Hydrogeochemical evolution in a tropical cave system; Batu Caves, Peninsular Malaysia

A hydrogeochemical evolution of tropical caves' drip water was carried out at Batu Caves, Peninsular Malaysia. Drip water was analysed to underscore the cave hydrogeochemical evolution. The karst system of Batu Caves is characterized by low water storage capacity due to the high porosity of its weathered soil. A monitoring program was performed on a fortnightly time scale. The tropical environment showed a significant variation on the hydrogeochemical processes in the karst system compared to temperate environments. Drip water was assumed to be fed from a matrix of soil water flow in the absence of recharge. A constant hydraulic pressure was expected as a result of high evaporation in the tropical region and rainfall throughout the year. The mean drip rates varied from 0.02 to 1.02 ml/s and increased coincidently with high water input, and vice versa. The drip water pattern suggested that the drip water differentiated in two flow regimes which are seasonal and seepage flow. Both regimes act differently and control the water infiltration process in the karst system. The water-rock interaction processes involved in the karst system are dissolution, dilution, ionic exchange and prior calcite precipitation (PCP) that lead to saturated SI calcite index. In general, drip water consists of Ca, Mg, and HCO 3 as dominant elements. The source rock deduction analyses strongly suggested that the hydrogeochemical properties of drip water originated from carbonate weathering. The drip and meteoric water were designated as Ca-HCO 3 and Na-CO 3 facies, respectively. Na ion in meteoric water replaced by Ca and SO 4 was exchanged with HCO 3 ; i.e., a Ca-HCO 3 as a result of the ionic exchange. The Ca and Ca:Mg determined that the dissolution rate declined as a result of the drip water reaching its saturation point against PCP. Most of the drip water reached oversaturated values ranging from 0.398 - 1.017. Each drip possessed a unique hydro-geochemistry characteristic which is significantly related to the host rock properties, flow path characteristic, fracture system behaviour, and volume of water input