A New Zealand freshwater nitrate data quality validation study using commercial laboratories and portable testing instruments

Nitrates are among the most pervasive contaminants in New Zealand freshwaters. Community-based testing such as mail-in programs and town hall events, offers a quick and popular way for residents to understand the extent of nitrate contamination in their region. An essential part of ensuring the robustness of citizen science results is using comparable sample collection, storage and analysis methods. A comparison of seven New Zealand commercial water testing laboratories was undertaken to evaluate their measurement accuracy and precision of a range of nitrate (NO₃-N) standards. A further 4 brands of portable nitrate testing instruments were also evaluated. Ten nitrate standards ranging from 0.5 to 25 mg/L NO₃-N were analysed by accredited New Zealand commercial laboratories, and 15 nitrate standards ranging from 0.5 to 100 mg/L NO₃-N were measured using portable instruments under non-laboratory conditions. All accredited laboratories performed within the expected accuracy criteria between 0 and 5 % of the standards, and the portable instruments had a similar accuracy range for standards between 1 and 10 mg/L NO₃-N. The nitrate ion selective electrode probe was found to be the most accurate portable instrument with a wider analytical range than the TriOS NICO, Hydrometrics GW50 and the RQflex, which required sample dilution or optical adjustment. However, from an 'ease of use' perspective, the TriOS NICO and Hydrometrics GW50 performed well for field-based groundwater measurements and required minimum operational training. A further aspect of this study examined the effects of sample storage and holding time on nitrate concentrations. Results showed no significant change in NO₃-N concentration over a 1-month period for samples stored in the fridge or at ambient temperature away from sunlight. The outcomes of this comparative study show that the use of portable nitrate measuring instruments for community-driven citizen science is a rapid, cost-efficient method to reveal freshwater nitrate concentration patterns for rural New Zealand. Moreover, when analysing water samples <10 mg/L NO₃-N, portable instruments can produce comparative results to commercial laboratory testing.