Abstract: An attempt was made to distinguish the natural and anthropogenic processes and factors responsible for groundwater quality.R-mode factor analysis was applied for the sets of chemical data,i.e.pH,EC,HCO-3,Ca2+,Mg2+,Na+,K+, SO2-4,NO-3 and Cl-in the Nandong underground river system.Groundwater samples and surface water samples were collected at 42 and 15 locations in 2008,respectively.Analytical results of these groundwater samples show that the Nandong underground river system has very variable chemical compositions and spatial variability.By applying the R-mode factor analysis,three factors were obtained,which explained the characterization of groundwater quality and identified the sources for the presence of ions and their geochemical processes.It was found that the anthropogenic contribution is responsible for Cl-,NO-3,SO2-4,Na+ and K+ in groundwater.The natural dissolution of existing rocks of carbonate is the common influencing source for Ca2+ in groundwater.However,the anthropogenic contribution is also responsible for Ca2+ in groundwater.The natural and human-influenced processes are responsible for pH,EC of groundwater.Mg2+,HCO-3 are influenced by the factors of soil and water-rock interactions,respectively.The concentrations of CO2 in soil and the water-rock interactions are responsible for Mg2+,HCO-3 in the groundwater.The anthropogenic processes are the most important influencing factors for groundwater quality in the Nandong underground river system.