Hydrological Research Bonaire
Fresh water on the semi-arid Caribbean island of Bonaire is scarce as groundwater is generally brackish (electrical conductivity 2,000 to 4,000 μS/cm) and, apart from the spring at Fontein, there are no perennial streams and springs. Until the 1960’s the drinking water supply of the island relied on a few fresh wells. Now drinking water for the 12,000 inhabitants is produced from desalinated seawater. Local people still use groundwater for relatively salt tolerant crops like maize and for goats in addition to rainwater harvested in reservoirs (tankis) during the rainy season which lasts from October to December. This form of subsistence farming has turned into a marginal activity as most people are employed by governmental organisations, tourist sector and local industries.
New initiatives for horticultural activities and tourist attractions have revived the interest in groundwater. The government of Bonaire, local entrepreneurs and the environmental organisation STINAPA requested the Acacia Institute of the Vrije Universiteit Amsterdam (VUA) and the Ecological Management Foundation (EMF) to carry out a hydrological reconnaissance study on Bonaire to get insight in the general hydrological processes on the island.
In November and December 2004 a field research was carried out on Bonaire. During this field research a well survey, meteorological and geophysical measurements were carried out. Based on this field research and literature a water balance was set up. On average about 5% of the annual 470 mm of precipitation recharges the groundwater (6.5 million m3/year), while 10% runs off to sea. Due to the high rates of evaporation and salt particles in the air the rainwater that reaches the soil is slightly brackish. The salinity of the groundwater shows great variations in space and time, both in the volcanic sediments, as well as in the (karstified) limestone terraces.
Most of the wells are brackish due to high evaporation amounts of the precipitation. Locally increased amounts of infiltration cause wells to be fresher. While on the other hand at other places wells are more saline due to the pumping of deeper saline waters, or due to high amounts of evaporation from surface ponds.
Groundwater use in the volcanic Washikemba formation is limited by the small permeability of the
material and the relative high salinities of the water. Near tankis or dams, where increased infiltration
takes place water of better quality may be found. Exploitation of groundwater in the limestone
formation is possible by applying a series of small wells or a horizontal gallery (tunnel). As the geology
varies locally exploration studies should be carried out on a detailed local scale.
Geophysical measurements carried out in this study, showed that local differences in salinity and
bedrock conditions for groundwater exploration can be measured with geophysical methods. However,
the number of soundings was too small and the spacing of the soundings was too large to indicate
favourable zones for groundwater exploitation.
Groundwater quality and quantity can be improved by retaining more rainwater. This will also decrease the amount of damage due to sedimentation to the coral reefs and will decrease the water nuisances caused by rapid runoff of rainwater. Building and maintaining dams and improving the vegetation cover by controlling grazing are ways to retain more rainwater.
More detailed research, including the monitoring of groundwater levels and quality over a longer period will give a better understanding of the groundwater system.
In general the problems on Bonaire require careful planning of land and water use, starting with an identification of the most critical areas. A geographical information system (GIS), as created for this project, can assist both governmental organizations and future research projects by making the information more available.