Using multiple experimental methods to determine fracture/matrix interactions and dispersion of nonreactive solutes in saturated volcanic tuff

The objective of this research was to investigate the effects of matrix dif fusion on solute transport in fractured volcanic tuff. Two tuff cores were studied, one with a matrix porosity of 0.27 and the other with a porosity o f 0.14. The matrix permeabilities of the cores were 4.7 x 10(-15) and 7.8 x 10(-19) m(2), 5 and 9 orders of magnitude less than the respective fractur e permeabilities. This suggested that the cores could be modeled as dualpor osity systems with no flow in the matrix but significant solute storage cap acity. Two types of tracer tests were conducted in each fractured core: (1) iodide was injected in separate experiments at different flow rates and (2 ) two tracers of different matrix diffusion coefficients (bromide and penta fluorobenzoate (PFBA)) were injected in another test. A difference in the m aximum concentrations of the solutes and the extended tailing of the breakt hrough curves were assumed to be indicative of diffusive mass transfer betw een the fracture and the porous matrix of the cores. Interpreting the resul ts from both methods allowed the identification of matrix diffusion and dis persion effects within the fracture by simultaneously fitting the data sets (with known constraints) using a relatively simple conceptual model. Estim ates of mass transfer coefficients for the fractured cores were also obtain ed.