Area of study

The vegetable and ornamental flower producing region of the Texmelucan Valley, Pue., is located at 19°17' N and 98°26' O at an altitude of 2,278 meters above sea level (masl). Its climate is warm and humid with a dry winter season (Cw), and an average yearly temperature of 17 °C. The rainy season begins in May and ends in October, with an average yearly rainfall of 801.6 mm and frequent rains in the afternoon. Between November and February, frosts are common (below 0 °C) and the weather is cold (5 °C) (^{García, 2004}).

Sampling

In 2011, samples were taken from the tributaries of Achichipilco, Santa María and San Felipe del Atoyac River, and points located in the municipal areas of San Felipe Teotlalcingo, San Matías Tlalancaleca, San Martín Texmelucan, San Salvador el Verde, and Santa Rita Tlahuapan. The sampling stations were established in different points of these municipal areas with the purpose of covering the largest possible areas in which irrigation is carried out with this water (Table 1). The sampling points were chosen using the criteria of accessibility to the sampling point, that water was used for irrigating crops, and that the sampled surface covered most of the area of study. Each sample was taken from the river in intervals of 2 to 5 km apart, and in deep wells, it was taken from the discharge pool (Figura 1).

Figure 1 Geographic location of the sampling points (see Table 1 for the corresponding identification). 

Sodium Adsorption Ratio (SAR)

The values of the Sodium Adosrption Ratio (SAR) were studied to determine the variations under different perspectives: observed SAR (SAR), (Gapón, quoted by ^{Ayers and Westcot, 1989}; equation 1), adjusted SAR (SARaj), which considers the formation of CaCO₃ precipitates, although the CaSO₄ precpipitates continue to be ignored, as well as to compare Ca and Mg (^{Bower & Wilcox, 1965}; equation 2) and corrected SAR (SAR^o) which considers the effect of the concentrations of carbonates (CO₃ ^2-) and of bicarbonate (HCO₃ ^-) and the salinity (ECa) on the processes of dilution and precipitation of calcium (^{Suárez, 1981}; equation 3).

Where Na = content of sodium in irrigation water in meq L^-1; Ca = content of calcium in irrigation water in meq L^-1; Mg = content of magnesium in irrigation water in meq L^-1; pHc = theoretical pH in which water would reach a balance with CaCO₃; and Ca^o = corrected content of calcium in irrigation water in meq L^-1.

The pHc was calculated using the equation pHc = (pK2-pKc) + p(Ca) + p(Alk).

Where pK₂ = the negative logarith of the second constant of dissociation of carbonic acid (H₂CO₃), corrected by the ionic strength of the solution; pKc = the product of solubility of CaCO₃, corrected by the ionic strength of the solution; pCa = the negative logarithm of the molar concentration of Ca²⁺; p(Alk) = the negative logarithm of the equivalent titratable concentration of CO₃ ^2- and HCO₃-.

The value of Ca^o is the content of calcium in irrigation water, corrected by the salinity of the water (CEa), by the content of bicarbonate ions in realtion to their own calcium content (HCO₃ ^-/Ca²⁺) and by the partial pressure of the carbon dioxide (CO₂) exerted on the first millimeters of soil (pressure equal to 0.0007 atm).

Distribution of anions and cations

The distribution of the cations and anions in the 28 points showed that the ion Mg covered the greatest surface, since it stood out in almost all sampling stations. This ion represented 44.57% of the total of cations, followed by Ca⁺², with 39.3% and Na⁺, with 12.44%; this was determined by the origin of the waters from melting and from the underground. The water presented a relation (Mg⁺² + Ca⁺²)/Na⁺ >3.5, which is why the use of this water in agricultural irrigation has, to date, not caused any problems in the physical and chemical properties of the soil. The content of Na indicates that the probability of reaching dangerous levels of exchangeable sodium is low, and therefore, in a medium and short term, this water is adequate for irrigation, from the viewpoint of this variable (^{Richards, 1982}).

The content of bicarbonate (HCO₃ ^-) ions in the water gave an average of 1.83 meq L^-1 and a ratio of HCO₃ ^-/(Cl^- + SO₄ ^-2) of 2.57. This ratio was influenced more by the biological activity, and less by the presence of Na ions; furthermore, the 1:2 ratio represents sodification processes in the soil. While the content of Ca⁺² ions was 0.98 meq L^-1 on average, and content of Mg reached a value of 1.11 meq L^-1. The variations in the concentration of the Ca⁺² and Mg⁺² ions modified the values of the sodium adosrption rate (SAR) favorably. Results coincide with those by ^{Paliwal & Gandhi (1976)} and by ^{Torres & Acevedo (2008)}, who mention that when the variations of Ca⁺² and Mg⁺² are balance, they avoid SAR from increasing and causing damage due to sodicity and salinity. The water used to irrigate ornamental flower crops contains a moderate amount of bicarbonate ions (<7.5 meq L^-1). This situation is favorable, since the bicarbonates have an influence on the development of flowers and, if they are excessive, white spots may appear on the leaves as a result of the aerobic and anaerobic biological activity which gives rise to most of the bicarbonate ions (^{Buyatti & Pilatti, 1997}).

Total concentration of dissolved solids

Values showed an EC of 0.7006 dS m^-1 obtained between the relation between total solids dissolved (TSD; mg L^-1) and EC (dS m^-1), indicating that the water of the Atoyac River tributaries contains bicarbonate ions (^{Ramírez, Ortega-Escobar, Rodríguez, Ramírez-Ayala, & Rone, 1989}). However, the EC values obtained were within those allowed by the (Diario Oficial de la Federación ^{(DOF), (1989)}, which is between 0.9 and 1.1 dS m^-1. The EC was determined by the origin of the water, since 92.8% of the water used to irrigate in the Texmelucan valley area are from melted ice, springs and underground well water, whereas only 7.14% corresponds to wastewater (points in which the drainages from factories and urban wastes discharge). Due to this, all the water samples presented a low EC and were classified as adequate for irrigation, considering only this variable (^{Olías, Cerón & Fernández, 2005}), with the exception of the points in which wastewaters are discharged.

In general, EC fluctuated between 0.109 and 0.875 dS m^-1 and the distribution of the samples was 64% between 0.1 and 0.25 dS m^-1, 28.57% between 0.25 and 0.75 dS m^-1, and 7.14% between 0.75 and 1.0 dS m^-1 (Table 4). It is important to point out that the highest percentages came from points 13 (San Martín Texmelucan), where wastewaters from textile factories and drainage are discharged; 25 and 26 (San Cristóbal Tepatlaxco), which incorporates wastes from textile factories, farms and urban wastes; 27 (City of Puebla), in which the main source of water is the discharge of the drainage that discharges all along the river, and finally, point 28 (Manuel Ávila Camacho dam), where all the waste incorporated along the Atoyac River, accumulate.

Concentration of boron

Boron is found in all natural water, and in the region studied, boron is an abundant element, since the water runs across volcanic soil surfaces and lithological strata (^{Rhoades, Kandiah & Mashali, 1992}). Out of all the samples taken, 85.71% had between 0.082 and 0.33 mg L^-1, and according to the norms by the National Water Commission (DOF, 1989), this concentration is damaging to very sensitive crops. The remaining 14.28% had between 0.33 and 0.67 mg L^-1. These concentrations damage crops such as avocado, lemon, raspberry, etc. (^{Ayers & Wescot, 1989}), while in ornamental flower crops (such as gladiola) damage only occurs when the tolerance limit of 2 mg L^-1 is surpassed. In general, we can say that the boron concentrations are found in amounts that have no toxic effects on most crops irrigated in the Texmelucan Valley and that this type of water may only damage crops that are very sensitive to this element (^{Torres & Acevedo, 2008}). A concentration of B of over 0.3 mg L^-1 causes toxicity in most crops, and in the area under study, only in four points was it found to surpass this concentration level. These points were located in the city of Puebla, San Cristóbal Tepatlaxco, San Martín Texmelucan, and the Manuel Ávila Camacho dam, which are places in which water from factories and urban waste are discharged.

In regards to the remaining 24 points, the boron concentration was found to change with the addition of wastewater. It began with a concentration of 0.082 mg L^-1, where the main source of boron were volcanic materials the water runs through (Ameyal), which emerges at the foot of the Popocatépetl volcano, while, in the path of the flow, the concentration of boron decreases, since part of this boron is fixed onto the clay complex. However, point four presented a considerable increase (0.63 mg L^-1) in relation to the initial concentration, mainly influenced by the incorporation of wastewater with high amounts of detergents in the town of San Felipe Teotlalzingo (28).

Concentration of sulfates

^{Ayers 6 Westcot (1989)} mention that the water originated from urban waste normally contain between 0.01 and 2.0 mg of SO₄ ^-2 L^-1, which has berought benefits to the soils on which this water is used, due to the sulfur in its different forms. The sulfate ion does not generally damage plants, although it does contribute to the increase in salinity in the soil solution (^{Glover, 1993}). In this study, 96.4% of the water had a SO₄ ^-2 concentration between 0.1 and 0.6 mg L^-1; only point 19, located in the Ex-hacienda de Chautla (3.57%) had a concentration of 1.01 mg L^-1. It is at this point that the discharge of wastewater from textile factories and drainages from nearby towns is incorporated. These concentrations lead to no phytotoxicity in the crops (^{Kowalska & Sady, 2003}) and therefore the concentration of sulfates is not a risk for the soil and/or crops irrigated with this water. On the contrary, they provide significant amounts of sulfur (SO₄ ^-2) to the soils irrigated with this water, so there is the advantage that the content of sulfur provided by the water to the different crops can be subtracted from the fertilization formula used by farmers.

Classification of the water in relation to the EC and SAR (SARc, SARaj, SAR^o)

The water samples from the Atoyac River tributaries were classified according to the original SAR, corrected and adjusted (Table 5). Out of all the samples, 64.28% displayed low salinity and sodium content, making them adequate for irrigation (C1-S1). However, problems may arise in soils with a low permeability and in sodium-sensitive crops; 28.57% was for water with a medium salinity and a low sodium concentration, also adequate for irrigation (C2-S1), although in certain cases, it may be required to use excessive volumes of water and use crops that are tolerant to salinity; also, some soils may present permeability problems. The remaining 7.14% was for water high in salinity (C3-S1), which may be used to irrigate soils with an adequate drainage, using excessive volumes of water to wash the excess salts from the soil and use crops that are tolerant to salinity.

These results indicate that the entire Texmelucan Valley production area has water of a decent quality, adequate for irrigation, and there are no sodicity problems. Only in points in which wastewater is incorporated, such as San Martín Texmelucan and the Manuel Ávila Camacho dam, could there be salinity problems in soils with a low permeability and in crops with a low tolerance to salinity. Results coincide with ^{Olías, Cerón & Fernández, (2005)}, who mention that there is no problem with the use of these waters due to their low EC (<0.75 dS m^-1) and SAR, and they can therefore be used in most soils, due to the low probability of reaching dangerous concentrations of exchangeable sodium. Meanwhile, ^{Levy, Goldstein, & Mamedov (2005)} mention that waters with a low SAR are ideal for agricultural use due to their minimum or null probability of causing salinity.

The results for the SAR obtained in this study in the different sampling points, with values that fluctuated between 0.035 (point 4: San Felipe) and 1.374 (point 28: Manuel Ávila Camacho dam), were very similar to those for SARc, with values ranging between 0.036 (point 20: Teotlatzingo) and 1.538 (point 27: Puebla de Zaragoza). However, in all cases, they differed significantly to the values for SARaj, (in the range of 0.063, in point 4: San Felipe, up to 2.706 in point 28: Manuel Ávila Camacho dam), which displayed values much higher than the first two variants of the Sodium Adsorption Ratio (SAR) parameter (Table 5). This coincides with ^{García (2012)}, who indicates that this was also proven in studies carried out by the Salinity Laboratory of the USDA.