N and P concentration-discharge relationships in streams: What can they tell us about transfer pathways?

Abstract

Waikato Regional Council operates a river water quality monitoring programme where samples are taken monthly at 114 sites and analysed for concentrations (C) of a range of water quality parameters. Water flow (or discharge, D) is measured at or nearby 26 of these sites, which allows concentration– discharge relationships (C-D relationships) to be established. The overall patterns of the C-D relationships for N and P were surprisingly similar across the region in spite of substantial differences in natural conditions, land use and the potential effect of point source discharges. Statistically significant C-D relationships were found at nearly all sites for total nitrogen, nitrate nitrogen and total phosphorus, although the corresponding coefficients of determination varied widely between nutrients and sites. All but two relationships were positive, i.e. concentrations generally increased with increasing discharge. Positive C-D relationships are often presumed to reflect that the stream water chemistry at low discharge is dominated by input from the older and less polluted groundwater reservoir, while shallower flow paths (interflow, artificial drainage, surface runoff) transferring younger water that is more enriched with nutrients become more important at higher discharge. We investigated two supplementary parameters (silica, electrical conductivity) to explore whether they can be used to corroborate the presumed flow path/water age relationship. Stream water silica concentrations can potentially be used as a crude proxy for water age, as the silica concentration of water generally increases with increasing contact time with silica-bearing minerals in the subsurface. Statistically significant negative C-D relationships were found at 18 of the 26 sites, i.e. supporting the presumed flow path/water age relationship. However, the small size of the available data sets (max. n=12 per site) prevented an in-depth analysis. EC is routinely measured and is in the absence of major point sources also primarily determined by the geology. Accordingly, statistically significant negative C-D relationships were found at 22 of the 26 sites, but sometimes low coefficients of determination also limited the utility of this parameter. Ongoing analysis aims at identifying the conditions under which silica or EC data can confidently be used