**5. Conclusions**

The majority of the population in Guinea-Bissau still retrieve water for daily needs, including drinking water, from shallow wells. The water was acidic to very acidic and heavily contaminated with faecal matter, and thus unfit for human consumption. This situation was present throughout the year, but deteriorated further in the wet season, associated with high water turbidity. The highest levels of contamination were associated with shallow wells and bucket water retrieval. Although the chemical contamination was less relevant, and no overall trend could be found, several of the surveyed water sources revealed high values of nitrogen species and heavy metals.

A national concert intervention on WASH infrastructure is essential to provide safe drinking water to the population, as recognised in the United Nations Sustainable Development Goals. In the short-term, the authors suggest the implementation of simple measures to improve water potability and reduce the disease burden associated with waterborne pathogens.

The results of this research fill the pressing need for scientific background knowledge concerning the water quality in Guinea-Bissau, decisive to help the design of sustainable mitigation strategies. Improving water quality could have a dramatic impact on the population health status, and consequently, on the development of this low-income country. **Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10 .3390/w14131987/s1, Figure S1: Example of typical water sources surveyed across Guinea-Bissau with the respective method used to retrieve water, Figure S2: Variation of key water quality parameters according to source type and method of water collection, Figure S3: Matrix of Spearman correlations between the physical–chemical quality parameters of the water sources across Guinea-Bissau, Table S1: Number of water sources surveyed according to the water collection system.

**Author Contributions:** Conceptualization, A.M. and A.A.B.; methodology, A.M., E.A. and A.A.B.; software, E.A.; validation, A.M., E.A. and A.A.B.; formal analysis A.M. and E.A.; investigation, A.M. and A.A.B.; resources, A.M. and A.A.B.; data curation A.M. and A.A.B.; writing—original draft preparation, A.M. and E.A.; writing—review and editing, A.M., E.A. and A.A.B.; visualization, A.M., E.A. and A.A.B.; supervision, A.A.B.; project administration, A.A.B.; funding acquisition, A.M. and A.A.B. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was partially funded through a fellowship to A. Machado (SFRH/BD/46146/2008), co-financed by POPH/FSE. This research was also partially supported by the Project BeachSafe (PTDC/SAU-PUB/31291/2017), co-financed by COMPETE 2020, Portugal 2020 and the European Union through the ERDF, and by FCT through national funds.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data generated or analysed during this study are included in this published article (and its supplementary information files). Notwithstanding that, the datasets used and/or analysed during the current study are available from the corresponding author upon reasonable request.

**Acknowledgments:** The authors wish to acknowledge the on-site logistic support of AIDA (Ayuda, Intercambio y Desarrollo), AMI (International Medical Assistance) and Cruz Roja Española. We thank Alfa and Samba, who assisted with sample collection and facilitated contact with the local population.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
