The "age" of very old groundwater: insights from reactive transport models

The distribution of radioactive and radiogenic isotopes such as Cl-36 and H e-4 provides important information about the residence time (or "age") and flow velocity of groundwater in sedimentary basins, but the relationship be tween the distribution of an isotope and the basin hydrology is not always clear. We use reactive transport modeling to investigate the controls on is otope distribution and groundwater residence time in hypothetical and real flow regimes. Our results show that by moving beyond the simple "piston flo w" concept, we can use isotope distributions to constrain internally consis tent descriptions of deep groundwater flow regimes within sedimentary basin s. (C) 2000 Elsevier Science B.V. All rights reserved.