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Ask the Chemist Vol. 2 - Dry Weight vs. Wet Weight Results

The report I received from one laboratory for my soil analysis cites the units µg/kg "wet." A report from a different laboratory stated "all results reported on an as-is basis unless otherwise noted." Yet a third laboratory reported the results as being "dry." Help!?

The answer to this question leads to many others, so we will address this in three parts:

  1. What is a "dry" versus "as-is" or "wet" value?
  2. How does the type of value used to report the concentration of the contaminant affect how I use the data?
  3. What are regulatory limits based on?
First the theory. What does it mean to have a contaminant concentration value that is "dry" versus "as-is" or "wet."

The initial contaminant concentration measured by the laboratory is considered an "as-is" or "wet" basis result, because no calculations have been made to compensate for the moisture content of the soil sample. When a "dry" value is reported, the laboratory has measured the moisture content of the sample, and calculated the concentration based on the percent solids present in the sample.

Example: The benzene concentration measured by the laboratory is 2.0 µg/kg (as-is value). After measuring the moisture content of the soil, it is determined that the percent solids is 80%. To calculate the concentration on a dry basis, the laboratory uses the following formula.

CD - concentration corrected for dry weight
CW - wet weight concentration

Using our example, the calculated concentration on a dry basis is 2.5 µg/kg.

How does the type of value used to report the concentration of the contaminant affect how I use the data?

As demonstrated in the example above, contaminant concentrations reported on a dry weight basis are higher than the same result on an as-is basis.

The uncorrected contaminant concentration is arguably biased low. Arguably, because the result may also include contamination that was present in the moisture or other liquid phase of the sample, and not just the solids portion.

The degree of "bias" is affected by the moisture content of teh sample. If your samples are relatively dry, you may not see a significantly higher result when reported as a dry weight basis. 

Working in sediments, however, can mean a significantly higher dry weight value than as-is value. Taking our example above and applying a percent solids value of 35%, the final dry weight concentration will be 5.7 µg/kg, more than twice the value of the as-is concentration, and, if your site is in Wisconsin, now over the Residual Contaminant Level of 5.5 µg/kg.

In practice, what is the implication to your project? Do you need wet weight (aka as-is basis) or dry weight corrected values? What are regulatory limits based on? Below, two state agencies answer this question.

Indiana Department of Environmental Management (IDEM)
Excerpt from Frequently Asked Questions

Q: "Should soil analytical results be reported on a dry or wet weight basis?"

A: "All soil analytical results should be reported on a dry weight basis. Dry weight results are necessary because dry weight is an integral part of the soil-groundwater partitioning model, which is utilized in the calculation of the RISC Closure Levels. Since dry soil bulk density is part of the partitioning equation, all soil sample results need to be reported on a dry weight basis. The laboratory should run EPA water Method 1684 to determine the Percent Moisture of each sample. The lab will then recalculate the sample result based on the dry weight by using the Percent Moisture result."

Louisiana Department of Environmental Quality (LDEQ)
Excerpt C-5 from RECAP Frequently Asked Questions

Q: "Should data be reported on a dry-weight or wet-weight basis?"

A: "Analytical laboratories routinely report data on a wet-weight basis. If requested, the laboratory can report the percent moisture of the sample so that the results can be converted to a dry-weight basis if desired or required. In general, exposure concentrations and hence, risk-based RS are based on wet-weight. However, the environmental fate and transport RS are based on dry weight. In general, most soils have a relatively low percent of moisture and the difference between wet-weight and dry weight concentrations is not usually significant. For soils with a high moisture content or for sediments, the weight-wet and dry weight concentrations may be significantly different. In this situation, the percent moisture should be taken."

In practice, these two agencies are slightly different. IDEM is bright-line in stating that its RISC Closure Levels are based on dry soil bulk density, so the contaminant concentration basis must mirror that and be reported on a dry basis. 

LDEQ, however, states that the exposure concentrations, and hence RECAP standards, are based on wet weight. LDEQ is not bright-line in requiring all sample results to be reported on a wet (or dry) basis. It states that the percent moisture should be determined for samples with "high moisture content or for sediments." The bottom line is that reporting requirements vary between state agencies, state and federal programs, contaminant types, and sample media. Familiarizing yourself with reporting requirements in your location, area of practice, and for the intended end use of your data will help you deliver the best results to your client.

Below are a few more links to documents from various regulatory programs which specify how they would like results reported.
Toxic Substances Control Act (TSCA) PCB regulations - 40 CFR Part 761

Wisconsin NR 720

Michigan DNR Guidance Document, Verification of Soil Remediation

California Department of Pesticide Regulation Environmental Monitoring Branch - Standard Operating Procedure, Calculating Pesticide Concentration in Dry and Wet Soil

In both our fixed-base and mobile laboratory settings, ECCS reports an as-is or dry weight corrected value based on your specific project
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