Introduction

The San Pedro river basin, located in the states of Durango, Zacatecas and Nayarit, encompasses the Sierra Madre Occidental and the Pacific Coastal Plain physiographic provinces. The first is one of the great siliceous igneous provinces of the Cretaceous-Cenozoic, where the main types of rocks are andesitic, dacitic-rhyolitic, ignimbrite and alkaline basalts, which lie on a Pre-Cambrian, Paleozoic and Mesozoic basement; in its central zone, all the rocks are calcalcaline and their composition varies from diorite to granite, with granidioritic being the dominant one (^{Ferrari et al., 2005}).

The Pacific Coastal Plain is a deltaic fluvial system formed by sediments from the San Pedro, Santiago and Acaponeta rivers and is related to marine transgressions during the late Pleistocene and Holocene (^{Curray et al., 1969}). The rocks that make up this plain are extrusive igneous rocks from the Tertiary, alluvial and marshy deposits, made up of sand, gravel, silt and clay from the Quaternary (^{Ferrari et al., 2005}). The main soils are cambisols and fluvisols (^{Bojorquez et al., 2006}).

The Coastal Plain is of great importance because it has a high agricultural activity, in the study area 125 856 ha are cultivated and of which 85% are irrigated, and it is where the water from the San Pedro river is used for the Irrigation of beans (Phaseolus vulgaris), maize (Zea mayz), sorghum (Sorghum bicolor L. Moench), tobacco (Nicotiana tabacum L.) and tomatillo (Physalis ixocarpa Brot. ex Horn.). When this water is used for this purpose, it is important to consider the quality, since according to this is the management that must be given to prevent the problems that could cause.

The elements contained in natural waters come from the dissolution or weathering of rocks and soils, and are transported by surface currents and deposited in the soils of the lower parts, either naturally or through irrigation (^{Can-Chulim et al., 2008}). From the ionic composition of the water, various criteria, index or associations are used, which allow classifying them, evaluating risks and determining their quality for agriculture.

Among these are the danger of salinization and that of sodification by means of the sodium adsorption ratio (RAS) and the percentage of exchangeable sodium (PSI), residual sodium carbonate (CSR) and others (^{Mandal et al., 2019}). Most focus on the Na⁺ content, on the concentration of Ca²⁺, Mg²⁺ and on CO₃ ^2- and HCO₃ ^-. If the concentration of Na⁺ is high, the danger of alkalization is high, this is magnified when there are high contents of CO₃ ^2- and HCO₃ ^-, due to the tendency of these ions to form precipitates with Ca²⁺ and Mg²⁺, in addition to being in suspension the most soluble substances, including Na₂CO₃.

^{Yaron and Tomas (1968)} demonstrated that high Na⁺ content in irrigation water considerably increases the PSI and when this happens they decrease the physical, chemical and nutritional properties of the soil. In this study, a hydrogeochemical classification was made, the quality of the water for irrigation was evaluated using the RAS, PSI, CSR indices, and the carbonate precipitation process using the San Pedro River water saturation index in Nayarit; with the aim of evaluating the adverse effects that the use of this water can cause in agriculture.

Materials and methods

Sampling design

Nine sampling sites were established on the bed of the San Pedro river, located according to the physiography, the communication routes and the agricultural area of the basin. The sites were El Rosarito (ER), San Pedro Ixcatan (SPI), El Venado (EV), Ruiz (RZ), El Tamarindo (ET), Tuxpan (TUX), El Mezcal (EM), Mexcaltitan (MEX) and Boca de Camichin (BC) (Figure 1).

Figure 1 Sampling sites in the San Pedro riverbed in Nayarit. 

Four samplings were carried out in the months of October 2016, April 2017, December 2017 and May 2018. Each site was georeferenced using a GPS with UTM coordinate system. Due to the formation processes that affect the basin, from Rosarito to Mezcal they are considered the first section and are waters from the natural channel of the San Pedro river. The Mexcaltitan and Boca de Camichin sites, being a transition zone between inland waters and marine intrusion waters, were considered the second section. Two samples were taken at each site, with 1 L polyethylene containers.

Results and discussion

Hydrogeochemical classification. Using the Piper diagram (Figure 2), MEX and BC are classified as chlorinated-sodium (Cl-Na) and are characterized by the intrusion of seawater, since they are in a transition zone between continental and ocean waters. In the ER to EM sites, the hydrogeochemical type was sodium-calcium-magnesium-bicarbonate (Na-Ca-Mg-HCO₃). According to ^{Madrigal-Solis et al. (2017)}, bicarbonated waters generally correspond to recent water, which has had a short time of permanence and interaction with rocks, this absence of changes was related to the altitudinal gradient that does not allow long periods of contact with geological material.

Figure 2 Hydrogeochemical classification of the San Pedro river water in Nayarit. 

The ionic composition obeys the following order: Na⁺ + K⁺ > Mg²⁺ > Ca²⁺; for anions it was HCO₃ ^- + CO₃ ^2- > Cl^- > SO₄ ^2-, while in MEX and BC it was Cl^- > HCO₃ ^- + CO₃ ^2- > SO₄ ^2- (Table 1). Low ionic concentrations are due to the silicate material being insoluble. The alkaline behavior is caused by the ignimbrite rocks and alkaline basalts of the Sierra Madre Occidental. Based on ^{Vidal-Solano et al. (2005)}, during post-subduction magmatism, ignimbrites in their composition have high iron and alkali contents up to Na₂O+K₂O= 8-10%, which explains the sodium behavior of water; also, according to ^{Aranda-Gómez et al. (2005)}, the SMO basaltic rocks are rich in MgO and SiO₂. Therefore, Mg²⁺ is one of the cations with the highest content.

Physical-chemical regime (pH). For the ER-EM section, in the rainy season, waters were obtained from neutral to moderately alkaline pH, with values from 7.3 to 8. During the dry season, the pH was from moderately alkaline to strongly alkaline, with values from 7.7 to 8.9, where the highest results were obtained in the fourth sampling. For the MEX and BC sampling sites, the pH turned out to be neutral to mildly alkaline, in MEX the pH_min and pH_max were 7.3 and 7.8, and in BC 7.3 and 7.7 (Table 1).

Most of the salts derive from a strong base like Na⁺, K⁺, Ca²⁺ and Mg²⁺ and a weak acid like CO₃ ^2- and HCO₃ ^-, according to ^{Raviolo and Farre (2017)} the hydrolysis of these produces basic aqueous solutions, while the NaCl and MgSO₄ derived from a strong base with a strong acid produce neutral solutions. The presence of Na₂CO₃ provides the water with high alkalinity (^{Sen, 2015}), for this reason the pH of the San Pedro river is above neutrality (pH> 7). Normal pH values in irrigation water range from 6.5 to 8.5, the waters of the San Pedro River in a dry period reached values up to 8.9, according to ^{Hong et al. (2013)} these will have implications on the availability and management of nutrients. If they are used for fertigation, it will be necessary to lower the pH between 5.5 and 6.5.

Electric conductivity. The EC_min and EC_max in the ER-MS section were 131 and 367 µS cm^-1, in MEX it was 2 530 and 16 330 µS cm^-1, and in BC 23 605 and 43 820 µS cm^-1. At the end of the rainy season, the waters of the ER-EM section were classified as C1; however, in the dry period, only RZ remained classified as C1, the other sites were classified as C2. MEX was classified in samples 1, 2 and 3 as C4, in sample 4 as all the samples from BC were classified as C5 (Table 1). Waters classified as C1 can be used for most crops, in almost any type of soil with very little probability that salinity will develop. Waters called C2 can be used as long as there is a moderate degree of washing, in almost all cases no special salinity control practices are needed. The water of the San Pedro river depending on its EC can be used for irrigation from ER to EM. For EC> 3 000 µS cm^-1 the degree of restriction is severe (^{Castellón-Gómez et al., 2015}), this situation occurs for MEX and BC, therefore it is not recommended for irrigation.

Sodium content. The values of RAS_or, RAS° and RAS_aj were less than 1.6 in the samples of the ER-EM section, and were classified as S1, both in the rainy season and in the dry season. They can be used for irrigation in most soils with little probability of reaching dangerous levels of exchangeable sodium. With the RAS_or MEX it was classified as S3 in samples 1, 2 and 3, while in sample 4, as well as in all BC samples, it was classified as S4. With RAS_aj, MEX and BC were S4 in the four samplings (Table 2). S3 and S4 waters are unsuitable for agricultural use, according to ^{Richards (1990)}; ^{Mandal et al. (2019)} can produce toxic levels of exchangeable sodium in soils and cause infiltration problems, particle dispersion and structure loss.

Ionic strength was calculated to determine the implicit pHc in RASaj. The formula to determine the activity coefficient of any ion in a solution is that of Debye-Hückel: -logγi= 0.51Zi2I1/2 , with the ionic strength (I) being the only variable I=12∑CiZi2 . According to the correlation between EC and I, the latter can be determined from the EC of any solution in its functional relationship I=αf(EC) , where α is a constant that depends on the ionic composition of the solutions saline, and EC is the experimental electrical conductivity.

Figure 3 shows the experimental relationship I=αf(EC) of the waters of the San Pedro river (a) for the ER-EM section with low electrical conductivity and, (b) for the MEX-BC section, waters with high electric conductivity. The value of α for the waters with low EC was α= 0.0129 and for those with high EC it was α= 0.011. These constants can be used for waters with low and high EC, however, the I is unique for a given composition, the value of which depends on the concentration of each ionic component and its respective valence.

Figure 3 Functional relationship of ionic strength with the electrical conductivity of the water of the San Pedro river, Nayarit. a) ER-EM; and b) MEX-BC. 

^{Leffelaar et al. (1983)} in 50 extracts at saturation of various soil profiles, obtained a coefficient of α= 0.0144 and ^{López-García et al. (2016)} in wastewater obtained an α= 0.0116. These activity coefficients turned out to be like those determined in the water of the San Pedro river, therefore, with the EC it is sufficient to obtain an ionic force with a high degree of approximation.

Exchangeable sodium percentage. According to the Gapon selectivity constants used and the different RAS conceptualizations, the PSI_min and PSI_max of the waters of the San Pedro river were obtained. For the ER-EM section with low EC, the maximum values of PSI, which can be reached by the river waters when applied to the soil, were below 3%. Mexcaltitan with the different RAS their PSI was from 13.3 to 51.8% and BC from 35.7 to 73.1%. Table 2 shows the PSI averages for each sampling site. The predominant salts of the San Pedro river water are sodium, calcium and magnesium bicarbonates, followed by NaCl, and when evaporation processes exist, sodium carbonate appears.

According to ^{López-García et al. (2016)}, due to the high solubility of sodium salts, these are dissolved even under evaporation conditions, and the CO₃ ^2--HCO₃ ^- system could be converted to CaMg(CO₃)₂, which increases the concentration of sodium in the solutions and in turn the PSI. ^{Elbashier et al. (2016)} mentioned that the PSI of the ground and the RAS are approximately the same. Figure 4 shows the PSI-RAS relationship for the different RAS formulations of the waters of the San Pedro river. It is observed that the variations of the PSI in the soils depend on the K_G values, if the relationship KG=RSIRAS , is analyzed, the ratio of exchangeable sodium RSI=SI(CIC-SI) , is a function of sodium exchangeable (SI) and cationic exchange capacity (CIC), and the latter, in turn, depends on the content of clays, or on the texture of the soil.

Therefore, when increasing the RAS, the magnitude of the PSI increase will be a function of the texture, as shown in Figure 4, where the PSI_max corresponds to a clay soil, while the PSI_min to a sand-clay soil-slimy.

Figure 4 PSI-RAS relationship of the water of the San Pedro river in Nayarit. 

The PSI values that can be reached by the waters of the San Pedro river in the ER-EM section do not present a risk for application to the soil; however, ^{Castellanos et al. (2000)}, mention that some soils can present permeability problems from 5% exchangeable sodium, especially if they are clayey. The agricultural area of the San Pedro river basin is characterized by having cambisol and fluvisol soils of coarse to medium texture (^{Bojorquez et al., 2006}) therefore, the PSI will be less than the calculated PSI_max. In practice, the actual PSI values in soil commonly turn out to be higher than those calculated by means of formulas, this is because the soil solution almost always has a higher sodium concentration than the irrigation water.

Residual sodium carbonate. The CSR indicates that the ER to EM and MEX sites in samples 1, 3 and 4 were classified as good quality water (<1.25). In sample 2, MEX is classified as conditional; BC in the four samplings was classified as not recommended (Table 2). Positive values indicate that there is a higher content of CO₃ ^2- and HCO₃ ^- than Ca²⁺ and Mg²⁺; however, this difference was not significant to restrict the use of these waters for irrigation. In the dry period, higher values were obtained than in the rainy season, this is because the flow rates are reduced and the concentrations of Ca²⁺ and Mg²⁺, due to their lower solubility than the Na⁺ salts, precipitate in the form of carbonates. ^{Dhembare (2012)} mentions that when negative values are obtained, they are due to the fact that Ca²⁺ and Mg²⁺ do not precipitate and remain in solution. ^{Castellón-Gómez et al. (2015)} mention that waters with high CSR values are not risky if used in fertigation, since CO₃ ^2- and HCO₃ ^- can be destroyed by adding acids.

Saturation index (IS). Negative values were obtained in the rainy season for ER to EM sites, indicating that the waters of the San Pedro river when applied in irrigation will dissolve CaCO₃ in the soil. In the dry period, in the second sampling, in the ER, SPI and EV sites, the water showed a tendency to precipitate CaCO₃ in the soil, this also happened in sampling 4 from ER to ET, the other sites obtained negative values. In the waters of MEX and BC, positive values were obtained, with the exception of MEX in sample 1 (Table 2).

The predominance of bicarbonates in the waters of the San Pedro river suggests a certain danger to be used for irrigation. According to ^{Hannam et al. (2019)}, waters with high concentrations of HCO₃ ^-, tend to precipitate calcium in the form of carbonates, as the soil solution is concentrated. Based on the solubility of the CO₃ ^2- and HCO₃ ^- salts, which can be formed with the majority cations in water, CaCO₃ will be the first to precipitate, followed by MgCO₃, processes that generally occur at pH> 8.2, by what when applying the irrigation with waters with this pH there will be precipitates. The IS indicates that in the rainy season the waters of the San Pedro river will dissolve calcite from the soil, however, in the evaporation process, the CO₃ ^2- and HCO₃ ^- will tend to precipitate, such as in the dry period. ^{López-García et al. (2016)}, mention that the precipitation of calcite causes Ca²⁺ to stop participating in the exchange complex, and Na⁺ to participate more. The presence of Na⁺ in the cation exchange complex has a decisive influence on the physicochemical properties of the soil, since a high content of this ion in water considerably increases the PSI and, in turn, the pH.

Conclusions

The water from the San Pedro river in Nayarit from the section El Rosarito (ER) to El Mezcal (EM) was of the sodium-calcium-magnesium-bicarbonate type and presented higher alkalinity (pH> 7); they are of low electrical conductivity due to the short residence time, the insolubility of the silicate minerals of the Sierra Madre Occidental and the absence of significant sources of contamination; they are of good quality for crop irrigation. In the MEX-BC section it was chlorinated-sodium due to the mixture with the marine intrusion water and they are not suitable for agricultural use.

The CSR and the saturation index indicate that these waters tend to precipitate CaCO₃ when the evaporation process occurs and in the dry season. The agricultural soils of the area are formed by alluvial sediments from the San Pedro River, therefore, over a geological-historical time span, the pedogeochemical profiles of the soils are constantly enriched with microscopic calcite.