GROUNDWATER RECHARGE TO A STRUCTURALLY COMPLEX ISLAND AQUIFER SYSTEM

This study quantifies groundwater recharge for sustainable groundwater development on the island of Tobago, Trinidad and Tobago, West Indies. Tobago is a developing island 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.

Components of Tobago’s water budget are quantified using historical precipitation and stream discharge data, as well as estimates of potential evapotranspiration from three sources. Mean-annual and monthly recharge values are calculated by taking the residual of these three quantities. Recharge is spatially distributed using groundwater recharge units, defined by refining watersheds at the sub-basin scale. Remote sensing data is used to identify structural features on the island, which greatly influence groundwater flow. To understand the hydrogeological system further, a steady state groundwater flow model was developed. Steady state hydraulic heads are predicted using groundwater models validated by hydraulic head observations and dry season stream flows.

We quantify the amount and spatial distribution of recharge, as well as the fraction of recharge being captured by groundwater pumping. Despite extensive freshwater withdrawals, current groundwater production reaches only ~7% of total groundwater recharge. Spatial analysis of the groundwater capture zones using groundwater recharge units suggests that some wells capture water from outside their topographic watershed. Capture zones predicted from modeling are consistent with hydrologic balance approach but are more robust and physically based. The location of significant fresh potable groundwater less than 1km to the coast with screens well below sea-level confounds the interpretation of the fresh water/salt water interface in aquifers dominated by fracture flow.