This is the peer reviewed version of the following article: Scheliga B, Tetzlaff D, Nuetzmann G, Soulsby C. Groundwater isoscapes in a montane headwater catchment show dominance of well-mixed storage. Hydrological Processes. 2017;31:3504–3519., which has been published in final form at DOI 10.1002/hyp.11271. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving.