Developing a low-cost nitrate and DOC sensor for natural water samples

Nitrate continues to be a major threat to drinking water resources, but rapid chanages in concentarions cannot be addressed by standard laboratory approaches. This study introduces a low-cost optical sensor for real-time, in-situ monitoring of nitrate (NO ₃ ⁻ ) and dissolved organic carbon (DOC) concentrations in natural water samples (soil water, groundwater and river water). Utilizing absorbance and fluorescence at specific wavelengths with LEDs and photodiodes, this sensorsystem offers a practical alternative to expensive and complex laboratory or in situ spectrometer methods. Our system comprises three modules designed for detection at specific wavelengths (UVA, UVC, and red), enabling precise measurements across various sample sources like laboratory standards, groundwater, stream water, and soil extractions. Initial tests with laboratory nitrate standard solutions achieved high accuracy, with a linear model exhibiting an R² of 0.99 and MAE of 2.63 mg/l. Although the sensor's accuracy does not yet fully match that of traditional laboratory analyses like ionchromatography or photogrammetric approaches, it maintains strong predictive capabilities with R² values exceeding 0.9 and and MAE of 4.2 mg/l NO ₃ ⁻ for a sample mixture of groundwater, streamwater and soil water. DOC can be predicted with an MAE of 2.2 mg/l. Challenges such as the interference of DOC and turbidity with the nitrate absorbance signal, intense calibration procedures and site-specific variability remain, necessitating further refinement. However, this sensor system provides a significant step toward accessible, continuous water quality monitoring and lays the foundation for linking nitrate concentrations to in-situ fluxes. These advancements are crucial for enhancing nutrient management and environmental protection practices.