Transient 3D simulation of 18O concentration by codes MODFLOW-2005 and MT3DMS in a regional-scale aquifer system: an example from the Estonian Artesian Basin

It is proved that a transient 3D distribution of 18O concentration in a regional-scale heterogeneous multi-layered aquifer system can be numerically simulated by codes MODFLOW-2005 and MT3DMS as a boundary problem. An optimum method of the transition of the observed negative δ18O values to respective positive units of the absolute 18O concentration needed for simulations has been substantiated. The practical applicability of the elaborated method has been verified by the reconstruction and interpretation of the geohydrological history of the Estonian Artesian Basin during the Late Pleistocene and Holocene. The adequacy of regional hydrodynamic calculations proceeded by eight consecutive modelling scenarios has been verified by a good correlation between the measured and simulated 18O values. The set of functionally interconnected groundwater flow and 18O transport models forms an integral hydrogeological model of the Estonian Artesian Basin for the last 22 ka. The paper contributes to a wider application of 18O concentration as a conservative tracer in the investigation of the complex problem of groundwater flow and transport in real-world conditions.