## 1 Introduction  
The aim of this study was to investigate the entity, distribution and cause of arsenic contamination in the Versilia plain in the Tuscany region of Italy and its potential impacts on human health and on the environment. Arsenic (As) is highly toxic to humans [1]; acute exposure effects range from gastrointestinal distress to death, depending on the dose, while long-term chronic exposure has been linked to cancer, cardiovascular diseases, diabetes, neurological diseases and skin lesions [2]. People may ingest As via direct routes, i.e., drinking contaminated water [3], inhaling As-contaminated dust [4] or eating contaminated food [5]. For example, As in rice from contaminated paddy fields [6] has recently been reported as one of the causes of chronic exposure with consequences to health. However, arsenic in drinkable water has the highest massive health impact; in fact, As concentration exceeding the 10 \(\upmu\)g/L concentration threshold recommended by the World Health Organization [7] has been reported in groundwater used as drinking supply in several countries, mostly in South America and Southeast Asia, but also in West Africa [8; 9; 10; 11; 12].  
Arsenic contamination is generally attributed to natural sources, such as the weathering of the most common As mineral oxides and sulfides, and/or related to specific geological settings, including hydrothermal and evaporative systems [13; 14; 15]. Human activities caused an increase in As contamination in the environment since arsenic has been extensively used in industry [16; 17] and in agricultural chemicals [18]. Among human-caused contamination, the hot-spots of As release in past mining settings are certainly of relevance and may cause severe impacts on the environment, as well as causing health problems [19; 20]. Indeed, in areas of former As-bearing or mining and processing, various mechanisms may be responsible for the spreading of arsenic in the environment and its secondary accumulation in soils and sediments [21; 22]. Arsenic released from geogenic and anthropic sources undergoes a wide range of biological and geochemical reactions in the environment [23; 24]. In reducing environments, As occurs as trivalent As(III); in oxidizing aquatic environments, arsenic primarily forms negatively charged pentavalent As(V) species, even if As(III) can be maintained in oxic conditions through biological reduction [25]. Inorganic arsenic may also undergo bio-transformations, producing a number of organoarsenic and methylated species [26]. Iron, Al and Mn oxyhydroxides in soils and aquifer sediments are the primary sorbents for both inorganic As(III) and As(V) and methylated As species, due to their surface complexation properties and widespread occurrence [27; 28; 29; 30; 31; 32; 33]. Peat may also be responsible for As immobilization through complexation reactions [34]. Redox-sensitive species, important for As sequestration, play a critical role in As release in the subsurface. Indeed, under anaerobic conditions, reductive dissolution of Fe and Mn oxyhydroxides carrying co-precipitated or adsorbed arsenic may occur, favoring the mobilization of As to soil and sediment porewater. Arsenic in soil may be taken up by plants [35; 36], adversely affecting plant metabolism and accumulating in the edible parts, posing a risk to humans and livestock.  
In this context, the main objectives of the present study were: (i) to establish the spatial distribution of As in groundwater and soil and the extent of translocation to vegetables; (ii) to assess the impact of past mining activities; and (iii) to assess the non-carcinogenic and carcinogenic human health risk for children and adults exposed to arsenic in a densely populated area.