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

• More than 1000 New Zealand freshwater samples were analysed for nitrate isotopes. • New Zealand nitrate isotopes are highly influenced by urine and urea due to farming. • Around 60 % of samples had NO3-N levels >0.9 mg/L. • Groundwater has a higher median NO3-N level than surface water. • Nitrate isotopes identify high-risk nitrate regions due to different landuse. A national survey identifies the extent and origin of nitrates in New Zealand's freshwater resources using nitrogen (δ15N) and oxygen (δ18O) 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 (NO 3 -N) concentrations >0.9 mg/L, a threshold noted for higher colorectal cancer risk. Groundwater (2.89 mg/L) had higher median NO 3 -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-Nitrogen-Retention (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 NO 3 -N concentration and dual isotope approach to identify high-risk sites. [ABSTRACT FROM AUTHOR]