Temporal changes in estuarine water quality during decentralized wastewater treatment implementation in the Paranaguá estuarine system, southern Brazil

This study investigates the temporal response of estuarine water quality to progressive implementation of decentralized wastewater treatment systems (DWWTS) in Eufrasina, a traditional community within the Paranaguá Estuarine System (PES), southern Brazil. Monthly monitoring was conducted over 20-months period (September 2024–April 2026) at four fixed sampling stations. Coastal Water Quality Index (CWQI), adapted from the Canadian Council of Ministers of the Environment framework, was applied to integrate physicochemical and microbiological variables and track temporal trends. Principal Component Analysis revealed seasonal structuring with summer campaigns associated with higher continental inputs and spring campaigns with greater microbiological contamination. CWQI showed a significant improving trend during the implementation phase (Mann-Kendall τ = 0.515, p = 0.024), rising from “Bad/Regular” to “Good/Excellent” as coverage expanded. Lagged correlations indicated that Escherichia coli and total coliforms had the strongest negative associations with cumulative sanitation coverage (lag 0–3), whereas enterococci, phosphate, and DO showed no significant associations, a pattern consistent with non-point sources and biogeochemical processes not addressed by household-level treatment. CWQI values declined episodically during periods of peak seasonal occupancy and rainfall, indicating that residual contamination. Water quality improved measurably even at early stage of DWWTS implementation, which highlights the value of monitoring designs that account for response lags when assessing delayed environmental responses to sanitation interventions. By showing that community-scale sanitation interventions can produce measurable reductions in estuarine pollution, this work provides evidence relevant to SDG 6 and 14, and United Nations Ocean Decade's call for science-based solutions to coastal and marine pollution.