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[1] Springs and gaining streams are locations where groundwater flow paths naturally converge and discharge as a flow-weighted mixture of water from the contributing aquifer. The age of that water is therefore a good measure of the mean transit time (MTT) of the contributing aquifer. The question examined in this paper is whether tritiogenic helium-3 and tritium ( 3 He trit -3 H) can be used to estimate MTT in these settings. To answer that question two factors must be considered: (1) the loss of 3 He from discharging groundwater as it becomes exposed to the atmosphere, and (2) the accuracy with which MTT can be determined from flow-weighted 3 He trit -3 H concentrations. These concepts were tested at the Fischa-Dagnitz system (springs and emerging stream), which is part of the southern Vienna Basin aquifer. Conducting a gas tracer test, gas exchange coefficients (l) were established for helium-4 (