Quantifying leakage and mixing in an alluvial aquifer system: a combined hydrochemical and hydrodynamic modelling approach constrained by isotopic evidence

摘要

Decline of groundwater quality has occurred at some sites due to induced leaching of salts in the over-stressed Lower Murrumbidgee alluvial groundwater system, Australia. Leakage and inter-aquifer mixing was assessed in detail for the 1998/99 irrigation season at the Tubbo site using both hydrochemical and hydrodynamic information. A local meteoric water line was established for the area to facilitate the identification of plausible leakage end-members. The most significant hydrochemical changes occurred in the middle aquifer within the first month of pumping. The water type converged with that of the deeper aquifer as waters became more depleted in δ~(18)O, δ~2H and then enriched in δ~(18)O, δ~2H and δ~(13)C. A radially axisym-metric FEFLOW model transient boundary conditions, is being developed to differentiate vertical leakage and lateral flow near the pumping bore. Good calibration could be achieved with observed hydraulic heads in the middle and deep aquifers either with spatially continuous or discontinuous silt units. Hydrodynamic data from the deep silt unit indicated that direct mixing occurred through aquitard windows, a finding consistent with hydrochemical and stable isotope evidence.