Statistical assessment of seasonal variation of groundwater quality in Çarşamba coastal plain, Samsun (Turkey).

Çarşamba aquifer is one of the most important coastal aquifers in Turkey. This aquifer is confronted by overexploitation due to the agricultural and industrial activities. The aim of the present study is to investigate the seasonal variations of hydrogeochemical parameters and to assess the suitability for drinking and irrigation of groundwater in the coastal aquifer of Çarşamba plain. For this purpose, in July and December of the year 2019, 33 and 30 groundwater samples respectively were taken from boreholes in the study area and for these samples, EC, pH, TDS, Na, Ca, K, Mg, CO, HCO, Cl, SO, NH, NO, and NO values were determined. Strong correlation was observed between Cl and Na during both seasons indicating the seawater intrusion on groundwater in the study area. Principal component analysis showed that in the study area, seawater intrusion, rock-water interaction, and anthropogenic activities from agricultural areas are the main factors that impact the groundwater chemistry. Seawater intrusion is the most important factor which affects the groundwater chemistry in July while in December, the main factor is rock-water interaction. In December, NO and NH values of most water samples exceed the authorized limits of Turkish Standard and WHO. Water quality index indicated that most of the water samples are suitable for drinking. Wilcox diagram and US salinity diagram used to evaluate the suitability of groundwater for irrigation suggested that in July, 87.87% (90% in December) (for Wilcox diagram) and 96.96% (100% in December) (for US salinity diagram) of the water samples belonged to the good to permissible class, and therefore are suitable for irrigation purpose. In addition, the EC, %Na, TH, RSC, SAR, PI, KI, and MH values of samples showed that during both seasons, most of the water samples are suitable for irrigation. However, in July, 51.52% (43.43% in December) of samples have extremely high potential salinity values, thus revealing the unsuitability of most groundwater samples for irrigation in the study area.