Isotope data guidance

This guidance is for data stewards uploading stable or radiogenic isotope data to DataStream to support a consistent approach when handling this data.

DataStream’s data schema (DS-WQX) aligns with the U.S. Environmental Protection Agency and U.S. Geological Survey developed Water Quality eXchange (WQX).

Data uploaded to DataStream needs to be formatted in the DataStream data structure (DS-WQX), which specifies required fields and allowed values for consistent vocabulary.

Parameters (“characteristics”) are reported using these four fields:

• Characteristic Name
• Method Speciation
• Result Sample Fraction
• Result Unit

Table 1: DataStream fields used to record a given characteristic.

Isotope measures are commonly reported in one of three ways: as a concentration, as a ratio comparing the heavier isotope to the lighter isotope or using delta notation (δ).

The delta notation (δ) is a way of reporting the isotope composition of a sample by comparing it to that of an international reference material or standard. Specifically, the ratio of the heavier isotope to the lighter isotope in the sample is compared to the same ratio in an international standard reference material and is reported using units of per mil (‰).

The following table provides the recommended Characteristic Name format of commonly reported isotopes and a list of appropriate Result Units for these cases. For the full list of DataStream Characteristic Names, including stable and radiogenic isotopes, please refer to the CharacteristicName LOOKUP tab of the DataStream Upload Template. 

Table 2: Recommended DataStream Characteristic Names and Result Unit formats for common isotope expressions.

*Note: The recommended DataStream Characteristic Names in Table 2 align with WQX Characteristic Name domain values. These are some examples of commonly measured stable isotope characteristics, for the full DataStream Characteristic Name list, including stable and radiogenic isotopes, please refer to the CharacteristicName LOOKUP tab of the DataStream Upload Template.

For many common isotope measurements, the compound (or species) that the isotope composition is being measured in needs to be defined to make the data useful and will determine how the results are interpreted (e.g., If Nitrogen-15 is measured in NO₃ versus NH₄). The MethodSpeciation field is used to define this and is a required field for isotopes that are measured in multiple species, which is often the case for stable isotope measurements in particular.

For isotope data, Method Speciation provides a definition of how the isotope is measured as part of a species, rather than being measured as that species (i.e., how MethodSpeciation is applied for other characteristics in the DataStream schema). Therefore, the “of” terminology, as seen in Table 3, is reserved strictly for characterizing isotope data.

The SampleFraction field can be used as necessary to define the portion of the characteristic being analyzed. Some common examples are outlined in Table 3. 

For example, if isotopic composition was being measured in particulate that was filtered from a water sample, you would use a sample fraction of “Non-Filterable (Particle)”. 

If the isotope is being measured in a dissolved or total compound in water, for example dissolved organic/inorganic carbon or total organic/inorganic carbon, you would define “Total” or “Dissolved” in the sample fraction.

For isotopes measured in a nitrogen or phosphorus compound, you would use the applicable sample fraction referenced in the DataStream Nutrient Data Best Practices Guide to define the filtration status. Depending on the species that the isotope is part of, the sample fraction may not be necessary to define (e.g., isotopes measured in H2O).

Table 3: Commonly reported isotopes with examples of potential Method Speciation and Sample Fraction.

*Note: This table lists only a few examples of commonly reported isotopes and their method speciation, and this principle may be applied to additional isotopes. For example, applicable species for Iron-57 may be ‘of Ferric Iron’ or ‘of Ferrous Iron’. Please contact us if you need assistance with submitting your isotope data.

Uncertainty in isotope measurements can be introduced at many points in the measurement process, from during sample collection, to sample analysis, to processing the data. If the uncertainty of an isotope measurement is quantified, this may be recorded in the ResultComment field of the DataStream schema (e.g., uncertainty +/- 1.2 per mil).

This guidance was prepared in collaboration with Dr. Megan Thompson (Thompson Aquatic Consulting) and Dr. Cynthia McClain (Alberta Biodiversity Monitoring Institute; University of Calgary).