Investigation of subsurface freshwater flow from southern Bonaire into Lac bay
In recent decades, concern about mangrove die-back has intensified. One of the last remaining mangrove areas in the Dutch Antilles is the mangrove forest in Lac bay on the island of Bonaire.
It has a relatively large biodiversity and is essential for many bird species. Not only the inhabitants of the forest itself – also many other species outside the forest – depend on it. This is because the filtering effect of mangrove roots prevents excessive sediment deposition on sea grass beds and coral reef. Unfortunately, extensive mangrove die-back in Lac bay has been observed in recent years. The hypothesis is that this die-back is caused by hyper-salinity. This study focuses on subsurface freshwater flow from southern Bonaire into Lac bay as a source of refreshment to the mangrove area.
In the rainy season 2012-13 (October to January), the hydrological system in the area surrounding Lac bay was intensively investigated. Measurements included rainfall, hydraulic conductivity, groundwater levels and open water salinity in the backwaters of Lac bay.
The geo-hydrological properties of limestone were found to largely determine groundwater flow on both regional and local scale. On a regional scale hydraulic conductivity and degree of karstification determines gradients, hydraulic contact to Lac bay and responses to rainfall. 4 and 5km north of Lac bay, hydraulic conductivities were found to be 1.3 and 2.7m d-1, respectively. The relatively low values are accountable for: 1) steep gradients of 300-380cm km-1 of the southward groundwater flow; 2) poor hydraulic contact to Lac bay; and 3) strong groundwater responses to heavy rainfall. For instance, a rise in groundwater level of 4m was observed in only 9 days.
Further south, extensive karstification exists. Here, a relatively high hydraulic conductivity of 12.5m d-1 was found, resulting in: 1) low gradients of 2-10cm km-1 of the eastward groundwater flow west of Lac bay; 2) high tidal impact on hydraulic heads; and 3) weak responses to rainfall.
On a local scale, karstified limestone makes groundwater flow complex, because it facilitates underground streams with unpredictable flow paths. 700m northwest of Lac bay, a northeastern flow direction away from Lac bay was found. This may explain the relatively high salinity greater than 50p.p.t. observed in the northwest backwaters of Lac bay.