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

Document Type


Publication Date


Digital Object Identifier (DOI)



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 Clconcentration 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.

Was this content written or created while at USF?


Citation / Publisher Attribution

Journal of Hydrology, v. 45, issues 1-2, p. 21-37