Assessment of flow paths and groundwater storage processes in an island fractured rock aquifer system using the stable isotopes of water (H2O) and strontium (Sr), and environmental tracers’ tritium (3H) and sulfur hexafluoride (SF6).

 

Access to fresh water is a critical humanitarian issue in many regions of the world and on the most important sustainable development goals. This crisis is exacerbated by the effects of climate change, pollutants, increases in demand and overuse. Fractured rock aquifers have been providing potable groundwater for many regions of the world, but there still many unknowns about the storage capacity, transit times and flow paths under changing climate scenarios. In this on-going study we aim to understand the magnitude of groundwater storage and Inter-basin flow for water supply development and sustainable use on the island of Tobago, WI. Samples of springs, surface water and production wells were analyzed for the stable isotopes of water (H₂O) and strontium (Sr), tritium(³H) and sulfur hexafluoride (SF₆). The stable isotopes of water δD_vsmow and δ¹⁸O_vsmow indicates that the groundwater in the northern region of the island is not hydrologically connected to the groundwater to the south because of their distinct D-excess signature.

⁸⁷Sr/⁸⁶Sr and Na-normalized strontium concentrations produces five possible mixing lines which radially increase from the lowest based value of ⁸⁷Sr/⁸⁶Sr ~ 0.70396 and Na/Sr (mg/L) ~0.00652. In detail, the maximum values of each line represent: extremes in Na concentration ⁸⁷Sr/⁸⁶Sr 0.70576 and Sr/Na 0.0008 mg/L, the groundwater to seawater mixing line⁸⁷Sr/⁸⁶Sr 0.70506 and Sr/Na 0.0023 mg/L, the precipitation to rock equilibration mixing line ⁸⁷Sr/⁸⁶Sr 0.70506 and Sr/Na 0.0023 mg/L, water located in silicate rocks to carbonate rocks mixing ⁸⁷Sr/⁸⁶Sr 0.70871 and Sr/Na 0.0085 mg/L, and wells that were once affected by seawater intrusion ⁸⁷Sr/⁸⁶Sr 0.70563 and Sr/Na 0.0692 mg/L.

Tritium results range from 0.02 to 0.60 TU and calculations suggest that wells contain a range of 2 to 44% modern groundwater. When compared to other islands at similar latitudinal locations, Tobago’s groundwater presents the lowest mean and median tritium values even though it is the closest to the equator. Basin flux and effective porosity were calculated for the 10 wells using the apparent age obtained from these results under the assumption of piston flow. It was found that all watershed volumes were magnitudes of orders larger than the sub-basins where the wells were located ranging between 0.09 km³ to 8.23 km³. Basin Flux and effective porosity also contain large range differences 1.40 *10⁵ m³/yr to 9.93 *10⁶ m³/yr, and 0.014 to 0.094, respectively.

This results also suggest that the groundwater in the southern regions of the island contains the oldest water with one well sample >60 years. SF₆ results reflect similar ages except for 3 wells samples which are suspected to be contaminated by excess air. This novel discovery illustrates that small, fractured rock island aquifers can possess structural complexities that lead to older groundwater ages and variances in basin characteristics.