​Physicochemical Compliance Does Not Ensure Microbiological Safety in Rural Wells of a Fractured Crystalline Aquifer

Groundwater is the main source of drinking water for many rural communities, and physicochemical compliance is often treated as an indirect indicator of microbiological safety. This exploratory baseline study examined whether this assumption holds in a rural fractured crystalline aquifer in the Monte Bonito district, Santa Bárbara Stream watershed (southern Brazil). Ten wells (shallow dug wells and semi-artesian boreholes) were sampled once, between September and October 2021, and physicochemical and microbiological parameters were compared against Brazilian drinking-water standards (Portaria GM/MS nº 888/2021). Five of the ten wells exceeded at least one physicochemical threshold, most often iron, manganese, turbidity, or apparent color. In contrast, total coliforms were detected in all ten wells and Escherichia coli in nine, including a 35-m semi-artesian well that met every measured physicochemical criterion. Consequently, none of the ten wells met overall potability criteria once the microbiological domain was considered, even though half were physicochemically compliant. Physicochemical results therefore did not identify microbiological non-compliance in this dataset. Elevated iron and manganese were consistent with water–rock interactions within the Pinheiro Machado Suite, while the widespread fecal indicator bacteria were consistent with surface-derived contamination linked to onsite sanitation, livestock activity, and inadequate well protection; specific sources and pathways could not be confirmed. These findings, from a ten-well pilot dataset and a single sampling campaign, indicate that physicochemical compliance did not reliably proxy microbiological safety and support the parallel interpretation of physicochemical and microbiological indicators, combined with sanitary inspection and wellhead protection, in rural communities relying on fractured hard-rock aquifers.