COMPARISON OF FLOW MODELS USED TO DELINEATE CAPTURE ZONES OF WELLS .1. LEAKY-CONFINED FRACTURED-CARBONATE AQUIFER

Analytical, semianalytical, and numerical flow models of a leaky-confined fractured-carbonate aquifer were used with particle-tracking/stream-function programs to delineate traveltime-related capture zones of a municipal wellfield and to assess "model" errors imposed by requisite assumptions associated with each flow model. The analytical flow model uses the Hantush-Jacob equation describing two-dimensional transient drawdown surrounding a well in a leaky-confined aquifer with superposition of a nonuniform regional flow field. The semianalytical flow model employs the Thiem equation describing two-dimensional steady-state drawdown surrounding a well in a fully confined aquifer with superposition of a uniform regional flow field. The numerical flow model uses a finite-difference solution describing three-dimensional steady-state flow and incorporates five model layers, specified-flux boundary conditions, spatially variable recharge, and recharge from a lake. Goodness-of-fit statistics show that the semianalytical flow model does not calibrate as well to measured heads as do the analytical and numerical flow models. Model errors caused by the inability of the semianalytical flow model to account for vertical leakage and by the required use of a uniform regional flow field cause the size of the five-year capture zone to be overestimated and its shape to be too simplistic compared to those of the analytical and numerical flow models. Polar plots of reverse-tracked pathline endpoints suggest that model error also produces significant differences in pathline trajectories. The results show that in this type of hydrogeologic setting, it is necessary to use a flow model that accounts for vertical leakage and for the nonuniform character of the regional flow field.