Hydrogeochemistry of tunnel seepage water along the contact of zone of metasedimentary and granitic rock within the Pahang–Selangor Raw Water Transfer Tunnel Project, Malaysia

Groundwater obtained from water inflows during the excavation of the Pahang-Selangor RawWater Transfer Tunnel (P-SRWTT) was analyzed to determine the characteristics of groundwater that flows along the main range granite and the metasedimentary geological boundary. A total of 61 tunnel seepage water (TSW) samples were immediately obtained after tunnel excavation. Hydrogeochemical analyses, including physicochemical and major ion analyses, were performed on TSW samples. Results show that the TSW sample in the P-SRWTT is meteoric water, which infiltrates at different elevations and flow paths. The prolonged interactions of groundwater in the metasedimentary rock and contact zones produced Ca²⁺- HCO₃ ⁻-SO₄ ²⁻, Ca²⁺-HCO₃ ⁻, and Ca²⁺-Na⁺-HCO₃ ⁻ in the water. Electrical conductivity (EC) values reach as high as 345.80 and 313.22 μs/cm in the metasedimentary rock and contact zones, respectively. Most water in these two zones present temperatures lower than 30 °C, which is comparable with the expected geothermal gradient of 35 °C/km. Ca²⁺-Na⁺- HCO₃ ⁻ is the dominant water type in the granite zone with an average EC of 288.37 μs/cm. Na⁺-HCO₃ ⁻ waters are generated through the interaction between Ca²⁺-HCO₃ ⁻ and Na⁺ in granitic rocks. The zone of warm Na⁺-HCO₃ ⁻ waters, presenting an average temperature of 35 °C at an overburden of more than 400 m, shows some anomalies from the expected geothermal gradient of 35°/km. The hydrogeochemistry analyses provide a good understanding of the complex hydrogeological system at the two major geological formations along the tunnel.