Nitrate contamination in New Zealand's domestic drinking water with a focus on rural groundwater-sourced self-supplies

Nitrate contamination is a major threat to freshwater quality with serious implications for ecological and human health. We report on New Zealand's largest drinking water nitrate investigation with more than 3800 samples collected nationally between 2022 and 2024. New Zealand's drinking water standards (NZDWS) do not require chemical testing of domestic rural groundwater self-supplies (from bores and springs), so are more vulnerable to nitrate contamination due to reliance on shallower groundwater and proximity to livestock in rural areas. This study overwhelmingly confirms that Canterbury has the largest percentage of elevated groundwater nitrates in New Zealand. NZDWS has a maximum acceptable value for nitrate‑nitrogen (MAV of 11.3 mg/L NO₃-N) which was exceeded in 6.8 % of rural samples from Canterbury, while 43.1 % exceeded ½ MAV (5.65 mg/L NO₃-N). Waikato and Southland (each with 15.0 % and 39.9 % of samples exceeding ½ MAV, and 2.9 % and 4.6 % exceeding MAV, respectively) had the highest maximum nitrate values (34.0 and 140 mg/L NO₃-N). The national database also signals emerging regions of concern, including Bay of Plenty, Hawke's Bay, Taranaki, Manawatū-Whanganui, Wellington, Nelson, Tasman and Otago with 9.4 % to 24.5 % of samples exceeding ½ MAV, and up to 7.5 % exceeding MAV. Using a dual nitrate isotope approach (δ¹⁵N_NO3 and δ¹⁸O_NO3), dairy effluent is identified as a primary cause of high nitrate levels found in these groundwaters. In this nationally significant study, 30.9 % of rural drinking water samples tested over ½ MAV and 5.1 % exceeded MAV, signalling a widespread and concerning threat to freshwater quality across multiple regions. The study identified 142 samples with nitrates exceeding NZDWS MAV, affecting more than 600 residents. Based on an estimated 646,600 rural residents in New Zealand using groundwater-sourced drinking water, there could be upwards of 21,200 people drinking nitrate contaminated water above MAV, and 101,000 people drinking water above ½ MAV across rural New Zealand.