Diagnóstico e monitoramento de águas subterrâneas urbanas para gestão de recursos hídrico do município de Canguçu, Rio Grande do Sul, Brasil

Abstract

Groundwater plays a strategical role to supply the demand of water around the world. In this context, the present study evaluated groundwater of five spring dammed in shallow wells in the urban area of Canguçu, by monitoring microbiological and physicochemical parameters in five surveys. The results were compared with secondary data from tubular wells made available by CORSAN. The water quality parameters analyzed were total coliforms, thermotolerant coliforms, Escherichia Coli, temperature, pH, dissolved oxygen, total dissolved solids, bicarbonates alkalinity, chloride, sulfate, calcium, magnesium, potassium, sodium, iron, manganese, and nitrate. The results were compared with the law of the ordinance no. 888 of the Health Ministry and with the guidelines of the World Health Organization. Afterwards, the exploratory analysis, variance analysis, graphical methods, principal component analysis, groundwater quality index, and stepwise multiple linear regression methods were carried out. The results of the microbiological parameters indicated the presence of thermotolerant coliforms and Escherichia Coli in the monitored wells, making the water unfit for drinking as established by law. Throughout the monitoring the variation of results for each parameter was temperature (13.2 to 28.10°C), pH (4.50 to 7.90), electrical conductivity (40.0 to 428.0 uS/cm), dissolved oxygen (0.98 to 9.20 mg/L), total dissolved solids (26.0 to 241.0 mg/L), bicarbonate alkalinity (2.20 to 41.60 mg/L), chloride (2, 0 to 48.5 mg/L), sulfate (0.10 to 15.48 mg/L), calcium (1.57 to 65.06 mg/L), magnesium (0.81 to 18.33 mg/L), potassium (0.48 to 18.31 mg/L), sodium (3.38 to 107.40 mg/L), manganese (0.01 to 0.26 mg/L), iron (0.12 to 17.08 mg/L), and nitrate (0.04 to 17.16 mg/L). The pH showed a value non-compliant with the standard in 72.0% of samples, manganese in 44.0%, iron in 41.6%, nitrate in 33.3% and potassium showed two samples with values above the recommended. The CORSAN wells recorded results for fluoride, iron and bicarbonate alkalinity in non-compliance with the standards. The hydrogeochemical facies of the waters are bicarbonate sodic and bicarbonate mixed. Principal component analysis explained 85.0% of the total variation of the results in three principal components, based on the relationship between the variables in different components, it was noted that the water quality of the monitored wells is influenced by geochemical processes and that the monitored parameters have different origins. The groundwater quality index classified the groundwater quality as excellent 10.0% of the samples, 45.0% as good, 25.0% as poor, 15.0% as very poor and 5.0% as unsuitable. The monitored wells P2, P3 and P4 showed the worst indices possibly due to the land use around these wells. The stepwise multiple linear regression analysis modeled two equations and identified: pH, manganese, potassium and calcium as the variables that most contributed to the index results. The results revealed the risk of water consumption for the health, thus a previous water treatment and wells maintenance is recommended.