Age dating base flow at springs and gaining streams using helium‐3 and tritium: Fischa‐Dagnitz system, southern Vienna Basin, Austria

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 ( λ ) were established for helium‐4 ( 4 He) and krypton‐84 ( 84 Kr), and derived for helium‐3 ( 3 He) and neon‐20 ( 20 Ne). By simulating measured groundwater inflow and gas transport in the stream, groundwater inflow concentrations for 3 He, 4 He, 20 Ne, and 84 Kr were estimated. Correcting for the various sources of He, the tritiogenic helium‐3 ( 3 He trit ) concentration of inflowing groundwater was estimated at 8.3 tritium units (TU). The flow‐weighted groundwater concentration of 3 H, determined from 22 stream water samples, was estimated at 9.8 TU. Assuming that the relationship between flow amount and transit time at Fischa‐Dagnitz is characterized by a hybrid dispersion–exponential age model, the 3 He trit – 3 H ratio (8.3/9.8 = 0.85) defines a MTT of 8 years. The validity of this estimate was evaluated by comparison to a long‐term 3 H time series that exists for Fischa‐Dagnitz. The likely range of MTT's derived from the measured 3 H time series is 11 to 14 years.