Sampling, water analysis and statistical analysis

To carry out this investigation, during September 2015 and April 2016, 188 wastewater, rivers and dam water samples were collected and analyzed from 135 sampling stations distributed in the Mexico City-Mezquital Valley drainage system (Fig. 1). Samples were collected according to NMX-AA-003-SCFI-1980, and considering the accessibility of the sites (^{SCFI, 1980}).

Figure 1 Location of the study area. Source: Author’s own elaboration. 

All the sampling stations (Table 1) were registered with a Geographic Positioning System (GPS Garmin^® Etrex Venture HC). In each sampling station, 2 liters of water were collected and distributed into 1 L polypropylene containers previously washed with an HCl solution at 10 % concentration, then rinsed with distilled water. In all the sampling stations the channels flow in the open, the water was taken from the central part of the drainage channels at an approximate depth of 30 cm using a plastic 10 L bucket. Later, the containers were washed three times with the same collected water (^{Sánchez-Bernal et al., 2014}).

The concentration of NO₃^-, PO₄^3- and B³⁺ was determined by spectrophotometry (JENWAY^® 7305 Spectrophotometer), with different dependent wavelengths based on the Beer-Lambert law, which indicates that by knowing the absorbance at a given wavelength it can be used to estimate the concentration (^{Rodger, 2013}):

Where: A is light absorption, ε is the coefficient of extinction or dependent wavelength (nm), C is the concentration (mol L^-1) and l is the length of the sample through which light passes (cm). NO₃^- was determined using the salicylic acid nitration method (^{Robarge et al., 1983}); PO₄^3-, with the ascorbic acid method (^{Eaton et al., 1998}), and to find the concentration of B³⁺, the H-Azomethine method was used (^{Rodier, 1978}). In all three cases, solutions of known concentrations were used to establish the calibration lines. The regression equations used were the following:

The ions Ca²⁺, Mg²⁺ were determined by volumetric titration with disodic EDTA (0.01 N), and the volumetric titration of Cl^- was carried out with silver nitrate (0.05 N). The concentration of K⁺ was determined with a flame spectrometer (Instrumentation Laboratory^® AutoCal Flame Photometer 643), the detection limit was 5 meq L^-1 of K⁺ [^{Eaton et al., 1998}; ^{Richards et al., 1982}]. The procedure to verify the correct analysis of water samples was the anion-cation balance criterion established in standard methods for the examination of water and wastewater (^{Eaton et al., 1998}).

Once all the results were obtained from the chemical analysis of water, a statistical analysis was carried out on each one of the variable, which consisted in determining: normality test using the Kolmogorov-Smirnov method, skewness, kurtosis, minimum, maximum, mean, median, standard deviation, range, coefficient of variation (CV), quartiles, and extreme values (^{Montgomery & Runger, 2015}). The software used was SAS^® version 9.0 and the graphs were created in SigmaPlot^® version 10.0.

To establish the risk of toxicity by specific ions, the concentrations of B³⁺, and Cl^- were considered according to the criteria established in different investigations (^{Ayers & Westcot, 1985}; ^{Maas, 1990}; ^{Richards et al., 1982}). The estimation of the nutrient content was carried out based on the results obtained from the concentrations of NO₃^-, PO₄³, B³⁺, Ca²⁺, Mg²⁺ and K⁺ using the dimensional analysis technique and considering 1 m irrigation sheet, which is normally applied on crops in the Mezquital Valley.

Nitrate, phosphate and boron content in wastewater

The concentrations of nitrate, phosphate, and boron in wastewater in the Mexico City-Mezquital Valley drainage network (Table 2) were determined. For the three ions, the coefficient of variation was found to be high, indicating the heterogeneity in the concentration of these ions in the wastewater.

Wastewater contains nitrogen, phosphorous, potassium, copper, iron and zinc and its use can reduce the need for fertilizers (^{Duncan & Cairncross, 1990}). However, in this study, the concentration of nitrate was attributed precisely to the use of fertilizers; the wastewater, in its course across the agricultural area known as the Mezquital Valley, becomes enriched with these ions that are lixiviate from agricultural soils and transported in drainage water. The continuous use of wastewater may cause problems in the natural fertility soil, and in the protection of water resources due to the high concentration of sodium, nitrate and phosphate salts (^{Belaid et al., 2012}).

The main drainage channel for Mexico City, through the 14-Peñón-Texcoco highway, had a nitrate concentration of 3.908 mg L^-1, and the 16-Tequisquiac tunnel, 2.180 mg L^-1 of nitrate. The highest concentration of this ion was found in the Tula River water in 17-Apaxco and 26-Atotonilco (153.380 mg L^-1 and 154.578 mg L^-1 respectively), in 112-agricultural drainage water (177.342 mg L^-1 of NO₃^-) and filtration water in 30-Atitalaquia (146.431 mg L^-1 of NO₃^-), indicating that wastewater coming from Mexico City, in its course through the State of Hidalgo is used in agriculture, and the excess of nitrogenated fertilizers is leached and evacuated by agricultural drainage, as well as the wastewater that is poured into the Tula River, so this River receives urban and industrial wastewater and agricultural drainage, which explains it nitrate concentration.

The data are asymmetrical (p-value < (); the third quartile (Q₃) had a value of 73.467 mg L^-1 of NO₃^-, and the median was of 57.185 mg L^-1 of NO₃^-. These data indicated that wastewater contain excess of nitrate (> 30 mg L^-1), 19 % of the water samples had between 5 mg L^-1 and 30 mg L^-1 of NO₃^-, and only six water samples contained lees than 5 mg L^-1 of NO₃^-.

The excess nitrate is due to the discharging of agricultural drainage into this hydrographic network, this occurs by the altitudinal gradient (from 2293 masl to 935 masl); finally, the flow of water is evacuated by the Moctezuma River. Use of the wastewater for agricultural irrigation may decrease the yield and quality of sensitive crops; their sensitivity to high ion concentration varies during the crop’s phenological stages, so as the crop’s needs decrease, high ionic concentration may be harmful (Ayers & Westcot, 1987). Figure 2 illustrates the data distribution on the nitrate concentration found in the Mexico City-Mezquital Valley drainage network.

Figure 2 Distribution of the concentration of nitrate in the Mexico City-Mezquital Valley drainage network. Source: Author’s own elaboration. 

On the other hand, water runoff and infiltration with high nitrate content due to fertilization practices, creates an important problem of widespread pollution in water resources, and if the nitrate reaches groundwater bodies, it can cause serious health problems for people, who consume this water (^{Aruzo et al., 2006}).

In Mexico, the highest nitrate concentration permitted in sources of drinking water is (10 mg L^-1 N = 44.26 mg L^-1 NO₃^-) (^{Secretaría de Salud, 2000}). This investigation did not analyze the quality of groundwater, but it is possible that the Mezquital Valley aquifer has infiltrations of wastewater, since more than 80% of the main channels are not revetted (^{Lesser-Carrillo et al., 2011}). In this sense, there is a possibility that a part of this excess concentration of NO₃^- can lixiviate into the aquifer (^{Belaid et al., 2012}), since irrigation sheet of over 1 m are applied in the crops irrigation, indicating that there is a leaching fraction greater than 0.45, it helps to keep low salinity levels (^{Hoffman, 1990}), but it increases the risk of ions lixiviation such as NO₃^- outside the root zone of the crops.

The concentration of nitrogen is obtained using the following equations:

Estimated nitrogen content was 137.01 kg ha^-1, for a 1 m irrigation sheet. The excessive concentration of salts and nutrients such as N are a risk for long-term agricultural production, although this conclusion must be verified for diverse types of crops and soils irrigated with wastewater (^{Belaid et al., 2012}).

Regarding phosphate (Fig. 3), a high coefficient of variation was found. Maximum PO₄^3- values were found in different irrigation channels: 65.776 mg L^-1 (46-irrigation channel 1, Morelos colony, Actopan-Ixmiquilpan road), 60.849 mg L^-1 (24-irrigation channel Atitalaquia), 36.899 mg L^-1 (23-irrigation channel Pemex, Atitalaquia-Tula road), 36.212 mg L^-1 (21-Teltipan-Tlaxcoapan road) and 36.091 mg L^-1 (11-Eastern emission channel, Ecatepec). The latter corresponds to the discharge of wastewaters from Mexico City. Infiltration water (in Progreso and Chilcuautla) and the Juchitlán spring, in this agricultural area, presented the lowest concentration values for PO₄^-3 (<0.083 mg L^-1), indicating a sanitation of wastewater through the soil. The mean value for PO₄^3- was 14.319 mg L^-1, and the median had a value of 15.490 mg L^-1 of PO₄^3-. Q₁ had a value of 3.754 mg L^-1 of PO₄^3-, and Q₃ had a value of 22.293 mg L^-1 of PO₄^3-; 95 % of the water samples had a concentration of PO₄^3- lower than 29.145 mg L^-1.

Figure 3 Distribution of the concentration of phosphate in the Mexico City-Mezquital Valley drainage network. Source: Author’s own elaboration. 

The concentration of phosphorous and phosphate is obtained using the following equations:

In this case, the concentration of phosphorus in the water is, on average, of 4.66 mg L^-1. This value indicates the hypertrophic condition in which the water is found (^{Moreno-Franco et al., 2010}); domestic and industrial wastewater are the main source of phosphorus, as well as agricultural drainage, and its main characteristic is that it is composed of detergents that derive from anthropogenic activity (^{Lizárraga-Mendiola et al., 2013}; ^{Ongom et al., 2017}) and the leaching of phosphated fertilizers. The phosphorus content was estimated in 46.6998 kg ha^-1 on average for a 1 m irrigation sheet, which was attributed to the discharge of wastewater and the leaching of phosphated fertilizers. Normal phosphate values in irrigation water are generally below 2 mg L^-1 (^{Ayers & Westcot, 1985}); in this case, the excess concentration of phosphate may be due to agricultural, domestic and industrial discharge (^{Velázquez-Machuca et al., 2010}).

Risk of toxicity due to the concentration of boron and chloride

The concentration of boron (B³⁺) was attributed to borax waste Na2B4O7·10H2O), which is widely used as a cleaning agent, and therefore present in domestic and industrial wastewaters (^{Hem, 1985}). This ion is important in agriculture, although amounts lower than 1 mg L^-1 are toxic to some crops such as citrus fruits and beans, hence boron is the most likely element to cause toxicity in crops (^{Page et al., 1990}).

Figure 4 shows the distribution of B³⁺ in wastewater. The highest values were the following: 1.881 mg L^-1 (14-grand drainage channel near the Peñón-Texcoco road), 1.568 mg L^-1 (113-agricultural drain, Nith-Debodhé road), 1.565 mg L^-1 (11-eastern emission tunnel, Tonanitla-Xaltocan road, in Ecatepec), 1.494 mg L^-1 (61-Boxtha drain in Actopan) and 1.461 mg L^-1 (4-Nextlalpan). The mean and median were 0.822 mg L^-1 and 0.777 mg L^-1, respectively. Irrigation with this water may cause decrease in the yield of sensitive crops to a concentration of B³⁺ of over 0.3 mg L^-1, whereas tolerant crops do not show symptoms at a concentration of B³⁺ in the soil solution of 4 mg L^-1 (^{Page et al., 1990}). It is recommended that the effects of irrigation water be measured directly on the soil and crops, yet it has been found that crop yields decrease as the concentration of B³⁺ increases to toxic levels (^{Pratt & Suarez, 1990}):

Figure 4 Distribution of the concentration of boron in the Mexico City-Mezquital Valley drainage network. Source: Author’s own elaboration. 

Where y is the relative yield of a crop (%); m is the decrease in yield per unit increase B³⁺ concentration; A is the maximum concentration of B that does not reduce yield (threshold); x is the B³⁺ concentration in irrigation water.

For one concentration of B³⁺ in irrigation water, there is a different concentration of B³⁺ in the root zone, depending on the leaching fraction. In this case, the leaching fraction is estimated in 0.45 and this value helps keep low salinity levels, and as the leaching fraction decreases, the salinity in soil water increases, due to the effect of concentration (^{Pratt & Suarez, 1990}) and this represents a risk of toxicity for crops, mainly for bean, which is considered sensitive to electric conductivity (EC < 1 dS m^-1) and B³⁺ (< 1 mg L^-1) (^{Maas, 1990}).

The risk of toxicity by chloride (Cl^-) was estimated with the ionic concentration of Cl^- in irrigation water, and according to the following values (^{Ayers & Westcot, 1985}): without restriction of use when the concentration of Cl^- is < 4 meq L^-1, use restriction is moderate between 4 meq L^-1 and 10 meq L^-1 of Cl^- and the use is not recommended when concentrations are over 10 meq L^-1 of Cl^-. The mean was 5.077 meq L^-1 of Cl^- and the median was 4.935 meq L^-1 of Cl^-. The coefficient of variation was 36 % and indicates very heterogeneous Cl^- values, Q₁ had a value of 3.610 meq L^-1 of Cl^-, the value in Q₃ was 6.215 meq L^-1 of Cl^-, and 95 % of the water samples had less than 7.960 meq L^-1 of Cl^-. According to the distribution of these data (Fig. 5), there is a risk of toxicity due to the concentration of Cl^- for sensitive crops such as bean (^{Maas, 1990}).

Figure 5 Distribution of the chloride concentration in the Mexico City-Mezquital Valley drainage network. Source: Author’s own elaboration. 

The extreme values for the concentration of Cl^-, higher than the upper limit, were found in drainage water: 12.98 meq L^-1 (4-Nextlalpan), 11.72 meq L^-1 (113-Nith-Debodhé road), 9.52 meq L^-1 (97-Bangandhó), 8.54 meq L^-1 (56-Actopan) and 8.27 meq L^-1 in the 129-Mercado-Ixmiquilpan sampling station.

Content of nutrients and organic matter in wastewater

Regarding the content o N, P, K, Ca, Mg, total solids (TS) and total volatile solids (TVS or total organic matter OM), Table 3 shows the corresponding data. The following sequence was found, from higher to lower concentration: Ca > Mg > K > N > P. This water contains organic matter (276 mg L^-1), as well as a considerable amount of total salts (>770 mg L^-1), mostly sodium and bicarbonate (^{Cuellar-Carrasco et al., 2015}; ^{López-García et al., 2016}), which creates the risk of salinization for soils and crops irrigated with this water.

According to the data obtained by ^{Can-Chulim et al. (2017)} the increase in the ionic concentration reduces the percentage of germination of Phaseolus Vulgaris and NaHCO₃ was the salt that caused the lowest percentage of germination, in this sense, a negative effect on the bean crops irrigated with wastewater in the Mezquital Valley is expected.

Regarding the content of organic matter, the remaining residue after calcination does not provide a precise distinction between organic and inorganic residues, given that the loss by calcination is not limited only to organic matter, but also includes losses produced by the decomposition of some mineral salts. It is recommendable to determine the chemical oxygen demand and biochemical oxygen demand in wastewaters (^{Eaton et al., 1998}). This investigation only provides an approximation to the organic matter content by determining the total volatile solids.