Environmental isotope techniques were applied to study the hydrochemical characteristics of groundwater in Soc Trang Province, Southern Vietnam, in frame of the project Improvement of Groundwater Protection in Vietnam (IGPVN). Groundwater samples were collected from various monitoring wells (newly drilled by the IGPVN project), national monitoring wells, private tube wells and production wells. Surface water samples were collected from rivers, ponds or canals. The aquifer system is more complex than presumed as the hydrochemical and stable isotope compositions of groundwater samples in the Middle-Upper Pleistocene (qp) aquifer differ significantly in lateral direction. Furthermore, observed changing redox reactions within the target aquifer from dry to wet season make it probable that some interaction with overlying aquifers exists. The stable isotope signatures of the qp groundwater samples can be divided into two distinct groups which, respectively, originated from paleo-meteoric water and either was located in paleo-salinized areas of the qp aquifer or resulted from evaporation effect of recharging water prior or during infiltration process. In fact, individual parts of "the same" aquifer seem not to be hydraulically connected to each other. The environmental isotope data provided neither evidences of hydraulic connection between the rivers and the qp aquifer nor of recent groundwater recharge in the Province. As a result, saltwater from the sea intruded inland to some extent via the Hau River during the dry season, but it did not affect the target aquifer. Any recharge from surface water to the qp aquifer in Soc Trang should occur outside the boundaries of Soc Trang Province. Considering the low groundwater transit velocities roughly estimated in this study (3.6 m/year and 7.8 m/year), it may take several ten thousands to hundred thousands of years for recharging water from beyond the Vietnam's national border to reach the qp aquifer in Soc Trang Province. Consequently, natural recharge cannot help to reduce groundwater declining in the short-to-middle term.
Recent exploration has revealed that deep-seated and large groundwater reservoirs in Africa's intracontinental basins can be regarded as an additional strategic resource for supply of drinking water. The origin, genesis and recharge of these groundwater reservoirs, however, are still poorly understood. A multidisciplinary approach involving remote sensing, geophysical surveys and hydraulic investigations, as well as hydrochemical and isotope studies, was pursued to gain better insight into the genesis and the potential of a recently discovered lower Kalahari aquifer (LKA) located in the Zambezi Region (Namibia). The study shows that regional tectonic activity associated with the propagation of the Okavango Rift Zone had a tremendous impact on the drainage evolution and hydrogeological setting of the region. Furthermore, there is geomorphological evidence that the LKA-prior to tectonic subsidence and burial-was part of a paleochannel of the upper Zambezi River. Hydraulic continuity could be confirmed by geochemical evolution down the flow path. Cation exchange combined with dissolution of calcite progressively produces alkalinity and sodium and consumes calcium in the north-south direction. Comparison of stable isotope content of the LKA with modern rainfall indicates that the recharge occurred under cooler climate conditions. Analysis of 14 C concentrations and 36 Cl/Cl ratios show that the age of the groundwater exceeds 100 ka and is hence older than presumed. It is concluded that the assessment of the sedimentology, tectonic structures and geochemistry are key factors for understanding both the paleoclimatic and modern recharge processes of deep-seated aquifer systems.
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