Stagnant groundwater stored in isolated aquifers: Implications related to hydraulic calculations and isotopic dating (Reply)

Not all groundwater flows, and traps that host stagnant groundwater may also occur, which are similar in nature to the better known traps of oil, gas and brine. Such traps are envisaged as mega-inclusions that are hydraulically isolated. They are under no hydrostatic head, as they are sealed from above and disconnected from recharge, but they are pressurized to various degrees by compaction. Hydraulic gradients based on data obtained from wells that belong to distinct aquifers that are hydraulically isolated from each other are false gradients. False hydraulic gradients applied in Darcian equations of flow velocity and water age lead to meaningless results (e.g. water ages that are too young). Criteria to establish hydraulic isolation of trapped groundwater include geological configuration (deep aquifer, engulfed by impermeable rocks), abrupt differences in water properties observed between adjacent wells (temperature, pressure, concentration of solutes, water age), and high groundwater ages (more than 107 years). The Great Artesian Basin, Australia, is discussed in some detail in the light of these guidelines, revealing a multitude of pressurized isolated aquifers. Calibration of groundwater dating methods based on hydraulic computations and isotopic methods (e.g. 36Cl and 4He) is discussed for the Great Artesian Basin, with reference to the competing models of dynamic and static groundwater aquifers