Mobilization of Heavy Metals in a Saline Confined Aquifer as a Consequence of Rainwater Injection: A Case Study in Southern Vietnam.

An artificial recharge test was performed in Ho Chi Minh City, Vietnam to see the geochemical response of a saline coastal plain aquifer to the injected rainwater. The results show that the rainwater injection can cause mobilization of heavy metals due to pyrite oxidation and this phenomenon can persist even after the full recovery of the injected water. In this study, a 30-m-deep well was installed in a confined aquifer. Pyrite framboids were observed in the sediment samples collected during the well drilling. A total of 400 L rainwater was injected into the well for 70 min. After waiting 63 h, the well was extracted at a pump speed of 2.7 L/min and the chemistry of the pumped groundwater was monitored for 10 h. The groundwater showed geochemical features close to rainwater at the early stage of pumping and gradually changed to those of the background waters, especially, in electrical conductivity and Cl- concentration, as the pumping proceeded. However, the groundwater pumped in the later stage showed much increased concentrations in SO42-, total iron (FeT), AsT, Ni, Mn and Zn relative to the calculated mixing concentrations due to pyrite oxidation even though NO3-, the pyrite oxidant, already had disappeared. It was revealed from the geochemical modeling that the persistent pyrite oxidation was the result of the reaction with ferrihydrite, which precipitated in pores of the sediment by the injection of aerated water. We believe our study is a good example showing the importance of careful design of the artificial recharge systems to avoid or minimize the geochemical disturbance of aquifer.

A multi-tracer approach to disentangle anthropogenic emissions from natural processes in the St. Lawrence River and Estuary.

The ability to differentiate anthropogenic signatures from natural processes in complex hydrological systems is critical for environmental regulation perspectives, especially to curb pollution and implement effective water management strategies. Here, we report variations in the concentrations of 57 chemical variables, including nutrients, major, trace and ultra-trace elements, as well as the concentrations of Escherichia coli in different water masses along the St. Lawrence River-Estuary continuum. The constant ratios among major elements indicate consistent carbonate and silicate weathering processes in the drainage basins. We also suggest applying Ce anomalies to trace waters of low alkalinity and low complexing capacity as the dominance of Ce3+ free ion could promote Ce oxidation, and thus negative Ce anomalies. Furthermore, the positive Eu anomalies and elevated Tl concentrations could highlight the cation exchange processes on clay particles. In the fluvial and estuarine sections of the St. Lawrence System, we demonstrate significant contributions of wastewater discharge and discuss the suitability of several wastewater tracers, e.g., excess of B, Na, K, as well as Rb/Sr and Gd anomalies. We also highlight the inputs of several minor and trace elements (e.g., Mn, Fe, Cu, Co, Ni) from south-shore tributaries to the St. Lawrence System. However, the complex anthropogenic activities in the watersheds did not allow clear source partitioning. Finally, increased mixing of different river water masses upstream of Quebec City, together with the estuarine salt front and suspended sediments, are also responsible for releasing these minor and trace elements into the aquatic media. The results presented here help support further environmental actions to curb the emission of contaminants in the St. Lawrence System and provide more robust tracers of natural and anthropogenic processes in aquatic environments.

Groundwater in Southern Vietnam: Understanding geochemical processes to better preserve the critical water resource.

A thorough understanding of groundwater geochemical characteristics and dominant hydro(bio)geochemical processes in the aquifers is valuable for sustainable groundwater protection. With this respect, this study provides a comprehensive assessment of hydrogeochemical characteristics of groundwater in sedimentary aquifers of the Southern region of Vietnam. The dataset comprised 291 water samples collected in rainy and dry seasons from 155 wells, and their chemical compositions of dissolved ions (Ca2+, Mg2+, Na+, K+, HCO3-, Cl-, SO42-, NO3-, NH4+, Fe, total dissolved solids) and pH. We calculated the groundwater quality index to determine the suitability of groundwater for drinking purposes. Accordingly, about 47% of groundwater samples range from poor to unsuitable quality for drinking purposes, in which total dissolved solid (TDS) and high iron concentrations are primary factors. We also examined hydrogeochemical characteristics by multivariate statistical analyses (Hierarchical Cluster Analysis and Principal Component Analysis). The results demonstrated four groups of water: fresh groundwater (TDS < 1 g/L) in the highland (Group 1), lowland fresh-to-saline groundwater (2a), lowland saline groundwater (2b), and lowland saline/acidic groundwater (2c). Although the highland area is currently not impacted by salinization, the contamination by nitrate and chloride associated with a dense urban population and agricultural practices is deteriorating its fresh groundwater resources. On the other hand, the lowland area of Southern Vietnam is highly vulnerable to seawater intrusion (groups 2b and 2c). Only 34% out of 191 samples collected in the lowland area of southern Vietnam remained fresh. In this light, this study depicted the specific geographical location for various groundwater groups in Southern Vietnam. This finding is significant to assist water scientists and decision-makers in implementing targetted groundwater management measures as prevention and protection strategies should be tailored to groundwater geochemical characteristics and the dominant hydro(bio)geochemical processes.

Occurrence of metal-rich acidic groundwaters around the Mekong Delta (Vietnam): A phenomenon linked to well installation.

Acidic groundwaters enriched with heavy metals are frequently observed in the coastal plain aquifers. The acidic pHs are observed even in the deep confined aquifers in southern Vietnam. This study geochemically explores the causes of these acidic groundwaters by investigating 41 groundwater samples, 4 soil samples and a 54 m long sediment core and the long-term monitoring data (4189 observations) obtained from 178 wells of the National Groundwater Monitoring Network for the South of Vietnam (NGMNS). The groundwater data show elevated Fe, Mn, Al, Pb, and Zn concentrations as the pH becomes acidic and suggest pyrite oxidation be the major cause for the groundwater acidification. This is further confirmed by pyrite framboids observed in the sediment or soil samples taken from the sites where strongly acidic groundwaters were observed. Results of leaching experiments using sediment and soil samples indicate that high metal concentrations in the acidic pH are associated with the increased metal solubility and mineral dissolution kinetics. The acidification of deep groundwaters is revealed to be associated with well installation, indicating the importance of proper well-installation techniques to protect water quality of deep confined aquifers.