Natural background level assessment in groundwaters: probability plot versus pre-selection method

•We apply two methods for the assessment of natural background levels (NBL).•We evaluate NBLs for As, F, Mn, Fe in a volcano-sedimentary aquifer.•The NBLs calculated with the two methods may range over two orders of magnitude.•Both Pre-Selection (PS) and Probability Plots methods have a degree of subjectivity.•PS appears more robust when supported by an adequate conceptual model of the aquifer.

The need for a sound methodology to establish a formal value separating natural geochemical background levels in groundwaters from the anthropogenic impact, is perceived especially from a legal point of view. In this research we compare the natural background levels derived for As, F, Mn and Fe in a volcanic-sedimentary aquifer in Central Italy, using probability plots and pre-selection methods. The derived values may differ by even two orders of magnitude for strongly skewed distributions. Results show that both methods have a degree of subjectivity, however the integration of the two methods reinforces the validity of the assessment. Critical issues include statistical data distribution choice, minimum population size, distance of the sampling sites from anthropized areas. Pre-selection methods appear more robust than probability plots when the hydrogeological asset is duly considered. The greater importance of the aquifer conceptual model of the system, than the choice of a statistical method, is pointed out.

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