The Deciding Factors! The What, How, and Why of Project Chemistries – Percent Solids/Moisture – September 2024

In an industry where time is money, choosing the right chemistries is necessary to conserve resources such as time and money. However, sometimes clients are at the discretion of the regulating body or sampling program, in which case, there is no choice but to include analyses that will use more budget, but only provide information that has low importance for the project. This is especially evident with inorganic and general chemistries, which typically have a low profit margin; therefore, do not add enough value to the project budget.

Out of curiosity, we decided to ask professionals in different fields of the environmental industry, to get a better understanding of the inorganic and general chemistries that are seen throughout projects and how they affect project goals.

Since there is so much information surrounding these chemistries, we are writing a series of articles that will each focus on a chemistry of discussion. In each article, we will provide an overview of the chemistry, and the method(s) used to perform the analysis. In addition, we will share the perspectives and information provided by individuals in different environmental fields.

The first chemistry we will discuss is percent solids/moisture and the many ways to determine this value.

Chemistry Background – Percent Solids/Moisture

Accurate determination of percent solids and moisture content is important for a myriad of environmental applications for soil/solid testing, including remediation, compound retention, chemical fate, and biosolid evaluation. These simple yet necessary calculations can be determined using a host of different methods, depending on use and matrix characteristics. Thus, variability in percent solids/moisture calculations can present some challenges.

Across the various methods used to quantify percent solids/moisture, in general, samples are weighed and dried, and dry sample mass is gravimetrically determined. At Environmental Standards, the two most common methods we see being used for determining percent solids/moisture are American Society for Testing and Materials (ASTM) Method D2974 and Standard Methods (SM) 2540G.

• Method ASTM D2974 – Measurement of percent moisture, ash, and organic matter in organic soils including clay, silt, and peat.
• Method A allows for moisture content to be determined by drying an organic soil sample at 105°C.
  • A minimum of 50 g of the soil sample is placed onto an evaporating dish (ED), after the weight of the ED has been measured and recorded to the nearest 0.01 g, and then the sample is placed uncovered into a 105°C drying oven for a minimum of 16 hours, until there is no change in mass between drying periods greater than 1 hour. Once stable mass is achieved, the sample is covered, transferred to a desiccator to cool, and mass is recorded. Calculated as follows:
    • % Moisture = [(soil wet weight) – (soil dry weight) × 100] / (soil wet weight) *OR
    • % Moisture = [(soil wet weight) – (soil dry weight) × 100] / (soil dry weight)
  • Method B allows for the removal of moisture by evaporating at room temperature followed by drying at 105°C; it is generally used for testing of nitrogen, pH, and cation exchange.
    • 100-300 g of sample is evenly placed across a large, tared baking dish and allowed to rest at room temperature, with occasional stirring, for a minimum of 24 hours, to reach “moisture equilibrium.” Once a constant mass is reached, the amount of moisture removed as a result of air drying can be determined. A portion of the air-dried sample is then ground in a blender for 1-2 minutes, and used to assess percent moisture, nitrogen, ash, and cation exchange capacity. The following equation is then used to determine the amount of ground and dried sample that is equal to 50 g of the initial sample:
      • % Moisture = [50 – (oven-dried sample mass)] × 2 *OR
      • % Moisture = [50 – (oven-dried sample mass)] × 100 / (oven-dried sample mass)
    • Method C is used to determine ash content by ignition of the sample at 440°C and is primarily used for geoscience purposes and classifications.
      • An ED is weighed, a sub-sample of oven-dried soil is added, its mass is determined, and the sample is immediately covered. The dish with sample is then placed into a drying oven, the cover is removed, and the temperature is slowly brought to 440°C. This temperature remains until no change in mass occurs (e., the sample turns to ash). The sample is then transferred to a desiccator for cooling and its mass determined. Calculated as follows:
        • % Ash Content = [(mass of ash) × 100] / (oven-dried sample mass)
      • Method D is used when peat is tested as a fuel source.
        • An ED is weighed, a sub-sample of oven-dried soil is added, its mass is determined, and the sample is immediately covered. The dish with sample is then placed into a drying oven, the cover is removed, and the temperature is slowly brought to 750°C. This temperature remains until no change in mass occurs (e., the sample turns to ash). The sample is then transferred to a desiccator for cooling and its mass determined. Calculated as follows:
          • % Organic Matter = 100.0 – (% ash content)
        • Method SM 2540G – Measures total, fixed and volatile solids content in both solid and semi-solid samples, including sediments and sludges.
          • Total Solids: An ED is heated to 103-105°C for 1 hour. For volatile solids, the ED must be heated to 550°C for 1 hour. The clean ED is then placed in a desiccator where it is cooled, weighed and stored. If a sample is moist enough to readily “flow,” it is stirred, 25-50 g are measured into a prepped evaporating dish, and the sample is evaporated dry using a water bath at 103-105°C for 1 hour, cooled in a desiccator, and weighed. The heating, cooling in a desiccator, and weighing steps are repeated until the change in mass is either < 4% or 50 mg (the lesser of the two). A minimum of 10% of all samples are evaluated as duplicates, and duplicate calculations must be “within 5% of their average weight.” Calculated as follows:
            • % Total Solids = {[(dried residue weight + dish) – (dish weight)] × 100} / [(wet sample weight + dish) – (dish weight)]
          • Fixed Solids (matter remaining after drying) and Volatile Solids: Once solids are dried as described for fluid samples, the samples are transferred to a cool oven and heated to 550°C for 1 hour, then cooled in a desiccator and weighed. (Samples containing a lot of organic matter may first be ignited over a burner under a hood if needed). Ignition is then repeated for 30 minutes, followed by cooling in a desiccator and weighing. This process is repeated until change in mass is either < 4% or 50 mg (the lesser of the two). Calculated as follows:
            • % fixed solids = {[(residue weight + dish after ignition) – (dish weight)] × 100} / [(dried residue weight + dish) – (dish weight)]
            • % volatile solids = {[(dried residue weight + dish) – (residue weight + dish after ignition)] × 100} / [(dried residue weight + dish) – (dish weight)]

Data Validation Issues

Throughout the years, data validators have noted where percent solids/moisture data corrections were necessary prior to reporting the data to the client. These disruptions lead to a significant increase in costs, as additional work is required to rectify the incorrectly reported results. In addition, distribution of the final report and exports is often delayed past original due dates. More often than not, these issues are due to inconsistencies in the dry-weight correction applied to the reported result; these incorrectly reported results are biased and not an accurate representation of the sample.

Three current examples are as follows:

• Soil/Solid data needing dry-weight corrected results to be reported had results only being partially dry-weight corrected, or not corrected at all, requiring multiple levels throughout the data validation process to correct and revalidate the data. In this case, the reported results were biased low.
  • In a similar but unrelated example, the dry-weight correction was not applied, thus causing disposal of the soil/solids material to be put on hold, because the disposal company was not provided with the necessary data to make their decisions.
• Soil/Solid data were dry-weight corrected for a calculated parameter, which should not have been applied due to the sample being dried prior to preparation and analysis. In this case, the dry-weight corrected results were biased high.

Geoscience Perspective

In addition, we asked members of the Geoscience Team of Montrose (Amy Graham and Joe Kraycik) some questions about this chemistry and how impactful it is in project decision making.

Here is what they had to say:

What information does this analysis provide the Geoscience Team during a project?

“Percent Solids/Moisture is primarily utilized to ensure consistent, accurate, and reliable soil/solid results regardless of the water content of the soil/solid material. Samples collected after a heavy rain would include more moisture in the sample versus a sample collected during a heatwave. The dry-weight reporting based on percent solids/moistures allows for the consistent reporting regardless of the moisture content of the samples.” – Amy Graham

On a scale of 1-10 (1=best), how would you rate the importance of this analysis?

“9, for overall consistency.” – Amy Graham

Do you have any example(s) of the analysis causing a tremendous headache?

A high precent moisture content increases the reporting limits, and sometimes will then exceed state- or site-specific criteria, especially sediment, and sludge matrix like samples.” – Amy Graham

“We sometimes have issues with elevated reporting limits associated with sediment samples due to high moisture content.” – Joe Kraycik

Is there a program you feel this analysis is most importantly tied to? Remediation, construction/development, mining, etc.?

“Remediation.” – Amy Graham

Is there a preferred method of analysis?

“No.” – Amy Graham

Conclusion

“Percent moisture is used in every soil/solid test we do, yet we don’t have a hard and fast procedure,” said David Blye, CEAC, Senior Principal Chemist at Environmental Standards, when asked about the varying methods and sample sizes among laboratories. With so many ways to determine percent solids, we support the idea that there should be some standardization across laboratories, to ensure the most accurate representation across samples and to decrease the number of instances where extra resources are required to correct mis-reported results.

Be sure to check out the next installment of this series in our next issue, which will be published in December