ESTIMATING RECHARGE RATES THROUGH UNSATURATED GLACIAL TILL BY TRITIUM TRACING

A tritium profile obtained from a core located on the Tipton Till Plain near West Lafayette, Indiana, showed a "tritium reversal" (corresponding to a low-high-low activity of tritium relative to depth) at 7 m, indicating the present position of recharge water derived from 1963/64 precipitation. The average recharge rate calculated using the tritium data is 3.5 cm/yr for the "mass-balance" method and 4.7 cm/yr for the "transit-time" method. The average of 4.1 cm/yr is in reasonable agreement with Arihood's (1982) estimate of 5.1 cm/yr for tills, based on numerical modeling of data from a water-budget study in the White River basin of Indiana. Two other cores, located on a nearby topographically lower slope, showed no definite reversal in tritium activity, probably because of lateral flow components at these locations. To be applied, the tritium technique should assess the contribution of summer and winter precipitation to recharge because tritium activities tend to peak in spring and summer precipitation. Contrary to the usual assumptions that recharge to aquifers in temperate zones of the northern hemisphere occurs between October and April, stable isotopic data of well water in the study area indicate that an appreciable amount of annual recharge (about 34%) actually occurs between April and October.