Processes Causing Strong Acidic Groundwaters in and Around the Mekong Delta Area

Extremely acidic groundwaters (EAG; pH < 3) are frequently observed in the Mekong Delta area. This study geochemically investigates the processes for this phenomenon. Forty-two groundwater and eight sediment samples were collected and performed chemical/mineralogical analyses and leaching experiments. Long-term data from 178 wells of National Groundwater Monitoring Network of the Southern Vietnam (NGMS) were also inspected. In the study area, groundwaters are characterized by increasing concentrations of various species such as SO₄, Fe, Mn, Al, Pb, Cr, Zn and U at low pH. SO₄ (up to 2959 mg/L) and Fe (up to 1679 mg/L) concentrations in the EAG are generally very high. Framboidal pyrite grains are typically observed in the sediment samples taken from the sites where acidic groundwaters were observed. The leaching experiments with those sediments using deionized water derived pH as low as 2.7 and high metal concentrations in the extractants. Heavy metal concentrations were also high in the extractants of pyrite-free sediments when the pH was adjusted to acidic range. Our results suggest that the high metal concentration in the acidic groundwater are closely related with the low pH induced by the pyrite oxidation. This study also shows that the acidic pH of groundwater is in part related with the well installation; that is, well installation chemically disturbs the confined aquifer, which was originally under reducing environments, and triggers the pyrite oxidation.