A national isotope survey to define the sources of nitrate contamination in New Zealand freshwaters

A national survey identifies the extent and origin of nitrates in New Zealand's freshwater resources using nitrogen (delta 15N) and oxygen (delta 18O) isotopes. A comprehensive database with 1042 samples investigates freshwater type, land-use, geology, soil type and geographic effects on nitrate isotopes across New Zealand. Nitrate contamination is the biggest threat to New Zealand's drinking water resources with almost 60 % of samples in the study having above-baseline nitrate (NO3-N) concentrations >0.9 mg/L, a threshold noted for higher colorectal cancer risk. Groundwater (2.89 mg/L) had higher median NO3-N concentration than surface water (0.65 mg/L). Traditional nitrate isotope biplots based on the Haber-Bosch cycle (which integrates synthetic fertilizers into the food chain), are reinterpreted to place New Zealand's unique farming nitrogen contributions from urea and ammonia into context. The majority of New Zealand's freshwater nitrate isotopes lie in a Normal-NitrogenRetention (NNR) zone which also acts as a Mixed Urea-Urine-Fertilizer (Mixed UUF) zone, transitioning nitrified fertilizers and animal waste via redox reactions, microbial action and/or dissipation towards a denitrification zone. Nitrogen 'hot spots' with strong urea/urine input can be differentiated from natural N cycling in the NNR/ Mixed UUF transition zone by a combined NO3-N concentration and dual isotope approach to identify high-risk sites.