Phreatic-confined discontinuities and restricted flow in confined groundwater systems

Six study cases are described in which a significant discontinuity is observed at the transition zone between phreatic and confined sections of an aquifer. The discontinuities are seen in the chemical composition, isotopic composition of D, 18O and 13C, in the concentrations of age indicators, such as tritium, 14C and 4He, as well as in the values of paleoclimate indicators, e.g. Ne, Ar, Kr and Xe or 18O. The age indicators are most useful in checking the degree of hydraulic communication occurring between phreatic and confined sections of an aquifer. The observations shed light on the importance of drainage as a factor regulating flow in confined systems. This explains reported discrepancies between hydrologically calculated groundwater ages, based on gradients and transmissivities, and much higher ages deduced from isotopic age indicators, e.g. 14C, 4He or 36Cl. (author). 9 refs, 14 figs, 1 tab