| The Gulf Islands include two aquifers: the granular and the fracture rock aquifer. The granular one consists of marine, fluvial and glacial material. It is generally thin (less than 18 m in thickness) or absent on the islands. Granular aquifers are found at local scale and aren't well known. Locally, perched aquifers may be observed. The regional fractured rock aquifer is widely exposed and consists primarily of interbedded sandstone-conglomerate and mudstone. The aquifer is consequently under unconfined conditions, showing moderate to very high vulnerability along the coast. The main threat is saline water intrusion, which is more present in the summer since the population increases during tourist season and the precipitations decreases. Surface water percolates in the topographic highs, flows down the rock fractures roughly following the surface topography. Water flows primarily from fractures as secondary permeability, reflecting the low primary porosity and permeability of the bedrock. All lithologies are highly cemented, dominantly by calcite. Sandstone-dominant unit with low fracture has low aquifer potential. Mudstone-dominant unit with high fracture density and elevated permeability shows high aquifer and good groundwater storage capacity. This aquifer represents the primary source of potable water for the islands. Groundwater is used for water supply for individual domestic use, small communities and for small mixed farming operations in the region. Groundwater quality is found to vary relative to the position. Recharge areas have fresh water, but deeper water is brackish. High fluoride concentrations are also observed. |
Groundwater Information Network