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Tania Karen Espinoza-Ju%u00e1rez et al. | 20731985; Armienta et al., 2001). Evidence of oxidation observed in cave rocks, which show high arsenic concentrations, together with water%u2013rock geochemistry, supports this mechanism. Arsenic transport occurs predominantly through fractures in Cretaceous limestones (Tamaulipas or Soyatal Formation) (Armienta et al., 2001), while the presence of scorodite suggests that its precipitation at low pH followed by incongruent dissolution may also contribute to arsenic in groundwater.Anthropogenic and natural sources of arsenic in Zimap%u00e1n have been confirmed based also on groundwater isotopic determinations. Point sources are linked to mining activities contaminating shallow wells located near tailings or former smelters, and diffuse sources are linked to regional flow systems in carbonate rocks with presence of arsenic minerals and iron oxides/hydroxides releasing As to the deep wells (Sracek et al., 2010), remarking that the diffuse source has significantly affected the health of the population in Zimap%u00e1n due to the consumption of water from the deep aquifer (Armienta et al., 1997b; Del Razo et al., 2011).In this context, a geophysical model of the study area was developed by analyzing the natural responses of the magnetic and Bouguer anomalies to deduce subsurface structures and lineaments that are challenging to detect through alternative methods, thereby elucidating the relationship between geological structures and groundwater chemistry, focused on arsenic occurrence.Given that geology (Figure 1) can be significantly affected by faults, fractures, folds, and thrusts, the gravimetric method is ideal for identifying these structures, as it relies on measuring the Earth's gravitational field to characterize subsurface features based on the density, to locate deep contacts, and determine their lateral and vertical extents (Toushmalani, 2010).The magnetic method facilitates the identification of changes in magnetic susceptibility related to changes in lithology, stratigraphic unit thickness, or the distribution of specific minerals that may influence groundwater chemistry (Dentith & Mudge, 2014); filters and corrections are employed to emphasize particular characteristics of the anomalies (Baranov & Naudy, 1964; L%u00f3pez-Loera & Trist%u00e1n-Gonz%u00e1lez, 2013; Milligan & Gunn, 1997; Fanton et al., 2014).While geophysical methods alone offer enhanced insights into structural geology and lithology, this data can be integrated with chemical analyses to improve comprehension of groundwater quality in the aquifer (G%u00f3mez-Hern%u00e1ndez et al., 2020; Alabi et al., 2020; Song et al., 2020; Eyankware et al., 2021). Electrical resistivity investigations are typically conducted alongside hydrogeochemical data for integrated analysis aimed at assessing groundwater quality, contamination and risk (Ismail et al., 2022). At the regional level, potential field methods facilitate the identification of anomalies and the assessment of the depths, geometry, and distribution of geological bodies that may influence the hydrogeochemistry (Andres Sanchez & Sanchez San Roman, 1998; Telford et al., 1990; Hsu et al., 1996; Rankin & Triggs, 1997).2. LocationThe study area involves the Zimap%u00e1n aquifer system, which is located within two main hydrological basins: the Moctezuma River basin and the Tula River basin, both belonging to the P%u00e1nuco hydrological region (CONAGUA, 2021). Zimap%u00e1n, the municipality that encompasses both basins, is in the state of Hidalgo, between 20%u00b0 34' and 20%u00b0 58' north latitude and 99%u00b0 11' and 99%u00b0 33' west longitude. Its altitude ranges from 900 to 2,900 meters, covering an area of 720%u00a0km%u00b2 (INEGI, 2010). The predominant physiographic provinces in the area are the Sierra Madre Oriental and the Trans-Mexican Volcanic Belt (Figure 2), which serve as watersheds and recharging zones for the Zimap%u00e1n aquifer system within the basins delimited by the Tolim%u00e1n stream, an effluent of the R%u00edo Moctezuma (CONAGUA, 2021).3. Geology3.1. StratigraphyMezozoicUpper JurassicIt consists of a sequence of limonites and shales corresponding to the Las Trancas Formation (Mora, 1998; Mart%u00ednez Flores & Ortega Gamboa, 2009).Lower CretaceousThe El Doctor Formation, or Tamaulipas Formation, comprises a sequence of massive batholithic and reefal limestone rocks banded with flint nodules.Upper CretaceousThe Soyatal Formation, dating from this period, overlies the El Doctor Formation and comprises flysch deposits characterized by an intercalation of calcareous shales, sandstones, limonites, and limestones (Mora, 1998; Mart%u00ednez Flores & Ortega Gamboa, 2009; Simons & Mapes, 1956), however, shales become predominant towards the upper strata (Simons & Mapes, 1956).CenozoicPaleogeneThe El Morro Formation is the predominant one in this age, encompassing the entire Eocene epoch and the lower portion