Using GIS methods for determining the spatial distribution of groundwater hydrochemical facies: a case study for a shallow aquifer in Fuyang, China

The determination of groundwater hydrochemical facies (GHF) is useful for analyzing the chemical composition of groundwater, tracking sources of groundwater recharge and assessing how its chemical composition changes along groundwater flowpaths due to water–rock interactions. This paper proposes a geographical information systems (GIS)-based method for identifying and mapping the spatial distribution of GHF categories in an aquifer. Several procedures including spatial interpolation, raster reclassification, raster-to-vector conversion, spatial union overlay, and generalization methods are coupled in GIS tools to determine the type and distribution of GHF categories. This method was implemented to distinguish shallow GHF categories for a shallow aquifer near Fuyang City in Anhui Province, China. The results obtained from the GIS assessment of water quality data from 30 wells show there were seven types of hydrochemical facies identified in the study area. The major GHFs categories are HCO3–Na + K + Ca + Mg, HCO3–Ca + Mg, HCO3–Na + K + Ca, and HCO3–Na + K, which cover 94% of the total area. The GIS method correctly identified the GHF categories determined in 80% of the wells in the area. The major GHFs categories progressively change in the direction of groundwater flow from HCO3–Ca + Mg to HCO3–Na + K+Ca + Mg, then to HCO3–Na + K + Ca, and then to HCO3–Na + K. The changes in the proportions of major ions in groundwater, coupled with salinity increases in the direction of groundwater flow are due to the combined effects of water–rock interactions in the aquifer and human activities. The method can relatively efficiently identify and map GHF zones for aquifers.