INTRODUCTION

Agriculture accounts for approximately 77% of water use in Mexico, followed by the public water supply system with 14%, electrical energy generation with 5% and industry with 4% (^{CNA 2011}). The quality of water for agricultural use and its availability is a topical issue from the social, productive and environmental point of view (^{Ayers and Wescott 1987}). Water pollution and scarcity has a severe impact on ecosystems and the survival of organisms (^{Galadima et al. 2011}). Therefore, the frequency and intensity of droughts in arid areas is increasing, resulting in intensive use of deep well water in northern Mexico’s irrigation districts (^{CNA 2015}).

The Comarca Lagunera covers an area of 220 thousand ha, with a maximum irrigable area of 105 thousand ha (^{García et al. 2006}), most of which is dedicated to fodder production. Alfalfa, maize and sorghum are the main fodder crops (^{Pedroza et al. 2014}). Water is a scarce resource in the Comarca Lagunera due to recurrent droughts and increased groundwater extraction, which generates overexploitation of aquifers by more than 480 hm³ (^{García et al. 2006}). Water extracted from wells at more than 200 m deep contains nutrients and pollutants (^{Sardiñas et al. 2006}), mainly in places where wastewater has been used in agricultural crops, with reports of an upward trend in heavy metal concentrations with a consequent impact on health and the environment (^{Mancilla et al. 2012}). Overexploitation of the aquifer has resulted in the extraction of poor quality groundwater, with a number of negative effects on the environment and health, such as: soil sodicity and salinization that make soils infertile and unproductive; damage due to the clogging of pressurized irrigation systems; plant toxicity damage; arsenic and heavy metal residuals for a high risk to animals and humans (^{Bonet et al. 2011}).

The quality of water for agricultural use is a function of the quantity and type of salts it contains (^{Sarabia et al. 2011}). The high concentration of salts in irrigation water reduces water availability for plants, which means that greater effort is exerted to absorb water, which causes physiological stress due to dehydration and affects their growth and development (^{Pérez-León 2011}). Therefore, the aim of this study was to make a residual soil and plant evaluation of fields dedicated to the cultivation of fodder maize irrigated with deep well water, as indicators of risk to health and the environment.

MATERIALS AND METHODS

Soil and plant sampling

According to the historical chemical water pollution database provided by the National Water Commission (CNA for its initials in Spanish), and by using digitized maps and Geographic Information System (GIS) tools, the area of influence of the present study was delineated. Wells (sites) that contained consistent information on water quality were identified, being those with identification number 2576, 2612, 1103, 1125, 1200, 3387, 950 and 760 (Figure 1) in the sites of La Loma, León Guzmán, La Gallega, Bermejillo, La Jarita, Venecia, La Rosita and La Purísima, respectively, which are representative of the Comarca Lagunera irrigation region.

Figura 1: Map of the sites where the main monitored wells are located for the current analysis of water for agricultural use. 

Soil sampling was carried out from April to July 2014 in a one-hectare area planted with fodder maize and irrigated with water from each deep well. For the extraction of the sample, a Van Walt (USA) Riverside soil auger was used. The samples were extracted in accordance with standard NMX-AA-132-SCFI-2006 (^{SCFI 2006}). Simple random sampling by means of a diamond-shaped path was used, taking five soil samples at a depth of 20 to 30 cm, and then mixing them to obtain a 1 kg composite sample. Each sample was packed and labelled in a polyethylene bag for transfer to the laboratory. The sampling of the plant material from May to July 2014 was done by taking one plant per soil sample extraction site. The samples were washed with distilled water and 5% HNO₃, dried in an oven for 24 h and then crushed, packed and labeled in a polyethylene bag.

RESULTS AND DISCUSSION

The quality of the water for agricultural use is a function of physico-chemical characteristics, such as the concentration of dissolved salts (EC), relative presence of sodium (SAR), content of carbonates and bicarbonates that condition the pH and the concentration of different cationic elements and heavy metals (^{Castellón et al. 2015}). It was found that the As concentration in the soil is above the limit allowed by NOM-147-SEMARNAT-2004, which establishes a maximum value of 22 mg kg^-1 in agricultural soil (^{SEMARNAT 2007}). This level represents a current and potential risk due to its permanent disposition, and also because it can accumulate in the plant; for example, from maize fodder, it can pass directly to fatty tissues and animal milk. The absorption of As from contaminated water has been reported for hydroponically-grown Lactuca sativa L. (^{Hüvely et al. 2011}).

Of the four cationic elements analyzed in the soil for agricultural use, it was found that Ca and Na were at higher concentration levels, with average values of 15.81 and 15.63 meq L^-1, respectively, whereas Mg and K had values of 5.59 and 0.92 meq L^-1, respectively. As and Pb were found at higher concentration, with values of 43.13 and 44.68 mg kg^-1, whereas Cd and Zn recorded concentrations of 5.42 and 1.28 mg kg^-1, respectively. The behavior without change over time of the cations of Ca, Mg, Na and K, as well as of the metals Cd, Pb and Zn, means that the concentrations remain stable but high from April to June, a period of time in the region when maize fields are irrigated with poor quality deep well water (^{Azpilcueta-Pérez et al. 2017}). The above is environmentally relevant, especially in the case of heavy metals, given that they are persistent environmental pollutants, with a minimum density five times that of water, so they cannot be metabolized by organisms, with severe toxicity consequences (^{Ehi-Eromosele and Okiei 2012}).

The cations of Ca, Mg, Na and K did not show temporal variation (p ≤ 0.05) in the analyzed months of 2014. By contrast, As had significant variation, with a higher concentration in the months of June and July (49.25 and 48.5 mg kg^-1), whereas the lowest concentration was recorded in April (31.5 mg kg^-1). Regarding Cd, Pb, and Zn, no variation was found (Table 1). The Bermejillo, La Gallega and La Rosita sites stand out with Ca concentrations in the soil of 20.36, 24.24 and 23.09 meq L^-1, respectively, while the rest of the sites had similar values, with the exception of La Loma, which had a concentration of 5.64 meq L^-1 (p ≤ 0.05). For Mg, Na and K there is similar variation among sites; La Gallega stands out with the highest Mg values (10.13 meq L^-1), whereas Bermejillo and La Rosita had high Na concentrations. The rest of the sites had significantly lower values of these elements. In relation to K, the Bermejillo region has a concentration of 1.85 meq L^-1 (Table 2).

Tabla 1: Temporal variation of the mean concentration of different cationic elements (meq L^-1) and heavy metals (mg kg^-1) contained in soil for agricultural use irrigated with deep well water. 

Tukey’s range test (p ≤ 0.05). Values with the same letters within the same column are statistically equal.

Tabla 2: Spatial variation of the mean concentration of different cationic elements (meq L^-1) and heavy elements (mg kg^-1) contained in soil for agricultural use irrigated with deep well water. 

Tukey’s range test (p ≤ 0.05). Values with the same letters within the same column are statistically equal.

Tabla 3: Temporal variation of the mean concentration of different cationic elements (meq L^-1) and heavy elements (mg kg^-1) contained in fodder maize plants irrigated with deep well water, at three sampling dates. 

Tukey’s range test (p ≤ 0.05). Values with the same letters within the same column are statistically equal.

K and Ca had higher concentrations, with average values of 1.064 and 0.563 meq L^-1 in the plant, respectively, whereas Mg and Na recorded lower concentrations with values of 0.25 and 0.072 meq L^-1, respectively. Pb and Zn were found with greater concentration in the Comarca Lagunera, with values of 26.86 and 41.58 mg kg^-1, whereas As and Cd were at lower concentrations, with values of 2.22 and 3.4 mg kg^-1, respectively. As varied significantly, with La Purísima having the highest concentration (61.58 mg kg^-1), whereas the rest of the sites had concentrations between 37.27 and 49.42 mg kg^-1, except for Bermejillo, which had the lowest concentration (27.88 mg kg^-1). Cd did not vary among sites and Pb was statistically higher in La Purísima (58.95 mg kg^-1). The concentration of Zn was greater in La Gallega, La Purísima and León Guzmán, whereas the rest of the sites had lower concentrations (Table 2). In most of the region, a high As concentration was recorded in the soil, above the limit established in standard NOM-147-SEMARNAT-2004 (^{SEMARNAT 2007}), although even in maize plants the concentrations found are above what is allowed (^{FAO 2009}), which implies a risk to health and the environment, due to the high toxicity effect of these elements (^{Sandeep et al. 2012}). The foregoing demands that this problem be addressed through better integrated water management actions in the region, in accordance with what is recommended by national and international organizations (^{ONU 2012}, ^{IMTA 2012}), or to carry out some treatment prior to using the water to irrigate crops (^{Bonilla et al. 2015}).

As and Cd have a residual effect due to their high concentrations in most of the sites studied, without significant variation, which indicates persistence, while Pb and Zn varied with a maximum concentration of 60.25 mg kg^-1 in May. Ca, Mg, Na and K did not vary among the monitored sites, which indicates that the contents of these elements remained stable during the study period. The concentration of heavy metals in the plant did not vary among sites; As was the only one that had significant spatial variation, which agrees with the spatial variation of this element in the soil. There is a high contamination of cationic elements and heavy metals in soils irrigated with deep well water and in fodder maize plants. Ca, As, and Pb had the highest concentrations in the soil, while As had the highest concentration in June and July.