## 1 Introduction  
The lack of water suitable for human activities is a matter of global priority, since water is an element of survival for living beings. Despite the vast amount of water on the planet, most of it (97.5%) is in seas with high salt content, rendering it unsuitable for human consumption, while the rest is found as fresh water (2.5%). Of this fresh water, 70% is frozen and about 30% exists in the form of moisture or in underground aquifers [1; 2]. Therefore, currently around 3.42 billion people experience acute fresh water shortage [3; 4]. In addition to the above, part of the available fresh water suffers from serious pollution rates, especially of fluoride and arsenic (As). For its part, As is present in the atmosphere as an element that interacts with air, water and earth through dust present in air currents, and is even present in living organisms [5]. Some human activities, such as metallurgy in mining extraction processes, lead to the presence of As in the environment [6]. Such is the demand of As in mining processes that Mexico produces 7% of arsenic trioxide worldwide [7; 8]. Arsenic toxicity is an environmental threat with health effects that include skin lesions, peripheral neuropathy, diabetes, cardiovascular diseases, damage to blood cells and cancer [9]. Human exposure to elevated levels of inorganic As occurs mainly through the ingestion of groundwater, foods prepared with this water, crops irrigated with water sources with high levels of As [10; 11], as well as through inhalation and the dermal route (contact with the contaminated soil) [12]. In Mexico, approximately 75% of the total population depend on groundwater for drinking, which is why some specifically arid and semi-arid regions have been diagnosed with HACRE (Chronic Endemic Regional Hydroarsenicism) and hydrofluorosis [13]. The state of San Luis Potosi, Mexico is recognised for its mining activity, specifically in Matehuala, SLP, a city whose main economic activities are mining and metallurgy, making it a high-risk area for population health, mainly that of children [14]. In Mexico, according to Official Mexican Standard NOM-147-SEMARNAT/SSA1 2004, safe concentration levels for residential users are 22 mg/kg in soils and 0.5 mg/L in drinking water for housing, livestock or agriculture [15; 16]. Studies on mining pollution have been conducted in the Matehuala region for decades. A study [17] carried out to estimate the risk of human exposure to As during sports activities in a soccer club where contaminated water is used regularly for the irrigation of the land and pastures showed average arsenic concentrations in shallow soils of 138.1 mg/kg, 6.2 times higher than that allowed in domestic soils (22 mg/kg). In addition, dermal contact of users exposed to contaminated soils resulted in a maximum carcinogenic risk value of \(1.8\times 10^{-5}\), an order of magnitude higher than the recommended risk value, while As concentrations in irrigation water were 6 mg/L, higher than WHO (World Health Organization) permissible limits in drinking water, which explains soil contamination after irrigation. The mobility of As from metallurgical waste in Matehuala [18] has been classified as an ultra-high concentration (4.8-158 mg/L) in water used for recreational purposes, as well as for corn cultivation. Local corn crops showed a high bioaccumulation of As, up to 2.5 times of bioconcentration and 45% of translocation. The results of this study show that irrigation with water rich in As represents a significant risk for the population that consumes the contaminated crops [16]. Undoubtedly, arsenic is a major pollutant of aquifers and in the Matehuala region, where levels of up to 158 mg/L have been found in water for human consumption, exceeding the Mexican standard. Water for human consumption in the region is obtained from a set of 12 deep wells (recharge zones) located in communities outside the city of Matehuala and stored in 12 regulation tanks (discharge zones) located in the zone of study.