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Water quality of lakes and rivers is heavily influenced by dissolved and particulate contaminants in the surface runoff and groundwater inflow from the surrounding catchment. Understanding the sources, both diffuse and point, of these contaminants is critical for improving the sustainability of freshwater systems.

Tracing The Sources Of Nitrate Using Isotopes

The use of nitrogen and oxygen isotopes in nitrate (δ15N-NO3 and δ18­­O-NO3, respectively) has developed over the past 30 years to provide powerful tracer value, identifying sources and tracking processes related to nitrogen fluxes through the atmosphere, biosphere and hydrosphere. Globally, the high δ18­­O-NO3 values associated with atmospheric sources and oxidation processes has been most powerful in source identification for polluted northern hemisphere regions. In these regions, the signal in δ18­­O-NO3 is immense but highly variable, and therefore obscures nuanced studies of nitrogen cycling. Over a decade of intensive New Zealand studies have shown signatures dominated by nitrogen cycle processes without substantial atmospheric inputs, therefore offering unique potential to drill into the detail of controls on ecosystem nitrogen retention.

New Zealand’s observed pattern of dual-isotope nitrate suggests that many pastoral soils have good nitrate retention, such that added urine or urea intensifies nitrate leaching that remains buffered through the soil organic nitrogen pool, as indicated by δ15N-NO3. Within a variability of 1–2 ‰, nationwide studies find nitrate matching these conditions sits on a 1:1 line in bi-plots (see Figure 1) from δ15N-NO3 = 4 ‰ and δ18­­O-NO3= 0 ‰ along a gradient of increasing agricultural intensity to δ15N-NO3 = 8‰ and δ18­­O-NO3= 4 ‰. This defines a denitrification and mixing (DAM) line for integrated soil processes characteristic of good ecosystem nitrogen retention.

In contrast, where highly permeable soils are found, the low δ15N-NO3 values of urea and urine (typically -2 to +2 ‰) can dominate localized sources and decrease the catchment integrated δ15N-NO3 in rivers and streams by 2 ‰ or more. Occasional shifts in localized sources to higher δ15N-NO3 and δ18­­O-NO3 reflective of denitrified effluents are also observed. These isotopic patterns are commonly associated with elevated nitrate concentrations that exceed thresholds for freshwater ecological health. When observed, this pattern points to opportunities to alter land use and management to reduce nitrate leaching reaching freshwater.

Adding recent data from the Rotorua Te Arawa Lakes catchments to datasets from Manawatu, Wairarapa, Canterbury and Southland reinforces these interpretations, suggesting that source signatures dominate integrated catchment δ15N-NO3 and δ18­­O-NO3. For inflows to Rotorua from pastoral areas on permeable pumice soils, δ15N-NO3 values are typically offset 2 ‰ lower than than the DAM line. Heavier soils associated with the ‘Rotomahana mud’ eruptives generally do not show this offset. Dual-isotope nitrate signatures associated with other distinctive sources are locally evident, including waster-water treatment, farm effluents, geothermal activity. Inflows from lakes where phytoplankton uptake has altered δ15N-NO3 and δ18­­O-NO3 can also be evident, and helpful in confirming the source of lake-fed springs.

Why do integrated samples from large catchments rarely show a ‘denitrification’ signal characterised by δ15N-NO3 and δ18­­O-NO3 increasing along a 2:1 line? Is this surprising when localised sampling from shallow groundwater bores or 1st-order streams shows this denitrification signature? The obvious answer is that the removal of the nitrate by denitrification tends to go nearly to completion, thereby removing the δ15N-NO3 to negligible levels when against background flows with appreciable concentrations. A secondary explanation in some environments could be that diffusion rather than denitrification could be the rate-limiting process suppressing the expected signal. Regardless, integrated source signatures are well-preserved in large catchments – enabling cost-effective characterisation and monitoring.

To better target mitigation, can dual-isotope nitrate measurements clarify hot spots and hot moments of nitrate production, mobilisation and loss? In soils, the δ15N-NO3 and δ18­­O-NO3 appear to go through a highly variable sequence following urine addition or wet up. But leached nitrate observed in flows from rain events appears to have relatively stable signatures that allows good soil nitrogen retention to be contrasted with other sources, such as direct outflow of urine/urea or effluent N isotope signatures. Nevertheless, high-resolution or continuous concentration measurements combined with the use of water isotopes or other tracers to confirm flowpaths may more directly target mitigation opportunities – augmenting the integrated potential of dual-isotope nitrate at catchment scales.

Figure 1. Indicative positions of dual-isotope nitrate categories