HYDROCHEMICAL ASSESSMENT OF SALINE GROUNDWATER IN AN ALLUVIAL FLOODPLAIN OF RED RIVER, VIETNAM USING ORTHOGONAL REGRESSION METHOD

An alluvial aquifer in a floodplain of the Red River delta, Vietnam, has been studied to examine the water quality status, especially salinity problems. For this study, we collected hydrochemical and isotopic data of groundwater samples from the Kien Xuong district of Thai Binh province. Three major hydrochemical processes were identified using the Principal component analysis (PCA) of hydrochemical data. These are: 1) freshening (i.e., salinity gradient), 2) water-rock interaction, and 3) redox process. The Hydrochemical Facies Evolution Diagram (HFE-D) and salinization index also indicated that freshening process is occurring. To scrutinize the freshening, orthogonal regression was attempted and could yield an orthogonal mixing line (OML) between fresh water and seawater. By applying the mixing line to the HFE-D and salinization index diagrams, we could produce a more reliable mixing line. By calculating residuals from the mixing line, the other hydrochemical processes occurring during freshening were evaluated. Sulfate and nitrate showed negative residuals, indicating their geochemical reduction (i.e., denitrification, sulfate reduction). Bacterial sulfate reduction was also indicated by increasing sulfur and oxygen isotope ratios of sulfate toward deeper groundwater. A few outliers of S and O isotope ratios represent the anthropogenic source such as chemical fertilizers, especially in shallower groundwater. Careful examination of the residuals of major cations, bicarbonate and chloride showed the gradual changes in the study area. Typical shallow groundwater belongs to the Ca-HCO₃ type and records the long-term freshening accompanying cation exchange, while deeper groundwater is typically of the Na-Cl type. This study shows an advantage of the use of orthogonal regression of hydrochemical data to interpret complex hydrochemical processes occurring in an aquifer.