A compartmental mixing-cell approach for the quantitative assessment of groundwater dynamics in the Otway Basin, South Australia

Transient groundwater fluxes, as calculated by a groundwater flow model (MODFLOW) are used as input into a compartmental mixing-cell (CMC) model to simulate the transport of hydrochemical and isotopic species in regional groundwater systems. Good agreement between modeled and measured concentrations of solutes in groundwater provides greater confidence in estimating aquifer parameters for the flow model. The CMC model is currently developed fur multi-layered groundwater systems and allows for both recharge to the uppermost aquifer and upward or downward leakage between individual aquifers. The combination of the CMC approach and MODFLOW allows chemical and hydraulic data to be analysed simultaneously to allow for improved conceptualization of the groundwater system. The use of transient fluxes reflects the temporal scale and variability of the chemical tracers as well as the temporal variability of hydraulic conditions. The model is applied to a transect in the Gambler Embayment of the Otway Basin in South Australia to quantify mixing between two main aquifer systems. This groundwater system is hydraulically connected to the ocean in the west and as a result would have been affected by sea level changes over the last 30000 yr. Radiocarbon simulations helped to quantify a major zone of downward leakage to the confined aquifer, with modeled rates of leakage ranging 2-9 mm/yr. (C) 1999 Elsevier Science B.V. All rights reserved.