Groundwater inflow represent a very important element in hydrological circulation. Water inflows in Pahang- Selangor Raw Water Transfer Tunnel have been analysed for stable isotopes of δ2H and δ18O. 61 samples were collected including Tunnel Seepage Water (TSW), surface water and hot spring water samples within the study area in order to understand the effect of multi topographical scale and geological characteristic along the tunnel alignment. Deuterium (δ2H) and Oxygen-18 (δ18O) contributes the understanding of the origin and flow paths of water in the mountainous region. The δ2H and δ18O data obtained from TSW samples ranging from −45.73%0 to −54.68%0 and −46.01%0 to −58.49%0 are clustered along the local meteoric water line (LMWL) indicating that the groundwater originated from meteoric water. d2H and d18O data primarily indicate the recharge altitude from 100m – 550m which control by the sub vertical flow path mainly from geological structures (i.e. fractures and faults) followed by the groundwater. The altitude effect is indicated by the relation between the stable isotope values and elevation in meters highlighting a depletion of heavy stable isotope with the increase of the tunnel overburden. The general trend obtained is δ2H and δ18O decreased with the increase of the overburden. Deviation of the δ2H and δ18O data from the expected trends may reflect the recharges are coming from the sub-horizontal flow path such as rivers or water infiltration from the valley. The results show that environmental isotopes indicates a better understanding of the complex hydrogeological system in a mountainous region and interaction between groundwater in granitic and meta-sedimentary rocks formation along the tunnel project.

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