Natural Background Levels of Potentially Toxic Elements in Groundwater from a Former Asbestos Mine in Serpentinite (Balangero, North Italy)

Massimo Bergamini,Elisa Sacchi,Jordi-René Mor,Elisa Lazzari,Elisa Pugliaro,Arianna Musacchio

doi:10.3390/w13050735

Abstract

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.

Highlights

Toxic elements (PTEs), known as trace or heavy metals [1], are always present in soils, being derived from the weathering of parent rocks that may contain them, and, depending on the hydrogeochemical context, may be found in groundwater too [2]
As this study deals with Potentially toxic elements (PTEs), water samples were first classified based on Eh and pH (Figure 2) [35]
We illustrated the procedure followed to derive PTE concentration values for Cr, Ni, Mn, Fe and Zn that can reasonably represent the natural background levels (NBLs) for the former Balangero asbestos mine, a “Contaminated Site of National Interest”. This area is characterised by the presence of ferromagnesian rocks, mainly serpentinite, and of sediments derived from their dismantling

Summary

Introduction

Toxic elements (PTEs), known as trace or heavy metals [1], are always present in soils, being derived from the weathering of parent rocks that may contain them, and, depending on the hydrogeochemical context, may be found in groundwater too [2]. High concentrations of PTEs in groundwater, exceeding the established TVs, represent a serious problem in many areas worldwide, affecting both hard rock [7,8] and sedimentary aquifers [9,10]. Some PTEs with high ionic potential (e.g., As, Cr, V) can form oxyanions, especially in their high oxidation state. This is the case of hexavalent chromium (Cr VI), which has many industrial applications and originates well-identified contamination plumes in urban and industrial areas [11]. Great concern is raised when Cr VI groundwater concentrations exceed the TV by reason of its high toxicity [12]

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Conclusion