Groundwater runs through pores and fissures of rocks that are below the ground ( Su et al., 2020), rain is the main source of food for this resource, whose particularity is that, being below the earth’s surface does not evaporate ( Zárate et al., 2021). In this sense, the impact of groundwater is considered as a safe source of human consumption, and also its treatment is economical ( Adimalla & Qian, 2019). In this context, 2.5 billion people meet their water consumption needs through the use of groundwater, the percentages of use of this type of water are considered to be 42 %, 36 % and 27 % for agricultural, domestic and industrial purposes respectively ( Sajjad et al., 2022).

In this context, it is important to know that not all groundwater is renewable, although in many parts of the world, humans use the resource in an accelerated manner without taking into account the time it takes to replenish this resource ( Agudelo Moreno et al., 2020). On a global scale, population expansion presents itself as a latent threat to aquifers, due to the development of human settlements, industries, food demand, increasing agricultural activities and the development of mining industries ( Anomohanran, 2015; Sajjad et al., 2022; Smith et al., 2020; Uddin et al., 2021).

On the other hand, groundwater quality is commonly associated with chemical concentrations of different types, so the presence of nitrates, heavy metals and pathogens represent some kind of contamination in the water ( Tafoya-Hernández et al., 2022). In this sense, agrochemicals bring concentrations of nutrients to the soil, these substances dissolve when water is infiltrated, and they will hit the aquifers, the concentration will depend on fluctuations in the water level, seasonality and uneven terrain ( Gutiérrez et al., 2021; Navarro et al., 2021). In Latin America there are examples such as the agricultural region of Mato Grosso, Brazil, where soybean, cotton and corn crops are grown, here studies were made in the drainage area of the São Lourenço river, finding pesticides and other compounds such as nitrates in rain and groundwater samples ( Casara et al., 2012). In Ecuador, studies have been made of the impact that nitrates could have on groundwater, but this in low basins such as the Daule River in the province of Guayas ( Ribeiro et al., 2017). In this sense, Ecuador has legislation in favor of the prevention and conservation of underground water resources, but, even when these laws exist, principles of protection and conservation are not taken into account ( Baque et al., 2016; Sandoval & Günther, 2013). Most of Ecuador’s efforts are in the study of surface waters ( Carrera-Oña et al., 2020; David et al., 2015; Guananga et al., 2022; Villamarín et al., 2014), leaving a significant gap in the study of groundwater in the Andean areas.

On the other hand, the Chambo aquifer located in the central area of Ecuador, provided 600 l/s of water for human consumption to the populations of Riobamba and Guano, most of the exploitation wells are in the sector of Llío and San Pablo, This area is characterized by being in the lower part of the slopes of the volcano Chimborazo, all this sector is of agricultural production characteristics, since there are aplias areas of crops and livestock ( Chidichimo et al., 2018).

In this context, this paper analyzes the possible sources of nitrate pollution in the groundwater of the Llío and San Pablo sector. This using water analysis data for the period 2016 to 2020.

Groundwater therefore represents an enormous resource that can only be managed if the behaviour of the aquifer with respect to the pollutants to which it is exposed is known, Therefore, this work analyzes nitrate concentrations in groundwater in the Llío and San Pablo sector in the period 2016-2020, to identify whether or not there is pollution with respect to Ecuadorian legislation.

The work was carried out in the volcano-sedimentary basin of the Chambo River, in the province of Chimborazo, in the so-called Chambo aquifer ( Figure 1).

To characterize the study areas, a cross-reference of bibliographic information was made, in addition to cartographic information, allowing us to know the areas of possible collection and also the flow of groundwater and the recharge zones of this ( Chidichimo et al., 2018; Mendoza, 2015; Procel, 2018). Once this area was identified, the geological and stratigraphic description was made, determining the type of structure and rocks found in the subsoil of the aquifer, allowing to relate the composition of the rocks with the presence of nitrates in the water of Llío and San Pablo. In addition, the results obtained in the study of the Guano and Chibunga rivers ( Mendoza et al., 2021), which determined the field and infiltration capacity, were used as a complementary part to understand the movement of water, to relate the presence of nitrate concentration in the infiltration process.

To define the anthropic areas and activities, the surrounding area was taken into account, indicating which are the direct and most intense anthropic activities that can influence the study area. These anthropic areas were determined by cartography, this information was used from several works carried out in the area of influence ( Chidichimo et al., 2018; Mendoza, 2015; Mendoza et al., 2021; Procel, 2018).

The water analysis results obtained from 2016 to 2020 were used to determine the behaviour of nitrate concentrations in water, taken from the analysis reports provided by the Environmental Services Laboratory of the National University of Chimborazo. In addition, laboratory analysis results of EP-EMAPAR from 2020 were used ( Mendoza et al., 2023).

The nitrate determination of groundwater by the Environmental Services Laboratory was carried out in accordance with the Standardized Methods for Examining Wastewater and Water ( Bluett et al., 2022; Bodrud-Doza et al., 2019; Daghara et al., 2019; Rice et al., 2017), and developed according to the following procedure ( Hach Company, 2000): 1) Shake the water sample for 1 minute to homogenize, 2) take an aliquot of 25mL of the sample to be analyzed in the glass cell (code HACH-20950-00), 3) add the contents of a bag of nitrate reagent powder NitraVer 5 (code HACH-14034-99) to the cell, cover, 4) shake the cell with the sample for one minute, 5) after shaking the sample let stand for five minutes, 6) take an aliquot of 25mL of deionised water in the glass cell (code HACH-20950-00) for use as a target, 7) place the analysis code for Nitrates N03-N on the HACH UV/VIS DR5000 instrument, leaving the wavelength of the equipment at 500nm, 7) place the cell with deionised water on the computer and press ZERO, 8) place the cell with the sample on the computer and press READ, 9) record the data.

Once the geological strata where the sampling points of the slope and the wells are located were identified, these were classified by the geological and stratigraphic composition. Afterwards, the results of water analysis of Llío and San Pablo were interpreted according to their geochemical composition, to determine the type of water and its origin, this through the diagram of Piper ( Albo & Blarasin, 2014) and geological description made by Salguero (2017) and Procel (2018). The anthropic activities present in the area of influence were also identified and the nitrate concentration data were obtained temporarily. At the end, the comparison was made with respect to the norms that show admissible levels of nitrates shown in Table 1, for water for human consumption that must meet quality standards to avoid risks of diseases ( Baque et al., 2016).

The work was carried out in the volcano-sedimentary basin of the river Chambo, in the inter-Andean depression, in this area the mesozoic basement is covered by sedimentary sequences, pleocene and pleistocene, constituted in the region by the Jurassic rocks of the Alao Paute Unit, Guamote, Daldal River, San Pablo de Sali and the Cretaceous rocks of the Peltetec Unit and Yunguilla Formation ( Chidichimo et al., 2018; Mendoza, 2015; Procel, 2018). The units that form the base of the lower basin of the Chambo River, are located in the Cordillera Oriental, with respect to the study area that is the sector of Llío and San Pablo.

As described by Procel (2018) in the area there are three aquifers, of the multilayer type, called Llío-Guano, Riobamba and Yaruquíes. The Llío-Guano Aquifer is located near the Chimborazo and Igualata volcanoes, this aquifer is of the groundwater type (free), with double porosity, average width of 200 m, associated with the volcanogenic deposits of the Chimborazo (upper) and Cizarán (lower) formations. The average flow rate is 0.27 m ³/s, of which 0.039 m ³/s correspond to the flow rates of the wells located in Llío and Guano. The rest, 0.23 m ³/s, corresponds to the source San Pablo, located approximately one km southwest of the wells of Llío. These wells and the spring are the source of water for public supply in the city of Riobamba and Guano. The hydraulic conductivity is in the order of 6.1x10 ^-6 m/s and the average water table level is 65 m, in the wells located in the Chimborazo Formation, and 140 m in the drilled in the Cizarán Formation. The predominant direction of water flow is from northwest to northeast (from Chimborazo to the Chambo River) and from north to south (from Igualata to the Guano River) ( Procel, 2018).

On the other hand, the stratigraphy of the area of Llío and San Pablo, shows that the area of the wells mostly consists of permeable areas, since it contains lithological profiles of sand, thick gravels and stones. In addition, it has semipermeable areas composed of gravels, clayey sandstones and quartzites, healthy schists and altered granite. Waterproof areas are presented in the form of healthy lavas and granites. The San Pablo slope presents permeable areas with the presence of medium and thick sand, pebbles, coarse gravel and sandy brown clay, in addition to waterproof areas that represent sands and gravels, clay sands and quartzites ( Mayorga, 2020). In this context it was identified that the main stratigraphy of the study area presents fine, medium and thick sands, coarse gravels, angular gravel, ridges, clay sandstones, sandy brown clay, quartzes, healthy schists, altered granite, healthy granites, coarse granite, medium granites, lavas ( Mayorga, 2020; Procel, 2018)

To describe the field capacity and infiltration, the area of influence was first identified ( Figure 2), crossing information between: type and use of the soil, vegetation cover, geology and geomorphology of the area. Determining according to the geomorphology there is incidence of two sources: from the volcano Chimborazo and from the volcano Igualata.

Identified two areas that do not allow infiltration such as bodies of water and snow. In addition, three types of areas in the upper parts of both volcanoes: paramo up to 4000 m.s.n.m, cultivated grass and short-cycle crops, this in the range of 4000 to 3200 m.s.n.m. In this context it is shown that there is great capacity and infiltration in this area ( Table 2) since we have an average of 112.04 mm/h ( Mendoza et al., 2021). The hydrogeochemical classification of groundwater considered three samples from Llío #5 well and San Pablo spring collected between December 2017 and August 2019 ( Table 3).

The contrast of information from the results of the physical-chemical and chemical analyses in samples from the well and the spring indicated the occurrence of a predominant type of water with the presence of calcium-magnesium bicarbonates in Llío and San Pablo ( Figure 3).

To analyze the variation in nitrate concentration, the pumped water collection system of EP-EMAPAR and the San Pablo spring were considered ( Figure 4). As seen in the figure the wells are located in an area with anthropic influence, since around the two sources there are areas of cultivation and pasture.

In this context, from January 2016 to December 2020, monthly sampling of water from wells 1, 2, 3, 4, 5, 6 and 7 of Llío and the San Pablo spring was carried out ( Table 4), these analyses were carried out in the Environmental Services Laboratory, EP-EMAPAR Laboratory. As shown in Figure 5, it is evident that there is variation in nitrate concentration, in addition the values do not exceed the maximum permissible limit of standards: INEN NTE 1108 (INEN, 2020), Guidelines for the quality of water for human consumption (WHO, 2018), Ministerial Agreement No 097 (MAE, 2016).

Although there are no values that affect the health of people who consume water from this pumping system, differences in nitrate concentration were found each year ( Figure 6), by 2016 there were a minimum of 1,74 mg/L and a maximum of 18,12 mg/L, for 2017 the minimum is 1,90 mg/L and the maximum is 21.96 mg/L, for 2018 the minimum is 1,21 and the maximum of 17.68 mg/L, in 2019 the minimum is 1,00 mg/L and the maximum of 25,90 mg/L. The average annual value is 7,4 mg/L among all the years of study, the standard deviation of the obtained results is 3,75 mg/L, this implies a very high value in relation to errors in the measurement in laboratory or in the collection of samples.

To establish the natural concentration of nitrates in the study area, we describe the microscopic analysis of the lithology of the influence zone described in the “Geological-structural levation of the area covered by the topographic sheet of Guano scale 1:50,000. Chimborazo Province” ( Salguero, 2017). First, lava flows are observed that have similar minerology, with serial dimensional porphyric texture and are composed of crystals of: 1.

“Plagioclase (30%) idiomorphic to subaiomorphic, with Carlsbad type maclas, with structures in gulfs and reaction edges, granulometric variable, in sizes from 0.6 to 2.5 mm and with a composition of andesin and oligoclase (for smaller crystals)”.

These andesitic lava structures are immediately arranged over the avalanche deposit of debris from the eruption of the Chimborazo volcano and contain only thin layers of regolith and soil. These remains of andesitic lava were deposited and filled with ravines and ancient valleys, formed inside the deposit of the avalanche of debris from the eruption of the Chimborazo volcano ( Procel, 2018) ( Salguero, 2017).

As shown in the description of Salguero (2017), the type of rocks and their composition shows mostly rocks composed of silica, calcium, sodium, aluminum, magnesium, manganese, lithium, iron, iron oxides. In addition, the geochemistry of the water samples presented in paragraph 4.6 also shows that the water present in Llío well 5 and San Pablo spring have an abundance of calcium bicarbonates-magnesium, so naturally Nitrates could not originate in the geological complex that was studied, passing the process of nitrogen formation by the presence of organic, atmospheric and anthropic components.

From the above it is evident that the major influence towards the area of Llío and San Pablo is given by the topography of the area. In this case, both the upper part of the Chimborazo volcano and that of the Igualata volcano influence the flow of surface and underground water, and therefore the infiltration of water in this area.

Figure 2 shows paramo areas in each area of the volcanoes which causes water retention and great infiltration power to the aquifer in this area, we could say that these two areas are the water recharge areas for this groundwater system. In addition, it is observed in the intermediate part before the upper limit of the aquifer of the same Figure 2, areas with cultivated grass and crops of short cycle. These areas have as main property the presence of organic matter in the upper part of the soil ( Mendoza et al., 2020). Thus, the aforementioned can be said that the nitrates present in the surface layers of the soil in the study area is due to the effect of the nitrogen cycle ( Holloway & Dahlgren, 2002). Therefore, the infiltration capacity of the study area allows the nitrogen compounds to move into the aquifer. In addition, the rainy season in the area should be taken into account ( Chidichimo et al., 2018), since from March to May there is a greater amount of precipitation so it influences the washing of the soil and the drag of all types of compounds into the water table. All this group of details present in the area are those that probably affect the groundwater and its concentration of nitrates of Llío and San Pablo.

Geology, stratigraphy, lithology in conjunction with the study of field capacity and infiltration allow to know the area of physical influence of Llío and San Pablo, as shown in the document volcanic structures are observed, lava flows and avalanche deposits that drastically influence the infiltration and movement of surface and groundwater. In determining the zone of influence, it was determined that the upper parts where the moors are present are the areas where the aquifer is recharged as they receive water from the precipitation and in addition to the melting of the Chimborazo volcano.

The geochemical study allowed to know the characteristics of the water obtained in the sector of Llío and San Pablo, according to the reported analyses and its interpretation in relation to the ions and cations present in this was determined to have water characteristics with bicarbonates of calcium-magnesium mostly.

The variety of nitrates in the groundwater of Llío and Sú Paulo shows that the concentrations of nitrates are lower than suggested by water quality standards such as INEN NTE 1108, Ministerial Agreement 097-A and those described by the WHO, therefore, in this sense the water maintains good quality with regard to nitrate concentrations. However, there have been strong variations over the period under study, reaching a figure of more than 20 mg/L in some months, which, although still within the standards, is worrying as nitrate levels rise at certain times of the year.

Therefore, according to what was studied in this work, it can be noted that the presence of nitrate concentration is mainly due to biological and geochemical effects that occur in the surface of the study area, mainly in areas where pastures are cultivated as there is presence of animals that contribute a good amount of nitrogen compounds such as urea. In addition, it is evident the presence of areas with short-cycle crops, in which natural and artificial fertilizers with nitrogen composition are used. In this context, soil studies are necessary to corroborate the information on the presence of nitrates. To all this, the rainfall regime in the Ecuadorian highlands is added and as shown the good infiltration capacity of the area, make the nitrogen cycle produce a good portion of nitrates that go to the groundwater.

The water that comes from Llío and San Pablo is of good quality according to the reports used in this research work, but it must be taken into account that the variation of nitrates found in it implies that anthropic pressure exists in the area of origin of this, so it should be considered to keep free of activities at least the area of moors in the recharge area to preserve this resource that is essential for the population of Riobamba and Guano.