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The definition of natural background levels (NBLs) for potentially toxic elements (PTEs) in groundwater from mining environments is a real challenge, as anthropogenic activities boost water–rock interactions, further increasing the naturally high concentrations. This study illustrates the procedure followed to derive PTE concentration values that can be adopted as NBLs for the former Balangero asbestos mine, a “Contaminated Site of National Interest”. A full hydrogeochemical characterisation allowed for defining the dominant Mg-HCO3 facies, tending towards the Mg-SO4 facies with increasing mineralisation. PTE concentrations are high, and often exceed the groundwater quality thresholds for Cr VI, Ni, Mn and Fe (5, 20, 50 and 200 µg/L, respectively). The Italian guidelines for NBL assessment recommend using the median as a representative concentration for each monitoring station. However, this involves discarding half of the measurements and in particular the higher concentrations, thus resulting in too conservative estimates. Using instead all the available measurements and the recommended statistical evaluation, the derived NBLs were: Cr = 39.3, Cr VI = 38.1, Ni = 84, Mn = 71.36, Fe = 58.4, Zn = 232.2 µg/L. These values are compared to literature data from similar hydrogeochemical settings, to support the conclusion on their natural origin. Results highlight the need for a partial rethink of the guidelines for the assessment of NBLs in naturally enriched environmental settings.

<p>Water pollution by asbestos may result from anthropogenic sources, such as water passing in cement-asbestos aqueduct pipes, or natural sources. Referring to this second case, pollution could be due to the flow of superficial water or groundwater into naturally occurring asbestos (NOA) in rock formations like green stones and serpentinites.</p><p>Asbestos-bearing rocks weathering is the principal natural cause of fibres water-dispersion. Despite the abundant occurrence of NOA rocks where water can flow (underground and superficially) in the North-Western part of the Alps, a few is known about the mechanism of fibres release in water and the correlation with the geolithological and hydrogeological characteristics of the area.</p><p>Moreover, the knowledge on the eventual noxiousness of waterborne fibres have still to be deepened: in fact, they can come into contact with human being as airborne fibres after water vaporization, or by ingestion, especially if fibres are present in drinking water. While a lot is known about disease caused by airborne asbestos fibres high-dose respiration, not enough has been yet comprehended about potential noxiousness of fibre ingestion. Following some in vivo studies, US-EPA (United States Environmental Protection Agency) defined a maximum contaminant level of 7x10<sup>6</sup> ff/l in drinking water, but this limit is not fully shared by the whole scientific community.</p><p>Against this background, it has become fundamental to clarify the main aspects related to waterborne fibres, in particular their natural occurrence in water and their transportation due to water flowing into NOA. Consequently, decision has been made to conduct a study on the former chrysotile mine of Balangero, in Piedmont (Italy), which was selected as a reference case study for its great significance in the North-Western Alps context. The case study was developed in collaboration with R.S.A. s.r.l., the company that is in charge of the site remediation.</p><p>A sampling and analysis campaign regarding the superficial hydrographic network of the area was settled: 5 different sampling points were selected, 2 of them inside the principal site perimeter and 3 in the villages situated downstream of the site. They have been monitored for about one year, to evaluate the seasonal variability.</p><p>The main aims of the research are:</p><ul><li>the evaluation of asbestos concentration in term of number of fibres per liter (ff/l);</li> <li>the correlation between the concentration variability and the precipitation pattern over the four seasons;</li> <li>the evaluation of asbestos concentration defined as mass per liter (pg/l), depending on fibres dimension;</li> <li>the study of fibres characteristics, such as their dimension, morphology and chemical composition;</li> <li>the study of a possible correlation between asbestos concentration in pg/l and ff/l;</li> <li>the potential presence of fibres bundles or aggregates which can constitute a problem in the evaluation of the asbestos concentration, in particular for the correlation between ff/l and pg/l.</li> </ul><p>Finally, an attempt to relate the number of waterborne fibres to those that can eventually be released in air is still ongoing.</p>

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