Matrix diffusion coefficients in volcanic rocks at the Nevada test site: Influence of matrix porosity, matrix permeability, and fracture coating minerals Paul W. Reimus a, ⁎ , Timothy J. Callahan b,1 , S. Doug Ware a , Marc J. Haga a , Dale A. Counce c a Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States b Department of Geology and Environmental Geosciences, College of Charleston, 66 George Street, Charleston, SC 29424, United States c Earth and Environmental Sciences Division, Los Alamos National Laboratory, Los Alamos, NM 87545, United States Received 9 February 2006; received in revised form 29 January 2007; accepted 30 January 2007 Available online 8 February 2007 Abstract Diffusion cell experiments were conducted to measure nonsorbing solute matrix diffusion coefficients in forty-seven different volcanic rock matrix samples from eight different locations (with multiple depth intervals represented at several locations) at the Nevada Test Site. The solutes used in the experiments included bromide, iodide, pentafluorobenzoate (PFBA), and tritiated water ( 3 HHO). The porosity and saturated permeability of most of the diffusion cell samples were measured to evaluate the correlation of these two variables with tracer matrix diffusion coefficients divided by the free-water diffusion coefficient (D m /D ⁎ ). To investigate the influence of fracture coating minerals on matrix diffusion, ten of the diffusion cells represented paired samples from the same depth interval in which one sample contained a fracture surface with mineral coatings and the other sample consisted of only pure matrix. The log of (D m /D ⁎ ) was found to be positively correlated with both the matrix porosity and the log of matrix permeability. A multiple linear regression analysis indicated that both parameters contributed significantly to the regression at the 95% confidence level. However, the log of the matrix diffusion coefficient was more highly-correlated with the log of matrix permeability than with matrix porosity, which suggests that matrix diffusion coefficients, like matrix permeabilities, have a greater dependence on the interconnectedness of matrix porosity than on the matrix porosity itself. The regression equation for the volcanic rocks was found to provide satisfactory predictions of log(D m /D ⁎ ) for other types of rocks with similar ranges of matrix porosity and permeability as the volcanic rocks, but it did a poorer job predicting log(D m /D ⁎ ) for rocks with lower porosities and/or permeabilities. The presence of mineral coatings on fracture walls did not appear to have a significant effect on matrix diffusion in the ten paired diffusion cell experiments. © 2007 Elsevier B.V. All rights reserved. Keywords: Matrix diffusion; Tracers; Matrix tortuosity; Matrix porosity; Matrix permeability 1. Introduction It is well known that in fractured rock groundwater systems, the diffusive mass transfer of solutes between flowing water in fractures and relatively stagnant water Journal of Contaminant Hydrology 93 (2007) 85 – 95 www.elsevier.com/locate/jconhyd ⁎ Correspoding author. Tel.: +1 505 665 2537; fax: +1 505 665 4955. E-mail addresses: preimus@lanl.gov (P.W. Reimus), callahant@cofc.edu (T.J. Callahan). 1 Fax: +1 843 953 5446. 0169-7722/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jconhyd.2007.01.017