Hydrogeochemistry and groundwater quality assessment using the water quality index and heavy-metal pollution index in the alluvial plain of Atibaia river- Campinas/SP, Brazil

Alluvial aquifers are highly vulnerable to contamination, especially in areas with peri-urban occupation and on-site sanitation systems. Due to the importance of the quality of groundwater in those areas, this article aims to understand hydrochemical characteristics and to evaluate groundwater quality using the Water Quality Index (WQI) and Heavy-Metal Pollution Index (HPI). Seasonal sampling campaigns of 20 samples of groundwater in hand dug wells were done during wet and dry seasons. The main hydrochemical facies determined in the Piper Diagram allowed to classify the waters as Ca-HCO3, Ca-Na-HCO3 and Na-Ca-HCO3 during the wet season, and Ca-HCO3, Na-Ca-HCO3 and Na-Ca-Cl during dry season, suggesting shallow waters and short residence time. The groundwater in the study area is mostly controlled by the incongruous weathering of silicates and the process of ionic exchange, as well as the anthropogenic sources that act as secondary contributions. The saturation index results show a state of supersaturation (SI > 0) for the following minerals: goethite, kaolinite and illite, and subsaturation (SI < 0) for the minerals: calcite and dolomite. The groundwaters were subsaturated in most of the samples, and supersaturated in other samples related to the mineral phase Fe(OH)(3). The significant correlations between NO3- and Cl- allowed to conclude that the study area is contaminated by domestic septic tanks, and that in the majority of the samplings natural iron rests above the limit for human consumption. WQI and HPI indicate that the majority of the samples showed excellent indexes and are adequate for consumption. However, in this study those indexes presented limitations, because they minimize some parameters that presents concentrations above the maximum value or evidences of contamination, as seen with Nitrate and Iron, respectively.