Factors causing groundwater acidification in the high land area of Ho Chi Minh City, Vietnam

Abstract

This study explores the causes of the acidic nature of the metal-rich, dilute groundwaters in the highland area of Ho Chi Minh City (HCMC), which is populated and mostly used for domestic, agricultural, and industrial purposes. The groundwater is generally very dilute (176 ± 128 µS/cm in electrical conductivity, 70 ± 220 µeq/L in alkalinity), but high in redox potential (343 ± 55 mV), and nitrate concentrations (19 ± 19 mg/L). Since the area corresponds to the highland and, thus, serves as a groundwater recharge zone. However, 53% and 90% of the investigated groundwater samples (n = 58) showed pH lower than 4 and 5, respectively, and, thus, 43%, 21%, and 7% of groundwater samples showed Al, Pb, and Cr concentrations exceeding their respective drinking water standards recommended by World Health Organization. Although nitrification is the most common acidification driver in the agricultural and/or urbanized lands, the nitrate concentration in this study area is strangely low compared to similar acidic groundwaters reported from other agricultural regions. To find out the causes of acidification, this study investigated the geochemical processes from the extensive groundwater chemistry data and performed geochemical simulations by changing water alkalinity and cation exchange capacity (CEC) of sediment and matching the results with the observed water chemistry data to confirm our hypothesis. Based on this approach, we could reveal that groundwaters of this study could become very acidic due to its dilute nature and low sediment CEC. Since groundwaters are generally very dilute in the recharge area, our finding provides another reason for the discreet management of highland areas where groundwater recharge is concentrated.