There has been a change to the Hydrology Nitrate Protocol. If there is low nitrite-N (less than 0.3 mg/L) in the water at your study site, then keep performing the protocol as you have been. However, if there is a significant amount of nitrite-N in the water (greater than 0.3 mg/L), then you need to do a pretreatment of the water for nitrite-N first, before measuring for nitrate-N.
A large amount of nitrite-N in the water can bias your nitrate-N results. There is not enough cadmium in the packets to react with the nitrate-N in the presence of high nitrite-N. Therefore the nitrate-N measurement value you obtain will be less than the true value in your water.
In order to perform a pretreatment of the water with high nitrite-N, you must purchase a pretreatment kit (available from Hach for $21.00). This kit will oxidize all the nitrite-N to nitrate-N so that the protocol you perform in the GLOBE manual will be more accurate, measuring the nitrate-N in your water without interference from large concentrations of nitrite-N.
If you do find nitrite-N concentration to be greater than 0.3 mg/L and you use the pretreatment kit, then you may proceed with the protocol written in the manual as usual.
The GLOBE Hydrology Team
University of Arizona