MODELING OF CONTAMINANT MOVEMENT NEAR PUMPING WELLS - SATURATED-UNSATURATED FLOW WITH PARTICLE TRACKING

A transient axisymmetric saturated-unsaturated numerical flow model wa s coupled with a particle tracking model to investigate the movement o f contaminants when a shallow unconfined aquifer is pumped at a consta nt rate, The particle tracking model keeps track of locations and mass es of solutes in the aquifer, and the time of capture by the well, At the end of each time-step the flow model solves the Richard's equation for the hydraulic head distribution from which elemental velocities a re calculated, Solutes are then displaced for a period equivalent to t he time-step using both the magnitude and direction of the elemental v elocities, Numerical experiments were performed to investigate effluen t concentrations in wells with screens of different length and in diff erent positions relative to zones of stratified contamination, At earl y times of pumping the effluent concentrations were similar to the con centrations adjacent to the well screen, but at late times, the concen trations approached the vertically averaged concentration in the aquif er, Time to attain the vertically averaged concentration was determine d by the well geometry, initial location of the contaminant plume in r elation to the well screen, and hydraulic properties of the aquifer, T he results are consistent with the hydraulics of now to a pumping well and of particular importance, they demonstrate that short-term pump t ests could give erroneous design concentrations for pump-and-treat sys tems. The model provides a means of quantifying arrival times and mixi ng ratios, It could therefore provide a useful means of designing prod uction wells in aquifers with stratified contamination and more effici ent recovery systems for aquifer remediation.