Limits of Detection (LOD) numbers are generally not permitted to be included as actual concentration values when calculating diagnostic ratios for polycyclic aromatic hydrocarbons (PAHs) in water samples. Diagnostic ratios should only use measured and reliably quantified values to ensure scientific and interpretative validity. This is why:

• Diagnostic ratios are commonly used to identify and distinguish PAH sources (e.g., petrogenic vs. pyrogenic origin) in various environmental matrices such as water, soil, and sediment. These ratios are based on actual measured concentrations of individual PAH compounds, assuming comparable physical and chemical behavior for the PAHs in question.
• LOD numbers represent the analytical threshold below which a substance cannot be confidently detected or quantified. Substituting LOD values for undetected PAHs (i.e., below detection) introduces artificial or arbitrary numbers into diagnostic ratios.
• The use of LOD values in ratio calculations can create systematic bias. For instance, a PAH may be reported as “below LOD,” meaning its real concentration could be anything between zero and the LOD. Including LOD (or substituting zero) distorts the true profile, especially when the diagnostic ratio denominator or numerator includes a non-detected compound.

Thank you, Peter Faber.

Since it appears you have knowledge about this I would have more questions.

1. Would this still apply to any type of water even if heavily polluted?

2. Assuming for a diagnostic ratio between BaA/(BaA+Cry) (on y-axis) vs Flt/(Flt+Ant) (on x-axis), I have some results that have a mishmash of some LODs results (either on one side for the above mentioned ratios) or no LODs on either side just whole numbers. How would one handle this exactly? I mean it is obvious that LODs don't represent true values or representatives for the compounds but anyway it is data that may still interpret the whole scenario.

3. How about LMW/HMW? Yes, as far as I know these "diagnostic ratios" do not have figures, but since I have LODs in my table would these be allowed to participate in the LMW/HMW ratio to determine at least a general view for the potential PAH source?

A recent and authoritative reference that describes the calculation method for leachate collected with lysimeters is the book chapter: "Use of lysimeters for monitoring soil water balance parameters and nutrient leaching" by Meissner, R.; Rupp, H.; Haselow, L. (2020), in the volume Climate Change and Soil Interactions, edited by Prasad, M.N.V. and Pietrzykowski, M., published by Elsevier. This chapter details various lysimeter types and explains the determination of soil water balance parameters, including the calculation and scaling of leachate quantities using area-based conversions, following the principles outlined in your previous question. This reference provides both the methodological background and practical calculation steps similar to what you described, and covers current best practices in lysimeter studies.

Hello, thankfully I was able to find out that it is still possible to use LODs within diagnostic ratios under this circumstance: If one site for the columns between BaA/(BaA+Cry) (on y-axis) vs Flt/(Flt+Ant) (on x-axis), either on one side is LOD and the side has just whole numbers. In addition to that, it is of utmost necessity to mention the case why LODs are present.