Hydrogeochemistry is a tool that provides useful information that can be used to study the groundwater of the karst aquifer. In this study, the temporal variation of different parameters and chemical composition of water from springs, dug wells, rivers, streams, drains, and wells during the period from 2018 to 2022 using annual hydrological data is used. Using the data available on the hydrogeological response to periods of rain and dry seasons, from the variations it was possible to evaluate the changes in hydrogeochemistry between the different sampling points within the karstic system, identifying the different lithologies through which the different flow patterns have traveled. Furthermore, it is possible to infer the origin and residence time of the water discharging at the site (Cardona et al., 2018).
The stable isotope study enabled us to define the groundwater recharge sites that discharge within the site for the different types of points that were sampled. The isotopic content of surface reservoirs such as rivers, streams, and wells show values below the LMLHZ, suggesting some evaporation (Fig. 6). This behavior is more noticeable for the November 2019 sampling, which corresponds to the dry season due to the decrease in atmospheric humidity. The same behavior was reflected in July 2020 (Centeno Herrera, 2023), where, although it was already the rainy season, the amount of precipitation was lower than in the other samples collected in the same season. The value of their slopes was 5.93 for November 2019 and 5.02 for the sampling done in July 2020. The increase in precipitation for the sampling conducted in September 2020 and June 2021 tends to produce an increase in δ18O content and a slight increase in δ2H, suggesting that re-evaporation occurs upon infiltration (Valenzuela et al., 2013; Rivera-Armendariz et al., 2024). It should be noted that enriched values in isotopic composition may also reflect deforested soils, high temperatures, and low atmospheric humidity (Dansgaard, 1964).
It can also be observed that some samples from springs and drains located above and near the LMLHZ represent the composition of recent recharge with values of δ18O (− 4.75 to − 4.20‰) and δ2H (− 29.12 to − 23.86‰). The regional system water flow is represented by the decrease in the most negative isotopic composition for δ18O (− 6.90 to − 6.65‰) and δ2H (− 41.37 to − 40.41‰), considering that this variation is given by the groundwater flow path having a longer path through the subsurface, causing the depletion of heavy isotopes in precipitation (Wassenaar et al., 2009; Wu et al., 2019; Melo and Caro, 2020; Paternoster et al., 2020). It also suggests that mixing may occur along the flow line, which has damped seasonal and spatial variations (Clark, 2015).
The excess deuterium (Fig. 7) means that for most of the samples, a value greater than 10‰ is observed, suggesting the recycling of moisture from tropical storms, which are common on the eastern slope. The main source of this is the Gulf of Mexico, where atmospheric humidity conditions are at 77%. Samples with values below 10‰ indicate a possible surface evaporation that is common for shallow environments, indicating a re-evaporation effect before infiltration. This implies that variations in atmospheric humidity play a fundamental role in the isotopic content in the region.
The elevation gradient was determined by comparing the weighted averages of the stations within the Huasteca region (Ro Verde, Tamasopo, and Ciudad Valles) to obtain the variation of isotopic composition with respect to elevation (Fig. 8). The elevation effect indicates that the higher the elevation, the more impoverished the δ18O (Clark and Fritz, 1997). The elevation gradient for this region is − 0.25‰ of δ18O/100 m, which is consistent with the value reported for the region (Esquivel-González,2022). This indicates that regional recharge can occur at elevations of approximately 1000 m in the Río Verde mountains.
In the karst area, usually the dissolution–precipitation of carbonates and sulfate minerals prevails, as shown in the chemical composition. Figure 4 shows that the dominant reactions are the dissolution of calcite, dolomite, and gypsum (Wu et al., 2014; Morán-Ramírez, 2016), the latter being responsible for dedolomitization processes that prevail during the dry season, indicating the presence of a base flow of deeper origin. This behavior is confirmed by the increase in SO42− concentration, being noticeable in Ca-SO4 water types (Yuan et al., 2017; Tang et al., 2021). This is also reaffirmed by the isotopic composition, as it is a clear example of the existence of a flow that has traveled deeper and further distances and is recharged in areas of higher elevation. Although the water in the basin is known for its Ca-HCO3 composition, recent observations have shown a connection between the dissolution of limestone rock formations and seepage. This suggests that the flow of water entering through vertical infiltration reacts quickly to precipitation events. The interaction between the CO2 in the soil and the biological activity of microorganisms decomposing organic matter creates a slight acidity that aids in the dissolution of limestone. This reaction also produces a buffering effect, making it a common occurrence in karst aquifers. A shorter and shallower groundwater path can be inferred from these discharges, which are directly related to the granular karst aquifer found in the El Abra Fm. and Tamasopo Fm. Consequently, Ca-mix water type is the result of a mixture of discharge waters with different compositions (deep origin and seepage), which is more perceptible in the rainy season (Bicalho et al., 2017; Liu et al., 2023).
The SI (Fig. 5) enables us to define the main dissolution–precipitation processes of karst groundwater. It can be inferred that water samples that are supersaturated with respect to calcite are due to two mechanisms. One is the degassing that is best distinguished in the rainy season, which is the tendency of the samples with Ca-HCO3 type water, where the water at the time of discharge loses CO2(g) in a natural way; however, in this season it is influenced by the fact that the water travels faster. Therefore, the time that the mineral has for reaching equilibrium is rather short, and it tends to precipitate. In addition to these reactions, the influence of the increase in SO42− concentration is also shown; in this case, it is seen how the oversaturation of calcite in the low water season shows that the Ca-SO4 and Ca-CHO3 type water, when receiving the base flow with SO42− concentrations, favors the dedolomitization process. However, gypsum undersaturation tends to equilibrium, which is independent of the season. Also, it is evident in the samples that are undersaturated with respect to calcite and dolomite that as soon as dolomite dissolution is higher, calcite begins to decrease its undersaturation, following the trend toward equilibrium and oversaturation. The same behavior can be observed in the samples with Ca-SO4 and Ca-mix compositions (Hassen et al., 2018; Zheng et al., 2018).
The PCA indicated that the hydrogeochemical characteristics of groundwater are typically controlled by water–rock interaction, including dissolution–precipitation limestone and evaporitic rocks, and anthropogenic activities such as the use of chemicals in agriculture. PCA analysis was carried out to evaluate the interelement relationship among major ions and field parameters such as temperature, CE, Ca2+, Mg2+, Na+, K+, HCO3−, Cl−, SO42−, F−, NO3−, Li+, Sr2+, pH, and the SI (calcite, dolomite, gypsum, celestite, and barite). Factors were extracted after varimax rotation using XLSTAT. This analysis was used to identify the dominant processes controlling the hydrogeochemical characteristics of groundwater in the study area (Morales-Casique et al., 2016; Tobin and Schwartz, 2016).
The classification obtained from the PCA represents defined characteristics for each type of process taking place at the site. The first two factors (1, 2) represent the geogenic conditions of the discharges at the site. The first component represents the flows that have remained in the subsoil for a longer time, including elements such as Li+, F−, characteristic of groundwater flows that have had a longer travel time and depth, giving them enough time in the subsoil to increase their concentration (Carrillo-Rivera, et al., 2002). Also in this same classification we find elements such as Ca2+ and Mg2+, whose increase is, as mentioned above, related to the dissolution of evaporite minerals; in this case gypsum (Edmunds et al. 1997; Kamel et al., 2013), which is grouped in this same factor. The EC associated with this is an indication of groundwater flows that have experienced longer water-rock interaction. The second component is related to recent infiltration flows. Calcite and dolomite SI are grouped in this component, corresponding to the lithology that is found outcropping at the site. The relationship of these with the pH resides first in the interaction of the CO2 in the soil particles with the water that falls on it, and this causes a slight acidity that helps the dissolution of this type of mineral phase. At the time when the dissolution process takes place, there is a buffer effect that originates from the presence of ions that are released when limestones dissolve (CO32−, HCO3−). The third component indicates a rather peculiar grouping, because while the association between Sr2+ and SI Cel continues to confirm the influence of discharges with longer water–rock interaction times, we also have NO3−, which, due to the quantities, may be related to diffuse sources associated with intensive agricultural activities occurring in the area, possibly indicating irrigation return water. The fourth component confirms the effect that may be caused from the use of chemicals at the site, since these elements may be nutrients related to the use of the site.
The conductivity and temperature measurements were useful for identifying different water sources and the influence of recharge, seasonal changes, and mixing conditions between flows discharging at the sites where the data loggers were situated, as well as identifying meteorological conditions and their contribution to the flow of the springs and stream, which is very useful for improving our understanding of karst systems.
It is assumed that during periods of low water, the EC content remains relatively constant due to the contribution of a deeper base flow with a SO42− composition, as well as a mixture with recently infiltrated water, defined as water with some HCO3− content. These mixtures suggest that dedolomitization processes are taking place, resulting in a decrease in EC during periods of high rainfall. The contribution of a base flow generated by a deeper flow is greater for groups 1 and 3, but for flows that discharge in group 2, it is inferred based on variations in EC content. It is primarily influenced by inputs of recently infiltrated water, which suggests this behavior is due to the HCO3− composition of the water at this location. It is inferred for this group that when EC decreases, it is caused by calcite precipitation, which is consistent with the short time that this mineral has for reaching equilibrium. It is also possible that the variations in water temperature are the result of the temperature-regulating effect produced by the solar radiation that affects the soil, and that the water, acting as a heat exchanger, can take in or give up heat, slightly dampening the environmental temperature (Matheswaran et al., 2014; Adji et al., 2017).
Hydrogeochemical conceptual model
From the analysis of the hydrogeochemical data, the existence of heterogeneity in the chemical composition of the water is determined. It shows that in the basin there are groundwater discharges from different sources that have different travel times, some of which have shallower circulation and recent infiltration and others that have traveled deeper and through the subsoil, which gives them specific characteristics that make it possible to infer their origin. It is therefore suitable to explain this type of behavior based on established models, such as that proposed by Tóth (1963, 2000), which exemplifies the different paths that water can take depending on topographic, climatic, and geological conditions, influencing the development of the flow systems that discharge at a given site.
Based on this model, three types of flow can coexist in a basin, local, intermediate, and regional. Each is associated with a different residence time and trajectory through the subsoil, from lesser to greater, which confers unique physicochemical characteristics. For the local flows, it is assumed that their discharges are distinguished by a lower content in EC values as a reflection of lower water–rock interaction and temperature close to the environmental average, and a recharge zone is inferred in high topographies close to sites where water emergence occurs. As the intermediate flows travel to an intermediate depth within the basin, the EC and temperature tend to increase. Regional flows are those that travel to greater depths, giving them more time for water–rock interaction, which generates greater dissolution of minerals, which can be seen reflected in the water–rock interaction (Tóth, 1963, 2000).
Considering this theory, we describe the hydrogeological functioning that is taking place in the karst terrain where the Ciénaga de Tamasopo is located, where the presence of three flow systems is assumed (Fig. 14). These are described as follows.
Figure 14
Local flow system
Water of Ca-HCO3 composition can be related to local flows that are of shorter relative residence time in the aquifer (Nuñez-Peña et al., 2015), which has its recharge zone in the upper parts of the basin corresponding to mountainous areas in the northern portion of the basin, where the karstification effect is greater (dolines, uvales), inducing rapid infiltration. Due to its rapid movement, its route through the karst is short, making it more noticeable in the rainy season, when the response of some springs located in the foothills is almost immediate. Therefore, at this site, it is characterized by shallow travel over the limestone rocks of the El Abra Fm. and Tamasopo Fm., which is evident in its chemical composition. The EC content in the rainy season is 428 µS/cm and in the dry season 543 µS/cm. In the rainy season the water–rock interaction time is faster, and dissolution–precipitation processes occur quickly, so the EC value is lower. A notable increase occurs during the dry season, so it is inferred that the water that remains stored in the karst aquifer continues to dissolve the limestone and generates an increase in EC during this season.
Intermediate and regional flow systems
For this case, we deduce the existence of a regional flow system circulating over the Guaxcamá Fm. that is composed of rocks containing evaporite minerals such as gypsum (CaSO4.2H2O), where from a small contribution of water that circulates through this and manages to ascend through faults or fractures, we assume a mixture between waters with a greater predominance of SO42−, which is evidence of a longer travel time. During its journey towards the discharge zone, this water mixes with water of HCO3− composition. This intermediate flow has EC for the rainy and dry seasons of 1381 µS/cm and 1454 µS/cm, respectively. The increase that occurs during the dry season may be related to an increase in the contribution of the regional system, or to climate conditions that favor the evaporation process and thus generate a concentration of salts, increasing the EC.
The mixing between different flows occurs mainly during the rainy season, being considerable in the water of rivers and streams, where the convergence of the different flows can have both concentrated and diffuse discharges, as evidenced in Ca-mix-type waters.
This intermediate flow is considered a base flow that could be corroborated in the various types of water usage points that were sampled: springs, wells, rivers and streams.
The incorporation of isotopic data allowed us to deduce that the recharge zone is probably located at an elevation of about 1000 m, typical of mountainous regions characterized by prominent karstification, such as areas with dolines or poljes. Most springs show no evidence of evaporation. Sites experiencing evaporation effects are observed in surface water bodies, where there is a greater indication of mixing of discharge flows. The isotopic content confirms that the water is of pluvial origin and the increases in EC are mainly due to the dissolution of salts, mainly gypsum, from the Guaxcamá Fm.
In discharges that are considered local, water with a δ18O content of − 6.74 predominates. The δ18O isotopic content of the intermediate/regional discharge is − 6.90, suggesting greater depletion of this isotope as the distance from the recharge site of the latter flow increases.