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| Titel |
Comparison of different methods to assess natural backgrond levels in groundwater bodies in southern Europe |
| VerfasserIn |
Elisabetta Preziosi, Daniele Parrone, Stefano Ghergo, Daniela Ducci, Mariangela Sellerino, Maria Teresa Condesso de Melo, Juana Oliveira, Luís Ribeiro |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250099066
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| Publikation (Nr.) |
 EGU/EGU2014-14811.pdf |
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| Zusammenfassung |
| The assessment of the natural background levels (NBLs) of a substance or element is
important to distinguish anthropogenic pollution from contamination of natural origin in
groundwater bodies. NBLs are the result of different atmospheric, geological, chemical and
biological interaction processes during groundwater infiltration and circulation. Rainfall
composition, water-rock interactions in both vadose and saturated zone, exchanges with other
water bodies and residence time also contribute to determine the groundwater natural
composition. Nowadays there are different methods to assess NBLs but the main concern is
that they may provide different results.
In the European legislative context, the Groundwater Directive (2006/118/EC) requests to
EU Member States to derive appropriate threshold values (TV) for several potentially harmful
substances, taking into account NBLs when necessary, in order to assess the chemical status
of groundwater bodies.
In the framework of a common project between Italy (CNR) and Portugal (FCT), several
groundwater bodies were taken into account in different regions of Italy (Latium and
Campania) and Portugal. The general objective is the definition of a sound comprehensive
methodology for NBL assessment at groundwater body scale, suitable to different
hydrogeological settings through comparing diverse case studies and different approaches.
The Italian case studies are located in volcanic or volcano-sedimentary geological contexts,
where high concentrations of substances such as As, F, Fe, Mn among others in groundwater
are well known. The Portuguese case studies are located in carbonate and porous media
aquifers.
Several data sets were explored with the use of statistical as well as mathematical
procedures in order to determine a threshold between natural and anthropogenic
concentration. Today essentially two groups of methods are proposed, the first ascribed to the
probability plots (PP method), the second based on the selection of the uninfluenced water
samples corresponding the natural population (Pre-Selection method, “PS”). PPs are
grounded on the principle that different sources generate different data populations which can
be separated by statistical procedures. Aligned points indicate samples belonging to a single
population that originated from a unique process, while changes in slope of the curves may
indicate passages between different populations. The PS method proposes to select only those
samples which are not, or very little, influenced by human activities, e.g. removing those with
high nitrate or other markers of anthropic contamination. In the residual data set
one value, usually one percentile is chosen as representative of the NBL, meaning
that all concentrations exceeding that level should be ascribed to anthropogenic
sources.
NBLs for several substances including As, F, Fe and Mn have been determined using both
PS methods and probability plots in linear and logarithmic scale. The comparison of results
shows, even for the same case study, a large range of values depending on the method used,
on the pre-selection criteria, on the chosen percentile especially in case of strongly skewed
distribution. The need for a sound methodology, to establish a formal value of the NBLs, is
perceived especially from a legal point of view both at the national and European level. |
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