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 st udied by simulating subsurface transport of environmental tritium. Sub surface transport included transport in a moderately deep unsaturated zone and in a relatively long cross section of the aquifer. The tritiu m data from the site enabled a four-step modeling analysis comprising (1) estimation of tritium content in the infiltration water, (2) trans port in the unsaturated zone, (3) estimation of flux-averaged tritium concentration in the recharge water, and (4) transport in the groundwa ter zone. The groundwater model simulations were sensitive to the long itudinal 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 l ikely 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)] t he two parameters being interdependent in that an increase in alpha(L) results in a decrease in alpha(T) and vice versa. The reported disper sivities 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 earl ier defined criteria, the range or the representative set of values th en represent the largest scale of earlier reported values. Including o ur range of alpha(L) in the set of reported dispersivities suggests th at alpha(L) does not increase indefinitely with scale.