Analysis of geochemical anomalies observed in accretionary prism environments yields important information about pore pressures and fluid migration pathways at convergent plate margins. We use a two-dimensional finite element flow and solute transport model to study the flow system in the Nankai accretionary complex. A diffuse zone of chloride dilution observed between 560 and 1200 mbsf at ODP site 808 may result from the production of fresh water as smectite is transformed to illite, migration of this fresh water tens of kilometers seaward along the decollement fault, and subsequent dispersion of fresh water away from the fault. We test this hypothesis by incorporating into our model both consolidation and clay dehydration sources, with sea water and fresh water concentrations, respectively. Simulations for steady state flow do not reproduce either the magnitude or broad distribution of observed pore water freshening. Simulations for transient fluid flow, in which decollement permeability is increased by 2 orders of magnitude at t = 0 to simulate the effect of hydrofracture, can reproduce the distribution of freshening. The shape of the modeled freshening profile depends primarily on the porosity of the underthrust section. On the basis of our results we conclude that the underthrust sediments must be underconsolidated by 7-15%, consistent with downhole and laboratory measurements. In order to produce the observed magnitude of freshening, we require over 35% smectite for a relative plate velocity of 2.0 cm/yr. With increasing convergence rates, less smectite is necessary. Because both the distribution and magnitude of pore water freshening are also sensitive to porosity changes in the underthrust section, it may be possible to use modeling results to constrain consolidation rates as rapidly buried sediments are transported arcward.