Evaluation of earth tide matrix dispersion

The impact of earth tide effects on species retardation migrating in fractured rock formations is investigated. One-dimensional oscillatory flow and solute transport between a fracture and the rock matrix is simulated for some realistic combinations of hydrogeologic parameters. While the time-varying flow field is modeled analytically the advective-disversive equation is solved by use the finite element method. The results are compared to solutions of the pure diffusion equation to discuss the importance of the earth tide induced matrix dispersion on the basis of an equivalent matrix diffusion coefficient. The calculations show that the mass retained by the matrix is directly proportional to: i) the earth tide amplitude difference between the matrix and the fracture and ii) the dispersivity of the matrix. A comparison with values reported in the literature shows that the additional 'tidal' transport coefficient may be as high as the conventional matrix diffusion coefficient. 5 figs., 1 tab., 9 refs