Dual porosity flow is an important mechanism for groundwater transport in fractured rock aquifers. However, quantification and characterization of fracture flow systems remains challenging, as it often involves complex procedures such as the injection of tracers. In this study we conducted single-well pumping tests in 11 uncased wells in a coastal fractured rock aquifer while monitoring in-well salinity and temperature gradients through downhole casts using a Conductivity-Temperature-Depth (CTD) logger. In this way, we aimed to observe how naturally occurring salinity gradients in the well become disturbed by induced groundwater flow to the well, and if these gradients may serve as natural tracers for fracture flow. Since natural temperature gradients in the wells are minimal, we applied point electrical heating at the bottom of the well to create a plume of slightly warmer water to migrate up the wellbore during pumping from the top. During the pumping tests in this set-up, repeated CTD casts suggest that groundwater flow to these wells is strongly focused along narrow zones and is occurring at various rates over a range of salinities and temperatures. Hence, the observed patterns in both salinity and temperature presumably reflect the presence of fracture zones, which could indeed be confirmed by downhole camera observations for some wells. Further data analysis resulted in detailed hydrogeological characterization of the 11 wells, comprising an assessment of the fracture density and hydraulic conductivity of the aquifers, as well as the origin of the inflowing water being meteoric mostly fresh water or deeper saline groundwater.

This study aimed to identify ocean-and land-based sources of nutrients to the coral reef communities surrounding the Southern Caribbean islands Aruba, Bonaire, and Curaçao. The composition of water masses around these islands were assessed to depths up to 300 meters and three distinct overlying water masses were identified, separated by mixing zones. A fluctuating pycnocline separating surface from deeper (> ~50 m) water indicated the presence of internal waves. Nutrient profiles were typical of tropical waters with oligotrophic waters occurring above the pycnocline and a deep chlorophyll maximum (DCM) just below it (~65 m). Concentrations of dissolved nutrients differed among islands. Inorganic nitrogen (DIN) and phosphate concentrations were respectively lowest around Bonaire and Curaçao. The spatial distribution of chlorophyll-a (indicative of phytoplankton abundance), rather than nutrient concentrations, suggested the presence of higher-than-average nutrient concentrations in islands with higher population densities, near urbanized/ industrial areas, and near upwelling areas.