Documentation and how-to

Metal-series data guidance

Best practices for submitting metal-series data to DataStream

Last updated: February 22, 2024

Introduction

This document provides guidance for data stewards uploading metal-series 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 standard. This guidance is also informed by the Best Practices for Submitting Metals Data to the Water Quality eXchange (WQX).

Scope

For the purpose of this guidance, the term ‘metal-series’ applies to metals (e.g., Fe, Cr, Cu, etc.), metalloids (e.g., As, Si, etc.), as well as additional environmentally relevant trace elements that fall outside these categories (e.g., non-metals, lanthanides and actinides).

Table 1: Commonly analyzed, environmentally relevant metal-series analytes.

Alkali metals

  • Cesium (Cs)
  • Lithium (Li)
  • Potassium (K)
  • Sodium (Na)

Alkaline earth metals

  • Barium (Ba)
  • Beryllium (Be)
  • Calcium (Ca)
  • Magnesium (Mg)
  • Strontium (Sr)

Transition metals

  • Chromium (Cr)
  • Cobalt (Co)
  • Copper (Cu)
  • Gold (Au)
  • Iron (Fe)
  • Manganese (Mn)
  • Molybdenum (Mo)
  • Nickel (Ni)
  • Palladium (Pd)
  • Platinum (Pt)
  • Ruthenium (Ru)
  • Scandium (Sc)
  • Silver (Ag)
  • Titanium (Ti)
  • Tungsten (W)
  • Vanadium (V)
  • Yttrium (Y)
  • Zirconium (Zr)

Post-transition metals

  • Aluminum (Al)
  • Cadmium (Cd)
  • Gallium (Ga)
  • Lead (Pb)
  • Mercury (Hg)
  • Thallium (Tl)
  • Tin (Sn)
  • Zinc (Zn)

Metalloids

  • Antimony (Sb)
  • Arsenic (As)
  • Boron (B)
  • Germanium (Ge)
  • Indium (In)
  • Silicon (Si)
  • Tellurium (Te)

Non-metals

  • Phosphorus (P)
  • Selenium (Se)

Lanthanides/Actinides

  • Cerium (Ce)
  • Lanthanum (La)
  • Niobium (Nb)
  • Thorium (Th)
  • Uranium (U)

Data format

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. Information on each characteristic (analyte) should be reported using the fields in Table 2.

Table 2: DataStream fields used to report a given metal-series characteristic.

Field name Description

CharacteristicName

Identifies the analyte being measured (e.g., Aluminum, Chromium). The name must have a match in the Allowed Values list (see ‘CharacteristicName LOOKUP’ tab in the DataStream Upload Template).

ResultSampleFraction

Describes the portion of the characteristic being analyzed (e.g. Dissolved, Extractable, Total).

ResultAnalyticalMethodID

Method ID number assigned by the analytic method publisher.

ResultAnalyticalMethodContext

Source of Analytic Method ID. Required if Result Analytic Method ID is populated. (e.g. VMV, APHA, USEPA).

ResultAnalyticalMethodName

Name of analytical method used by the lab. Can be used when MethodID and MethodContext are not available.

LaboratoryName

Name of Laboratory responsible for the result.

ResultComment

Allows for the entry of comments about the result.

Choosing the correct characteristic name

The CharacteristicName identifies the characteristic (analyte) being measured. The full list of characteristic names is available in the DataStream Upload Template (CharacteristicName LOOKUP tab). For most metal-series analytes, the CharacteristicName is the analyte’s elemental name (e.g., “Aluminum” or “Lead”).

If you don’t see a CharacteristicName appropriate for the analyte in your dataset, let us know and we can add it to the list.

Redox-active species

For redox-active species with multiple stable oxidation states, the ionic form can be incorporated in the characteristic name (e.g. “Chromium (III)” and “Chromium (VI)”). If the oxidation state is unknown, the elemental name is acceptable (i.e., Chromium).

Isotopes

For metal-series analytes that exist as stable or radioactive isotopes, please refer to our Best Practices for Reporting Isotope Data.

Metals and ions

Some elements (e.g. Calcium, Magnesium, Sodium, and Potassium) can be measured either as ions, or in their elemental form in a metals-series. In both cases, the same CharacteristicName is applicable, and methods are differentiated in the ResultAnalyticalMethod field. For example, ion chromatography is a common method to measure ion concentrations, while mass spectrometry (e.g. ICP-MS) is a common method to measure elemental concentrations.

Phosphorus

The nutrient Phosphorus is a non-metal that is sometimes measured in its elemental form in a metal-series analysis suite. When this is the case, the preferred DataStream CharacteristicName is “Phosphorus, elemental”. Otherwise, in the majority of cases, the CharacteristicName for Phosphorus should be chosen based on the guidance in DataStream’s Best Practices for Reporting Nutrient Data.

Choosing the correct sample fraction

Sample fractions describe the portion of the characteristic (analyte) that has been measured in a sample. An accurate representation of the sample fraction is important for interpretation and re-use of data.

Samples may be prepared using physical (e.g. filtration, decantation, and/or centrifugation) and/or chemical (e.g. extractions) techniques prior to an analysis protocol that quantifies or detects the analyte.

To select the correct sample fraction term, both the preparation and analysis method(s) should be considered.

The method(s) or protocol(s) reported by the laboratory may only include information about the analysis method, and not the preparation method. Because information about the analysis method only may not be sufficient to determine the sample fraction, we recommend consulting with the laboratory to identify which fraction term is appropriate in these cases.

To promote consistency and avoid ambiguity, DataStream requires the use of specific allowed values for sample fractions, based on the metal-series terminology recommended by WQX.

DataStream’s preferred sample fraction terms for water and sediment samples are explained below, and their definitions are provided in Tables 3 and 4.

Water samples

For filtered water samples, the term ‘Dissolved’ refers to the fraction of the sample that passed through a filter (i.e., the filtrate). ‘Pot. Dissolved’ (i.e., potentially dissolved) is for samples that were filtered after treatment with nitric acid.

For unfiltered water samples, DataStream recommends using the terms ‘Extractable’ or ‘Total’ to indicate the fraction of the sample that has been analyzed following a chemical digestion or solubilization procedure. 'Total’ may be used if the procedure is expected to yield an analyte concentration that is representative of the whole sample, whereas the term ‘Extractable’ may be used in instances where weaker chemical extraction procedures were conducted.

Particulates in water samples

The particulate portion of a water sample refers to the suspended particulate fraction that can be physically separated from the aqueous solution (e.g. by filtration, centrifugation, or decantation). For metal-series characteristics, we recommend using the sample fraction ‘Suspended’ for this particulate fraction of a water sample.

Sequential extractions, which may performed on the suspended particulate, will be discussed in a following section.

Sediment samples

Sediment samples should be indicated in the ActivityMediaName either as “Surface Water Sediment” for suspended and/or shallow surface bed sediments or as “Subsurface Soil/Sediment” for deeper sediment (e.g. bed sediments or sediment cores). For more details, see the glossary tab of the DataStream Upload Template.

Metal-series characteristics (analytes) in sediments may be analyzed in their solid state, or they may be dissolved via an extraction process where they are exposed to reagents (e.g. salts or acids) that modify their solubility.

For sediment samples analyzed with a single (unique) extraction reagent, use the sample fractions ‘Total’ or ‘Extractable’. As with water samples, ‘Total’ is indicative of a strong (complete) extraction procedure, while ‘Extractable’ indicates a weaker (partial) extraction.

Sequential extractions, which may performed on sediment samples, will be discussed in the following section.

Table 3: Recommended sample fraction terms for submitting metal-series data to DataStream.

Recommended sample fraction Media type Definitions

Dissolved

Water

The portion of the analyte found in the liquid medium, which cannot be removed by filtration. DataStream uses this term for metal-series analytes and other non-nutrients to indicate that the water sample was filtered.

0.45 µm filters are commonly used for filtration. Filter pore size should be indicated in the ResultComment field if it is different than 0.45 µm.

Pot. Dissolved

Water

The portion of the analyte measured from the filtrate of a water sample that was treated with nitric acid prior to filtration. As such, samples solubilized with nitric acid reported as “Acid soluble” can be assigned the fraction “Pot. Dissolved”.

Filter pore size should be indicated in the ResultComment field if it is different than 0.45 µm.

Total

Water & Sediment

The total of all fractions of the analyte in an unfiltered water sample, or a sediment sample, obtained by a strong digestion that produces a concentration value that can be assumed to be representative of the entirety of the analyte in the sample.

Applies to unfiltered samples reported by the laboratory as “Total recoverable”.

Extractable

Water & Sediment

The portion of an analyte that is solubilized from unfiltered water samples or solid samples (e.g., sediment), using a weaker or partial digestion.

Applies to unfiltered samples reported by the laboratory as “Acid soluble”.

Suspended

Water

The portion of an analyte suspended in a water sample, either as (or adsorbed to) particles, which has been physically separated before analysis.

Sequential extractions

Metal-series characteristics may be analysed in a series of sequential extractions, where the sample is exposed to multiple reagents, and the analysis is repeated following each exposure. The measured value therefore indicates the amount of analyte associated with specific chemical fractions in the whole sample.

Sequential extraction analyses are most likely to be conducted on solid samples (e.g. sediments), or particulates in water samples.

Reagents may include salts (e.g. MgCl2), organic acids (e.g. acetic acid), or inorganic acids (e.g. HCl). DataStream’s recommended terminology for describing sample fractions for sequential extractions (Table 4) is based on common operationally-defined terms from the literature (Tessier et al. 1979; Gleyzes et al. 2002).

For sediment samples, the sequential extraction process would be entered in the ResultSampleFraction column.

For the particulate portion of a water sample, the recommended ResultSampleFraction term is ‘Suspended’. As such, we recommend providing additional information about further sequential extraction procedures in the ResultComment column.

Table 4: Recommended terms for sequential extraction protocols.

Preferred term * Definition

Extractable, exchangeable

Pertains to species that are weakly sorbed on sediment surfaces by relatively weak electrostatic interactions, or that are released by ion-exchange processes.

Reagents include but are not limited to salts such as MgCl2, CaCl2, KCl, or CaNO3.

Extractable, CaCO3-bound

Pertains to analytes that are assumed to be co-precipitated with carbonate minerals (also known as the acid-soluble fraction).

Reagents may include acetic acid/sodium acetate solutions.

Extractable, oxide-bound

Pertains to analytes that are assumed to be associated with iron and manganese oxides (also known as the reducible fraction).

Common reagents include oxalate/oxalic acid (Tamm reagent), hydroxylamine, and dithionite.

Extractable, organic-bnd

Pertains to constituents that are bound to or form complexes with organic matter (also known as the oxidizable fraction).

Common reagents include H2O2, NaClO, Na4P2O7 and NaHCO3.

Extractable, residual

Pertains to analytes present in the crystal lattice of minerals that are dissolved by strong acids.

Reagents may include HF, HClO4, HCl, HNO3, or aqua regia.

Extractable, other

Pertains to analytes that are extracted using extraction methods not listed in the above terms.

If possible, the reagent(s) used in the extraction should be indicated in the ResultComment.

* For sediment samples, these terms would be entered in the ResultSampleFraction column. For the particulate portion of water samples, these terms would be specified in the ResultComment column because the ResultSampleFraction should be reported as ‘Suspended’.

Result comments

DataStream recommends using the ResultComment column to provide additional observation-level information, which can include further details regarding sample preparation methods and analyzed sample fractions.

For example, sample filtration most commonly occurs with a filter pore size of 0.45 µm. If the filter pore size was not 0.45 µm, this should be indicated in the ResultComment.

This field may also be used in instances where physical and chemical sample fractions cannot be accurately described using only the ResultSampleFraction field. For example:

  • If ResultSampleFraction is ‘Suspended’ (i.e. the particulate portion of the water sample was analysed), ResultComment may be used to indicate additional chemical extraction procedures, if applicable (e.g., ‘Total’, ‘Extractable, oxide-bound’, etc.).
  • If the ResultSampleFraction is ‘Extractable, other’, the ResultComment may provide further details on the reagent(s) used in the analysis

Detection limits

Lower detection limits are the lowest quantity of a substance that can be distinguished from the absence of that substance (in the case of upper detection limits; the highest quantity of a substance that can be reliably measured). When a result is below a detection limit, its exact amount cannot be determined, but it is still important to report this result as it conveys that the substance is at a low concentration, or not present at all. If left out from a dataset, only values over the detection limit will be displayed, skewing any statistical analyses of that substance.

When a result is below or above a detection limit, the ResultValue field is left blank. The result detection condition (Table 5) and quantitation limit measure, unit and type must be inputted to adequately describe the result.

Table 5: Detection condition terms used in DataStream that are applicable to metal-series data.

Result Detection Condition Description

Above Detection/Quantification Limit

For lab data where results exceed defined laboratory detection/quantification/reporting limits, e.g., colony count and rare occasions where the result is above highest calibration curve point concentration

Below Detection/Quantification Limit

For lab data where results are less than defined laboratory detection/quantification/reporting limits

Not Reported

For instances where a lab is not able to report a result of any type (for example, where the sample was lost/destroyed or where quality assurance requirements are not met)

This document was prepared in collaboration with Dr. Megan Thompson of Thompson Aquatic Consulting.

References

Gleyzes C., Tellier S., Astruc M. 2002. Fractionation studies of trace elements in contaminated soils and sediments: a review of sequential extraction procedures. TrAC Trends in Analytical Chemistry 21(7): 451-467.

Tessier A., Campbell P.G.C., Bisson M. 1979. Sequential extraction procedure for the speciation of particulate trace metals. Analytical Chemistry 51 (7): 844–851.

USEPA, 2018. Best Practices for Submitting Metals Data to the Water Quality eXchange (WQX). https://www.epa.gov/waterdata/wqx-metals-best-practices-guide.



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