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

Abstract

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 (3Hetrit–3H) can be used to estimate MTT in these settings. To answer that question two factors must be considered: (1) the loss of 3He from discharging groundwater as it becomes exposed to the atmosphere, and (2) the accuracy with which MTT can be determined from flow‐weighted 3Hetrit–3H 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 (4He) and krypton‐84 (84Kr), and derived for helium‐3 (3He) and neon‐20 (20Ne). By simulating measured groundwater inflow and gas transport in the stream, groundwater inflow concentrations for 3He, 4He, 20Ne, and 84Kr were estimated. Correcting for the various sources of He, the tritiogenic helium‐3 (3Hetrit) concentration of inflowing groundwater was estimated at 8.3 tritium units (TU). The flow‐weighted groundwater concentration of 3H, 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 3Hetrit–3H 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 3H time series that exists for Fischa‐Dagnitz. The likely range of MTT's derived from the measured 3H time series is 11 to 14 years.