Nitrate source evaluation of surface water and groundwater in the Franklin area using a dual stable isotope approach

Summary

GNS Science was contracted by Auckland Council to interpret the nitrate (NO3-N) concentrations and nitrate isotope signatures of 36 water samples collected in 2014 and 2015 in the Franklin area, Auckland, to identify possible sources and transport environmental processes in the context of elevated nitrate concentration in groundwater.

The sample set consists of eight surface water and four groundwater samples collected in 2014 to investigate elevated nitrate concentration in the Franklin area and 24 vadose zone leachate samples from a farm in Franklin collected to understand the impact of fertiliser application on soil leachate in 2015. Analysis for nitrate concentration and isotopes was undertaken and reported as analytical results at the time of sampling, and this interpretive report is based on these results.

Using a dual nitrate isotope approach, the findings include:

• The key contaminant source was identified as synthetic fertilisers rather than farming effluent due to their isotopic signature falling into the fertiliser and Mixed soil nitrogen (Soil N) and fertiliser zones.
• The samples had NO3-N concentrations ranging from 0.002 to 69.6 mg/L and nitrate isotope values from -6.7 to 9.2 ‰ for δ¹⁵N and -2.0 to 10.0 ‰ for δ​¹⁸O.
• Surface water samples with high NO3-N concentrations (>5 mg/L) indicate contamination from recent fertiliser application, as the nitrate concentrations are above expected background levels (of <1 mg/L).
• Groundwater samples with higher NO3-N concentrations (>5 mg/L) indicate fertiliser breakthrough into the aquifer and/or subsurface transport of leached nutrients from soils.
• Groundwater with lower nitrate concentrations (<3 mg/L), suggesting denitrification (based on isotopes) or lower contamination levels and sufficient assimilation within the ecosystem to assimilate any excess nutrients. There is only one site (LB02, a 47-m-deep bore from Fielding Road) where NO3-N concentrations are very low, with low dissolved oxygen and higher NH4-N levels, suggesting that it has probably reset to natural background levels due to denitrification.
• δ¹⁵N and δ​¹⁸O values reflect the nitrate source and its subsequent fate. Key processes observed in these samples involve mineralisation and transformation of nitrogen sources through to the denitrification zone. If further fertiliser pulses or applications occur, the cycle can reset, and nitrate isotopes reflect the new input. Only when the removal process rate from denitrification or ecosystem uptake exceeds the additive rate will nitrates decline through ecosystem assimilation or denitrification.

This information will contribute to an ongoing research strategy by Auckland Council to improve understanding of the dynamics and interactions of groundwater and surface water nitrate in the Franklin area. The findings will help inform future management strategies that aim to reduce these nitrate concentrations over time in order to meet the requirements of the National Policy Statement for Freshwater Management 2020.

GNS Science report, 2022/02​

February 2022