Frequently Asked Questions on Nitrate Source Tracking

1. What pretreatment is required for the samples?

In our laboratory, first we filter the water samples using 0.2 𝞵m filters to remove any particulate. In order to analyze the sample we must chemically convert aqueous NO₃ into headspace N₂O. Nitrite (as another species along the reduction pathway) is first removed from the solution via Sulfamic Acid. As water sample chemistry can affect results samples are matched to standards of matching salinity and NO₃ concentrations. Concentration is determined by spectrophotometry. Samples are stoppered and crimped in gas-tight acid-washed vials purged with an inert gas. Following the incubation of the sample with the reducing reagent the sample is ready for analysis.

2. How are the Nitrate Source Tracking results reported?

Isotope ratio data are reported as delta (δ) values in units of parts per thousand (per mill) (‰). Nitrogen isotope ratios are reported relative to N₂ in air and oxygen isotope ratios are reported relative to VSMOW reference water and normalized. The results are also presented graphically on a plot which includes representative areas of the isotopic composition (δ¹⁸O and δ¹⁵N) of various nitrate sources.

3. Which contamination sources can I identify from the Nitrate Source Tracking reports?

At the moment, the Isobar NST report includes a plot with 6 potential sources: NH₄ in fertilizer and rain, NO₃ fertilizer, Precipitation NO₃, Marine NO₃, Soil N and manure sewage.

4. I would like to identify the contamination origin but the potential sources are different from those presented in the Isobar reports. Can I still proceed with the analysis?

The source origins identified in the Isobar reports are the most common, but will only provide you with general results. It’s possible to identify the exact sources if you provide samples of each origin to be used as a baseline.

5. What is Enhanced Nitrate Source Tracking (ENST)?

In cases where the source regions of contaminants overlap in terms of their oxygen and nitrogen isotopic signatures – making it difficult to identify a single contamination source – boron isotopes (δ¹¹B) can be used to enhance the source tracking.

6. Can I differentiate between manure and sewage sources?

Manure and sewage sources can only be differentiated using enhanced nitrate source tracking (ENST) – through the analysis of δ¹⁸O, δ¹⁵N and δ¹¹B.

7. What type of samples do you accept for NST and how much sample is required?

We currently only accept water samples. 40mL is required. Samples must be acidified upon collection to prevent any changes in the chemistry due to potential microbial activities.
NOTE: if you plan to use the ENST service by measuring δ¹¹B, a SEPARATE 100 mL of water sample needs to be collected in a plastic container. DO NOT acidify this sample as addition of any external chemicals, e.g. acids, will change the chemistry of boron isotopes in the sample.