Groundwater investigation of a coastal aquifer in Brunei Darussalam using seismic refraction

Seismic refraction has been utilized widely in the past to characterize shallow aquifers and subsurface layers. This study applies the method on a water-shed scale to get information on the regional groundwater flow system. Furthermore, the application of seismic refraction for groundwater and hydrogeologic studies has not been done before in Brunei Darussalam. The country of Brunei Darussalam, located in the North of Borneo, uses more than 99% of surface water resources for its industry and public consumption, with the remaining groundwater resources currently unexplored. However, the ever-increasing population and national industrialisation are putting more pressure on Brunei’s water resources. In view of finding new groundwater reserves, we incorporate seismic refraction methods to characterize the local subsurface layers and to delineate the groundwater levels at the Berakas Forest Reserve, located in the North-East of Brunei. The site is chosen based on its surface geological suitability for groundwater reservoirs as well as the occurrence of springs and ephemeral streams in the area. Initial investigations indicate that the near-surface lithology of the study area has good aquifer potential, with the occurrence of conglomerate and sand deposits formed by braided and meandering rivers during late Miocene to Pliocene. We carried out four seismic surveys at strategic locations with different elevations and distances to the sea. The interpreted water table elevations from the velocity-depth profiles are compared with the actual ground elevations of observed springs as well as an ephemeral stream. The interpretations from the velocity profiles were verified using 1D multichannel analysis of surface waves at selected locations. In addition, seismic tomography was used to investigate the capillary fringe zone, indicating the presence of clayey and silty sediments in the saturated and vadose zone. Our interpretations show that the water table varies with topographic elevation and distance from the South China Sea, as the overall flow direction is towards the sea. The hydraulic gradient is calculated to be 0.04. Water samples collected from springs indicate fresh water based on density of 1.00 g/cm3 and chloride content below 150 ppm. However, the shallow aquifer is impacted by low pH values and high sulphate concentrations likely due to local acid sulphate soils present in the study area. No saltwater intrusion is expected which is investigated using the Ghyben–Herzberg relationship.