Contrasting Groundwater and Streamflow Ages at the Maimai Watershed

Understanding the links between groundwater age, runoff generation processes, and their effects on stream water transit time (TT) is a major research challenge. Here we present new tritium age dating and hydrogeological characterization from 40 bedrock wells drilled at the intensively studied Maimai Experimental Watershed in New Zealand. We investigated the extent, dynamics, and age of the groundwater in a 4.5 ha headwater catchment over a 400 day period. In particular, we explored the controls of bedrock structure on aquifer dynamics, the aquifer flow domain and its influence on time-varying stream water TT. We show that low permeability hillslope bedrock minimizes deep recharge, thereby regulating groundwater age, stream water MTT, and surface water-groundwater interaction. Two distinct hydrologic units can represent catchment storage: shallow young soil storage and deep much older bedrock groundwater. Groundwater ages near discharge zones were up to 23 years compared to soil water ages that ranged between 0.1 and 0.5 years. This difference in age for the two main storages resulted in contrasting seasonal stream water TT response. During the 8 month wet season, stream water TT was young and stable while stream water TT in the slightly drier summer season was highly dynamic. These qualitative field observations are a process exemplar that support the Berghuijs and Kirchner (2017) quantitative descriptions of preferential release of young streamflow; and for the Maimai catchment, support the notion that most groundwater is exchanged only slowly with the surface and is therefore relatively old.