Studying Transformations of the Hydrogeochemical Conditions of Pliocene Aquifer System (Mekong R. Delta, Vietnam) Using Thermodynamic Modeling

Abstract

The results of earlier studies of stable and radioactive isotopes have shown that groundwater of the Middle Pliocene aquifer system form mostly due to the infiltration of meteoric water and its mixing with sedimentation water of marine genesis. Thermodynamic methods of studies were used to identify the main factors governing the formation of the hydrochemical pattern of groundwater and to determine the zones of its widest occurrence. Thermodynamic calculations established the level of geochemical equilibrium between the groundwater and the host rocks; determined the conventional boundary of the occurrence of interaction processes between infiltration water and host rock minerals and the processes of its mixing with sedimentation water; demonstrated the considerable role of host rocks as a source of additional ions of calcium, magnesium, and sulfates in groundwater chemistry; specified the mineral composition (secondary mineral phase) of the host rocks; and identified the minerals (microcline, laumontite, muscovite, as well as gypsum and calcite) that determine the final equilibrium chemistry of the aqueous phase. The obtained equilibrium–kinetic model of the Middle Pliocene aquifer system (including the zones of groundwater recharge and transit flow), along with data of earlier isotope studies, were used for zoning the study area by groundwater formation conditions (in terms of hydrodynamics and hydrochemistry) and established that groundwater chemistry formation reflects the effects of climatic (precipitation chemistry and volume), geological–hydrogeological (occurrence depth, recharge area, groundwater flow velocity) geochemical (host rock composition), and tectonic (the distance from the faults and their directions) conditions.