Does Tritium Counts Help Quantify Residence Time of Recharge Waters Today?

Abstract:

Concentration of tritium in waters sampled from cold, warm, and thermal sources contained in shallow and deep aquifers, and springs were analyzed to characterize and model the residence time of recharge waters in fractured, unconsolidated, and recent volcano-clastic aquifers in the southwestern part of the Main Ethiopian Rift System. The measured tritium concentrations were interpreted (i) with reference to historical records at three nearby GNIP (Global Network of Isotopes in Precipitation) stations in Ethiopia, (ii) in conjunction with hydrochemical variables that can imply residence time of water in aquifer formations and unsaturated layers, and (iii) in association with some of the major basin attributes that could potentially influence on the process of natural recharge, hence, age of aquifer waters. The measured tritium counts in 102 water samples generally fall between 2.5 and 4 TU. Measurements portrayed no spatial patterns and defined relations with variables including concentration of chloride, molar ratios of Mg²⁺ to Ca²⁺ and Na⁺ to Cl^-, NO₃^-, electrical conductivity, dissolved oxygen, chemical water types, fracture density, static water level, depth to water table, depth of well, depth of precipitation, degree of aridity, drainage density, and surface slope. This study shows the limited application of tritium to quantify the residence time of recharge waters and conceptualize the mechanism of recharge in the study area. Keywords: Tritium, residence time, recharge, Main Ethiopian Rift