Low-Cost Optical Sensing for Nitrate and Phosphate: Advancing Community Freshwater Monitoring

High-resolution water quality data are fundamental to achieving the objectives of the European Water Framework Directive (WFD) and the United Nations Sustainable Development Goal 6 (SDG). While community-based monitoring projects provide valuable data, concerns remain regarding the accuracy and reliability of those data. This study addresses these challenges by developing an affordable, reliable, and open-source optical sensor for monitoring nitrate and phosphate concentrations in freshwater environments. The sensor was developed to support citizen scientists and community monitoring and consists of a 3D-printed case made of polylactic acid, a light-emitting diode (LED), and a commercial ambient light detector managed by a Raspberry Pi Zero W. Data can be stored offline or transmitted in real time via Wi-Fi. The analytical performance was evaluated in laboratory and operational conditions using standard and natural river samples, performing a comparative analysis with a laboratory-based spectrophotometer. Results showed that the sensor provides accurate and repeatable measurements with a significant improvement over conventional colorimetric methods. The sensor technology follows open science principles as the 3D design, operating software, and user guidelines are freely available online to support further advancements and its application in community-based water quality monitoring.