Methane Analysis in Waters

Ask the Expert Question:

Water samples were collected from wells that were believed to be impacted by methane due to hydraulic fracturing and analyzed by two different laboratories. Two different analytical methods were run, RSK-175 and EPA Method 18. The results from the same sample source were about an order of magnitude different. Both methods used the analysis of headspace. Do you know why such a large difference would occur?

Experts Response:

In response to your question about large differences in results from two methods - RSK-175 and EPA method 18, I suspect that the differences relate to differing approaches in purging methane from each sample. RSK-175 is intended to measure dissolved gases, such as methane, in water samples, whereas EPA method 18 is designed for analyzing volatiles in air samples. The results for EPA 18 will depend on the how the laboratory created the headspace vapor sample, as that step is not defined in the method.

I suggest you evaluate how the methane in the water sample was converted to a gaseous sample, and also evaluate how standardization was performed for EPA 18. Another contributing factor is how the integrity of the water sample was maintained from collection to time of analysis. Unless the samples remained tightly sealed until analysis was performed, some methane could have been lost prior to analysis.

In summary, both methods should provide accurate methane analysis, so the main issues to investigate are the sample preservation and handling prior to analysis, and particularly how the EPA 18 headspace was created.

View Dr. Jack Tuschall's expert profile

Question on Incremental Sampling Methodologies

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One of the main tenants of the Incremental Sampling Methodologies approach is the analysis of a larger analytical subsample. For organic analysis, this doesn't seem to be a problem, but it is for metals. The digestion blocks that are the norm in environmental labs basically fix the analytical sample at 1 or maybe 2 grams. In fact, the official digestion methods are only validated for these small sample sizes. Is pulverization the only way to really get around the limitation on the analytical subsample size?

Experts Response:
You are correct that using large subsamples is a common part of Incremental Sampling Methodology (ISM). For organics using 10-30 g is already the norm so this isn't really a change for organic extractions. As you point out most metals digestion methods are optimized for 1-2 g aliquots. There are two common ways to address this limitation.

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Sampling Protocols for Collecting a Tap Water Sample for Lead Analysis

Ask the Expert Question:
What are the sampling protocols for collecting a tap water sample to analyze for lead?
Expert Response:
There are 2 guidelines for collecting a sample for the testing for lead.

According to EPA's lead and copper rule for drinking water, it is recommended that you collect a first-draw sample from a tap where the sample has stood in the pipes for at least 6 hours (e.g., no flushing, showering, etc.). First draw samples collected at single-family residences should be drawn from the cold-water kitchen tap or bathroom tap. Collect the sample into a 1-liter poly container. The laboratory can acidify the sample upon receipt for preservation.

For water quality monitoring, the sampling guideline indicates that the water source should be fully flushed for 10 minutes. Then collect the sample into a 500ml (or 250ml) poly bottle preserved with nitric acid.

View Cam Pham's expert profile