THE ROLE OF FRACTURE PERMEABILITY AND STORAGE IN A CRYSTALLINE ROCK ISLAND AQUIFER

Climate change has progressively impacted water resources worldwide initiating studies of untapped sources of potable water. Fractured bedrock aquifers accounts for a fifth of the world’s aquifer systems yet significant questions of how to characterize and quantify the magnitude of its subsurface storage in the context of water supply development remains to be understood. In this work, we describe the results of an on-going study to understand inter-basin flow, the magnitude of groundwater storage, and water supply sustainability of one such aquifer system on the island of Tobago. The island is predominately composed of highly fractured Mesozoic igneous and metamorphic rocks with a well-developed saprolite soil cover, and a carbonate platform to the south. Prior work has established that the fractured bedrock receives recharge of ~400 mm of the ~1900 mm total annual precipitation.

Using established geochemical methods we evaluated the abundance of solutes provided by saprolites from groundwater throughout the island. We also analyzed selected wells for tritium (³H) analysis to establish the age of these groundwater samples. The results suggest that fractures have provided flow paths that enabled groundwater mixing from the elevated central region of the island towards the southern regions, across significant topographic watershed divides. This supports observations that groundwater production exceeds calculated recharge inputs in some sub-catchments in these regions of the island.

⁸⁷Sr/⁸⁶Sr and Na-normalized strontium concentrations suggest five possible mixing lines all with the same based value ⁸⁷Sr/⁸⁶Sr ~ 0.70396 and Na/Sr (mg/L) ~0.00652, and the maximum values of each line that represents mixing for: extremes in Na concentration ⁸⁷Sr/⁸⁶Sr 0.70576 and Sr/Na 0.0008 mg/L, groundwater to seawater mixing ⁸⁷Sr/⁸⁶Sr 0.70506 and Sr/Na 0.0023 mg/L and precipitation to rock equilibration mixing 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 also suggest that the groundwater is older in the southern regions of the island with values ranging from 0.02 to 0.60 TU. On-going measurements of CFCs, and SF₆ will be used to validate tritium results and applied to groundwater models to calculate mean transit times, and the aquifer’s storage response under different climate scenarios.