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.

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.