Ever wonder what all the extra info in your stormwater analytical laboratory reports is about? Know what that “J” next to your Oil & Grease (O&G) result means? Today, we are breaking down some of the basics of reading an analytical laboratory report and going from behind the scenes of sample analysis.
First off, you will likely receive a PDF via email from your lab, containing a cover page, some lab intake information that was collected when you dropped your sample off, the results, quality control data, and any laboratory notes/qualifiers. For simplicity sake, we are going to look at the results and a common qualifier in this post.
If you are not submitting the samples yourself to the lab (maybe you have a consultant doing it), you should check that the date and time sampled in the lab report (circled below) matches when you or your consultant collected the sample. You can cross check this against your visual observation form and/or the chain of custody. In addition, you should check the date the stormwater sample was received by the lab (in this example it is right below the date sampled). As we posted previously, some consultants are holding onto stormwater samples far too long and exceeding hold times, which potentially invalidates the results….
Diving a little deeper, we check that the field ID (example X-1) is the same as what is on your site map, in your Stormwater Pollution Prevention Plan (SWPPP), and what you collected during stormwater sampling in the field. This quick is a good habit to get into to ensure consistency and accuracy. Note that consistency in naming is a big problem when tracking sample results over multiple storm events, years, and even sites. It is good practice to adopt one identifier and stick with it, so you are always comparing S-1 to S-1 (and not S-1 to Outfall 1 to S001 to Old-1/DP-2 to Discharge #2).
Okay, the big reveal! Let’s check out what the analytical result for O&G is for our stormwater sample……1.98 J. Huh? 1.98 of what? What does that J mean?
First, let’s figure out 1.98 of what. The units (see picture below) and method (also below) give us the “what” of the result. For O&G it is typically milligrams per liter (mg/L) as in our example lab report.
YO! Check your method used in your lab report and make sure it is what your permit requires.
For example, the California Industrial General Permit (IGP) requires Environmental Protection Agency (EPA) method 1664A for O&G analysis (Table 2 of the IGP).
Okay back to what the heck is that J doing next to my 1.98 mg/L of O&G!
A J included with the result means that the analytical laboratory has flagged it (or J-flagged) that the value being reported is an estimate. Wait! You mean I paid for a result that is only an estimate? Yup.
You see, the analytical laboratory has analytical equipment for measuring concentrations in stormwater samples down to a certain limit. That is called the Reporting Limit (RL) or sometimes called the Practical Quantitation Limit (PQL), just depends on the analytical laboratory. This is the lowest limit possible that the laboratory feels that they can give you an accurate result.
In our case, the RL for O&G is 5.49 mg/L. But kinda like limbo, the permit (e.g. IGP) says “go as low as you can go.” For the more technical version, check out page 41 of the IGP below. Item 11.b says that you (the facility) should measure down to the Method Detection Limit (MDL) and report it in SMARTS.
This means you need to have your results reported to the MDL. The MDL (1.7 mg/L in our example below) is the lowest possible reading a laboratory can make using this analytical method. However, the laboratory is not completely confident in measurements made between their RL and the MDL. Therefore, they estimate the value (such as 1.98 mg/L) and flag it (aka J-flagged).
If the laboratory could not detect it at all above 1.7 mg/L in this example (the MDL), they would say it is non-detect and put the result down as ND.
IMPORTANT: Write on your Chain of Custody (COC) in the notes section “Report to MDL” otherwise the laboratory usually defaults to just reporting to the RL. If there is no MDL section on your lab report and the result says ND, then you may still have a result above the MDL but below the RL. The key is asking for it to be reported to the MDL to ensure compliance with the permit. Asking for it to be reported to the MDL doesn’t cost extra!
Bonus feature! Ever look at a bunch of stormwater sampling lab reports and wonder why the same lab for the same analytical method (maybe even during the same day) has different RLs or MDLs? The precision and accuracy of some instruments are affected by the presence of other “stuff” in the water sample and/or the amount of potential pollutant in the sample. Therefore, labs need to dilute samples in order to not break their equipment or mess it up. You know if a sample has been diluted due to the dilution factor being greater than 1.0. This happens frequently with metals analysis, especially if the water is at all saline or brackish (dissolved ions mess up analytical equipment at really low levels of analysis).
Well, there you have a basic break down of an industrial stormwater sample’s laboratory report.
For those of you not so lucky to be ND after sampling, we have more on Numeric Action Levels (NALs) and stormwater exceedances.