Current State of Arsenic, Fluoride, and Nitrate Groundwater Contamination in Northern Mexico: Distribution, Health Impacts, and Emerging Research

Abstract

The plateaus of north-central Mexico have an arid to semiarid climate and groundwater naturally contaminated with inorganic arsenic (iAs) and fluoride (F). Like other arid and semiarid areas, this region faces great challenges to maintain a safe supply of drinking and irrigation water. Studies conducted in the past few decades on various locations within this region have reported groundwater iAs, F, and nitrate-nitrogen (NO₃-N), and either their source, enrichment processes, health risks, and/or potential water treatments. The relevant findings are analyzed and condensed here to provide an overview of the groundwater situation of the region. Studies identify volcanic rocks (rhyolite) and their weathering products (clays) as the main sources of iAs and F and report that these solutes become enriched through evaporation and residence time. In contrast, NO₃-N is reported as anthropogenic, with the highest concentrations found in large urban centers and in agricultural and livestock farm areas. Health risks are high since the hot spots of contamination correspond to populated areas. Health problems associated with NO₃-N in drinking water may be underestimated. Removal technologies of the contaminants remain at the laboratory or pilot stage, except for the reverse osmosis filtration units fitted to selected wells within the state of Chihuahua. A recent approach to supplying drinking water free of iAs and F to two urban centers consisted of switching from groundwater to surface water. Incipient research currently focuses on the potential repercussions of irrigating crops with As-rich water. The groundwater predicaments concerning contamination, public health impact, and irrigation suitability depicted here can be applied to semiarid areas worldwide.

1. Introduction

Groundwater scarcity results from depletion of aquifers and from contamination that renders the water unusable for drinking and/or irrigation [1,2]. Arid and semiarid regions are particularly vulnerable to both depletion and contamination of aquifers, a situation that is becoming critical in many regions, especially those with intensive agriculture [3,4]. Jasechko et al. [3] report declining groundwater levels in 24% and 8% of the aquifer systems in arid and semiarid climate zones, respectively, plus an accelerated decline in the agricultural areas under these climates. To make things worse, longer droughts and more intense storms are an expected effect of climate change in arid and semiarid areas [4,5,6], as are the increased evaporation [4], higher groundwater temperatures [7], and the potential for catastrophic floods [8].

Like other arid and semiarid areas, northern Mexico faces a plethora of challenges to maintain a safe supply of drinking and irrigation water to its inhabitants. An additional hurdle is that the groundwater in part of this region is naturally contaminated with inorganic arsenic (iAs) and fluoride (F). This contaminated region is an elevated plateau flanked by a mountain range to the east and one to the west. The co-occurrence of iAs and F in this region of Mexico is well-known [9,10,11,12,13]. The relatively flat topography and semiarid climate of this central region form endorheic basins in about half of its territory, allowing the enrichment of these contaminants as compared to the better-drained coastal areas next to it [11]. iAs and F contamination has also been reported in other world regions, including China, India, and Argentina [9,10]. Recent studies recognize iAs, F, and NO₃-N as the most concerning contaminants in drinking water affecting human health, and this has resulted in multiple studies on their occurrence, governing mechanism, and health risk [11,14]. While NO₃-N is now a global concern and its content is associated with anthropogenic sources, iAs and F sources differ depending on location: northern India and regions with humid climates and delta regions report the source to be associated mainly with the presence of clays and industrial waste. In contrast, Argentina, northern Mexico, and the southwestern U.S. associate their content with the presence of silicic volcanic rocks such as rhyolite and ignimbrite [9,10,11,14]. Similar to other regions contaminated with iAs, F, and NO₃-N, studies in northern Mexico have multiplied in the past few decades, reporting their concentrations in groundwater, potential sources, enrichment processes, health and environmental impacts, exposed population, and/or cost-effective removal technologies [11,12,13,15]. These studies have gradually formed a more precise picture of the origin of these contaminants and their enrichment processes; whereas older studies reported arsenopyrite, fluorite, and industrial wastes as main contributing sources to iAs and F, the large contribution of iAs and F by silicic volcanic rock and volcanic glass became evident under repeated scrutiny, as well as geogenic nature and less industrial waste being the predominant nature of iAs and F contamination in groundwater [9,10,11,12]. Thus, the studies condensed here each constitute a piece towards understanding the processes responsible for iAs and F concentrations, and this, in turn, is a step forward to better managing the water resource to supply water for drinking and irrigation purposes sustainably. Nitrate (NO₃−, or as nitrate-nitrogen NO₃-N), a parameter that groundwater studies should include due to its widespread presence, was observed in high concentrations in groundwater of the studied region; therefore, it was included in this study. A compilation and in-depth analysis of groundwater quality studies (iAs, F, and NO₃-N) conducted in north-central Mexico is presented here to better appraise the present situation of groundwater availability and identify the most affected areas within this region as well as those areas in which environmental and public health information are scarce. Due to the widespread occurrence of these contaminants around the world and their relevance to public health, these results can be applied to other semiarid areas elsewhere facing similar challenges.

The specific objectives of this review are as follows:

• Provide an overview of iAs, F, and NO_3-N contamination of groundwater in north-central Mexico, an area where high concentrations have been reported.
• Map the areas where studies have been conducted to identify the distribution of high and low concentrations, as well as areas where monitoring data are lacking.
• Provide updated information about the effects on public health of the ingestion of these contaminants via drinking water to the local population.
• Compile the current situation on water treatment for drinking water supply and document the recent changes in water management that will alter the percentage of surface to groundwater in both drinking water and irrigation water, a change that may negatively affect irrigation water quality.

2. Description of the Study Area

The study area comprises the north-central Mexican states of Chihuahua, Coahuila, and Durango, which enclose a high plateau and the highest concentrations of groundwater iAs, F, and NO₃-N in Mexico [14], although the concentrations are highly variable. The region occupies a surface area of 225,243 km².

2.1. Hydrogeology

The north-central portion of Mexico has a mountainous landscape and high plateaus (Figure 1). Climatically, the region belongs to the Chihuahuan Desert and geologically to the Basin and Range province. The latter geological province resulted from Cenozoic volcanism and its related extensional stresses [16]. The volcanism that formed the Sierra Madre Occidental occurred during the subduction of the Farallon Plate in the Oligocene. The Sierra Madre Occidental is composed of a thick mass of volcanic rocks of rhyolitic composition, which host numerous mineral deposits, including gold, silver, lead, zinc, and fluorite. The mining of the latter deposits has been an important historic economic activity of the region. Weathering of the volcanic rock produces rock fragments, minerals such as clays, and ions in solution. The weathering process is facilitated by the smaller size and thus increased surface area of rock fragments that fill intermontane basins.

The eastern edge of the study area is flanked by another mountain range, the Sierra Madre Oriental, which is composed of folded sedimentary rocks (limestones, shales, sandstones, and evaporites) of Paleozoic age. The area between these mountain ranges contains elongated hills trending SE to NW, forming intermontane basins where alluvial aquifers and endorheic basins are common [17]. The intermontane basins function as unconfined alluvial aquifers [17] for the most part, although clay lenses may produce confinement conditions in places. The study area contains a total of 137 alluvial aquifers: 61 in Chihuahua, 48 in Durango, and 28 in Coahuila. About half of the territorial expanse consists of endorheic basins. Alluvial aquifers have an irregular geometry and variable depth, with an estimated average depth of 800 m. Groundwater flows towards discharging streams in exorheic basins, which ultimately discharge to either the Gulf of Mexico or the Pacific Ocean, or towards the center of endorheic basins [11,12].

Figure 1. Map of the study area showing the areas encompassed by the various groundwater studies (a to n) reporting [18,19,20,21,22,23,24,25,26,27,28,29]. iAs, F, and NO₃-N contamination within the Mexican states of Chihuahua, Durango, and Coahuila. The Comarca Lagunera is shown in pink. Map modified from NOAA Global Relief Model [30].

Figure 1. Map of the study area showing the areas encompassed by the various groundwater studies (a to n) reporting [18,19,20,21,22,23,24,25,26,27,28,29]. iAs, F, and NO₃-N contamination within the Mexican states of Chihuahua, Durango, and Coahuila. The Comarca Lagunera is shown in pink. Map modified from NOAA Global Relief Model [30].

2.2. Climate and Land Use

Once historical mining decreased, economic activities shifted to agriculture and livestock after dams were built on key rivers [31]. The region has a variety of climates depending on elevation; however, the aquifers are restricted to the basins, where alluvial aquifers form. The region faces recurrent droughts that can last up to twelve years [32,33]. Most recently, it has experienced two exceptional droughts, one in 2012 and another in 2024 [34].

Paradoxically, arid areas can be important agricultural regions [35]. The agricultural areas in northern Mexico are top producers of foodstuff (groundnuts, peppers, and onions) and forage (oats, alfalfa, and corn) and manifest the problems typical of arid climates: overexploitation, soil salinity, and groundwater contaminated with agricultural wastes (nitrates, herbicides, etc.) [13,31]. Overexploitation of the aquifers has produced declining water levels and cones of depression under urban centers [18] and agricultural areas [13].

3. Groundwater Quality

Contaminants of concern include naturally occurring iAs and F, which have been extensively studied since the occurrence of fluorosis or arsenicosis first appeared in the region [36]. The volcanic rocks of rhyolitic composition that outcrop in the region are a known source of iAs and F, which are released into the water upon weathering of the rock fragments. Other potentially toxic contaminants that occur at lower concentrations and under localized conditions include uranium, selenium, and sulfate [24,26,37,38]. High concentrations of sulfate, exceeding 500 mg L⁻¹ SO₄, have been reported in the eastern part of the study area, including the Comarca Lagunera [26,27] and northern Coahuila [24]. A contaminant increasingly present in aquifers worldwide and in this region is nitrate, which is discussed in Section 3.2. The aforementioned studies report oxic conditions in the aquifers within the study area and circumneutral to slightly alkaline pH values [11,12,17,18,20].

3.1. iAs and F

The groundwater of the study area contains iAs and F in a wide range of concentrations, many of them at toxic levels [11], and a presence associated with natural sources [17], in agreement with other regions where these solutes co-occur [9,10]. Water quality studies within the study area target locations where these contaminants have been observed in high concentrations, among them urban and agricultural centers [12,13,17,18]; although some studies have been conducted on non-irrigated rural regions as well [15]. Both iAs and F are considered incompatible in magma and to become enriched during the late stage of magma crystallization [39]; therefore, they have been traced to the weathering of rhyolitic volcanic rocks and desorption from their secondary minerals (clays and iron oxyhydroxides) [11,17]. Their distribution is reported as variable and highly dispersed over the area [12]. The release of iAs and F from the rock depends on conditions such as the composition of the rock, temperature, and presence of other substances in solution. The dispersed iAs and F concentration pattern has been attributed to the (a) heterogeneity of the alluvial fill and (b) presence of mineral deposits and mining tailings [12]. Various factors of enrichment have been identified, including the amount of clay in the alluvial fill [23], residence time [21], and evaporation [11,12,26]. Concentrations may also be locally impacted by the presence of wastes or hydrothermal fluids [12,27,36].

3.2. Nitrate

Among the anthropogenic contaminants affecting groundwater systems, nitrate has received much attention in the last few decades, as it is affecting aquifers worldwide [40,41]. Nitrate is labeled an anthropogenic contaminant because its sources have been intrinsically related to an excess of N-based fertilizer and treated or untreated sewage, although in some desert regions, undisturbed deposits of N-salts may have formed naturally [42]. Nitrate concentrations are expressed as NO₃ or as nitrate-nitrogen (NO₃-N); the former is more commonly used in the Americas and the latter in Europe and Asia. Values of NO₃-N concentration can easily be obtained after multiplying NO₃ values by 0.2258. Nitrate acts as an indicator of many other contaminants associated with its sources, including herbicides in the case of fertilizers and pharmaceuticals, hormones, and other emergent contaminants in manure or sewage. Contaminants in groundwater flow towards its discharge, where they enter receiving streams and/or lakes. This input has been identified as an important contributor to stream contamination [2].

3.3. Contaminant Levels and Spatial Distribution

The concentrations of iAs, F, and NO₃-N reported by recent studies are listed in

Table 1. Average and range of values of As, F, and NO₃-N as reported by selected studies, including the number of aquifers N_aq, wells N_wells, and measurements N_data. n.a. = not applicable.

Location, Naq, Nwells	Ndata	iAs µg L−1	F mg L−1	NO3-N mg L−1	Reference
State of Chihuahua
El Sauz-San Diego, 4, 45	45	38.5 (1.5–344)	1.49 (0.02–9.7)	12.9 (0.5–30.7)	[18]
Chihuahua, central, 2, 15	45	24.4 (1.0–226)	2.73 (0.37–20.7)	3.96 (0.18–23.7)	[20]
Chihuahua City, 1, n.a	92	n.a.	1.64 (0.46–3.60)	2.53 (0.48–10.6)	[19]
Irrig. District 005, 1, 63	63	n.a.	1.58 (0.62–4.8)	7.5 (0.7–23.2)	[21,22]
Irrig. District 005, 1, 40	40	86.9 (7.0–489.0)	n.a.	n.a.	[23]
Chihuahua, south, 8, n.a.	445	48 (0.1–419.8)	1.30 (0.05–11.8)	n.a.	[15]
Jiménez-Camargo, 1, 30	30	150 (10–800)	n.a.	7.6 (0.5–32.5)	[43]
State of Durango
NW Durango, 14, 44	286	27.6 (0.7–369.5)	1.53 (0.02–9.1)	2.1 (0.004–15.4)	[12]
SW Durango, 16, 64	266	47.0 (0.7–386.5)	3.94 (0.02–45.2)	2.1 (0.009–18.5)	[12]
NE Durango, 7, 153	412	40.0 (0.7–462.8)	0.85 (0.02–3.6)	11.1 (0.008–127.7)	[12]
SE Durango, 11, 58	273	48.0 (0.7–310.0)	1.75 (0.10–15.8)	4.3 (0.005–81.9)	[12]
Durango City, 1, 9	9	32.0 (n.a.)	3.89 (n.a.)	8.7 (n.a.)	[28]
Valle del Guadiana, 1, 40	40	41.1 (1.5–199.8)	0.17 (0.01–0.58)	0.16 (0–0.62)	[29]
Comarca Lagunera
Comarca Lagunera, 6, 31	31	70.0 (10–350)	0.88 (0.12–3.1)	7.7 (0.01–45.0)	[27]
Recharge zone, 6, 29	29	n.a.	1.1 (0.12–4.5)	20.2 (1.5–109.0)	[25]
Transition zone, 6, 9	9	n.a.	0.6 (0.25–1.1)	4.3 (0.14–4.3)	[25]
Discharge zone, 6, 15	15	n.a.	1.0 (0.31–3.1)	1.2 (0.01–2.6)	[25]
Comarca Lagunera, 6, 55	55	51.9 (5.0–349)	0.98 (<0.2–4.54)	12.3 (<0.02–109)	[26]

. Their methods of sampling and analytical determinations consistently report that they have followed standard procedure as well as strict quality assessment and quality control guidelines (e.g., letting a casing volume of water run prior to taking the sample, maintaining the sample cool during transport to the laboratory, use of grade quality reagents, replicates, blanks, and standardization). The areas encompassed by these studies are shown in Figure 1 to allow a visual appraisal of their location, extent, and distribution within north-central Mexico.

As observed in Table 1, all iAs, F, and NO₃-N concentrations vary widely in each studied area. However faintly, patterns of concentrations can be recognized, showing the parts that are richer in one contaminant: The highest iAs concentrations, up to 800.0 mg L⁻¹ iAs, are found in southern Chihuahua and the Comarca Lagunera. The highest F concentration, 45.2 mg L^–1 is reported in SE Durango, whereas NO₃-N concentrations peak in the Comarca Lagunera and eastern part of Durango, with values up to 127.7 mg L⁻¹ NO₃-N. Furthermore, the highest concentrations of iAs and NO₃-N are found in endorheic basins under arid climatic conditions, which are conducive to salt accumulation [12,44], whereas the highest F concentrations correspond to the foothills of the Sierra Madre Occidental [12]. Roughly, and in agreement with the lithology of the eastern and western edges of the study area, iAs and F concentrations diminish with increasing distance from the Sierra Madre Occidental, while other solutes increase, e.g., calcium (Ca²⁺), sulfate (SO₄²⁻), and bicarbonate (HCO₃⁻) [12,13,24,27].

As shown in Figure 1, the surface area encompassed by areas reported in studies a-m (except study k) comprises each a small fraction of the total surface area. These studied areas concentrate around large urban centers (Chihuahua, Torreón, Gomez Palacio, and Durango) or areas with intensive agriculture and livestock farming (irrigation districts). In these areas, groundwater levels are rapidly declining, an average of 0.4 m year⁻¹ in Chihuahua and 1.5 m year⁻¹ in the Comarca Lagunera [31]. The depiction of the areal extent occupied by these studies exposes significant gaps in data coverage, e.g., north and northeast Chihuahua and northern Coahuila. The similar hydrogeology and climate within the study area are conducive to assuming that the non-studied areas will also contain groundwater contaminated with iAs and F; however, sampling wells in the non-studied areas would be needed to ascertain groundwater contamination.

To better visualize the dispersion of concentrations for each parameter (iAs, F, and NO₃-N), boxplot diagrams for the different geographic locations within the study area were constructed. The results are depicted in Figure 2. The southwest quadrant of Durango [11] was selected as representative of Durango in this comparison. A noticeable enrichment of iAs (mean concentrations at about the 10 µg L⁻¹ guideline) is observed, with concentrations observed in order of higher to lower for iAs: Southwest Durango = Comarca Lagunera > Chihuahua; F: Southwest Durango > Chihuahua > Comarca Lagunera; and NO₃-N: Comarca Lagunera > Chihuahua = Southwest Durango. This graph includes the guidelines (red lines) to facilitate the identification of regions where the median concentrations surpass the guidelines, e.g., iAs and F in southwest Durango and As and NO₃-N in the Comarca Lagunera.

Under the arid conditions and an enrichment of bicarbonate ions, calcite may reach saturation conditions, which has been reported for endorheic basins in the eastern part of the study area, where carbonate rocks are exposed [12,13]. Under these conditions, F concentrations decrease, a behavior explained as the possible co-precipitation of F with precipitating calcite [12].

Groundwater iAs and F concentrations vary widely with geographic location [11,12]. However, their content within a particular well varies only slightly with respect to time [45,46,47]. The high variability in concentrations is explained by the heterogeneity of the alluvial fill and from differences in climate and topography, land use, and groundwater extractions [11,12,18,25]. To better address these variations, some studies divided their study area into sub-areas according to recharge, transition, and discharge areas [25] or by geographic quadrants [12].

Trends in the concentration of the above contaminants have been reported only in a few studies [46,47]. A robust statistical trend requires a minimum of ten years of data for each season, a drawback for regions where monitoring data are fragmented and scarce, as is the case in arid and semiarid areas of developing countries, including northern Mexico. Nevertheless, these few studies coincide that concentration trends are generally stable in groundwater quality with respect to time, with a few wells experiencing a slightly increasing or decreasing trend in one or more water quality parameters [22,46,47]. This result suggests that, except for a few aquifers, the groundwater systems have not yet reached a critical level and may be able to take in the increase in temperature and evaporation expected because of global climate change. Of course, the current trend towards reforestation and other strategies implemented in northern Mexico towards the protection of water quality may result in groundwater quality improvement, as happened in Denmark after Europe’s Nitrate Directive and policy action plans were implemented [48]. A concerning fact in Mexico is that since 2020, monitoring by the Mexican national water office, CONAGUA, has diminished significantly [45], thus affecting access to water quality data that is vital to the proper management of water for drinking, irrigation, and ecological purposes—data especially important nowadays when the quantity and quality of groundwater cannot be taken for granted.

Conceptual models of the groundwater flow and contaminant transport reported for the alluvial aquifers of northern Mexico agree that the recharge areas are within the foothills, their alluvial fans, and streams, while the discharge areas are streams in the case of exorheic basins and the lowest elevations in the case of endorheic basins [18,20,21,26,38]. The groundwater flows (e.g., surficial and deep) acting in these aquifers seem to be affected by temperature and concentration differentials, with the highest concentrations observed at the discharge points and not at greater depth [21,25]. A simplified sketch (Figure 3) condenses the lithology and hydrology of the study area, although one has to keep in mind that there are more than 130 alluvial aquifers (bolson type, some of them hydraulically connected) within the study area and that the lithology in this model is overly simplified. The model (Figure 3) depicts the main controlling processes described above, including the weathering of exposed rocks through hydrolysis of the silicic volcanic rocks that comprise the Sierra Madre Occidental (releasing iAs and F⁻) and the dissolution of the sedimentary rocks exposed in the Sierra Madre Oriental (releasing Ca²⁺, Mg²⁺, Na⁺, SO₄²⁻, and HCO₃⁻). Hydrolysis of silicate minerals results in the formation of clay and iron oxyhydroxide minerals, which become enriched in the western part of the study area and are transported downhill by wind and water. Once released from volcanic rock fragments, iAs (as arsenate ion HAsO₄²⁻) and F⁻ follow different paths; while iAs adsorbs preferentially onto clay minerals and iron oxyhydroxides, F⁻ remains in solution and only adsorbs or co-precipitates with calcite under Ca²⁺ and HCO₃⁻ concentrated conditions and high pH values [12].

4. Safe Limits and Drinking Water Regulations

The World Health Organization (WHO) recommends the following maximum limits for drinking water: As 0.01 mg L⁻¹, F 1.5 mg L⁻¹, and NO₃-N 11 mg L⁻¹ [49]. In Mexico, new maximum permissible limits by the Norma Oficial Mexicana (NOM) for drinking water were issued in 2022 (NOM-127-SSA1-2021) [50], replacing those issued in 2000 (Mod.NOM-127-SSA1-1994). The present limits are 0.01 mg L⁻¹ for As, 1.0 mg L⁻¹ for F, and 11 mg L⁻¹ for NO₃-N [50]. A lower F limit was set to protect public health by compensating for the higher water consumption in fluoride-rich areas, especially those in hot, arid climates [11]. To prevent excessive F intake and reduce the risk of dental and skeletal fluorosis, as well as other potential health effects, a lower limit was established than that recommended by the WHO.

Regarding nitrate, the permissible limit was increased from 10 mg L⁻¹ to 11 mg L⁻¹ based on an analysis of public health effects and a review of international recommendations established by the WHO. This adjustment, according to the Mexican officials, does not compromise health safety, aligns with international standards, and facilitates regulatory compliance in areas where natural nitrate concentrations are high [50].

5. Health Effects of Drinking Water Contaminated with iAs, F, or NO₃-N

The consumption of drinking water contaminated with iAs, F, or NO₃-N represents a serious public health problem in northern Mexico [14,28] and various other regions around the world [10,13]. Studies reporting iAs, F, or NO₃-N contamination generally include health effects or health risk assessment of the exposed population [15,51,52,53,54,55,56,57]. While studies on the effects of either one contaminant are common, the number of health studies addressing co-exposure (two or more contaminants) are much scarcer.

The toxicity of iAs depends on the oxidation state; As(V) is less toxic than As(III) [51,52]. Chronic ingestion of water with high F concentrations can lead to dental fluorosis as well as skeletal fluorosis. Severity of health effects depends on the concentration as well as body weight and amount of daily water intake [58]. Nitrate (NO₃-N) in drinking water affects especially vulnerable groups such as infants and pregnant women, e.g., methemoglobinemia, “blue baby syndrome”, which is the most documented effect of NO₃-N exposure in infants under six months of age [59,60]. Recent studies point out, however, that at smaller concentrations (>2 mg L⁻¹ NO₃-N), a variety of adverse effects such as cancer and birth defects may occur [61]. Prolonged consumption of water with high NO₃-N concentrations has been associated with an increased risk of gastric and colorectal cancer since nitrite can react with compounds in the stomach to form nitrosamines, substances potentially carcinogenic [62]. Recent studies report that excess NO₃-N concentrations can interfere with iodine uptake by the thyroid, affecting thyroid hormone production. In adults, this may contribute to hypothyroidism [62,63].

Health Studies in North Mexico

In Mexico, approximately 3 million people are exposed to co-occurring iAs and F above the guidelines, and 9 million people are exposed to iAs [14]. These studies have mainly been conducted in northern and central Mexico, where iAs and F levels are elevated.

Table 2. Selected health studies related to exposure to iAs in northern Mexico (study area plus state of Sonora).

Health Concerns/Health Issues	Target Population	Reference
Chihuahua
Association between body mass index and urinary arsenic metabolites	Adult	[64]
Diabetes, elevated triglycerides and cholesterol, and cardiometabolic risk	General	[65,66,67]
Durango
Apoptosis of peripheral blood mononuclear cells in As-exposed children	Children	[68]
Exposure to As and Cr is associated with kidney injury molecule-1	Children	[69]
Urinary arsenic and fluoride in mothers in rural areas and their newborns	Mothers Newborns	[70]
Comarca Lagunera
Increased prevalence of diabetes after exposure to arsenic in water	General	[51]
Association of obesity, diabetes, and hypertension with arsenic in water	General	[71]
Lung inflammation biomarkers and lung function in children chronically    exposed to arsenic	Children	[72,73]
Prenatal arsenic exposure and functional changes in gene expression in    newborn cord blood and subsequent birth outcomes	Women, Newborn	[74,75]
Telomere length analysis in residents of a community exposed to arsenic	Children, general	[76,77]
Sonora
Inflammation biomarkers associated with arsenic exposure by water    and respiratory outcomes	Children	[78]
Decreased DNA repair in vitro and in individuals exposed to As	General	[79]

,

Table 3. Selected health studies related to exposure to F in northern Mexico (study area plus State of San Luis Potosi).

Health Concerns/Health Issues	Target Population	Reference
Chihuahua
Vascular and kidney injury biomarkers in Mexican children exposed to    inorganic fluoride	Children	[80]
Evaluation of kidney injury biomarkers in an adult Mexican population    environmentally exposed to fluoride and low arsenic levels	Adult	[54]
Durango
Urinary arsenic and fluoride in mothers in rural areas and their newborns
San Luis Potosi
Fluoride-induced disruption of reproductive hormones in men	Men	[81]

and Table 4 list recent health studies conducted in northern Mexico on iAs, F, and NO₃, respectively.

As is evident from Table 2 and Table 3, the majority of the health research has focused on iAs and less on F and NO₃, a fact that is probably explained by a larger population being exposed to iAs concentrations above the guidelines, as mentioned above, as well as a common public perception that iAs is much more toxic than F or NO₃.

Nitrate (NO₃⁻) in drinking water has gradually increased in concentration and is now found in groundwater in all states of Mexico [44]. Concentration prediction models list the affected areas around large urban centers of northern Mexico (Monterrey, Torreón, and Chihuahua), where about 8 million people are at risk of consuming groundwater with NO₃-N concentrations above WHO guidelines (11 mg L mg L⁻¹ NO₃-N) [44].

Health studies have intensified after evidence of adverse effects at much lower concentrations than the WHO (and Mexican) guideline, especially in the Durango part of the Comarca Lagunera, where concentrations have been reported to have a median value of 12.26 mg L⁻¹ NO₃–N [30], more than five times the median concentration of the whole study area of 2.92 mg L⁻¹ NO₃-N [82]. Health studies require a long time to collect all the information required before an association with a contaminant can be established. Table 1 shows studies conducted in the Durango part of the Comarca Lagunera addressing some of the health impacts experienced by exposure to NO₃-N, while other studies (e.g., effects on women) continue being investigated [59].

Table 4. Selected health studies related to exposure of the population to nitrate in northern Mexico.

Health Issue	Target Population	Reference
Durango-Comarca Lagunera
Subclinical hypothyroidism due to chronic consumption of nitrate-contaminated  water in rural areas with intensive livestock and agricultural practices  in Durango, Mexico	Families, General	[59]
Health risk and methemoglobin in children ingesting nitrate-contaminated  Water	Children	[83]
Methemoglobin and Heinz bodies as biomarkers in children exposed to nitrate	Children	[84]

Table 4. Selected health studies related to exposure of the population to nitrate in northern Mexico.

Health Issue	Target Population	Reference
Durango-Comarca Lagunera
Subclinical hypothyroidism due to chronic consumption of nitrate-contaminated  water in rural areas with intensive livestock and agricultural practices  in Durango, Mexico	Families, General	[59]
Health risk and methemoglobin in children ingesting nitrate-contaminated  Water	Children	[83]
Methemoglobin and Heinz bodies as biomarkers in children exposed to nitrate	Children	[84]

On a note about prediction of health risk, high NO₃-N concentrations have been related to agricultural land use and concentration by evaporation (either semiarid climate or by boiling water in homes) [44]. Because of prevailing high NO₃-N concentrations and a climate conducive to the ingestion of more water, the guideline of 11 mg L⁻¹ NO₃-N seems insufficient to protect the population.

In sum, the above studies report evidence that drinking water with a high concentration of one or more iAs, F, and NO₃ is affecting the health of the population accordingly. iAs has been associated with the diseases type 2 diabetes and hypertension [80], NO₃-N has been linked to a high frequency of subclinical hypothyroidism, and F causes dental fluorosis and neurological problems such as a reduction of IQ. Studies also report a relation between chronic ingestion leading to more severe health effects, e.g., chronic F intake causing cardiovascular and kidney dysfunction and a decrease in the production of reproductive hormones in men [54,81]. Interestingly, public health studies report a continued increase in metabolic diseases among the population and point out that most people are unaware of the acute adverse effects of ingesting iAs, F, or NO₃-N [70,79,80].

6. Removal Technologies

The quality of groundwater can be enhanced by applying appropriate treatment technologies. The simultaneous implementation of programs of community education about contaminants and water treatment as well as relying on clear policies is recommended [43,57]. Globally, many iAs and F removal technologies have been developed to reduce their content in water enough to meet WHO guidelines [85,86].

A large variety of technologies to reduce the concentration of iAs, F, and NO₃-N from drinking water exist, each of which presents advantages and disadvantages [58]. A detailed description of removal technologies is beyond the scope of this review; however, Abascal et al. [41], Shaji et al. [58], Maity et al. [85], and Sanchez et al. [86] provide excellent summaries.

Several cost-effective technologies that would be applicable to the conditions of northern Mexico include adsorption using activated alumina, biochar, or chitosan [87] and electrodialysis [28]. Each of these studies reports a removal efficiency of approximately 90% of the target contaminant. Nevertheless, all but one of the treatments remain at a bench or pilot scale. The one treatment utilized on a large scale is reverse osmosis, which removes arsenic, fluoride, and other contaminants. Reverse osmosis filtration is based on the application of pressure to water with a high salt content so that water molecules are forced to pass through semipermeable membranes, retaining larger particles in another section of the system (reject water) [88]. In the state of Chihuahua, reverse osmosis units have been installed on about 320 community wells [11,88]. The filtered water is provided separately from the water distribution system at strategically located sites where the user fills a container with filtered water. The user will most likely use this water for drinking and cooking and not for any of the other household uses. By filtering only the water that will be used for drinking and cooking, this method of distribution benefits a larger number of people (estimated at 500,000) at a lower cost and generates the lowest possible amount of waste (brine). Although this measure has largely solved the compliance with drinking water regulations in many communities within the state of Chihuahua, there are other challenges to be solved, including the disposal of process waste in an environmentally friendly manner [88].

To provide safe water quality and mitigate aquifer decline, especially in groundwater containing high levels of iAs and F in the Comarca Lagunera and the city of Durango, a government plan to switch the domestic water usage from groundwater to surface water, the water supplied to the distribution network, has been recently implemented [89]. The newer plan of potable water supply in the Comarca Lagunera consists of harnessing surface runoff from the Nazas River, thereby suspending the usage of 160 wells that had supplied drinking water to a population of two million for decades. The implementation of this plan raises questions about future groundwater availability; for example, how much water will be extracted from these suspended wells, the impact of the impending technification of irrigation on groundwater levels, and the creation and enforcement of policies intended to regulate them. As the scarcely available surface water is diverted to the drinking supply network, farmers will likely supplement the water needed to irrigate crops with groundwater. The outcome of this approach will become known in the next few years, i.e., if enough water for both drinking and irrigation will be available and how this change will affect agriculture. A similar project will be implemented shortly in the city of Durango. In both cases, a drinking water treatment plant will treat the iAs-, and F-free surface water using conventional flocculation and filtration methods.

7. Emerging Research

One concern that is increasingly addressed in areas where contaminated groundwater is used for irrigation is the accumulation of its contaminants in crops [90,91,92,93,94,95]. Until recently, irrigation water in agricultural areas within the study area was provided primarily by surface water, with a small percentage coming from groundwater, but this proportion is rapidly changing to more groundwater as surface water becomes scarcer. Irrigation water with a larger percentage of groundwater represents a larger amount of iAs and F available to crops. As a result, studies on the impact of groundwater contaminants on food are being undertaken. The studies focus on crops that are either economically important to the region or crops that are staple foods. Among the contaminants studied, arsenic has received more attention [90,94,95,96,97], although fluoride [91] and lead [95] have also been investigated for some crops. Recent studies conducted in northern Mexico on arsenic accumulation by various crops are listed in

Table 5. Accumulation of arsenic (As) in crops growing in northern Mexico.

Location	Crop	Studied Part	As mg kg−1	Reference
Chihuahua	Jalapeño pepper Serrano pepper Onion	Fruit Fruit Bulb	0.016 0.050 0.38	[94]
Chihuahua	Barley Oats	Grain Stems Grain Stems	0.20 0.62 0.55 1.53	[95]
Chihuahua	Alfalfa	Leaves and stems	<0.02	[97]
Durango	Corn tortilla	Food item	0.14–0.54	[92]
Durango	Melon plants	Leaves and Stem Root	5.2 34	[96]
Comarca Lagunera	Maize	Grain	0.12	[92]
Comarca Lagunera	Alfalfa	Leaves and stems	2.2–23.3	[98]

.

The results obtained within the study area show that the edible parts of barley, alfalfa, and peppers contain safe levels of As, below the recommended limits by the FAO’s Codex Alimentarius of 0.2 mg kg⁻¹ [99], while oats and onions are slightly above it. However, the above crops were sampled within a small area and are not representative of the totality of the study area, which implies that additional studies are needed to verify these levels as well as their dependence on other factors, including the contaminant content of soil, climate, and agricultural practices.

8. Conclusions

The numerous groundwater quality studies that have been recently undertaken in north-central Mexico have shed light on the source of iAs, F, and NO₃-N concentrations and have identified hot spots. However, visual appraisal of their mapped study areas shows that significant portions of the surface area have not been studied. In addition, questions remain about concentration trends and the extent to which conditions such as evaporation, drainage (endorheic or exorheic basins), and residence time influence such concentrations.

Health risk assessment studies point to a large amount of the population being exposed to unsafe levels of these contaminants, and while the guideline levels for iAs and F seem to be effective, the guidelines for nitrate are lax, according to health studies that report health issues at concentrations much smaller than the 11 mg L⁻¹ NO₃-N guideline. The larger and broader population exposure has been reported for iAs, whereas the exposure of F becomes higher near the Sierra Madre Occidental in the southern part of the study area, and the exposure to NO₃-N is more prevalent in the Comarca Lagunera.

Multiple treatment techniques to reduce the concentration of these contaminants in drinking and irrigation water have been investigated, from filtration to adsorption to electrocoagulation, and although many yield promising results, they remain at bench scale. Chihuahua offers its population filtered water (reverse osmosis) at strategic locations. A recent plan to switch groundwater for arsenic-free surface water in the drinking supply of two former hot spots may provide safe drinking water to a large population. Due to surface water scarcity, more groundwater is used to irrigate crops, increasing an interest in the impact that groundwater rich in iAs and F will have on typical crops, e.g., maize, onion, melon, barley, and oats.

Monitoring, treatment, and improved groundwater management remain urgent necessities to improve water quality. Aquifers’ decline affects water quality and can be ameliorated by implementing water management practices that focus on effectively curbing groundwater overexploitation and deforestation and by promoting aquifer recharge.