Do you know which soil test protocol to use? And when?

Posted by on Nov 12, 2013 in C&T Thought Leadership

Curtis & Tompkins tests a lot of soil samples for leachable Lead using both the STLC and TCLP extractions. Have you wondered when you use one vs. the other? Or wondered why the results are often so far off from one another? In fact, it’s common for the STLC result to be ten times higher than the TCLP result. Why is that?

There are three main reasons:

1. Sample to Leaching Fluid Ratio. The Waste Extraction Test (WET), used to generate a leachate for STLC determination, uses 50 grams of sample combined with 500 mL of leaching fluid (1:10), whereas the TCLP ratio is 100 grams to 2L of extraction fluid (1:20). With nothing else different, the TCLP result would be half the STLC concentration. More on those ratios below…

2. The WET extraction takes place over a 48-hour period; the TCLP extraction is 18 hours.

3. Most importantly, while both leaching fluids are approximately pH 5, the STLC fluid is more highly buffered, which allows it to remain at this pH for a longer time, in turn leading to a more aggressive extraction.

Making the STLC & TCLP Decision

Curtis & Tompkins has created a handy reference for review of your total metals results to see if you need to run STLC or TCLP, available here  on our website.

The STLC extraction is called for when the total result is greater than 10 times the STLC regulatory limit; for TCLP it’s 20 times. Those are the same ratios of sample to leaching fluid described above.

As an example, if we had a sample containing 40mg/Kg of lead (100% extracted into the leachate) and we wanted to check on the proper test, we could look on page 4 of the decision guide. There we see that we would need to run an STLC on lead (Pb) if there was more than 50 mg/Kg of lead in our sample. So, with only 40mg/Kg, lower than the regulatory level, there is no STLC needed.

If you have soil samples that need testing and want some help, either check out the decision guide  or call your project manager at C&T!