Descriptions of the surface-groundwater interactions are required for enhanced water resource management in the irrigation canals of the Kerian Scheme. The current study aims to find the hidden aquifer, determine the water loss/gain in an irrigation system, and identify the hydraulic interconnection between aquifer systems using geophysical, water balance, and stable isotope methods. The Resistivity Image Profiling (RIP) method is a common geophysical survey technique used to find a potential groundwater bearing zone in the study area. RIP results show that the thickness of the aquifer varies between 5 and 10 m at a depth of 10-30 m. The results are compared against borehole drilling data. Water loss and/or water gain in an irrigation system is an important component of the water balance for planning and operation. The water level at the Selinsing Canal system increases, clearly indicating that the catchment is very responsive to rainfall events. The use of stable isotopes to trace water movement in hydrology, including surface water and groundwater, is an important task within the environmental field. Thus, the results plotted for both water samples from Selinsing Canal and groundwater are linearly similar to those in the Global Meteoric Water Line and the Malaysian Meteoric Water Line. Thus, the samples are revealed to come from evaporated samples. This indicates that there is a connection between the surface and the groundwater in the catchment area.

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Groundwater inflows represent a very important element in hydrological circulation. Water inflows in Pahang-Selangor Raw Water Transfer Tunnel have been analyzed for isotopes stable of δ2H and δ18O. 61 samples were collected, including Tunnel Seepage Water (TSW), surface water and hot spring water samples within the study area which to understand the effect of multi topographical scale and geological characteristic. 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%0to-54.68%0and-46.01%0to-58.49%0are clustered along the local meteoric water line (LMWL) indicating that the groundwater originated from meteoric water. δ2H and δ18O 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 the 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. The 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 rock formation along the tunnel project.