Groundwater Flow System in Klang River Watershed, Kuala Lumpur, Malaysia

This study aims to clarify the groundwater recharge and flow processes in a basin located in a tropical humid area with a complex geological setting. We conducted intensive sampling campaigns for river water and groundwater during three different time periods in the relatively dry season (September 2019, January-March 2020, and August-September 2022). The 40 river water samples and 25 groundwater samples were taken from the upper-stream area with an altitude of 527 m to the downstream area with an altitude of 10 m. The oxygen-18 (δ¹⁸O) and deuterium stable isotopic compositions and inorganic constituent concentrations were determined on all water samples. We also used the monthly stable isotopic compositions in rainwater observed at a location with an altitude of 26 m, 15 km apart from the Klang River basin, by the Global Network of Isotopes in Precipitation database (GNIP), IAEA. These chemical compositions were used as tracers to investigate the groundwater recharge and flow system. The deep groundwater shows a lower δ¹⁸O than the volume-weighted mean of rainwater and higher ion concentrations, whereas the shallow groundwater shows a higher δ¹⁸O and lower ion concentrations. This suggests that the deep groundwater with low δ¹⁸O seems to be recharged in the mountainous area with an altitude ranging from 70 to 1421 m. Additionally, we conducted a principal component analysis and cluster analysis using inorganic constituent concentrations and stable isotopic compositions, showing that the deep and shallow groundwater samples are classified into two groups. This shows that the deep groundwater in the downstream area is recharged mainly in the mountainous areas with the highest altitude of 1421 m, and the shallow groundwater is recharged partly in the hilly areas with the highest altitude of 250 m. We believe our study serves new findings on the groundwater flow system in mega-cities of tropical climate regions.