Long term infiltration and tracer transport in fractured rocks: Field observations and model analyses

This paper presents modeling analyses of long term infiltration and tracer tests in fractured tuffs at Yucca Mountain, NV, USA. The experiments were conducted on a 20 m thick tuff section in a flyover formed by two exploratory tunnels. The infiltration test last for 870 days. Both measured infiltration and seepage show significant temporal and spatial variations. The tracer test used inorganic tracers (I−, Br−, F−) and organic tracers (fluorobenzoic acids) released 559 days after the infiltration test started. Leaching from dry salts from fracture walls was found to have affected tracer breakthroughs. The unsaturated flow was evaluated by optimizing 45 parameter values in a three-dimensional model, which accounts for fracture–matrix interaction and heterogeneous hydraulic properties in a column-based scheme. The field data are valuable asset to evaluate the modeling approaches for fractured rocks and the relative importance of the matrix diffusion process. Results show that matrix diffusion is an important process for transport, and that effective matrix-diffusion coefficients at the field-scale are larger than those at the laboratory-scale for the solutes.