Estimation of recharge rates to the sand and gravel aquifer using environmental tritium, Nantucket Island, Massachusetts

Estimation of the average annual rate of ground-water recharge to sand and gravel aquifers using elevated tritium concentrations in groundwater is an alternative to traditional steady-state and water balance recharge rate methods. The Nantucket tritium recharge rates clearly are higher than rates determined elsewhere in southeastern Massachusetts using the tritium, water table fluctuation, and water balance methods, regardless of the method or the area. Because the recharge potential on Nantucket is so high (runoff is only 2% of the total water balance), the tritium recharge rates probably represent the effective upper limit for groundwater recharge in this region. The accuracy of the tritium method is dependent on two factors: the accuracy of the effective porosity data, and the sampling interval. For some sites, the need for recharge rate data may require a determination as statistically accurate as that which can be provided by the tritium method. However, the tritium method is more costly and more time consuming than the other methods. For many sites, a less accurate, less expensive, and faster method of recharge rate determination might be more satisfactory. 40 refs., 13 figs., 5 tabs