TCLP, STLC, TTLC… What Does It All Mean?

March 1, 2021

By: Ranjit Clarke

If you manage hazardous waste in California, you may have heard the terms “TTLC, STLC, and TCLP”, but what do these mean and when do you need to use them?

When hazardous waste is disposed of in a landfill, some of the toxic constituents in the waste may leach out into the soil and groundwater. To limit the risks associated with toxic substances leaching out of the waste, the EPA requires generators to show that the amount of hazardous chemicals that may leach out into the environmental is below their thresholds.

Toxicity Characteristic Leaching Procedure (TCLP) is required for Federal hazardous waste characterization.  This list of 40 compounds must be analyzed by extracting the sample with a 20:1 ratio of acetic acid solution to sample.  This procedure is used to simulate a landfill environment and to determine how much of a regulated compound will leach out into the environment.  The time of extraction for this procedure is 18 hours.  The TCLP extraction is the only method approved for use under the Federal RCRA hazardous waste regulations.  If any compound fails the TCLP test, the waste is listed as a RCRA hazardous waste.

Soluble Threshold Limit Concentration (STLC) is a California requirement for hazardous waste.  This test is also referred to as the Waste Extraction Test (WET).  This list of 35 compounds must be analyzed by extracting the sample with a 10:1 ratio of citric acid solution to sample.  This procedure is also used to simulate a landfill environment and to determine how much of a regulated compound will leach out into the environment.  The time of extraction for this procedure is 48 hours.  The STLC extraction is not accepted for federal sites, but is necessary for disposal in California.  The citric acid extraction solution used for the STLC extraction is much more aggressive than the acetic acid extraction solution used for TCLP.

Total Threshold Limit Concentration (TTLC) is a California requirement for hazardous waste.  The extraction solvents used for testing TTLC are not intended to simulate a landfill environment, but rather to determine the total amount of a compound in the sample.  The list of compounds covered by the TTLC list encompasses the same compounds as the STLC list, however, additional compounds are usually reported as well (typically the lab’s full list for the specific method).  When any target analyte exceeds the TTLC limits the waste is classified as hazardous and its waste code is determined by the compounds that failed TTLC.

The results from the TTLC analysis, assuming no exceedance of the TTLC limits, are used to determine if an STLC test is required.  Using the “10x rule” (based on the 10:1 extractant to sample ratio), if the result of a particular compound is 10x or higher than its STLC limit, that compound will require STLC analysis.

These results can also be used to determine if a TCLP analysis would be required.  Using the “20x rule” (based on the 20:1 extractant to sample ratio), if the result of a particular compound is 20x or higher than its TCLP limit, that compound will require TCLP analysis.

Resources
Click the links for more information on the specific regulations:

STLC/TTLC (CA)

TCLP (Federal)

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Ranjit Clarke
Senior Project Manager
Ranjit has over 24 years of experience in the environmental industry. He has worked in various departments within the lab ranging from bench chemist up to his current position as Senior Project Manager. Ranjit holds a B.S. in Biology with an emphasis on chemistry.

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