Hydrology of Small Oceanic Islands — Utility of an Estimate of Recharge Inferred from the Chloride Concentration of the Freshwater Lenses

In Bermuda, as in other coastal and oceanic-island environments, rainfall has a significant chloride concentration. As a result, there is a relatively straightforward way of estimating groundwater recharge by considering the Cl− ion as a tracer which is concentrated by evapotranspiration. The Cl− concentration of rainfall in Bermuda is about 15 ppm. That of the freshest part of the largest Ghyben-Herzberg lens is about 60 ppm. Taking the 60 ppm value as an indicator of the Cl−concentration of recharge, the average recharge rate is estimated at 0.25 of the 147-cm/yr. average rainfall, or about 37 cm/yr. This estimate is similar to two other estimates of recharge in Bermuda, each derived from hydrogeologic field data: (1) A 33-cm/yr. estimate inferred from a 2·106-m2 area of diversion in which: (a) outflows (extractions) are 2870 m3/day; and (b) the change in storage is estimated at 1100 m3/day, from the rate of lowering of the water table. (2) An estimate of 35 to 45 cm/yr. resulting from combination of: (a) the ratio of recharge to hydraulic conductivity of the Paget Formation (12·10−6), determined from the configuration of the Ghyben-Herzberg lenses; and (b) the hydraulic conductivity of the Paget Formation (85–100 m/day), estimated from the behavior of the water table. The agreement between the three estimates of recharge indicates that the rather simple and inexpensive technique of calculating recharge from Cl− content of rainfall and fresh groundwater can be a useful addition to the arsenal of techniques by which recharge of small oceanic islands can be evaluated