stagnant water zones on solute transport in realistic large-scale Discrete Fracture Network-based models. In this study, we present an efficient and flexible algorithm for the simulation of transport in fractured rock with diffusion into stagnant water and rock matrix. The algorithm is a generalization of a previously developed numerical framework for time domain particle tracking in sparsely fractured rock. The key of the generalization is that total time in fracture (τ_f) is first evaluated using a Monte Carlo sampling and then a second sampling is performed conditioned on τ_f. The algorithm has been successfully validated against existing independent solutions and the implication of diffusion into stagnant water and secondary diffusion into the matrix has been assessed for a realistic modeling scenario. The results show that, due to diffusion into stagnant water, contaminants are more strongly retarded. This increased retention is more significant for sorbing species, as a larger number of sorption sites is accessible. A high sensitivity to the flowing channel/stagnant water zone geometry has also been observed.

• Research Organization: Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States)
• Sponsoring Organization: USDOE
• Grant/Contract Number: AC05-00OR22725
• OSTI ID: 1649437
• Journal Information: Water Resources Research, Vol. 56, Issue 4; ISSN 0043-1397
• Publisher: American Geophysical Union (AGU)Copyright Statement
• Country of Publication: United States
• Language: English