Assessment of shallow groundwater quality for drinking purposes: an integrated approach based on the water quality index and the use of multivariate statistical analyses, southern Brazil

Groundwater plays a strategic role supplying the worldwide demand of water. In Cangucu town, groundwater is used as an alternative source for domestic supply. The present study evaluated urban shallow groundwater quality through the use of water quality indexes and a multivariate statistical approach. The following parameters: thermotolerant coliforms, Escherichia coli, pH, total dissolved solids, calcium, magnesium, potassium, sodium, bicarbonate, chloride, sulfide, and manganese were obtained from four surveys from spring five sampling points during the period 2020-2021. The results were compared with the WHO drinking water guideline limits, then a drinking groundwater quality index DGQI was calculated. The data were evaluated through the use of a stepwise multiple linear regression, and a principal component analysis was carried out. Microbiological results demonstrated that groundwater analyzed is not safe for drinking without disinfection. Physicochemical results indicated that the parameters pH, manganese, and potassium often did not comply with the WHO guideline limits. The DGQI indicated that 55% of the samples were classified as having "good" to "excellent" water quality, 25% had "poor" water quality, 15% had "very poor" water quality, and 5% were considered to be unsuitable for drinking purposes. Stepwise multiple linear regression analysis modeled two equations and identified that sodium, pH, manganese, potassium, and calcium as the variables that contributed most to the DGQI results. Principal component analysis demonstrated the predominance of the ion exchange process in controlling groundwater chemistry in the area. The present study is the first groundwater quality study of shallow groundwater in Cangucu town, Pelotas Batholith, and demonstrated the patterns of groundwater and the urgently necessity of previous treatment of groundwater, as well as the great hazard of drinking untreated water.