Arsenic Results - Differences among Approved Analytical Methods?

[zabrab]

My question concerns differences in Arsinic levels using EPA approved analytical methods. The water system serves < 10,000 and is in NJ (As MCL is 0.005 mg/l)

Three years ago, As testing was done and Std Method 3113 B was used. Of the 4 source wells, 1 was clearly over the MCL (>0.010 mg/l) 1 was just over (around 0.0056 mg/l) and 2 wells were below the MCL (<0.004 mg/l). 1st quarter 2006 testing was performed using EPA 200.8. All results are approximately 2x the 2003 results; so instead of treating two wells it looks like all four will require treatment.

I can’t believe there that much variation between approved analytic methods, but, to be frank, I don’t know. I’ve called the testing lab twice last week and asked this via voicemail, but no one is getting back to me.

With all that as background information, my question is:

Can the differences between the 2003 and 1st quarter 2006 results be attributed to the difference in the analytical methods used?

 

[ccor]

I can't say for sure for arsenic, but it may be like ammonia which can be reported as total ammonia or as ammonia nitrogen (NH3-N). The molecular weight of arsenic acid H3AsO4 is 147 about twice the 75 for As. You must also make sure which format the MCL is in. It may be worth looking into.

 

[kenvlach]

Method 200.8 is more sensitive to the valence state of the As (must be oxidized to +5), so sample preparation is critical for accurate results. See footnote 14 at

http://www.epa.gov/OGWDW/methods/inch_tbl.html

But, you really need to have more testing done, with split samples going to different labs. Maybe even split 4-ways & have done by both methods to settle the issue.