STEADILY DISCHARGING ECCENTRIC WELLS: COMPARING NUMERICAL SIM-ULATIONS TO EXISTING EXPERIMENTS

OpenAlex · English

Abstract Operation of eccentric wells in a circular island unconfined aquifer under steady state flow conditions was addressed in this study by comparing the results of experimental, analytical and developed numerical models. MODFLOW was used in the developed numerical models. The results include simulations of various refined meshes. Such relationships are of practical importance in operating eccentric wells in unconfined radial flow systems. The objectives of this study were: 1) to determine the drawdown ranges, in terms of their agreement, among the numerical, experimental, and analytical solutions; 2) to test whether the MODFLOW (using PCG2 and PCGN solvers) provides accurate results for such wells; and 3) to investigate the sensitivity of the numerical solutions to mesh size and eccentricity. The results showed that all solutions performed better for smaller drawdowns in eccentric wells than they do for larger drawdowns; and for smaller drawdowns, analytical and PCG2 models provided good agreement taking into consideration root-mean-square error as evaluation criteria. For larger drawdowns, analytical and PCG2 either grossly overestimated the drawdown values or became inapplicable as the hydraulic head in well dropped to zero. PCGN solver performed best for both smaller and larger drawdowns, but the parameter selection needs care. Eccentricity and mesh size have effects on the solutions, especially for parameter estimation in PCGN, and the consistency of outputs requires investigation of optimal mesh size depending on eccentricity. When compared to wells with larger eccentricity, linear PCGN (PCGN1) performed better for large eccentricity regardless of mesh size, while PCG2 performed better for wells closer to the concentric. Numerical methods predicted larger drawdowns for finer mesh spacing size. Based on these results, numerical solutions could be used to adequately describe the drawdown for a well placed eccentrically within a circular island unconfined aquifer under steady state flow conditions provided the modeling parameters are dealt with scrutiny.