In 1975, Foster described the “Tritium anomaly” in the British Chalk, where very low levels of post-bomb tritium were observed in groundwater, as compared with concentrations in the vadose zone above it. This was hypothesised to occur due to the concentration gradient between rapidly recharging water through fissure, and relatively immobile pore water. Since then, research in the UK has focused on this process, and more recent work has highlighted the critical role that dual porosity/permeability mechanisms play in controlling the “nitrate load to come”.
Simplistically, when recharge events occur, water moves through high permeability pathways (fissures), and diffusion occurs from this high nitrate recharge into the low concentration matric water. As the concentration in the matric water increases over time, the concentration gradient can be reversed, and concentrations in the recharging water may then be enhanced (load to come). This has been observed in the UK Chalk, with the matric water becoming a ‘source’ of nitrate as land use changes have resulted in lower nitrate concentrations in recharge. Similar processes occur in the saturated zone, and have been observed through failed aquifer storage and recovery trials, and tracer experiments.
In New Zealand alluvial gravel systems, as in the Chalk, there are two contrasting components of flow, with the same opportunity for movement of solutes between the two phases. The fine-grained matrix is almost always close to saturation, but has very low intrinsic permeability, whereas the open framework gravels (OFGs) within the gravel sequence, as with the fissures in the Chalk, transport most of the flow but are usually unsaturated in the vadose zone.
This paper explains the process of nitrate transport in dual porosity/permeability systems, providing evidence from the UK and New Zealand. Whilst the work in the UK has been carried out over several decades, quantification of the nitrate load to come has not yet started in New Zealand. There is a crucial need to fully quantify the extent to which nitrate has been retained in immobile pore water. Until we measure this, we cannot have confidence in modelling the impacts of land use change on groundwater and surface water quality.