Citation: Guerrieri, N.; Lami, A.; Musazzi, S.; Austoni, M.; Marchetto, A.; Orrù, A.; Giacomotti, P.; Tartari, G.; Lucchini, G.; Scaglioni, L.; et al. Arsenic Distribution and Pollution in Three Mountain Streams (Anzasca Valley, Italian Central Alps). Sustainability 2023, 15, 14217. https://doi.org/10.3390/ su151914217 Academic Editor: Elena Cristina Rada Received: 31 August 2023 Revised: 21 September 2023 Accepted: 23 September 2023 Published: 26 September 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). sustainability Article Arsenic Distribution and Pollution in Three Mountain Streams (Anzasca Valley, Italian Central Alps) Nicoletta Guerrieri 1, * , Andrea Lami 1 , Simona Musazzi 1 , Martina Austoni 1 , Aldo Marchetto 1 , Arianna Orrù 1 , Paola Giacomotti 1 , Gabriele Tartari 1 , Giorgio Lucchini 2 , Leonardo Scaglioni 2 and Gigliola Borgonovo 2 1 National Research Council, Water Research Institute, Largo Tonolli 50, I-28922 Verbania, Italy; andrea.lami@cnr.it (A.L.); simona.musazzi@cnr.it (S.M.); martina.austoni@cnr.it (M.A.); aldo.marchetto@cnr.it (A.M.); arianna.orru@irsa.cnr.it (A.O.); paola.giacomotti@cnr.it (P.G.); gabriele.tartari@cnr.it (G.T.) 2 DeFENS Department of Food, Environmental and Nutritional Sciences, Via Celoria 2, I-20133 Milano, Italy; giorgio.lucchini@unimi.it (G.L.); leonardo.scaglioni@unimi.it (L.S.); gigliola.borgonovo@unimi.it (G.B.) * Correspondence: nicoletta.guerrieri@cnr.it Abstract: Anzasca Valley is part of the Monte Rosa gold district located in the Italian Central Alps. Since we do not know the effects of arsenic on the aquatic environment in Anzasca Valley, we investigated the biofilm of three streams. The three perennial streams studied are in the Anza catchment but with different lithology. Rio Rosso flows out of an ancient gold mine (Miniera dei Cani); its waters, acid mine drainages, are rich in iron and arsenic. Rio Gattera, a small stream adjacent to the Rio Rosso, flows through metamorphic rocks with mixed composition. Its waters are not acidic but contain a small amount of arsenic. Rio Roletto is upstream, with respect to the others, and drains different metamorphic rocks without arsenic. We analyzed the chemistry and the metals of the water, characterized by microscopic analysis and HPLC, the phytobenthic community living in the biofilm, and in the Rio Rosso, we measured the arsenic adsorbed in the biofilm. Sampling was performed between 2012 and 2014, and arsenic in the biofilm of Rio Rosso was measured in different seasons. In the three streams, the carotenoids of the biofilms showed the different stability of phytobenthic communities (Bacyllariophyceae vs. Cyanobacteria): in Rio Roletto and Rio Gattera, the ratio between the communities did not change; in Rio Rosso, the ratio between the communities changed completely, probably due to the peculiarity of water composition and presence of arsenic. Keywords: arsenic; carotenoids; biofilm; gold mine; phytobenthic communities; acid mine drainage 1. Introduction Arsenic is a ubiquitous element on the earth and a metalloid. Several hundred arsenic minerals are known, but only a few are relatively common in crustal rocks, sediments, and soil. Pyrite is present in sediments of many rivers, lakes, oceans, and aquifers and plays a very important role in the geochemical cycles of various elements [1] often associated with arsenopyrite [2]. Minerals and other inorganic solid arsenic molecules may be classified into five groups: elemental, arsenides, arsenosulfides, arsenites, and arseniates [2]. Arsenite, As (III), exists in low-oxygen (reducing) groundwater and hydrothermal waters [3]. Arsenate, As (V), is present in oxidizing groundwater and surface waters [2]. Arsenic toxicity depends on its oxidation state: the reduced forms As (III), arsenite, are more toxic than those oxidized As (V), arsenate [4,5]. The water solubility of the arsenic molecules is very different and modifies their bioavailability, i.e., the possibility of spreading along the food web [1,2]. Arsenic is one of the most toxic metalloids in the natural environment [2], and the toxicity of the organic species is lower than that of the inorganic ones [6]. Arsenic is present worldwide [7–9], from natural (volcanism, erosion) and anthropogenic sources (industrial activity, pesticides) [10]. In Italy, arsenic is naturally present in significant quantities Sustainability 2023, 15, 14217. https://doi.org/10.3390/su151914217 https://www.mdpi.com/journal/sustainability