Anthropogenic Contamination in the Free Aquifer of the San Luis Potosi Valley

doi:10.21203/rs.3.rs-2476817/v1

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Anthropogenic Contamination in the Free Aquifer of the San Luis Potosi Valley

https://doi.org/10.21203/rs.3.rs-2476817/v1

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The valley of San Luis Potosí is an endorheic basin formed by three aquifers: shallow hanging, alluvial material on a hardened horizon known as "tepetate", two deep, free and confined type. Population growth has led to gradual contamination of the surface aquifer. Recent studies documented anthropogenic, biogenic and heavy metal contamination. A recent phenomenon of settlement has affected houses and buildings in the urban area, extending to parts of the valley that are still used for agriculture, developing "faults" affecting the tepetate that supports the superficial aquifer, facilitating the infiltration of contaminated water into the aquifer. free type. This investigation reports detection of nitrogen (N-NO₃), with a concentration of 15 mg/L (milligram liter) its maximum allowable limit is 10 mg/L. Twelve samples with fecal coliforms from 5 to 17 MPN/100 (the most probable number in one hundred milliliters of the sample). Thirteen samples showed total coliform concentrations, one sample reported a too high concentration of 150 NMP/100, compared to the rest with concentrations of 5 to 17 NMP/100, indicating less contamination. Some trace elements with concentrations in parts per million (ppm) that were also detected were manganese (Mn), arsenic (As), cadmium (Cd), and mercury (Hg). Results that give the first sign of attention to the contamination of the deep free aquifer, derived from anthropic activity in the valley of San Luis Potosí.

contamination

aquifer

faults

ground settlement

Groundwater is of essential importance to our civilization because it is the largest reservoir of drinking water in the regions inhabited by humans. Although drinking water can occur at the surface in the form of springs, or be extracted by wells, groundwater is preferable because it does not tend to be contaminated by waste or microorganisms. Although groundwater is less polluted than surface water, the increasing and diverse anthropogenic activity due to urban growth has gradually accelerated the quantity and diversity of pollutants in wastewater, both from urban, mining, and industrial activities, exceeding the capacity of treatment plants, so that untreated water could contaminate groundwater, which is why this resource, which has become a priority in industrialized countries, must be taken care of. For example, (Mushak et al., 1992) commented that human beings exposed to chemical and organic pollutants contained in groundwater may be related to the outcome of cancer, stomach disease, or malformations in babies. (Honqi Wang et al., 2013), documents that in China more than 400 cities exploit groundwater using more than 1/3 of the total water resources, as the only source of water supply, so a series of problems with groundwater utilization gradually arise. One of the best-known examples is the contamination with organic solvents and dioxins of ¨Love Canal¨, New York in 1978, to which high rates of cancer and an alarming number of birth defects were attributed (Rodriguez et al., 2007).

In Mexico, agriculture is the human activity that pollutes water the most, but the greatest damage is caused by urban and industrial waste (Shortle et al., 1986). Some 30 million tons of garbage buried in eastern Mexico City contaminate the region's groundwater (Pacheco et al., 2001). According to (IPN 2009), 1,500 km away from the capital, in the Yucatan Peninsula, most of the cenotes are affected by sewage and garbage. Diarrhea is the fifth leading cause of infant death in Mexico and is associated with the water consumed (Pacheco et al., 2001). In the future, it is expected that groundwater sources will increasingly become the only source of additional water for community development, particularly in more remote areas. However, the quality and quantity of these resources are constantly threatened by the activities inherent to the growth and development of urban centers.

In the San Luis Potosi Valley (VSLP) recent water quality studies documented that the water contains high concentrations of fluoride (assigned to the substrate draining surface water and water-rock interaction) and arsenic, assigned to mining activity (Estrada 2013), in addition to other elements derived from anthropogenic activity. Currently, anthropogenic activities have a high impact on the quality of groundwater in the VSLP (housed in three aquifers; one hanging and two deep aquifers) with the increase in population, change in land use, and expansion of industrial and mining activities. These factors were responsible for the contamination of the hanging aquifer with heavy metals, major ions, nitrates, fats, oils, and grease, and bacteriological contamination. The low average annual precipitation does not contribute substantially to surface water recharge, but it is considered that in times of rain, the contribution is by infiltration in the margins of the valley, and feed the hanging aquifer mainly and the deep free aquifer (Lopez 2013) Fig. 1b).

Aguirre and Martinez (1984) determined the areas of the valley where pronounced depletion cones are manifested by the pumping of deep wells, and point out that this highlights the overexploitation of this aquifer.

Another important factor to examine is the subsurface geology of the valley. (Aguirre 1992), based on the information of lithological sections from deep wells, documents the structural and geological situation of the subsoil of the VSLP and Villa de Reyes (Fig. 1a). With this, the author verified the existence of basement blocks delimited by staggered faults, with their course and range of settlement, thicknesses, and continuity of the lithological units of the subsoil, fundamental data to define the dimensions and real geometry of the deep aquifer of that valley. The Risk Atlas for the municipalities of San Luis Potosi (capital) and Soledad de Graciano Sanchez, documents land settlements (Fig. 1a) that coincide with the limits of some of the basement blocks in the subsoil reported by (Aguirre 1992), suggesting that the settlements are controlled by these structures.

The water quality records where contamination was detected in the shallow aquifer (hanging type) coincide with the surface areas where ground settlement and faulting (with rupture of the hardened soil substrate or tepetate- that supports it) have been reported in the Valley of San Luis Potosí (Risk Atlas of the Municipalities of San Luis Potosí and Soledad de Graciano Sanchez, 2018). This paper argues that anthropogenic contamination of the shallow aquifer widely documented (Lopez, et al., 2013; Moran, 2010; Estrada, 2013), is manifesting, incipiently, in one of the two deep aquifers (free type) are linked to the possible deterioration and rupture of drainage pipes (especially in old drainage pipes), because the settlement and faulting of the valley terrain are creating conduits for percolation of contaminated water into the deep free aquifer. The importance of these studies is that a large part of the population of the municipalities of San Luis Potosí capital, Soledad of Graciano Sanchez, S.L.P., and Mezquitic de Carmona, S.L.P., receive water for human consumption from the aquifer examined here, so it is a priority to evaluate the risks that this implies for the health of the population, and to take the necessary measures to prevent them.

This work documents that contamination of trace elements, with potentially harmful effects on health, have percolated from the contaminated shallow aquifer to the deep free aquifer. This process is assumed to be of a temporary resent nature, due to ground settlement processes and fracturing of the tepetate layer that supports the shallow aquifer.

Location

The state of San Luis Potosí is located north of the Central Plateau, in the central-eastern part of Mexico, between the parallels 21° 09' 35'' and 24° 33' 09'' north latitude and the meridians 98° 19 ' 52' '' and 102° 17' 51'' west of Greenwich (Fig. 2). It is divided into four natural regions: Altiplano, Centro, Zona Media and Huasteca (INEGI 2009). It is divided into two contrasting hydrogeological regions: the Salado region, located in the north-central portion of the state and with an area of 35,164.19 km², presents a dendritic endorheic hydrological pattern where surface water currents are intermittent, scarce and with little flow., flowing only during the rainy season in summer and occasionally in winter. The second is the Pánuco region, located in the south-southeast portion of the state, with an area of 27,140.55 km2 and a dense fluvial network of perennial streams that form some rivers that flow into the Gulf of Mexico (CRM 1992).

The San Luis Potosi aquifer, defined with the code 2411 in the Geographic Information System for Groundwater Management (SIGMAS) of the (CONAGUA 2020) Fig. 2.

There are three aquifers in the San Luis Potosí Valley (VSLP). One is of the hanging type, with a shallow depth of only approximately 40 meters (CONAGUA 2020), with a stratum of low hydraulic conductivity; it is recharged from precipitation in the valley and its behavior is very dynamic. It has diffuse induced recharge (occasionally by drinking water and/or drainage leaks) and by irrigation water returns (Lopez 2013). Irrigation return flows are responsible for the deterioration of groundwater quality in a large number of countries, particularly in semiarid and arid regions (UNESCO 2004). Several studies have detected the presence of organic and bacteriological contaminants in the water that captures this aquifer, which has been attributed to the shallow depth at which it is located, and the fact that most of the crops are irrigated with untreated municipal sewage; in addition to contamination by some metals that can be linked to anthropogenic activities (Carrillo and Armienta 1989; Carrillo et al., 2008; Moran 2010; IPICYT 2010). The aquifer material has textural variations, which influence its characteristics as an aquifer. Towards the Sierra de San Miguelito (SSM), conglomerates immersed in a sandy-clay matrix predominate, and towards the northeast of the east valley, silts and sands predominate. The direction of flow of the hanging aquifer is from southwest to northeast (Ramos et al., 2007), and its recharge depends on conditions such as precipitation, evapotranspiration, runoff, vegetation, presence of soil layer, the slope of the terrain, and permeability of the material. The main recharge in the aquifer takes place in the southwestern and western flank of the VSLP, from the streams that flow down from the SSM towards the Valley (Ramos et al., 2007). In this region, the boundary of the hanging aquifer is located towards the La Palma locality, where the granular material is wedged and its contact with the fractured volcanic material is closer. The aquifer of the granular medium feeds the wells and rests on the hardened soil or “tepetate”.

The other free type (and deeper) aquifer, which is hosted in the granular alluvial material that filled the tectonic trenches now buried by the valley fill, has been the most exploited. Geological structures, such as faults, control the distribution and thickness of this aquifer unit, which is composed mainly of Paleogene and Quaternary clastic materials. The aquifer is currently exploited by wells that reach depths of up to 350 meters of sedimentary material. Its upper limit is approximately 100 to 150 meters deep. It is confined in the center of the valley by a sedimentary layer that is not very permeable. Its thickness ranges from 100 to 200 meters (IPICYT 2007).

Another aquifer, of the confined and deeper type, is hosted in alluvial material that was covered by a pyroclastic unit (an ignimbrite) that constitutes its seal and separates it from the free aquifer. Most of the wells drilled in this aquifer have depths of 350–450 m, although there are some of 800–1000 meters (Cardona 2007). The depth of the potentiometric surface is greater than 150 m, depending on the location within the valley.

Geology

The aquifer studied is located in an endorheic tectonic valley, whose lowest topographic parts are in its northeastern part. The various studies carried out to date (Labarthe et al., 1982; Aguirre 1992; Lopez and Tristan 2013), have fully established the geology around the valley, which is outlined below. The basement is a sequence of calcareous and calcareous-marine clay-calcareous rocks (basin and platform) of Mesozoic age, outcropping in the Sierras of Alvarez and The Coro, which limit the valley in the eastern and northeastern parts. The north, west and southwest limits are: La Melada and San Miguelito mountains (respectively), formed by Paleogene (Oligocene-Miocene) volcanic rocks of rhyolitic and dacitic composition. The valley was formed by extensional faults in two main systems, those of NW-SE orientation (north and southwest limits of the valley), and those of N-S orientation, in the other limits.

Geological events that shaped the valley generated a series of tectonic blocks, which remained under the alluvial fill, and have been determined by the coincidence of lithological columns cut by numerous wells at different depths, and evidenced by magnetometry geophysical studies (Lopez and Tristan 2013). Thus, the aquifers have been documented: the deep confined in clastic deposits between the Mesozoic basement and pyroclastic deposits (ignimbrites) from the middle Oligocene that outcrop regionally; intermediate in the fill of alluvial material deposited on the ignimbrites, and upper "pendant" (5–30 m higher) that has its base on the hardened layer called "tepetate", typical of the arid and semi-arid regions of Mexico. Due to its characteristics, this shallow aquifer is the most susceptible to being impacted by anthropic activity, the object of this research study, especially its influence on the incipient contamination of the deep free aquifer.

According to the topography of the study area, the recharge and discharge zones were identified, the selection of the points of wells and waterwheels was carried out. The samples of wells that correspond to the deep free aquifer have an average depth of 260 m. In the shallow aquifer, the wells have a depth between 3 m and 10 m.

Sample bottles are washed with 10% hydrochloric acid (HCl) and phosphate-free detergent, rinsed more than three times with distilled water, and allowed to dry.

In the hydrogeochemical sampling campaign, 29 groundwater samples were taken from 22 wells and 7 wells. The sample was taken using a peristaltic pump with a hose with two ends; the sample was passed through one end and the other end was passed through a filter of point forty-five microns (0.45 µ) and deposited in the polyethylene container, duly labeled.

In situ physical parameters such as pH, T (temperature), EC (electrical conductivity), DO (dissolved oxygen), ORP (oxide reduction potential) and total dissolved solids (TDS) were performed with a Hanna HI 9829 multiparameter; and sample alkalinity with HI3811 test kit with 10 mL phenolphthalein indicator, 10 mL bromophenol blue indicator, 120 mL alkalinity titrator. The samples taken for cations and trace elements were preserved with concentrated nitric acid (HNO₃) until pH < 2 and kept in a cooler at 4°C for transfer to the laboratory.

Sample analysis

The analysis of the samples was carried out at the Geochemistry laboratory of the Geology Institute, Universidad Autonoma of San Luis Potosi. The analysis consisted of determining the concentrations of the major ions [sodium (Na⁺), potassium (K⁺), calcium: (Ca^+ 2), magnesium (Mg^+ 2)] using an optical emission spectrophotometer with plasma coupled by atomic absorption induction (ICO - OES). Induction Coupled Plasma Inductively Coupled Mass Spectrometer (ICP-MS) was used to determine trace elements such as mercury (Hg), barium (Ba), strontium (Sr), cadmium (Cd), lead (Pb), silver (Ag), rubidium (Rb), cobalt (Co), copper (Cu), iron (Fe), arsenic (As), lithium (Li), nickel (Ni), manganese (Mn), chromium (Cr), zinc (Zn), aluminum (Al).

The UV-Vis method with the HACH DR/2000 equipment was used for the analysis of nitrates (N-NO₃⁻), sulfates (SO₄ ^− 2), fluorine (Fˉ); the volumetric method for bicarbonates (HCO₃⁻), and carbonates (CO₃ˉ²); the argentometric method for chlorides (Cl⁻), and the analysis of pH, electrical conductivity, alkalinity, total dissolved solids, fats and oils (GA), total coliforms (CT) and fecal coliforms (CF) was performed in the groundwater laboratory of the Faculty of Engineering of the UASLP.

In February 2020 the sampling campaign of the VSLP was carried out, obtaining its physicochemical parameters in situ (in the field) table I, and, in addition, the characterization of the chemical composition of the groundwater of the valley was carried out in table II.

These parameters are expressed in a 10.57 km physicochemical section (Fig. 3) of the VSLP; It begins in the southwestern part of the valley, with sample 3P located along Hernán Cortés avenue, crosses the valley towards the center of the city with samples (1P, 10P, 9P), heading north of the valley ( 4N, 6P,8N,7P), and at the end of the stretch, the sample (2P), this development is the furthest from the VSLP where the volcanic rocks at the base of this aquifer are approximately 360 m deep.

The pH at the beginning in the 3P sample is 7.7, it increases, and reaches values of 8 in 5P, maintaining little variation until Tangamanga park; however, in the end, values of 8.54 are reached in 2P (Fig. 3 and table I). The temperature starts at 38°C and throughout the stretch it remains intermittent and increases until it reaches 26°C towards the end of the stretch in El Saucito sample 2P (Fig. 3 and table I).

The average electrical conductivity in the waters is 488 µS/cm (microSiem/centimeter), in the entire section. It starts low at 3P (769 µS/cm), the highest points are located almost in the center of the section in the valley at 4N (808 µS/cm) and 8N (930 µS/cm), due to the chemical components dissolved in Water; and the value decreases at the end of the section in the 2P samples, which suggests that the waters interacted with volcanic rocks (Fig. 3 and table I). Alkalinity starts with values of 156 mg/L at 3P, increases slightly towards the center of the valley to 210 mg/L at 9P, 10P, and increases to 348 mg/L at 4N and 8N; this increase is associated with the interaction of water with carbonates in the sands and silts of the fill material (Fig. 3 and table I).

Regarding the total dissolved solids (TDS), it begins with a concentration in 3P (206 mg/L), increasing towards the center of the valley in 1P to (391 mg/L), in 4N (403 mg/L) and 8N (465 mg/L), decreases at the end, the concentration changes in 4N and 8N that are in the shallow aquifer reflect the contamination of the shallow aquifer, added to the above, this is consistent with the CE ion network and the salts, minerals, metals and any other organic or inorganic compound (Fig. 3 and table I).

The oxidation / reduction potential in units of milliVolts (mV) starts low at 3P (209mV), the high values are represented at the point 5P (284 mV), 4N (210mV), 8N (312 mV) and concluding the last point with a low concentration in 2P (132mV), see (Fig. 3 and table I).

The samples analyzed in the laboratory must comply with the principle of electroneutrality, the sum of the charges of all the cations must be equal to the sum of the charges of all the anions, and they must be electrically neutral (Fitts, 2002). To check the accuracy of the analysis of the majority ions, the ionic balance is performed (Eq. 1)

\(\% Electroneutralidad=\frac{\left(\sum Cationes-\sum Aniones\right)}{\left(\sum Cationes+\sum Aniones\right)}\times 100\) (Eq. 1)

Cations and anions are expressed in meq/L. (Appelo 1996; Postma 2005). For the analysis of a sample to be valid, the amplitude of variation of the electroneutrality percentage must be ± 5%; although an amplitude of up to ± 10% can be accepted (Freeze and Cherry 1979). In this case, the interval ± 10% was considered (table II).

The predominant water family of the analyzed samples presents 55.2% of bicarbonate-calcium composition (Ca-HCO₃), bicarbonate-sodium (Na-HCO₃) second place represents 37.9% of the total samples, and 6.89% of chloride-calcium composition (Ca-Cl) (table II), in coherence with the nature of the fractured and carbonate volcanic rock of the main permeable materials existing in the center of the VSLP.

The aforementioned water families reflect that the main supply corresponds to the Santiago River and the salt content corresponds to the interaction of bicarbonate-sodium water where the water movement is in contact with the fractured volcanic material, sand, and silt, so the flow of this may have hydraulic communication with the aquifer of the granular medium, which feeds the wells and rests on the hardened soil or “tepetate”. The bicarbonate-calcium family may represent the interaction of the water with the granular material that filled the tectonic pits and the chloride-calcium family may be due to the agricultural activity developed in that area.

The chemical composition of groundwater is the result of continuous processes of interaction between the meteoric water that infiltrates the ground and circulates through the different subsoil materials. Part of the chemical components is acquired in the recharge zone, while others are acquired along the way and are captured by wells and wells in the discharge zone, resulting in different hydrogeochemical signatures, known as water families (types), which are represented in Piper's diagrams (1994). In this diagram, it is also possible to recognize water-rock interaction processes and, in some cases, ion exchange, especially in the samples from the deep well regions. The bicarbonate-calcium family is represented by the pink diamond, located in the east and center of the VSLP, indicating samples that passed through carbonate rocks (1P, 2P, 3P, 7P, 11P, 12P, 13P, 14P, 16P, 19P, 20P, 21P, 22P), (4N, 23N, 26N and 27N), processes of water mixtures found in the center of the diamond, and samples with a direction towards the south of the gray diamond are also observed: this represents an ion exchange corresponding to samples (3P, 5P, 6P, 10P,15P, 17P, 28P, 29P), (8N, 24N, 25N,) from deep wells and only two chloride-calcium samples correspond to (9P and 18P) from 280 and 350 m depth cutting granular material that filled the valley, well 9P is located in the center of the city of San Luis Potosí and well 18P to the NE of the valley in the Agronomy school (Fig. 4).

Ionic ratios are used to support the identification of possible groundwater origins or mixing processes occurring in the subsurface, the Na^+ 2K⁺ vs. Cl⁻+SO₄ ^− 2 ratio is proposed by Mifflin (1988) to identify the presence of regional flows, as well as in the characterization of some evolutionary processes of geogenic or anthropogenic origin, so it was used in the identification of these for the study area (Fig. 5). In this study, most of the samples are found in the local flow area and 5 in the intermediate flow, which means that the water is of recent infiltration and that it has had little chemical evolution as well as its water-rock interaction.

Table III shows the results of nitrogen and phosphorus contents of the sampled works (wells and wells) and localities referred to in the maps. Table IV, the results of bacteriological contents, and table V, the contents of trace elements and metals.

Nitrogen and phosphorus content

Sample 8N (table III) is the only one with anomalous nitrogen content and belongs to a 3 m deep private well located in settlement 3era Chica in the El Saucito zone, north of the VSLP. It presents a concentration of 15 mg/L of nitrate nitrogen (N-NO₃), which exceeds the NOM-127-SSA1-1994 since its maximum allowable limit is 10 mg/L.

Bacteriological Results

Regarding the biogenic content in the sampled waters related to anthropogenic activity, the results are shown in table IV.

Of the seven wells sampled, only three show high biogenic contents related to anthropogenic activity. These are noria 4N with a concentration of 2 NMP/100; norias 24N and 27N with concentrations of 11 NMP/100 and 5NMP/100 respectively.

Concerning the wells, the presence of fecal coliforms is documented in wells of the deep free aquifer of the VSLP. Of the 29 wells studied (100%) 12 samples (41%) were found to be out of the norm, These are: well 2P with a concentration of 3 NMP/100 (most probable number per 100 ml); well 3P with a concentration of 8 NMP/100 (from Morales park); well 12P with concentration 11 NMP/100, wells 15P and 16P with a concentration of 11 NMP/100; well 17P with a concentration of 14 NMP/100; wells 18P and 19P with a concentration of 5 NMP/100 and 17 NMP/100, respectively; well 20P with a concentration of 11 NMP/100; well 22P with a concentration of 5 NMP/100; and well 28P, with a concentration of 11 NMP/100.

Fats and oils

The measurement of fats and oils does not measure a specific substance, but a group of substances with the same physicochemical characteristics (solubility). Therefore, the measurement of fats and oils includes fatty acids, soaps, fats, waxes, hydrocarbons, oils, and any other substance that can be extracted with hexane, and its permissible limit must be 26 mg/L according to NOM-002- SEMARNAT-1996.

Therefore, almost all the samples show low concentrations of fats and oils, although they present a small amount.

Metals and trace elements

Table V documents the contents of metals and trace elements measured in the sample studied. In this table, it is highlighted that the samples out of range correspond to manganese, arsenic, cadmium, and mercury.

For manganese, only one sample exceeds this permissible limit, and that is well 18P, with a concentration of 0.84mg/L.

For arsenic: well 24N (in Soledad) with a concentration of 0.05 mg/L, wells 12P with a concentration of 0.126 mg/L, and 18P with a concentration of 0.09 mg/L, are the ones with contents that exceed the permitted levels (Fig. 6a).

For Cadmium: the samples that exceed the limit are: wells 24N and 26N. 24N and 26N with concentrations of 0.0067mg/L and 0.0119mg/L respectively. Wells 11P with concentration of 0.0169mg/L; 12P with concentration of 0.0206mg/L; P14 with concentration of 0.005 mg/L; 17P with concentration of 0.0085 mg/L; 20P with concentration of 0.01068 mg/L; and 29P with concentration of 0.00524mg/L. (Fig. 6b).

For Mercury, the samples that fall outside the norm are: wells 23N with a concentration of 0.004mg/L and 25N with a concentration of 0.002mg/L. Wells 11P with a contraction of 0.002mg/L; 18P with a concentration of 0.0015 mg/L; 20P with a concentration of 0.002 mg/L; and 22P with a concentration of 0.0012mg/L (Fig. 6c).

(Moran 2010) and (Lopez et al., 2013), studied water quality in this shallow (hanging) aquifer, detecting, in general, significant levels of nitrates, sulfates, chlorides, and, in the urban zone, punctual anomalies of heavy metals (mercury, barium, strontium, cadmium, lead, phosphorus, and silver). These authors report the highest anomalies in heavy metals for the industrial zone, and in places where irrigation is done with recycled water from a water storage tank from the industrial zone. The distribution of the wells sampled in these studies coincides, roughly, with the distribution of the ground settlement faults that have been documented in the valley (Risk Atlas of the Municipalities of San Luis Potosí and Graciano Sánchez; Institute of Geology, UASLP), which suggests that, if the lower boundary of this aquifer has been fractured, water contaminated with these heavy metals is possibly infiltrating into the deep free aquifer housed in the granular medium that fills the valley.

The results of the sampling and analysis of water from 22 wells and 7 wells in this study show anthropogenic contamination. The anthropogenic contaminants (fecal coliforms, total coliforms, and trace elements) have a direct relationship with drainage systems, and the sites where contents exceeding the permissible limit by NOM-127-SSA1-1994 for human consumption were detected in groundwater may indicate areas where there is an advanced deterioration of the drainage pipes, and which also have conduits for percolation to depth.

In the area of El Saucito, one of the settlement structures that have been monitored for almost 30 years is located, and affects a large number of neighborhoods in the northern part of the city, some of which were urbanized using drainage materials that are very susceptible to fracture in the event of a land settlement phenomenon, which could be a source of severe contamination to the shallow hanging aquifer, and, according to these results, infiltrate to the deep free aquifer. On the other hand, the Santiago River with a distance of approximately 12.7 km (Cuevas, 2015) is the natural riverbed of the city, which historically represented the natural edge that limited the growth of the urban stain and since then flooding causes road safety and environmental health problems; the fracturing of the land within this river plays an important role in flooding time. During the rainy season, part of the water infiltrates into the San Miguelito mountain range and the other surface water flows down and feeds the Santiago River following its course from east to northeast, connecting with the rest of the territory and the wastewater treatment plant of the Tenorio Tank located in the industrial zone, joining in a single flow to the northeast, passing by the side of the Agronomy School where well #19P and well #18P are located, These wells are contaminated with fecal coliforms and total coliforms, and the wells located in Soledad also have anthropogenic contamination.

It is safe to assume that both factors: land settlement and rupture of old drainage may be contributing to the contamination of two aquifers in the San Luis Potosi Valley. In this case, the ground settlement structures that fracture the cemented and hardened material that supports the shallow hanging aquifer are more towards the central-eastern part of the valley, which could facilitate the infiltration of contaminated water towards depth.

Nitrogen and Phosphorus

Nitrate and nitrites ion occur in soils and water as part of the nitrogen cycle in the earth. Nitrate constitutes the major total amount of nitrogen available in surface waters. Nitrogen occurs naturally in soils, typically bound to organic and mineral matter in the soil. Life depends, among other things, on the proportion of nitrogen (N) and phosphorus (P) that is available in the medium. Normally there is much more nitrogen than phosphorus Yan et al. (2016). However, at concentrations that exceed the norm of nitrates, they can cause methemoglobin, that is, oxygen deficiency in the blood, causing death. In this study area, only one 8N sample presented a concentration of 15 mg/L as shown in table III.

The biogenic contaminants have a clear relationship with human wastes that are normally channeled through the drainage system and are driven to their dispersion towards the NE part of the VSLP in the cultivated areas in that sector of the valley. In this sense, the dispersion of organic pollutants should be preferentially manifested towards that part, and nevertheless, the location and the values found in the wells of the central part, where most of the population and commercial activity is concentrated, allow us to suppose that there is also percolation of drainage wastes towards the hanging aquifer and that this occurs due to rupture of the drainage pipes. A major problem of this situation lies in the possibility that this biogenic contamination penetrates the deep free aquifer because part of the distribution of drinking water comes from that aquifer; This possibility has been confirmed in this study, at least in one of the wells (well 2P), which contains a high concentration of total coliforms (up to 150 NMP/100), and according to NOM-127-SSA1-1994, the permissible limit for total coliform organisms in water for human consumption is 2NMP/100 (most probable number per 100ml), although most of the wells sampled have contents of < 3 NMP/100. This contamination is already reflected as increasing in 13 more wells (values between 5 and 16 NMP/100; see Table IV), covering a very wide area. The contrast of the area where this type of contamination with a clear anthropomorphic relationship is detected, with the system of ground settlement faults in the VSLP, suggests that both phenomena have a connection, given that this differential subsidence is linked to rupture of the "tepetate" layer that supports the shallow hanging type aquifer that is known to be contaminated (Moran 2010, Lopez et al., 2013). The percolation conduits to the deep aquifer are constituted by these faults. The well with a high value of these contaminants is located in an area where the greatest development of ground settlement has been detected, the most representative evolution of which is the Aeropuerto fault, monitored for more than 20 years. Table IV presents the results of total coliforms in the wells and wells studied in the VSLP, and these values show that only one well (well 2P) contains a high concentration of total coliforms (up to 150 NMP/100), and according to NOM-127-SSA1-1994, the permissible limit for total coliform organisms in water for human consumption is 2NMP/100 (most probable number per 100ml). However, 13 more samples (44%) have concentrations between 5 and 16; 8 of these (wells 15P, 16P, 17P, 18P, 19P, 20P, and 22P) have concentrations between 11 and 17 NMP/100. The other 15 samples have < 3 NMP/100. The bacteriological results of the studied samples reflect the low quality of water for supplying the population in the valley. As for the wells, well 24N has a concentration of 11 NMP/100 and well 27N has a concentration of 5NMP/100, both located in Soledad.

Permissible limits Non-detectable fecal coliform organisms NMP/100ml

For example, well 3P with a concentration of 8 NMP/100, well 12P with a concentration of 13 NMP/100, wells 15P and 16P with a concentration of 11 NMP/100 and 16 NMP/100, well 17P with a concentration of 14 NMP/100, in well 18P with concentration 10 NMP/100, in well 19P with concentration 17 NMP/100, in well 20P with concentration 16NMP/100, in well 22P with concentration 7 NMP/100, and well 28P with concentration 11NMP/100. (Table IV). The other 15 samples have < 3 NMP/100.

The bacteriological results reflect the low quality of water for supplying the population in the valley. The noria 24N has a concentration of 11 NMP/100, and the noria 27N has a concentration of 5NMP/100.

Concerning fats and oils, their detected content varies in value from 2.65 to 7.7, and the concentration increases in the wells towards the center of the valley following the direction of the subway flow. Of the eight wells where fats and oils were detected in the water, all have low concentrations (between 2.65 and 7.70 mg/L) concerning the NOM-002-SEMARNAT-1996 standard, which establishes 26 mg/L as the permissible limit. Only four of the water samples have concentrations above 6.2 mg/L, but all of them show contamination in the process because since fats and oils are a product of the anthropogenic activity, the fact that they are present in the wells reveals that they have percolated into the deep aquifer. This is supported by two important factors: the central zone of the VSLP has the highest concentration of ground settlement faults. This is logical since in the center of the city of San Luis Potosí there are restaurants, hotels, public bathrooms, the train station, etc., and the concentration increased towards the wells in the center following the direction of the subway flow.

Another factor to consider is the presence of trace elements (with some heavy metals) in the wells of the deep free aquifer. The values detected are presented in table V.

Manganese (Mn) is considered a mineral associated with igneous and metamorphic rocks containing divalent manganese as a minor constituent; in particular, it is significant in basalt, due to its dominant mineralogy of olivines, pyroxene, and amphibole. Small amounts are present in dolomites and limestones replacing calcium (Hem 1985). Being the previous source, the main responsible for the contribution of Mn in the study area. This element is normally found in the organism, being an activator of certain enzymes. When ingested in large doses, it is a poison that mainly affects the central nervous system; in appreciable quantities, it produces an unpleasant taste in the water, which makes its presence noticeable when drinking and its toxic action more easily avoided (Catalan 1981).

The permissible limit for human consumption according to NOM-127 is 0.15 mg/L; one sample exceeds this permissible limit; well 18P, with a concentration of 0.84 mg/L (Fig. 6a).

Arsenic is important in water chemistry, especially since the modern use of pesticides has become widespread since these products contain this element. Arsenic is found free in nature as a steel-gray, brittle solid (Catalan 1981). High concentrations of arsenic are commonly associated with sediments partially derived from volcanic rocks of acid or intermediate composition (Appelo and Postma 1996). The presence of this element is likely due mostly to the occasional input of arsenic-containing fertilizers; however, it may also be due to the dissolution of arsenic-bearing volcanic rocks within the study area.

Long-term exposure to arsenic via drinking water at concentrations of 0.05 mg/L and even lower, causes skin, lung, bladder, and kidney cancer and skin alterations such as pigmentation changes and thinning of the skin. Immediate symptoms of acute poisoning include vomiting, abdominal pain, and hemorrhagic diarrhea (Revuelta 2003). The permissible limit established for arsenic (As) in NOM127-SSA1-1994 is 0.05 mg/L. The samples out of range are: the 24N Soledad waterwheel with a concentration of 0.05mg/L. Wells 12P with a concentration of 0.126mg/L, 18P with a concentration of 0.09mg/L. (Fig. 6a)

Cadmium is not considered, biologically, neither beneficial nor essential for man, but a toxicant that acts on the kidney and liver producing nausea and vomiting. Cadmium poisonings produce arterial hypertension and it is a proven carcinogen (Catalan, 1981).

As for its presence in groundwater, its probable source is of external origin and it is not so much associated with the dissolution of minerals containing cadmium in their composition. Water contaminated by cadmium generates corrosion of the pipes used because cadmium is a contaminant of galvanized iron and zinc (Catalan 1981).

The permissible limit for Cadmium (Cd) in human consumption is 0.005mg/L according to NOM-127-SSA1-1994, the samples that exceed the limit are the waterwheels. 24N with a concentration of 0.0067mg/L and 26N with a concentration of 0.0119mg/L. Wells 11P with a concentration of 0.0169 mg/L, 12P with a concentration of 0.0206 mg/L, P14 with a concentration of 0.005 mg/L, 17P El Ranchito with a concentration of 0.0085 mg/L, 20P with a concentration of 0.01068 mg/L, and 29P with a concentration of 0.00524 mg/L. (Fig. 6b).

These values indicate that for the shallow aquifer, the zone where the N24 wells are located, the contamination by this metal in the shallow aquifer is only slightly higher than the standard (0.0067 mg/L), while for N26 it is double this value (0.0119 mg/L). Wells P14 and P29 show incipient values, or rather, at the limit of the standard (0.005 and 0.00524 mg/L, respectively). Wells P11, P12, and 20 show values twice to four times the norm (0.01068, 0.01690, and 0.02060 mg/L), which is highly worrisome.

Mercury is one of the most widely distributed metals in the environment and is known for its high toxicity (mainly methylmercury); it is harmful to the environment and can bioaccumulate. It can come from both natural and anthropogenic sources. It is released naturally by mobilization generated in the earth's crust, by volcanic activity, or by rock erosion. Anthropogenic sources are associated with the use of fossil fuels and the mining industry. An important source is currently represented by the remobilization and release of waste deposited in soils, sediments, water bodies, and garbage dumps, as well as the incineration of municipal, medical and hazardous waste, in addition to cremations and releases to the ground in cemeteries. (Zambrano et al., 2014).

The maximum allowable limit for Mercury (Hg) is 0.001 mg/L. The samples that exceed this limit are wells 23N with a concentration of 0.004mg/L and 25N with a concentration of 0.002mg/L. Wells 11P with a contraction of 0.002 mg/L, 18P with a concentration of 0.0015 mg/L, 20P with a concentration of 0.002 mg/L and 22P with a concentration of 0.0012 mg/L (Fig. 6c). From the distribution of the wells and wells, and the values recorded for these trace elements in the sampled waters reported here, it is inferred that they come from the leaching of sediments derived from the historical metallurgical processes of mining in Cerro de San Pedro due to the distribution of elements (Mn, As, Cd, Hg) that were detected in this area.

In the Risk Atlas for the municipalities of San Luis Potosi and Soledad de Graciano Sanchez (the northeastern part of the valley), a series of ground settlement fault segments are recorded, which may represent infiltration conduits for water into the deep aquifer. Given that the maximum concentration detected (0.004 mg/L) is from well 23N, and that well 20P located in the same area of the valley has (0.002 mg/L), double that allowed by the standard, it can be argued that there is infiltration of this element from the shallow aquifer to the deep aquifer. However, for well 11P located in the NW of the VSLP, outside the influence of the zone of influence mentioned here, the possible contamination factor is the Peñazco municipal dump, located in that sector, given that land settlements also occur in that area, which is why it is known as the "Tierra Rajada" site (to NW of the valley).

The most common damages occur in: the nervous system, affecting brain functions, which can cause degradation of the ability to learn, personality changes, tremors, vision changes, deafness, muscle incoordination and memory loss; DNA and chromosomes; allergic reactions, skin irritation, tiredness, and headache; negative effects on reproduction; with changes in sperm, birth defects and miscarriages (Zambrano et al., 2014).

The detected values of the aforementioned trace elements represent contamination levels because they exceed the permissible limits of the NOM-127-SSA1-1994 standard, the results of trace elements represent a danger to groundwater, nitrates, fecal coliforms, and total coliforms, with concentrations lower than those reported (Moran 2010, Lopez et al., 2013) in the superficial aquifer, are evidence of infiltration of water from the hanging aquifer into the deep free. These results corroborate that part of the nitrate, fecal coliform, total and trace coliform contaminated waters in the shallow hanging aquifer reported in previous studies (Moran, 2010, Lopez et al., 2013), are infiltrating into the shallow free aquifer, through the lattice of ground settlement faults in the valley. The extensive study of the waters of the deep aquifer wells allowed us to verify that the waters of the intermediate-free aquifer are showing signs of anthropogenic, biogenic, and heavy metal contamination. This is of great importance especially if the use of water from these wells is reviewed about human consumption.

The trace elements in the sampled waters reported here are inferred to come from the leaching of sediments derived from the historical metallurgical processes of mining in Cerro de San Pedro due to the distribution of elements (Mn, As, Cd, Hg) detected in this area. It is assumed that there may be natural recharge to the deep aquifer in the northeastern margin of the valley, but the record of recent settlements that have been documented allows us to suppose that this process may also be recent or its dilutional effect may be added if the first possibility has been active. That will require other studies not carried out here, which we can assume soon.

The results obtained in the present study document that there is evidence of contamination derived from anthropogenic activity in the waters of the deep free aquifer of the San Luis Potosi valley. Organic contaminants, E. colli, and total coliforms, which are widely present in the shallow aquifer, have percolated into the deep aquifer; this contamination is particularly prevalent in well 2P, located in the area with the longest history of land settlement in the VSLP.

Contamination by fats and oils, being products directly derived from the anthropogenic activity and with a strong presence in the shallow aquifer, their presence in the wells (although with discrete values), indicates that their infiltration into the deep aquifer is already in process.

Nitrate levels in drinking water are due to groundwater contamination from animal waste or water spills from dairies or livestock, excessive use of fertilizers, or infiltration of human sewage from septic tanks. Microorganisms present in the soil, water, and sewage transform nitrates into nitrites.

The incipient and localized manganese contamination in groundwater in the VSLP is inferred to come from water-rock interaction by rainwater drainage in the limestones and dolomites that outcrop extensively in the Sierra de Alvarez, which borders the valley on its eastern side.

The presence of arsenic in the groundwater of the VSLP may be contributed by the interaction of water with arsenic-bearing volcanic rocks within the study area, or, arsenic residues in the "Jales" derived from mining activity (e.g., old tailings from the Cerro de San Pedro mining center). Another common source is the occasional contribution of fertilizers containing arsenic in their composition.

Cadmium contents in three of the sampled wells (wells P11, P12, and P20) show values twice to four times the norm (0.01068, 0.01690, and 0.02060 mg/L), which is a priority to be taken into account as a prevention to avoid possible effects on human health.

From the distribution of mercury concentrations in the wells and wells, in the sampled waters reported here, it is inferred that the contamination of this element comes from the leaching of sediments derived from the historical metallurgical processes of mining in Cerro de San Pedro due to the distribution of elements (Mn, As, Cd, Hg) that were detected in this area. Other possible anthropogenic sources would be the re-mobilization and release of contaminants by the passage of water through sanitary waste tanks, and by the incineration of municipal, medical and hazardous waste, in addition to cremations and releases to the ground in cemeteries (Zambrano et al., 2014).

In summary, groundwater contamination that has been widely documented for the shallow aquifer is generating the contamination in the deep free type aquifer, from which part of the drinking water needs for the population is covered. The results reported here document that contaminants are of two main types: biogenic and trace elements. The former is directly related to the area occupied by the urban zone, and may be related to the rupture of pipes due to the natural wear of old construction materials for sanitary drainage; the latter, to factors that include anthropogenic industrial activity, natural drainage of old tailings from mining activity, the influence of sanitary landfill sites, and the influence of agricultural activity zones.

Combined with the results of the present study with previous studies reporting water contamination in the shallow aquifer hanging in the valley, and, related to the documentation and/or mapping of numerous ground settlement faults in the central part of the valley, it is possible to assume (which is proposed here) that if there is infiltration of contaminated water from the shallow aquifer into the deep aquifer through these faults that control ground settlement, which was initially considered unclear or of no concern, it is argued here to be a priority issue because of the possible implications for both the drinking water supply to the population and the long-term (or short? ) in public health. Therefore, a broader study of the waters of the latter aquifer is considered pertinent for more complete documentation of what is reported here.

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Tables I to V are available in the Supplementary Files section.

No competing interests reported.

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Anthropogenic Contamination in the Free Aquifer of the San Luis Potosi Valley

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Abstract

Figures

Introduction

Location

Geology

Materials And Methods

Sample analysis

Results

Nitrogen and phosphorus content

Bacteriological Results

Fats and oils

Metals and trace elements

Discussion

Nitrogen and Phosphorus

Conclusions

References

Tables

Additional Declarations

Supplementary Files

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