A NUMERICAL 3-DIMENSIONAL CONDITIONED UNCONDITIONED STOCHASTIC FACIESTYPE MODEL APPLIED TO A REMEDIATION WELL SYSTEM/

In this study a three-dimensional stochastic facies-based aquifer mode l was developed. The model can be used to numerically simulate flow an d solute transport in heterogeneous groundwater aquifers. The stochast ic generation process can be conditioned by using available facies inf ormation in one vertical plane or two orthogonal vertical ones. In thi s study the information was obtained from a facies interpretation of a vertical georadar profile in a natural gravel formation in Switzerlan d. In the domain outside the known profile, unconditioned lenses and l ayers were generated at random according to statistical information on coherent sedimentary structures based on observations in adjacent gra vel pits [Jussel et al., 1994a]. The method was applied to a single ex traction well designed to capture an initially block-shaped contaminan t plume. A total of 80 conditioned and unconditioned synthetic aquifer s was generated. The flow and transport simulations were performed usi ng a finite element how model and a random walk transport model. The r esults are presented as the ensemble of integral solute mass recovery curves of single realizations. One would expect conditioning to reduce the bandwidth of the recovery curves representing the uncertainty, bu t the results show that the bandwidth even increased. This effect was attributed to a discrepancy in the mean volumetric fraction of the dif ferent facies types in the conditioned and the unconditioned cases. Mo reover, a simulation using a homogeneous model with constant equivalen t flow and transport parameters overestimated the remediation efficien cy. The influence of a linear, reversible equilibrium sorption on the remediation well efficiency was taken into account by an uncorrelated random field of the retardation factor based on values from the litera ture. However, the impact of the variability in hydraulic conductivity clearly exceeded the effect of the variability in the retardation fac tor.