Search

Groundwater has been the primary source of daily water supplies for people living in the Red River Delta, the second largest delta in Vietnam. For this reason, identification of hydrogeochemical properties of the groundwater is indispensable for sustainable utilization of groundwater sources. In this study, the spatio-seasonal hydrogeochemical characteristics of groundwater in the unconfined aquifer of the Red River Delta have been investigated by systematically applying self-organizing maps (SOM) and Gibbs diagrams. The groundwater chemistry dataset used in the analysis is composed of eight major dissolved ions (i.e., Ca2+, Mg2+, Na+, K+, HCO 3 − , Cl−, SO 4 2 − , and CO 3 2 − ) and total dissolved solids that are collected from 47 groundwater monitoring wells within the study area during the dry and rainy seasons. The SOM application classified the hydrogeochemical data into five clusters, which revealed three basic representative water types: high salinity (one cluster), low salinity (two clusters), and freshwater (two clusters). The spatial distribution of clusters and water types were identified. In particular, the low-salinity type was found not only in the downstream area but also in the northeastern parts of the upstream and middle-stream areas, where the groundwater was mainly classified into one specific cluster, in which agricultural activities were considered to influence groundwater chemistry. Cluster changes from the dry to rainy seasons were detected in approximately one-fifth of the observations wells. Dilution by surface water may significantly affect the chemical characteristics of the unconfined aquifer during the rainy season. Based on Gibbs diagrams, rock weathering was found to be the main process in the evolution of chemical composition of freshwater type, whereas the chemical structure of the low- and high-salinity types was primarily controlled by saltwater intrusion or anthropogenic activities.

Summary The Red River Delta (RRD) is the second largest delta in Vietnam, and its local communities depend on groundwater sources for water supply. A clear understanding of the groundwater hydrogeochemical properties, particularly their changes from the dry to rainy seasons and spatial characteristics, is invaluable and indispensable for the management and protection of this important water resource. In this study, self-organizing maps was systematically applied for the first time to investigate the seasonal and spatial hydrogeochemical characteristics of groundwater in the Pleistocene confined aquifer of the RRD. The hydrogeochemical characteristics clustered by SOM were further examined using the Gibbs Diagrams. The groundwater chemistry dataset used in the analysis comprised eight major dissolved ions (i.e., Ca 2+ , Mg 2+ , Na + , K + , HCO 3 − , Cl − , SO 4 2− , and CO 3 2− ) and total dissolved solids that were collected from 52 groundwater monitoring wells within the study area during the dry and rainy seasons. Based on the results, the hydrogeochemical groundwater data of the confined aquifer monitoring wells for the delta were classified into 8 clusters, which revealed three basic representative water types: high salinity (2 clusters), low salinity (3 clusters), and freshwater (3 clusters). The high-salinity types were located in the middle-stream and coastal areas of the RRD, while the low-salinity types were observed near the western and northeastern boundaries of the delta. Cluster changes from the dry to rainy seasons were detected in approximately one-third of the observation wells. The increase in groundwater recharge during the rainy season is the main reason for these changes. Based on Gibbs diagrams, the source of soluble ions in the groundwater of the freshwater types was found to be the weathering of rock-forming minerals, while evaporation and marine activities (leaching from salty paleowater and salt water intrusion) were found to be the main factors affecting the chemistry of the groundwater characterized by the low- and high- salinity types, respectively.

In the Red River Delta, situated in the northern part of Vietnam, nearly its entire population depends solely on groundwater for daily water consumptions. For this reason, groundwater quality assessments must be carefully carried out using hydrogeochemical properties, to ensure effective groundwater resource planning for the Delta’s present and future groundwater use. In this study, the spatial and seasonal changes in the hydrogeochemical characteristics of groundwater in the two main aquifers of the RRD were investigated by analyzing the physicochemical data obtained in 2011 from 31 conjunctive wells in the Delta’s Holocene unconfined aquifer (HUA) and Pleistocene confined aquifer (PCA) using the Piper diagram and the Gibbs diagram. Results of the data analysis show that the groundwater in both aquifers in the upstream area of the delta is dominated by the [Ca 2+ –HCO 3 − ] water-type, while the [Na + –Cl − ] dominates along the middle-stream and downstream areas. Seasonal changes in the hydrogeochemical facies in both aquifers, comparing the results for the dry and the rainy seasons, were detected in about one third of the sampling wells, which were mainly located at the upstream portion of the Delta. The hydrogeochemical facies of HUA were different from that of PCA by about 45% of the sampling wells in both the dry and the rainy seasons, which were found mostly in the upstream and middle-stream areas.

Groundwater from the Holocene unconfined aquifer (HUA) and Pleistocene confined aquifer (PCA) is the major source of drinking and domestic water in Hanoi, Vietnam. A clear understanding of the groundwater hydrogeochemical properties, particularly their changes during the dry and rainy seasons, is invaluable for the management and protection of this important water resource. In this study, the changes in the hydrogeochemical properties of groundwater during the dry and rainy seasons were investigated by analyzing the major ions (Ca2+, Mg2+, Na+, K+, HCO3 −, SO4 2−, Cl−) that were recently obtained in 2011 from 13 conjunctive sampling wells for HUA and PCA in Hanoi. The hydrogeochemical assessment was carried out using the Piper and Gibbs diagrams. Based on the results, at least 30 % of the sampling wells in both aquifers exhibited changes in the hydrogeochemical facies during the dry and rainy seasons. Most of the changes occurred for the cation-type facies, while the anion type remained unchanged. Moreover, the hydrogeochemical facies of HUA was found to be different from that of PCA by approximately 50 % of the sampling wells for both the dry and rainy seasons. In addition, the Gibbs diagram showed apparent differences in the weight ratios Cl/(Cl + HCO3) between PCA and HUA. The results revealed that rock weathering is the main process involved in the evolution of chemical composition of groundwater, but agricultural activities and salty paleowater are the other important factors that affect the groundwater chemistry in some parts of the study area.

A groundwater-monitoring network has been in operation in the Red River Delta, Vietnam, since 1995. Trends in groundwater level (1995–2009) in 57 wells in the Holocene unconfined aquifer and 63 wells in the Pleistocene confined aquifer were determined by applying the non-parametric Mann-Kendall trend test and Sen’s slope estimator. At each well, 17 time series (e.g. annual, seasonal, monthly), computed from the original data, were analyzed. Analysis of the annual groundwater-level means revealed that 35 % of the wells in the unconfined aquifer showed downward trends, while about 21 % showed upward trends. On the other hand, confined-aquifer groundwater levels experienced downward trends in almost all locations. Spatial distributions of trends indicated that the strongly declining trends (>0.3 m/year) were mainly found in urban areas around Hanoi where there is intensive abstraction of groundwater. Although the trend results for most of the 17 time series at a given well were quite similar, different trend patterns were detected in several. The findings reflect unsustainable groundwater development and the importance of maintaining groundwater monitoring and a database in the Delta, particularly in urban areas.

Predictions in Ungauged Basins (PUB) mainly focus on surface water, but there are much higher prediction uncertainties inherent in groundwater. Hanoi depends entirely on groundwater for its domestic water supply. However, the characteristics of the entire Hanoi aquifer system remain poorly understood due to the lack of available data. Recently, we were nominated to construct a hydrogeological database. Using the valuable data contained in this database, this paper comprehensively analyzed the best number of 240 boreholes including well logs and their hydrogeological parameters obtained from pumping tests for the first time in order to identify the entire Hanoi aquifer system and characterize the hydrogeological conditions for potential groundwater resources. Great efforts have been made to establish and analyze the hydrogeological maps, cross sections and the isopach maps of main aquifers’ thickness and transmissibility. As for the results, we found that groundwater mainly exists in the topmost Holocene unconfined aquifer and the shallow Pleistocene confined aquifer (PCA), while cleft and karst water exists in the Neogene water bearing layer and the Mesozoic fractured zones. These aquifers are adequately, quantitatively characterized and evaluated from the viewpoint of potential groundwater resources. We found the PCA serving as the highest groundwater potential and the most important aquifer for the water supply. The findings are indispensable for further groundwater analyses contributing to ensuring the sustainable groundwater development not only in Hanoi but also in poorly gauged or ungauged neighboring basins. Copyright © 2012 John Wiley & Sons, Ltd.

The Red River Delta is one of two biggest deltas in Vietnam. People living in the delta depend entirely on groundwater for their domestic water. However, the aquifer system in the whole Red River Delta remains poorly understood due to the lack of available data. Recently, we were nominated to construct a hydrogeological database. Using these valuable data contained in this database, this paper comprehensively analyzed the best number of 778 boreholes including well logs and their hydrogeological parameters obtained from pumping tests for the first time in order to identify the entire aquifer system and characterize hydrogeological conditions in the whole delta for potential groundwater resources. Great efforts have been made to establish and analyze hydrogeological maps, cross sections, and contour maps of main aquifers’ thickness and transmissivity. As for the results, we found that groundwater mainly exists in Quaternary unconsolidated sediments as porous water forming the topmost Holocene unconfined aquifer (HUA) and the shallow Pleistocene confined aquifer (PCA) sandwiching the Holocene-Pleistocene aquitard (HPA), while cleft and karst water exist in consolidated Neogene formations and Mesozoic rocks constituting the Neogene water bearing layer (NWL) and Mesozoic fractured zones (MFZ), respectively. PCA is almost entirely distributed over the delta. It serves as the highest groundwater potential and the most important aquifer for water supply. HUA is also widely distributed about 88% over the delta and has a high groundwater potential. NWL and MFZ, placed below PCA but exposed on the surface outside the delta, are minor sources for local domestic water supply only. These findings are indispensable for further groundwater analyses needed to ensure the sustainable groundwater development for the high-security water requirements in the delta, but have never been completed sufficiently before due to the unavailability of large-scale basic data sets.