Hydrochemistry of the natural low pH groundwater in the coastal aquifers near Beihai, China

Abstract

Natural weak acidic groundwater occurs in the unconfined and confined aquifers consisting of Quaternary and Neogene unconsolidated sediments near Beihai in southern Guangxi, China. Under natural conditions the groundwater has low TDS (less than 200 mg L−1) and low concentrations of trace elements (less than 100 µg L−1) with a deceasing tend in contents of the Lanthanides (rare earth elements, less than 1 µg L−1) towards higher atomic number. The groundwater ranges in pH from 3.33 to 7.0 with an average value of 5.12 (even lower than that of local rainwater, 5.88). pH values in the groundwater are a bit higher in rainy seasons than those in dry seasons and do not show significant increasing or decreasing trend with time. The average pH value in groundwater in the confined aquifers is even a bit lower than that in the unconfined aquifer. Comprehensive analyses of the groundwater environment suggest that H+ in the groundwater may be derived from dissociation of H2CO3, release of the absorbed H3O+ in clay layers and the acidity of rainwater. The H2CO3 in the groundwater may be formed by dissolution of CO2 (g). Minerals in the unconsolidated sediment are predominated by quartz with small amount of clay minerals. The sediments undergoing a long-term weathering contain low levels of soluble constitutes. Lack of alkaline substances in the groundwater system is also helpful in the accumulation of acidity of the groundwater.