[A-HW19] Tracer Hydrology: Advances in Measurement and Modelling

[AHW19-02] Groundwater flow system revealed by multi-tracer method in the Klang River Watershed, Malaysia

*Mariko Saito¹, Maki Tsujimura¹, Kaori Kobayashi², Sumire Torimaru¹, Taiga Suzuki¹, Syahiran An³, Mohd Muzamil Mohd Hashim³, Norsyafina Roslan⁴, Faizah Che Ros⁵, Ismail Yusoff⁶ (1.University of Tsukuba, 2.Nippon Koei Ltd., Inc, 3.Malaysian Nuclear Agency, 4.Universiti Kebangsaan Malaysia, 5.Universiti Teknologi Malaysia, 6.Universiti Malaya)

Keywords:groundwater flow system, multi-tracer approach, humid tropics

We investigated the groundwater flow system in the Klang River basin, Malaysia, which has high precipitation, large topographic gradients, and is underlain by folded and faulted bedrock, by applying multi-tracer methods such as dissolved ion constituents and stable isotope components of hydrogen and oxygen (δ²H and δ¹⁸O). We conducted five sampling campaigns between September 2019 and December 2022, collecting 44 river water and 34 groundwater samples from upstream to downstream. The dissolved ion constituents and δ²H and δ¹⁸O in the water samples were analyzed in the laboratory. δ¹⁸O in precipitation and hydraulic head of groundwater were collected. Considering the spatial distribution of δ¹⁸O, dissolved ion constituents in river water and groundwater, and the spatial distribution of hydraulic head, two types of flow systems are suggested: (1) an intermediate flow system, where the recharge area is the midstream hilly area, and the discharge area is the nearby midstream valley, and (2) a regional flow system, where the recharge area is the upstream mountainous headwaters, and the discharge area is the most downstream. Surprisingly, the δ¹⁸O in the mainstream of the Klang River decreased from -7.2‰ to -7.6‰ as the river flowed from the midstream to the downstream, suggesting that the deep groundwater recharged in the headwaters, which is composed of low δ¹⁸O ranging from -8.3‰ to -7.6‰, is discharged to the downstream river with high flux. Thus, our study shows that the groundwater flow is driven by the topography, where there is high precipitation and large topographic gradients.