Aquifer parameter estimation for a constant-flux test performed in a radial two-zone aquifer

A patchy aquifer or an aquifer with a finite thickness skin can be considered as a radial two-zone aquifer system, which can be characterized by five parameters, i.e., the thickness of the first zone and four aquifer parameters including the transmissivity and storage coefficient for each of the first and second zones. This paper proposes an approach based on an analytical solution of a constant-flux pumping in a confined two-zone aquifer and the simulated annealing algorithm to determine the five parameters simultaneously. The estimated results for the five parameters are fairly good even assuming the aquifer as a single-zone system at the beginning of the data analysis. The estimated results indicate that the first-zone parameters are much more difficult to accurately identify than the second-zone parameters due to insufficient early-time data and high correlation of the sensitivities among the first-zone parameters. However, the problem of inaccurate results obtained at the first-zone can be significantly improved if more densely temporal drawdown measurements are used.