Abstract
Groundwater in the Red River’s delta plain, North Vietnam, was found in Holocene, Pleistocene, Neogene and Triassic aquifers in fresh, brackish and saline types with a total dissolved solids (TDS) content ranging from less than 1 g L−1 to higher than 3 g L−1. Saline water exists inHolocene aquifer, but fresh and brackish water exist in Pleistocene, Neogene and Triassic aquifers. This study aims at the investigation into genesis and processes controlling quality of water resources in the region. For this isotopic, combined with geochemical techniques were applied. The techniques include: (i) measurement of water’s isotopic compositions (δ2H, δ18O) in water; (ii) determination of water’s age by the 3H- and 14C-dating method, and (iii) chemical analyses for main cations and anions in water. Results obtained revealed that saline water in Holocene aquifer was affected by seawater intrusion, fresh water in deeper aquifers originated from meteoric water but with old ages, up to 10,000–14,000 yr. The recharge area of fresh water is from the northwest highland at an altitude of 140–160m above sea level. The recharge water flows northwesterly towards southeasterly to the seacoast at a rate of 2.5m y−1. Chemistry of water resources in the study region is controlled by ferric, sulfate and nitrate reduction with organic matters as well as dissolution of inorganic carbonate minerals present in the sediment deposits. Results of isotopic signatures in water from Neogene, Triassic and Pleistocene aquifers suggested the three aquifers are connected to each other due to the existence of faults and fissures in Mesozoic basement across the delta region in combination with high rate of groundwater mining. Moreover, the high rate of freshwater abstraction from Pleistocene aquifer currently causes sea water to flow backwards to production well field located in the center of the region.
Highlights
The Red River Delta plain (RRDP), covering an area of approximately 15,000 km2 is situated across the northern part of Vietnam and is one of the two most productive deltas in the country
Pleistocene and Neogene aquifers in study region ranged from 0.01 to 0.57, which is much lower compared to that ratio for water in borehole GV01 where groundwater is purely fresh and of Ca-HCO3 compared to that ratio for water in borehole where groundwater is purely fresh and of Ca+ ] GV01 type for which the equivalent [Ca2+ ] to [Na ratio was 28
This indicates the excess of sodium over chloride concentration that is due to the release of sodium from aquifers sediment by Ca2+/Na+ exchange as it was explained by otherresearchers, e.g., [48]
Summary
The Red River Delta plain (RRDP), covering an area of approximately 15,000 km is situated across the northern part of Vietnam and is one of the two most productive deltas in the country. In RRDP, water is used for irrigation, industrial and domestic purposes, out of which irrigation consumes around. In the RRDP, groundwater is the main source of domestic usage water. Reports state that the groundwater storage capacity of the RRDP is over 23 billion m3 , Water 2019, 11, 2120; doi:10.3390/w11102120 www.mdpi.com/journal/water out which, roughly is located in the 20%. Nam part of the overof billion m3, out20%. Of which, roughly is located in the southern part,Dinh i.e., and ThaiaBinh, Nam Ninh. Dinh Binh and aprovinces part of the[2,3]. Ninh Binh provinces [2,3].
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