Marsha Kezia Allen, David F Boutt

Understanding the residence time, flow velocities and storage volumes in fractured rock aquifers is essential for assessing the sustainable use of groundwater in island aquifer systems. Environmental tracers such as tritium (3H) and sulfur hexafluoride (SF6) that enter the aquifer systems through natural hydrological processes are effectively used to quantify the “apparent age” of young groundwater (<60 years) to calculate these aquifer characteristics. The island of Tobago is predominantly a fractured rock aquifer system that contains groundwater sub-basins that produce between 100 to 1000% more than their calculated recharge. In this study, we analyzed ten samples from groundwater wells throughout the island for environmental tracers to establish the apparent age of the groundwater. We then estimate the active flux and storage volume of the aquifer. The 3H results of ten wells throughout the island range from 0.02-0.60 TU. The groundwater samples to the south of the island possess more pre-modern groundwater ages (older ages) compared to samples in the north of the island. These tritium values reveal age ranges between 18 to 60 plus years This suggests that fractures create flow paths oriented towards the southern parts of the basin that enabled the transport of groundwater across significant topographic boundaries and watershed divides. Additionally, the data may support that southern wells are connected to isolated old sources of groundwater. SF6 values range between 0.91 to 7.97 (part per trillion volume) with interpreted age range of between 18 to 39 years. Both tracers support the original hypothesis that longer residence time waters are located to the south of the island except for three SF6 samples which are believed to be affected by excess air.