From: B.Hamilton@irl.cri.nz (Bruce Hamilton)
Subject: Detection limits, was Re: Mercury
Date: Thu, 04 Apr 1996 19:55:35 GMT
Kurt_Wright@msn.com (Kurt Wright) wrote:
>RE: Municipal wastewater treatment plants (aka Publicly Owned
>Treatment Works or POTW)
>The current EPA method for analyzing the level of mercury in POTW
>effluent samples can detect down to 0.20 micrograms per liter. A
>state has established an effluent limit for a particular POTW below
>the detection level (viz. 0.017 micrograms per liter). There have
>been interpretations of samples measuring below the detecable level
>as violations of the effluent limit.
I can't see how samples below the detection limit can be violations if the
effluent limit is the detection limit.
Frankly, this trend is very annoying. Ideal for lawyers, bad for the public and
the "polluters". If any organisation wants to establish pass/fail criteria it
has to have a valid measurement procedure. Setting the detection limit as the
pass/fail criteria is a nonsense, because the detection limit is really only
indicating where quantitation can start. They should invest money and assess
analytical procedures, and select one that has a detection limit well below
their pass/fail point. Working at the detection limit is always fraught with
problems, look at the history of the analysis of Dioxin. As the procedures
move from ppm to ppb to ppt to ppq the scatter of analytical results increased
as labs kept refining their own procedures. It's only when a fully-validated
standardised procedure is available that the scatter is reduced. Note that
the procedure may not give the "correct" answer, but should always give a
consistent answer. The ultimate consequence of the Dioxin situation was that
specific analytical facilities were recognised as providing meaningful
measurements, and many others had to forgo the work as their results
were consistently outside the accepted reproducibility as determined by
It should be up to both the regulators and the regulated to meet and negotiate
the test procedures and limits. working at the limit of detection inevitably
results in unacceptable variation for important analyses. If a procedure does
not exist, then they should fund modifications such that they can be assured
that they are working significantly above the detection limit of the method.
>EPA is developing permit guidance for reporting below the method
>detection level and they are developing a new procedure that can
>measure below 0.20 micrograms per liter (Atomic Fluorescence Mercury
Crazy. If they have validated a method, and arrived at a detection limit. they
should stop. If they wish to measure lower levels, they should modify the
method ( or choose a new method ) and revalidate. I can see my reports now.
Certificate of Analysis Stupid1 1 April 1996
Sample Aquifer Aqueous Water
Hg 0.0007 ug/l*
Note: This result was obtained using EPA Method Foolish1 ( 1996 ), which
has a reported detection limit of 0.2 ug/l.
What is a Detection Limit, if not a detection limit for that procedure?.
>I would appreciate technical information you may have relative to
>this topic and/or actual experience with effluent limits below the
>current detectable level. For example, if you know of a POTW with an
>effluent limit below the method detection level, how were results
>below the detecable level reported and how were those reports
>interpreted by the regulating authority?
Any number below the formal detection limit of a method is reported
as below the detection limit of the method. If the procedure is modified
at the client's request to measure lower concentrations, then the results
are reported as being from modifications to the standard method that
have not been fully validated.