CONVECTIVE DISPERSIVE STREAM TUBE MODEL FOR FIELD-SCALE SOLUTE TRANSPORT .2. EXAMPLES AND CALIBRATION

The use of the stream tube model developed in the first part of this s tudy is illustrated for several examples with a stochastic pore-water velocity, nu, and distribution coefficient, K-d, The model allows quan tification of the concentration variance in the horizontal plane to ev aluate models for transport in heterogeneous fields, Increased vertica l solute spreading due to stochastic local-scale parameters is accompa nied by increased horizontal variations of the held-scale mean concent ration. Solute application at the surface is modeled as a boundary val ue problem (BW) and an initial value problem (IVP), The field-averaged concentration vs. depth exhibits more spreading for the BVP than the IVP since a variable solute mass is applied to each stream tube in the latter case, Flow is also modeled by a lognormal probability density function for the saturated conductivity, K-s, and the unit gradient as sumption instead of nu. The use of a random nu instead of K-s is prefe rable for small variations in water content, Results of the stream tub e model are compared with those of a one-dimensional macroscopic conve ction-dispersion equation (CDE) with effective parameters (i.e., depth -dependent constants), When these constants are determined from time m oments of the held-scale flux-averaged concentration, (c) over cap(f), for the BVP, the stream tube model and the macroscopic CDE will give different results if the effective parameters are used to model other transport scenarios, Finally, the stream tube model was fitted to the concentrations obtained from a detailed numerical simulation of flow a nd transport in a (hypothetical) heterogeneous field, The (simple) str eam tube model appears to provide a sensible description of the held-a veraged concentration and variance.