As and Pb Presence within the Meoqui-Delicias Aquifer, Chihuahua, Mexico

Abstract

This study aimed to determine the amount of As and Pb in the water in the Meoqui-Delicias’ aquifer and their spatiotemporal dynamics. Twenty-one water sampling points were selected. Seventeen samples were from wells and four were from surface water; two were used for human consumption and the rest for agricultural use. The samples were taken from May 2019 to January 2020 in four sampling events, one for each climatological season of the year. The studied geochemical anomalies seem to be linked to the nature and mechanism of volcanic emplacement. Several samples exhibited high concentrations of arsenic ranging from 1.20 to 156.54 ppb, unlike lead, with low values being the maximum value of 26.32 ppb. These elements (As and Pb) are in the water in Naica, part of the mining district where tons of Au, Ag, Pb, Cu, and Zn were obtained. From a geographical standpoint, it is impossible to establish that these elements are related, even though these elements (As and Pb) are present in the water in Naica, a mining zone where tons of Au and Ag were historically mined.

1. Introduction

Groundwater serves as a crucial natural water resource that plays a vital role in drinking and irrigation, particularly in arid and semi-arid regions where surface water sources are limited [1]. Groundwater constitutes the largest reserve of drinking water in the world. Therefore, it is essential to conserve it and keep it free of anthropogenic pollution [2]. However, the geological environments of different regions of the planet determine the behavior in groundwater of various elements and inorganic compounds that are potentially harmful to humans, in the case of being ingested in high doses, as is the case for arsenic [3,4,5]. Groundwater represents the only permanent water source available for many arid and semi-arid areas throughout the world, and more specifically, in central and northern Mexico. More than half of the country’s territory is dominated by dry climatic conditions. Mexican aquifers contain a considerable reserve of water. It is being exploited extensively however, with a massive increase in extraction in the past few decades due to the availability of new and cheaper drilling and pumping technologies [6]. Hydrogeologists refer to this drastic change in groundwater utilization as ‘the silent revolution’ since it has occurred in many countries in an unplanned and totally uncontrolled manner [7]. Providing safe drinking water to the world’s 7.8 billion people is one of the greatest challenges of the century [8]. Water quality is another critical issue, as it is closely related to the type of hydrogeological environment. For example, in several hundred meters-thick aquifers, especially in fractured media, porosity and permeability decrease with depth due to lithostatic load. The recharge of deep aquifers is so slow that groundwater is considered to be a ”fossil.” In this case, long-term water–rock interactions increase the salinity of groundwater, the latter not being recommended for most of the most common uses [9]. Arsenic in groundwater is characterized by significant spatial variability. The factors which determine the contamination of arsenic in groundwater can be divided into natural and anthropogenic factors. There are several natural factors, including geology, geomorphology and hydrogeochemistry. The anthropogenic factors taken into account are the direct emissions of arsenic-bearing sewage and the release of arsenic from arsenic-bearing sediments and minerals caused by anthropogenic activities [10]. Heavy metal contamination in soil and groundwater is an environmental issue [11]. The widespread presence of arsenic (As) in groundwater poses substantial risks to human health on a global scale. This element has been identified as the most prevalent geogenic contaminant in groundwater in northern Mexico [1]. Lead (Pb) is a toxic heavy metal that can enter the human body through inhalation and ingestion from a variety of sources such as contaminated air, water, soil, and food [11]. Previous studies indicate that the Meoqui-Delicias aquifer’s (MDA’s) groundwater presents chemical parameters whose concentrations exceed the permissible limits according to the Mexican Standard NOM 127-SSA1-1994. Mining and ore processing generates a large amount of metal-rich dust, and the fallout produces soil pollution; additionally, the remaining tailing deposits became a source of water pollution [12]. Mining represents a significant source of metal contamination in soil. The surrounding environment of mining areas is highly susceptible to contamination [13]. That is why it is interesting to investigate the behavior of arsenic (As) and lead (Pb) concentrations in water, both surface and subterranean, to determine these element’s behavior in water bodies. Four samplings were carried out over a year to collect groundwater and surface water samples from the study area. The arsenic (As) and lead (Pb) concentration were measured; sample collection sites were chosen for analysis based on groundwater (wells for drinking and irrigation water) and surface water (the San Pedro River, the Francisco I. Madero dam, and irrigation canals) located within the aquifer area. The goal of this study was to characterize and evaluate the behavior of arsenic (As) and lead (Pb) in the MDA area to discover potential sources of this element in the water and its distribution, evolution, and spatiotemporal mobility.

2. Materials and Methods

Area of Study

The study area is located in the southern section of the Meoqui-Delicias aquifer (MDA) (Figure 1) in the Mexican state of Chihuahua. It is bound by the parallels 27°31′ to 28°35′ north latitude and the meridian 105°45′ to 105°00′ west of Greenwich. The climate in the area is classified as BWhw (semi-arid) according to the Köppen classification, modified specifically for Mexico by García [14] with an annual average temperature between 18 °C and 22 °C, with summer rains, and a percentage of rain in winter ranging from 5 to 10.2% of the yearly precipitation. In its southern region, the prevailing climate is classified as being BSohw, arid and semi-hot, with temperatures ranging from 18 to 22 °C with the same winter rainfall regime [14,15].

Physiographically, the area is located within the Mexican Basin and Range Province. The northern region of this province is characterized by its desertic landscape, with scattered, folded, and faulted mountain ranges are separated by vast valleys made up of continental and lacustrine sedimentary deposits. Igneous, sedimentary, and metamorphic rocks make up mountains with elevations ranging from 1350 to 1850 m.a.s.l. [16].

The soils are mostly alluvial in origin, they contain some calcium carbonate and sodium salts due to poor drainage, common within this area, collecting deposits of soluble elements and giving rise to saline and salty soils. More than 14,000 hectares are affected by salinity and sodicity to different degrees, with a pH higher than 7.5 [15].

The most crucial stream crossing the Meoqui-Delicias area is the Conchos River, a tributary of the Rio Grande [16]. The Conchos River originates in the south-central region of the Sierra Madre Occidental and flows northeast to its confluence with the Rio Grande. The Florido River sub-basin constitutes the southeastern region of the Conchos River basin, and originates in the state of Durango, Mexico. The San Pedro River originates in the central region of the state of Chihuahua, and its sub-basin constitutes the western region of the Conchos River basin [17]. After pumping the stored water for slightly over 100 km of the previously lined Main Channel, the Boquilla Dam gives rise to the Irrigation District 05-Ciudad Delicias. The Francisco I. Madero Dam, which manages the San Pedro River’s discharge, providing additional storage. It is emplaced within the Rosales municipality, about 35 km west of Ciudad Delicias, before its confluence with the Conchos River [15].

According to the piezometric condition of the study area, the valley aquifer is considered to be an unconfined or phreatic type, except for some local semi-confinements, in areas where clay bodies occur at shallow depths, such as in the northwestern zone in Meoqui [18]. Such confinements find outlets in multiple wells, most of them intended to irrigate plots. Aquifer recharge is mainly due to the infiltration of surface waters within the valley, such as distribution channels and, at the parcel level, the infiltration of irrigation surpluses. This water is surficial in origin (the San Pedro River and the Conchos River), constituting the irrigation system. There are also multiple runoffs and streams, coming from surrounding mountains, converging precisely in the basin’s middle portion. Even though rainfall values are poor, due to the almost impermeable nature of the igneous rocks making up the surrounding mountains and the scarce, or relatively no forest vegetation, once it rains, both the liquid and the sediment end up deposited in the valley. Thus, direct infiltration during rainfall may be minimal, due to the high evapotranspiration in the area and the moderate rainfall [19].

Sedimentary and volcanic rock (Figure 2) outcrops are abundant. The volcanic rocks (rhyolitic tuffs and rhyolites flows) of Tertiary ages, and limestones (Aurora formation, Lower Cretaceous), with secondary permeability due to fracturing and dissolution (outcropping in the Naica mine and the mountain ranges located south of the aquifer), are the most critical hydrogeological units. The other hydrogeological unit is made of the alluvial materials that, together with igneous material intercalations, fill out the valley’s subsoil, where the vast majority of the wells drilled in the plain of the Irrigation District are found [19].

The stratigraphic series comprises marine and continental sedimentary and igneous rocks with a chronostratigraphic record ranging from the Paleozoic era to the Quaternary, as stated in chronological order below, starting with the oldest unit (see Figure 2, Figure 3, Figure 4 and Figure 5). The Paleozoic era is represented in the Sierra del Cuervo by an outcrop of quartzites and reduced-extended sandstones. However, its age has been a reason for debate, and its diminutive dimensions detract from its geohydrological importance since it is also practically impermeable. The Jurassic rocks make up La Casita formation [15].

Due to their granulometry and cementation, a sequence of shales, sandstones, and limestones are considered impermeable units for hydrogeological purposes. Upper Cretaceous in age, they are Las Vigas, Cuchillo, and Aurora formations. The first comprises shales, sandstones, and limestones that function as practically impermeable rocks [15]. The Cuchillo formation overlies Las Vigas formation. It comprises gypsum, limestones, and sandstones, impermeable due to their granulometry and degree of cementation. The Aurora formation comprises a series of limestones with flint nodules, concretions, and fossils, affected by fractures and ducts, that sometimes form true caverns. Its outcrops are isolated, predominating in the southern region. In the central part of the aquifer, they are part of the Alamillo and Savonarola Mountain ranges and the San Antonio and Naica mines, where they cause serious problems to dislodging groundwater due to the significant volumes of groundwater they provide [15,16]. The Aurora formation has a tremendous geohydrological interest due to its potential as an aquifer. It functions in some parts as a confined aquifer and as an unconfined one when it outcrops. It is even semi-confined when it is covered by less permeable materials from other lithological units, such as the conglomerates located at the base of the limestone mountain ranges. Lower Cretaceous in age, an alternation of shales and very low permeability sandstones sometimes rest concordantly over the Aurora formation. These sedimentary lithological units are made up of materials with varied permeabilities, based on their granulometric characteristics, consolidation, and cementation [15]. Due to fracturing, rhyolitic tuffs, rhyolitic, andesitic-basaltic, and basaltic flows have high permeability values. Continental conglomerates, gravels, sands, limestones, lacustrine limestones, and rhyolite flows have medium permeability due to fissures. Finally, the low permeability units are the lacustrine deposits and the rhyolitic tuffs. It should be clarified that the continental conglomerates outcropping between the Ciudad Camargo and La Perla mine, outside the aquifer area, are considered to be practically impermeable due to their consolidation and cementation. Quaternary materials are distinguished: (a) gravels in old alluvial fans, exposed on the banks of the Chuviscar River, making up the Ranges foothills, (b) coarse colluvial deposits at the foot slopes of the mountains, (c) fluvial deposits, (d) alluvial deposits, and (e) lake deposits and flood plains. All these units have variable permeabilities between medium and high, except for the lacustrine deposits, with medium-to-low permeability; they can function as aquitards that transmit water to the units that underlie them. In summary, and according to what has been described above, two types of aquifers are distinguished: (a) those constituted mainly of granular/clastic materials filling out the Meoqui and Delicias valleys as well as Loma Larga, Moncayo, Santa Rita, and Naica, with the latter being of tectonic origin, as well as (b) fractured volcanic rock aquifers [16].

Groundwater flows within the study area are described by employing historical piezometric database dating back to 1972. The aquifer’s recharge is mainly due to the surface water infiltration from streams and distribution channels and, at the parcel level, the infiltration of irrigation surpluses. This surface water comes from the rivers entering the aquifer (the San Pedro and Conchos Rivers). There are also multiple runoffs and streams, coming from the perimetral mountains, that converge precisely at the center of the valley. Even though rainfall volumes are poor, due to the almost impermeable nature of the igneous rocks making up surrounding mountains and the scarce, or relatively no forest vegetation, once it rains, both the liquid and sediments end up deposited in the valley. Direct infiltration during rainfall may be minimal due to the high evapotranspiration in the area and moderate rainfall. The main discharge is given due to the extraction made by the wells, the base flow that leaves through the San Pedro River, and the underground flow that leaves to the north [20].

The Meoqui-Delicias aquifer (MDA) is generally unconfined, with local semi-confinement conditions, due to the presence of clay-rich soils or compact volcanic rocks. It is mostly made of clastic sediments of varied granulometry, reaching a thickness of up to 600 m in its central and northern regions. Limestones underlying the clastic deposits constitute another potential aquifer, not yet explored, but known from mining works developed within the region (see Figure 4) [15].

Figure 3. The Meoqui-Delicias aquifer’s lithostratigraphic column [21].

Figure 3. The Meoqui-Delicias aquifer’s lithostratigraphic column [21].

Figure 4. Lithological cross-sections in the Meoqui-Delicias aquifer [15]. See Figure 2 to validate where the sections A to F are located.

Figure 4. Lithological cross-sections in the Meoqui-Delicias aquifer [15]. See Figure 2 to validate where the sections A to F are located.

Figure 5. Lithological sections drawn based from INEGI’s ([22,23] geologic charts (Figure 2)). See Figure 2 to validate where the sections G to K are located.

Figure 5. Lithological sections drawn based from INEGI’s ([22,23] geologic charts (Figure 2)). See Figure 2 to validate where the sections G to K are located.

With the help of the G13-02 [23] and H13-11 [22] geological charts from the Mexico Geological survey (SGM), as well as the H1311 and G1302 topographical charts from Mexico’s National Institute of Statistics and Geography [24] and previous geological work [24,25,26], a geological analysis of the area was performed to determine if the presence of the elements in groundwater was geogenic or due to anthropogenic causes. Two lithological cross-sections were carried out in MDA’s southern part, to better understand the study area (Figure 5) [22,23,24,27].

Twenty-one sampling points (17 groundwater samples and 4 surface water samples) were selected; out of the 17 groundwater samples, 2 were from wells used for human consumption and the rest were for agricultural use. Sampling occurred from May 2019 to January 2020, with four sampling seasons/year: spring (May 2019), summer (August 2019), fall (October 2019), and winter (January 2020) (

Table A1. The sampling sites’ coordinates, season of sampling, and water use.

No.	ID	X	Y	Spring	Summer	Autumn	Winter	Use
1	PA01	452,097	3,084,798	9 May 2019	8 August 2019	3 October 2019	10 January 2020	Agricultural
2	PA02	451,301	3,085,203	9 May 2019	8 August 2019	3 October 2019	10 January 2020	Agricultural
3	PA03	451,306	3,085,900	9 May 2019	8 August 2019	3 October 2019	10 January 2020	Agricultural
4	PA04	450,511	3,085,031	9 May 2019	8 August 2019	3 October 2019	10 January 2020	Agricultural
5	PA05	450,301	3,087,001	9 May 2019	8 August 2019	3 October 2019	21 January 2020	Agricultural
6	AP01	448,932	3,101,792	28 May 2019	14 August 2019	10 October 2019	21 January 2020	Drinking Water
7	AP02	428,273	3,071,545	9 May 2019	8 August 2019	3 October 2019	10 January 2020	Drinking Water
8	PA06	430,944	3,073,332	9 May 2019	8 August 2019	3 October 2019	10 January 2020	Agricultural
9	PA07	439,286	3,079,426	9 May 2019	8 August 2019	3 October 2019	21 January 2020	Agricultural
10	PA08	415,804	3,068,585	28 May 2019	8 August 2019	3 October 2019	21 January 2020	Agricultural
11	PA09	421,566	3,070,606	28 May 2019	8 August 2019	NA	21 January 2020	Agricultural
12	PA10	436,382	3,074,631	28 May 2019	14 August 2019	8 October 2019	10 January 2020	Agricultural
13	PA11	459,947	3,075,377	28 May 2019	14 August 2019	8 October 2019	10 January 2020	Agricultural
14	PA12	453,510	3,069,379	28 May 2019	14 August 2019	8 October 2019	10 January 2020	Agricultural
15	PA13	443,118	3,067,889	28 May 2019	14 August 2019	8 October 2019	10 January 2020	Agricultural
16	PA14	420,866	3,083,338	NA	8 August 2019	NA	21 January 2020	Agricultural
17	PA15	444,909	3,103,926	NA	8 August 2019	8 October 2019	21 January 2020	Agricultural
18	AS01	448,389	3,107,565	NA	19 August 2019	NA	NA	Agricultural
19	AS02	438,688	3,115,681	NA	19 August 2019	10 October 2019	21 January 2020	Agricultural
20	AS03	452,841	3,126,203	NA	19 August 2019	10 October 2019	21 January 2020	Agricultural
21	AS04	456,599	3,132,179	NA	19 August 2019	NA	21 January 2020	Agricultural

, Appendix A). The sampling procedure was carried out following the NOM-014-SSA1-1993 standard [28], which establishes the sanitary procedures for the sampling of water for human use and consumption in public and private water supply systems. Plastic containers with a capacity of 1.0 L and coolers were used to conserve the samples. Sampling was carried out according to the following procedure:

• The drainpipe valve was opened, and the water was allowed to run for about 3 min.
• To avoid interference with the sample, the container was opened near the outlet hole, and the stopper was held down.
• The sample was taken quickly, leaving free space to allow agitation.
• The samples were stored in a cooler and kept at a temperature of about 4 °C.
• Samples were identified with the following data:
  • Sample name
  • Date and time
  • Location coordinates of the sampling point and observations.

The procedure for the digestion of water analysis was developed using the wet hot plate technique:

• A measures of 100 mL of each sample is taken, which must be at room temperature to avoid variations in volume.
• A measure of 45 mL of sample is taken and placed in a beaker (previously washed with deionized water).
• Add 5 mL of nitric acid (HNO₃) and mix.
• The samples are left for 2 h on the electric grill at a temperature of 90 °C. The temperature should not be exceeded because some elements to be analyzed may volatilize.
• Filter the samples into a 100 mL flask.
• After filtering, make up to 100 mL.

Samples were analyzed using the inductively coupled plasma emission spectrometry (ICP-OES) technique at the Chemical Analysis Laboratory of the Advanced Materials Research Center (CIMAV). The results were organized in tables and concentration isoline diagrams to relate them geographically and to analyze their behavior. In Figure 6, the locations of sampling points and uses are shown.

The results of the chemical analysis were organized in tables, maps, and concentration isoline diagrams to corelate them geographically and to observe the spatial behavior of the analyzed elements. These maps were processed in Surfer (11.0.642) software [29].

All material and equipment used along this investigation was provided by the Chemical Analysis Laboratory at CIMAV, Chihuahua, Mexico.

3. Results and Discussion

Results are shown in

Table 1. As and Pb (ppb) concentration results.

No.	Element	As				Pb
	NOM-127	25 (ppb)				10 (ppb)
	NOM-0001	200 (ppb)				500 (ppb)
	Station	Spring	Summer	Autumn	Winter	Spring	Summer	Autumn	Winter
1	PA01	68.7801	70.3980	61.7422	27.5270	1.4247	3.9208	ND	7.7424
2	PA02	54.7624	57.6879	60.6778	42.1852	2.0608	2.2614	ND	6.8736
3	PA03	44.9838	47.1107	46.2778	2.9683	2.6138	3.8866	26.3200	6.8144
4	PA04	71.2084	70.3250	55.8822	27.5643	1.4972	2.5678	ND	7.5011
5	PA05	35.4676	35.8022	36.8356	27.7268	3.4182	4.5639	ND	8.6339
6	AP01	23.8593	26.9457	27.7467	28.7897	6.1027	3.1531	ND	4.3952
7	AP02	19.4893	23.1434	16.6644	17.2978	1.4002	ND	9.3956	6.8941
8	PA06	1.1999	6.9885	ND	6.9695	1.1903	0.9825	ND	4.0515
9	PA07	98.3359	100.7361	156.5444	52.3509	0.4633	2.3283	ND	7.3497
10	PA08	10.6329	11.1670	11.7689	10.6047	5.2683	1.4696	17.8311	8.3421
11	PA09	10.7063	11.0089	NA	11.3220	2.3719	2.7092	ND	8.0657
12	PA10	23.5760	9.6176	26.5867	28.7430	0.9794	5.3584	2.7533	6.0670
13	PA11	32.5038	33.2515	29.3089	30.9716	0.6096	2.7160	ND	22.2817
14	PA12	15.7230	17.0210	14.5467	16.5281	0.2126	1.2732	ND	5.8739
15	PA13	12.7527	11.5096	8.9311	14.1539	2.6198	4.3784	ND	7.9070
16	PA14	NA	46.1733	NA	42.2855	NA	3.4774	ND	4.1821
17	PA15	NA	95.4749	71.0044	68.6418	NA	5.0414	ND	4.6709
18	AS01	NA	10.8156	NA	NA	NA	ND	NA	NA
19	AS02	NA	13.4156	25.2867	20.4510	NA	ND	ND	2.7585
20	AS03	NA	11.5778	16.6267	16.1863	NA	ND	ND	5.6596
21	AS04	NA	48.2267	NA	72.6581	NA	ND	NA	7.4740
	MIN.	1.1999	6.9885	8.9311	2.9683	0.2126	0.9825	2.7533	2.7585
	MAX.	98.3359	100.7361	156.5444	72.6581	6.1027	5.3584	26.3200	22.2817

Notes: NA—Not applicable. Some samples were not taken at all four events due to road conditions to access the venue. In the cases of the surface water intakes (AS01 and AS04), the samples were not taken since the event was outside the irrigation season, so the channels did not carry water.

. The drinking water supply wells (AP01 and AP02) results were compared with the Mexican Std NOM-127-SSA1-1994 [30,31], indicating a maximum permissible concentration of 25 ppb for arsenic and 10 ppb for lead. Agricultural wells and surface water samples (AS01, AS02, AS03, and AS04) were compared with the Mexican Std NOM-0001-SEMARNAT-1996 [32] which indicates an allowable limit for arsenic (As) of 200 ppb and 500 ppb for lead (Pb).

In Figure 7, as in both surface and groundwater, concentrations of arsenic can be observed, which is very common in this aquifer, as previous work has shown [15,16,19,20,33,34,35,36]. Most of wells are for agricultural use, and all of them are within the NOM Std values in terms of arsenic and lead (Figure 7b), so there is no risk to human health. The AP01 well slightly exceeds the permissible limits for As in the drinking water (Figure 7a). Previous work has discussed the possibility that such concentrations are due to chemical weathering (hydrolysis in silicate minerals) of rhyolitic rocks initiated by significant flows of CO₂ from the mantle [37]. Hydrolysis in silicates usually releases sodium and bicarbonates in solution and solid weathering products (e.g., clays and oxides). This promotes ion-exchange reactions, removing Ca⁺² and Mg⁺² ions from the solution, causing a more significant release of Na⁺, and groundwater of a typically sodium-bicarbonate composition [37]. Work in the Ethiopian rift [38] found that felsic rocks are the product of fractional crystallization. It caused a progressive enrichment of incompatible elements (e.g., halogens and As) in the residual portion of the magma; due to the above, elements such as As were later enriched in differentiated volcanic products (rhyolites). During explosive eruptions, halogens and other elements that show an affinity for volatile phases (e.g., As) can be released and trapped in the ash (tephra) [37]. It is possible that the source of As is inextricably related to the nature of the magmas in the valley and, more importantly, its location. Furthermore, the contribution of arsenic-containing minerals (e.g., pyrite, arsenopyrite, arsenogoyazite, etc.) leaching upon contact with water should be considered. Another hypothesis mentions that arsenic, after weathering, is trapped in the valley’s clays, after agricultural work and irrigation, it is released and mobilized into water.

In Figure 8, Figure 9, Figure 10 and Figure 11, the As concentration behavior in the timeline (spring, summer, autumn, and winter, respectively) can be seen, where it is observed that the highest concentrations are in the southwestern part of the study area, finding remarkable variation between each season (e.g., autumn to winter). During the spring season (Figure 8), there is a clear high concentration area in the southwest area; however, since samples were not taken in the northern zone of the aquifer, we cannot determine if there are high concentrations in this area, as it became an interpolation to determine the approximate concentration in that area.

Suppose enough surface water samples were taken in the northern part of the aquifer during the summer sampling. In that case, it can be seen in Figure 9 that arsenic is present in the surface water, used for irrigation at a relatively high concentration, but it still poses no risk to human health. For the spring sampling season (Figure 8), the highest As concentrations were found in the southwestern part of the study area, presenting more wells with higher concentrations, with P07 having the higher concentration. It can be noted that, for the most part, the wells presented an increase in concentration from spring to summer, except for wells PA04, PA10, and PA13.

In the autumn sampling (Figure 10), P07 is again showing the highest arsenic concentration, exceeding 150 ppb and approaching the permissible limit according to the Mexican standard NOM-001-SEMARNAT-1996 [39], which indicates a maximum concentration of 200 ppb for agricultural water, repeatedly the area with the highest concentration is in the southwestern part. Only two surface water samples were taken during the autumn, AS02 and AS03, presenting a relatively high concentration (greater than 25 ppb) of the AS02 sample. There is no noticeable change from the summer season to the autumn season. Most wells remain relatively stable, except for the PA07 well, which increases considerably.

For the winter sampling season, there is a significant decrease in concentrations. Although these concentrations are still relatively high, there is a noticeable decrease in the areas, especially in well PA07. In general, the wells have lower concentrations, except for wells PA08, PA11, PA12, and PA13, for agricultural use and the wells of drinking water (AP01 y AP02) that remain relatively stable as shown in Figure 11.

In Figure 12, the Pb concentration levels are shown. In Figure 12a, sample AP02 is very close to the permissible limit, with a concentration of 9.3956 ppb (the maximum limit being 10 ppb for drinking water). Considering agricultural water, for both groundwater (Figure 12b), the maximum permissible limit is 500 ppb, so all samples are well below that limit.

In Figure 13, Figure 14, Figure 15 and Figure 16, the Pb concentration behavior throughout in the timeline (spring, summer, autumn, and winter, respectively) is observed.

In the spring and summer sampling seasons (Figure 13 and Figure 14), concentrations of Pb remained relatively stable at low concentrations (less than 10 ppb). However, there is a radical change during fall (Figure 15), with two wells having concentrations greater than 15 ppb (PA03 with 26.32 ppb and PA08 with 17.8311 ppb). Although the points are not relatively close, it delimits two different zones: one in the south-central part of the aquifer and the other in the adjoining aquifer to the west of the aquifer. During winter (Figure 16), Pb concentrations increase, with the majority remaining below 10 ppb and one well with a concentration of 22.2817 ppb (PA11).

In a previous study related to mineral Naica ore deposits [25], an abundance of sulfide minerals was noted: galena (PbS), present in the chimneys and sills, as well as arsenopyrite (FeAsS), which is found in sills in a disseminated form. The study area is located within the world-famous Naica mining district, where tons of Au, Ag, Pb, Cu, and Zn minerals have been extracted historically [40]. The high amounts of Pb and As in water and soil at a local level may be linked to the mine’s existence, since, aside from intrusions, the area lacks igneous rocks. The above information is important because at the regional level, a previous study [20] has proposed that dissolved As in groundwater is related to igneous rocks (rhyolites, andesites, and basalts) being weathered. Locally, Naica’s mineral deposit is the main source of Pb and As in the water in the studied area. However, the regional trend, which becomes more critical as one proceeds away from the mine due to the high levels of As detected in the Meoqui-Delicias aquifer, should not be underestimated.

4. Conclusions

The water extraction depths in wells have increased due to the area’s climatic conditions and agricultural expansion, this could explain the increase in As concentration since there is a consensus that at greater depth the groundwater has a longer interaction time with the aquifer, thus, the water quality worsens. The geochemical anomalies studied seem to be linked to the nature and mechanism of volcanic emplacement. There are several samples with concentrations above the NOM std for As. Unlike Pb, they are comparatively low, so it can be concluded that both elements do not carry a direct relationship with each other. Because Naica is located within the same-named mining district, tons of Au, Ag, Pb, Cu, and Zn [41] have been extracted; therefore, the presence of lead in the water can be linked to the presence of the mineral deposit and its exploitation. On the other hand, the As source is considered to be the igneous rhyolitic rocks, which are highly reactive. In addition, the weathering and leaching products of these geological materials (volcanic/fluvial/lacustrine sedimentary environments) can concentrate both elements. At the same time, Pb is attributed to the mine’s exploitation and the water infiltrating through the sediments in the tailing dams and the mining works in general, towards the aquifer. Rock and sediment characterization, as well as leaching experiments on the same materials (e.g., rocks, volcanic glass, oxides, organic matter, clays, etc.), should be carried out in the future to better determine the specific role of each as possible sources of these geochemical anomalies.