Characterization of groundwater salinity by hydrogeochemical and multivariate statistical analysis in the coastal aquifer of Nagapattinam district, Southern India

•Characterisation of groundwater quality in Nagapattinam coastal area carried out.•Hydrochemical facies were identified using Piper diagram and scatter plot.•Factor and Cluster analysis was carried out in identifying the saline water intrusion.•Three factors explaining 84.02% and 76.67% of the total variance in Uppanar and Cauvery river basin.•More salinity observed in Uppanar river basin than Cauvery river basin. The impact of seawater intrusion from coast to inland terrain in the Cauvery River Basin (CRB) and Uppanar River Basin (URB) was evaluated based on major ion groundwater chemistry. TDS ranges from 229-2260 mg/l, and 408-3732 mg/l; Na+ range from 67-560 mg/l, and 74-1600 mg/l, and Cl- range from 120-906 mg/l, and 110-3260 mg/l for CRB and URB respectively. Piper Diagram, Hydrochemical Facies Evolution Diagram (HFE-D), rock-water interaction (Gibbs Plots), various bivariate plots viz., TDS vs. Cl-; Na+ vs. Cl-; Ca2+ vs. Cl-; Ca2+ vs. SO42-; TH vs. TDS and Principal Component Analysis (PCA) (Cluster and Factor analysis) were used to identify the seawater intrusion from coast to inland aquifers and to understand hydrogeochemical characterization and salinization processes. Piper diagram shows that most of the samples are Na+-Cl- type, HFE-D diagram also shows that most of the samples were saline intrusion type and mixing behavior, while TH vs. TDS plot shows hard fresh to hard brackish type from both the basins. PCA results clearly show the three factors, explaining 84.02% and 76.67% variance in URB and CRB. Factor-1 records 53.03% alteration, with a strong confidence loading of TDS, Na+, Cl-, Ca2+, K+, SO42, Total Alkalinity (TA), and Total Hardness (TH) in URB indicating saline nature. A total variance of 46.23% in CBR is more positively loaded with TH, Mg2+, Ca2+, and SO42- indicating rock-water interaction. Cluster analyses of these parameters illustrate the cluster distribution in CRB and URB. In URB, TDS, Na+, and Cl- ions make a cluster with a linkage distance of 5000m, whereas in CRB, the TDS, Na+, Cl-, and TA ions make a cluster with a linkage distance of 2800m. The factor and cluster analysis fetched out an effect of intensive use of fertilizers, aquaculture activities, and excessive groundwater exploitation. This technique gave the relationship between various chemical parameters in groundwater. Factor and cluster analysis have proven highly effective in groundwater quality studies. The study concluded that the study area has the threat of saline water intrusion in shallow aquifers with continuous influences of seawater mixing. The impact of seawater intrusion from coast to inland terrain in the Cauvery River Basin (CRB) and Uppanar River Basin (URB) was evaluated based on major ion groundwater chemistry. TDS ranges from 229-2260 mg/l, and 408-3732 mg/l; Na+ range from 67-560 mg/l, and 74-1600 mg/l, and Cl- range from 120-906 mg/l, and 110-3260 mg/l for CRB and URB respectively. Piper Diagram, Hydrochemical Facies Evolution Diagram (HFE-D), rock-water interaction (Gibbs Plots), various bivariate plots viz., TDS vs. Cl-; Na+ vs. Cl-; Ca2+ vs. Cl-; Ca2+ vs. SO42-; TH vs. TDS and Principal Component Analysis (PCA) (Cluster and Factor analysis) were used to identify the seawater intrusion from coast to inland aquifers and to understand hydrogeochemical characterization and salinization processes. Piper diagram shows that most of the samples are Na+-Cl- type, HFE-D diagram also shows that most of the samples were saline intrusion type and mixing behavior, while TH vs. TDS plot shows hard fresh to hard brackish type from both the basins. PCA results clearly show the three factors, explaining 84.02% and 76.67% variance in URB and CRB. Factor-1 records 53.03% alteration, with a strong confidence loading of TDS, Na+, Cl-, Ca2+, K+, SO42, Total Alkalinity (TA), and Total Hardness (TH) in URB indicating saline nature. A total variance of 46.23% in CBR is more positively loaded with TH, Mg2+, Ca2+, and SO42- indicating rock-water interaction. Cluster analyses of these parameters illustrate the cluster distribution in CRB and URB. In URB, TDS, Na+, and Cl- ions make a cluster with a linkage distance of 5000m, whereas in CRB, the TDS, Na+, Cl-, and TA ions make a cluster with a linkage distance of 2800m. The factor and cluster analysis fetched out an effect of intensive use of fertilizers, aquaculture activities, and excessive groundwater exploitation. This technique gave the relationship between various chemical parameters in groundwater. Factor and cluster analysis have proven highly effective in groundwater quality studies. The study concluded that the study area has the threat of saline water intrusion in shallow aquifers with continuous influences of seawater mixing.