CONTROLS ON THE GEOCHEMICAL EVOLUTION OF GROUNDWATER ON TOBAGO

This study investigates the geochemical evolution of surface and ground waters of Tobago to understand timing of recharge, nature flow paths, and rates of solute flux. Tobago is a developing island in the Caribbean, whose aquifers are highly heterogeneous and anisotropic, composed of metamorphic and crystalline rock with structural controls on permeability. Hydrogeologic analyses of the water budget suggest that despite the low matrix permeability of the host rocks, the island is underlain by a prolific fractured rock aquifer system, the characteristics of which allow groundwater to bypass major topographic boundaries. Understanding the hydrogeology of small islands is critical to evaluating groundwater resources, especially in the Caribbean which experiences strong seasonality in precipitation.

In March and December 2014, we sampled 32 groundwater wells, 36 surface water sites (with discharge measurements), 5 springs, and ocean water. Eight months of precipitation data (June 2014 – January 2015) from 16 meteoric stations on the island were also collected. All samples are analyzed for stable isotopes of oxygen and hydrogen of the water molecule, major and minor elements, and ⁸⁷Sr/⁸⁶Sr isotopes. This extensive collection of samples and robust geochemical data is uncommon among coupled hydrologic and weathering studies.

Initial findings constrain temporal controls on recharge, and what appears to be seawater signatures in groundwater wells. Stable isotope signatures of groundwater are uniform spatially and seasonally, and similar to the signature of wet season rains. This data supports conceptualization that wet season precipitation dominates recharge and groundwater chemistry, despite the fact that dry season precipitation contributes significantly to total precipitation amounts. Interpretation of major elements and ⁸⁷Sr/⁸⁶Sr isotopes indicate seawater mixing in groundwater wells, some of which are screened below a brackish water bearing confining unit. We distinguish the source of the seawater signature from saltwater intrusion, or from downward leaching of brackish water. The location of significant fresh potable groundwater less than 1km to the coast with screens below sea-level confounds the interpretation of the fresh water/salt water interface in aquifers dominated by fracture flow.