Comparison of Methods Used to Delineate Capture Zones of Wells: 2. Stratified‐Drift Buried‐Valley Aquifer

Analytical, semianalytical, and numerical models of the flow system in a stratified‐drift buried‐vailey aquifer were used with particle‐tracking/stream‐function programs to delineate traveltime‐related capture zones of a municipal wellfield and to assess conceptual errors imposed by requisite assumptions associated with each flow model. The analytical flow model uses the Theis equation describing two‐dimensional transient drawdown surrounding a well in a fully confined aquifer and superposition of a nonuniform regional flow field. The semianalytical flow model implicitly uses the Thiem equation describing two‐dimensional steady‐state drawdown surrounding a well in a fully confined aquifer and superposition of a uniform regional flow field. The numerical flow model uses a three‐dimensional steady‐state finite‐difference solution and incorporates four model layers, specified‐flux and head‐dependent flux boundary conditions, spatially variable recharge, and head‐dependent leakage to and from streams. The predictive ability of the flow models is based on comparison of mean absolute errors and root mean squared errors between measured and simulated heads as well as comparison of pathline distributions defining the one‐year capture zones. The comparisons show that the uniform flow field and uniform transmissivity required of the semianalytical flow model cause it to be less accurate than the analytical and numerical flow models. The inability of the analytical flow model to incorporate spatial variations in transmissivity causes it to be less accurate than the numerical flow model. These conceptual errors associated with use of the analytical and semianalytical flow models in this hydrogeologic setting cause their one‐year capture zones to be less reasonable than those from the numerical flow model.