Formation of Groundwater Resources in the Nam Bo Plain, Vietnam

Van Canh Doan1, Duc Huy Trieu2
1 Vietnam Association of Hydrogeology
2 National Center for Water Resources Planning and Investigation - NAWAPI

O 10.7 in Wasser und Umwelt in Südostasien

24.03.2018, 11:00-11:15, 2

The origin, age and salinization of groundwater in the Nam Bo Plain, Vietnam were investigated using isotopic and geochemical techniques. Groundwater was sampled along three transects. Transect AA crosses the North through South starting from HCM City to Nam Can in Ca Mau province. Transect BB crosses through the southeast Nam Bo. Transect CC crosses through the southwest Nam Bo, between the Tien and Hau Rivers. Water samples were taken from 5 aquifers: middle Pleistocene (qp2-3), lower Pleistocene (qp1), upper Pliocene (n22), lower Pliocene (n21), and Miocene (n13) which are important in respect to the supply with fresh water for drinking and domestic use of the local population. The origin of groundwater in the study region was investigated based on the composition of water stable isotopes (d2H and d18O). The age of water samples was estimated by the tritium (3H) and radiocarbon (14C) dating method with the use of the stable carbon composition (d13C) in total dissolved inorganic carbon. The salinization process occurring in the aquifers was studied by using the chloride concentration in water in conjunction with the d18O in respective samples. Results of the d2H and d18O analysis suggested that groundwater in the Nam Bo region was originated from the local precipitation, but has paleo-ages. The d2H and d18O in water were spread relatively close to the regional meteoric water line and the age of water ranged from older than 100 a BP up to older than 40,000 a BP. Statistical treatment using the Mann-Whitney method without preliminary hypothesis, based on the mean value of d18O and its standard deviation in water from the Tien and Hau River and that in the qp2-3 showed that the groundwater was not or very weakly recharged from the Rivers as the mean d18O value of the two water groups was significantly different from each other as Z-value of the test was 2.81 higher than z-critical of 1.96 at a = 0.01. This means that the deeper aquifers did not or very weakly receive recharge from the surface, but water there likely was connate, so that fresh groundwater resources in the region seem to be limited. It was observed that groundwater in the same aquifer, but in one location was older, meantime in other sites it was younger than the age that could be expected from the common trend that the deeper the well the older water is. This fact was explained either by the inter-aquifer leakage or up-coning effect resulting from the over abstraction from certain aquifers. Several samples were contaminated with chloride ion but the salinization in most aquifers was thought to result from the migration of saline water entrapped in sediment pore to recharged fresh water. Meanwhile, in several locations mixing between fresh water with seawater was observed as evident from the d18O vs. [Cl-] relationship. Particular measures must be developed for better management of groundwater in the Nam Bo Plain to have sustainable resource of freshwater for the region.

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