MODPATH, MT3DMS, and lumped parameter models (LPMs) allow us to estimate transit times of nitrate to streams while only LPMs can be applied over a large regional scale with minimal time or cost investment, albeit in an approximate manner. In New Zealand, all three models have been calibrated to tritium measurements in streams, allowing us an evaluation of different methodologies to obtain nitrate concentrations. In this talk, we discuss some of the issues inherent in using LPMs to assess subsurface transport of nitrate to streams. We demonstrate that LPMs cannot account for spatially variable nitrate inputs; in particular when high nitrate concentrations are exclusive to a specific band of transit times of groundwater (i.e. only short or only long). Therefore, by estimating isochrones, the spatial application of nitrate can be simulated more effectively with LPMs. We also highlight the importance of tritium when investigating the delivery of nitrate to streams. Tritium, which is a component of meteoric water, decays with a half-life of 12.32 years, is inert in the subsurface and streams, and can provide information on transit time of the water through the subsurface, which will reduce the degrees of freedom when calibrating to nitrate levels in a stream. |