Diphofe, KhutjoDiamond, RogerKotze, Francois2026-03-172026-03-172025-08-02Diphofe, K., Diamond, R. & Kotze, F. Quantifying Baseflow with Radon,Hand O Isotopes and Field Parameters in the Urbanized Catchment of the Little Jukskei River, Johannesburg. Hydrology 2025, 12, 203:1-20. https://doi.org/10.3390/hydrology12080203.2306-5338 (online)10.3390/ hydrology12080203http://hdl.handle.net/2263/109036DATA AVAILABILITY STATEMENT : The data for this study is given in tables within the article.Understanding groundwater and surface water interaction is critical for managing water resources, particularly in water-stressed and rapidly urbanizing areas, such as many parts of Africa. A survey was conducted of borehole, spring, seep and river water radon, δ2H, δ18O and field parameters in the Jukskei River catchment, Johannesburg. Average values of electrical conductivity (EC) were 274 and 411 μS·cm−1 for groundwater and surface water, and similarly for radon, 37,000 and 1100 Bq·m−3, with a groundwater high of 196,000 Bq·m−3 associated with a structural lineament. High radon was a good indicator of baseflow, highest at the end of the rainy season (March) and lowest at the end of the dry season (September), with the FINIFLUX model computing groundwater inflow as 2.5–4.7 L·m−1s−1. High EC was a poorer indicator of baseflow, also considering the possibility of wastewater with high EC, typical in urban areas. Groundwater δ2H and δ18O values are spread widely, suggesting recharge from both normal and unusual rainfall periods. A slight shift from the local meteoric water line indicates light evaporation during recharge. Surface water δ2H and δ18O is clustered, pointing to regular groundwater input along the stream, supporting the findings from radon. Given the importance of groundwater, further study using the same parameters or additional analytes is advisable in the urban area of Johannesburg or other cities.en© 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license.Urban hydrologyBaseflowStable isotopesStable isotopesRadonJohannesburgArticle