Fractured bedrock aquifers account for 20% of the world’s aquifer systems, yet significant questions of how to characterize and quantify the magnitude of subsurface storage in the context of water supply development remain to be understood. These aquifer types are the only water source for many regions of the world, and it is imperative to understand their hydrologic sustainability under the threat of changing climate. In this work we describe the results of an on-going study to understand the magnitude of groundwater storage and water supply sustainability of one such aquifer system on the the island of Tobago. The island is predominately composed of highly fractured Mesozoic igneous and metamorphic rocks with well-developed saprolite soil cover. Prior work has established a robust hydrologic budget of the entire aquifer system, with the major conclusion that the fractured bedrock receives recharge of ~400 mm of the ~1900 mm total annual precipitation. This study aims to use geochemical and environmental tracers to further confirm, constrain, and refine this conceptual framework.
We evaluate the distribution and abundance of solutes provided by both weathered rocks (e.g. saprolites) and non-weathered fractured crystalline rocks during the storage and transport of groundwater. Preliminary results suggest that fractures have provided flow paths and enabled groundwater mixing due to increases of major elements, compounds and stable isotopes from the central northeast region of the island towards the south-southeastern region, across significant topographic watershed divides. This is supported by the observations that groundwater production exceeds calculated recharge inputs in these sub-catchments. Strontium concentration also increases by an order of magnitude towards the southeast creating three distinct water groups: fresh water (87Sr/86Sr ratio range 0.70400 to 0.070750 and 0.05 to 0.50 mg/Kg), fresh water (87Sr/86Sr ratio 0.70450 to 0.70650 and 0.5 to 2.0 mg/Kg) and brackish water (0.70550 to 0.70650 and 2.0 to 5.0 mg/Kg). Stable isotopic measurements of ground and surface water suggest a well-mixed large-subsurface storage reservoir. On-going measurements of tritium, CFCs, and SF6 will establish mean transit times of groundwater that can assist in estimating the groundwater response time of the aquifer under different climate scenarios. Together, this study will established the magnitude of groundwater storage in the complex fractured bedrock aquifer system, which is essential for management of this freshwater finite resource.