DELINEATION OF TRAVELTIME-RELATED CAPTURE AREAS OF WELLS USING ANALYTICAL FLOW MODELS AND PARTICLE-TRACKING ANALYSIS

Traveltime-related capture areas of wells can be estimated in hydrogeologic settings with predominantly two-dimensional flow regimes using image-well theory, the principle of superposition, an appropriate well-hydraulics equation, and particle-tracking analysis. These analytical methods can be used to simulate the response of an aquifer to pumping stress by incorporating site-specific values of hydraulic properties, the effects of geologic and hydrologic boundaries, nonuniform regional hydraulic gradients, and well interferences. This is done by developing a conceptual model of the flow system, incorporating an image-well solution (if needed), computing drawdowns at the intersections of a rectangular grid superposed over the area of interest, and subtracting computed drawdowns from regional prepumping head values assigned to the grid intersections. The resulting flow model can be calibrated by comparing predicted heads with measured heads. The calibrated head distribution then is used in a particle-tracking analysis to determine flowpath trajectories and traveltime-related capture areas of wells. Analytical flow models based on the Theis equation and the Hantush-Jacob equation were constructed from conceptual flow models based on geologic, hydrologic, and pumpage data from wellfields completed in a confined/unconfined stratified-drift aquifer and in a leaky-confined fractured-carbonate aquifer. Predicted head distributions from the calibrated flow models were used in a particle-tracking analysis to compute flowpath trajectories and traveltime-related capture areas. The results compare favorably with known flowpaths from sources of contamination proximal to each wellfield.