Sewer leakage as the main cause of multi-decadal nitrate plumes in a small tropical city

Abstract

Groundwater provides a critical buffer for urban water security, yet contamination by nitrate from deteriorating sewer networks poses a persistent and globally underestimated threat. Unlike agricultural pollution, urban-derived nitrate is sustained by diffuse, chronic leakage from buried infrastructure, producing plumes that can persist for decades. Here, coupled groundwater flow and solute transport models (MODFLOW–MT3D) were applied to simulate 30-year scenarios of aquifer response in a Brazilian city where a nitrate plume exceeding 1.5 km 2 has been sustained for over 20 years. Modeled results revealed that maintaining current leakage rates (~ 11%) stabilizes contamination at concentrations above drinking-water standards, while increases to 15–20% accelerate plume migration into deeper aquifer zones, further compromising supply wells. Conversely, halving leakage reduces nitrate below potability thresholds within 17 years, while complete source cessation leads to near-total aquifer recovery within 23 years. Managed pumping from intermediate zones enhances vertical mixing, dilutes concentrations, and slows plume expansion, highlighting the dual role of infrastructure maintenance and groundwater operations in restoring water quality. These findings demonstrate that urban aquifers are not irreversibly degraded, but their recovery depends on proactive interventions. By linking long-term contaminant fate with actionable management, this work provides a transferable framework for assessing the resilience and recovery potential of nitrate-impacted aquifers worldwide.

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