Unconfined and confined aquifers occur in the unconsolidated sediments in the coastal plain near Beihai, Guangxi. Groundwater in the aquifers is of low pH ranging commonly from 4.0 to 6.0. Minerals in unconsolidated Quaternary sediments are predominated by quartz with small amount of clay minerals. SiO2accounts for most of the chemical compositions of the sediments. The sediments containing mainly insoluble constitutes and dissolution of a long time lead to low TDS of the groundwater. Under the natural conditions, H in the groundwater is derivedfrom dissociation of H2CO3, release of the absorbed H2O inclaylayers and the acidity of rainwater. The H2CO3 in the groundwateris formed by dissolutionof CO2. The CO2comes mainly from biologicalcontribution. Lack of alkaline substances in the groundwater system is also helpful in the accumulation of acidity, resulting in a decrease in pH of the groundwater.