Regional-Scale Effects of Glacial Till Aquitard Vertical Heterogeneity on Transient Well Flow and Groundwater Budget With Variable Discharge in Leaky Aquifer Systems

Abstract

In Pleistocene glacial regions, glacial till aquitards typically exhibit a significant regional-scale decrease in hydraulic parameters, such as specific storage (S_s) and hydraulic conductivity (K), with depth due to increased sediment compaction, reduced porosity, and lower pore connectivity. Pumping rates also decrease over time due to equipment wear and hydraulic friction. This study developed a novel analytical model for variable groundwater discharge in a leaky aquifer system, incorporating the often-overlooked vertical heterogeneity of aquitards. The model integrates two exponential decay mechanisms: one for the aquitard S_s and K with depth, and one for the pumping rates over time. Special cases addressing only depth-decaying S_s, depth-decaying K, or constant rates were also derived. New analytical solutions investigate how the decay exponents of aquitard S_s and K, variable pumping rates, and aquitard thickness influence drawdown and groundwater budgets. Results reveal that variable pumping rates cause local maxima and minima in aquifer drawdown, especially with higher decay of S_s and K. While aquitard thickness and pumping rates affect depletion and leakage, changes in the physical properties of aquitards, such as the decay of S_s and K, are more critical in determining the actual patterns of depletion and leakage fractions. Larger S_s decay or smaller K decay enhance early peak depletions caused by time-decaying pumping rates. These results highlight the importance of prioritising depth-dependent parameters in groundwater management, particularly in regions with significant vertical heterogeneity, like glacial deposits. The study offers valuable insights for hydrological assessments and optimising groundwater resource management in similar settings.