The short half-life of tritium (12.7 years) means the signal due to production from nuclear bomb testing in the 1950s and 1960s has essentially been lost from the atmosphere but can still be seen in young (<60 year) groundwater and provides a tracer of groundwater contribution to stream baseflow in shallow, active systems. Back-calculating the mean age of baseflow in stream samples, combined with knowledge of aquifer hydraulic parameters provides a means of assessing the relative contribution of different source areas and the likely distance of provenance. This can aid in the conceptualization of groundwater-surface water systems and provides constraints on relative contributions of groundwater sources to surface water systems.
We have used tritium measurements in surface and groundwater samples across the Upper Waikato region of New Zealand to assess relative contributions of shallow and deep groundwater sources and the potential zones of contribution and to test conceptualisations of nutrient pathways and attenuation. Mean groundwater ages were calculated using known tritium decay curves for New Zealand and the USGS software package TracerLPM, comparing piston flow models with exponential mixing models to assess the most likely age distribution. This was carried out for both groundwater and surface water (baseflow) samples. A range in flow distances were then calculated to provide indicative travel paths and for comparison of data from bores on expected flow transects.
The results provided additional evidence that the surface waters are supplied predominantly by shallow, relatively fast-moving groundwater in this area and deeper groundwater is unlikely to provide a significant contribution to baseflow in this part of the catchment. This analysis lends support to nitrate attenuation estimations that suggests nitrate transport to streams is dominated by shallow flow-paths and significant denitrification prior to discharge to local waterways.