Large-scale dispersion in a sandy aquifer: Simulation of subsurface transport of environmental tritium

Large-scale dispersion in a sandy unconfined aquifer in Denmark was studied by simulating subsurface transport of environmental tritium. Subsurface transport included transport in a moderately deep unsaturated zone and in a relatively long cross section of the aquifer. The tritium data from the site enabled a four-step modeling analysis comprising (1) estimation of tritium content in the infiltration water, (2) transport in the unsaturated zone, (3) estimation of flux-averaged tritium concentration in the recharge water, and (4) transport in the groundwater zone. The groundwater model simulations were sensitive to the longitudinal and transverse dispersivity parameters, alpha(L) and alpha(T), as a set of parameters, but a model sensitivity analysis showed that it was not possible to identify a unique set of parameter values. A likely range of variation for the two parameters could be identified: (alpha(L), alpha(T)) is an element of [(1 m, 0.005 m); (10 m, 0.0 m)] the two parameters being interdependent in that an increase in alpha(L) results in a decrease in alpha(T) and vice versa. The reported dispersivities represent a scale of 1000 m, the approximate travel distance from the water table to the observation wells. If the estimated alpha(L) can be regarded as being of intermediate reliability following earlier defined criteria, the range or the representative set of values then represent the largest scale of earlier reported values. Including our range of alpha(L) in the set of reported dispersivities suggests that alpha(L) does not increase indefinitely with scale.