Sewage water

There are four sources of wastewater: domestic or urban, industrial, agricultural uses and rain. Although most (about 90%) comes from the first two, the urban agricultural and rainwater uses acquire ever greater importance due to runoff of fertilizers (phosphates) and pesticides are the main causes of aging of lakes and marshes , a process called eutrophication.

The wastewater is a mixture of organic and inorganic materials suspended or dissolved in water. Most of the former are food and vegetable waste, feces, mineral salts, soaps and synthetic detergents. Greater use of wastewater in agriculture is carried out in the Irrigation Districts 03 and 100 (Tula and Alfajayucan, respectively) located in the Mezquital Valley, Hidalgo, where it is recorded that 120 years began to use these waters from the metropolitan area of Mexico City.

Today the cultivated area is 91 036 ha and fodder crops such as corn, vegetables, grasses occur, etc. Each year the availability of residual water and irrigated area (^{CONAGUA, 2008}) increases. Although Tulancingo water pollution has not yet acquired the dimensions of the problems are in large cities, should not be overlooked that there are contaminants that if they are not served with skill can become a difficult problem solve.

Tulancingo has insufficient sanitary sewer system during the rainy season usually f looded some areas the water sprout public sewers and even within homes. It is recorded that from about 50 years ago the DR 028 makes use of wastewater for irrigation has 997 forage crops such as alfalfa, grasses and corn. Today is third surface of irrigated with wastewater from Hidalgo (^{CONAGUA, 2009}).

Soil contamination

Although the use of waste water for agricultural irrigation has advantages such as increased nutrients such as nitrogen (N), phosphorus (P), potassium (K) and organic matter (MO), in addition to providing moisture to crops has disadvantages when its use is not considered inappropriate or soil conditions on which they are used. Another form of soil contamination by wastewater use is the possible accumulation of heavy metals and salinization (^{Montoya, 2010}).

^{Carrillo et al. (1992)} indicate that the use of wastewater in the Valle del Mezquital causes the introduction of the following heavy metals to soil: lead (Pb), chromium (Cr), cadmium (Cd), nickel (Ni), copper (Cu) , manganese (Mn) and zinc (Zn) in different concentrations, which, when compared with the values of the heavy metals found in the well water, the excess over ten times concentration.

Regarding the type of crops that could be irrigated with treated water are known to the fruit, due to its high sensitivity to certain elements such as boron, chlorine and sodium, they are "prohibited" in irrigated areas with water containing high concentrations of the elements aforementioned.

Vegetables have a higher tolerance range wastewater, however, many of them that have direct contact with the waste water, such as lettuce, carrots, beets and radish, among others, can reach contaminated with heavy metals and plant pathogenic organisms (such as Erwinia spp.) and enteric bacteria, which are those found in the human intestines and cause gastrointestinal diseases (^{Sáenz, 1998}).

The potentially toxic elements (EPT) are those who are involved in biochemical and toxicological processes (^{Tiller, 1989}). For better location, are those metals of the periodic table with atomic number greater than 20, excluding the alkali metals and alkaline. This term is used to classify metals are environmental contaminants among which are: Pb, Cd, Cu, Zn and Ni.

According to ^{Gómez (2002)}, there are two sources of heavy metals in the soil: natural and anthropogenic. Heavy metals contained in the original material or unweathered, are concentrated in soils. Very high concentrations in soils may cause some metal accumulation in plants and cause toxic effects in animals that consume them. They should also be considered volcanic activities that emit heavy metals such as arsenic (As), mercury (Hg) and selenium (Se) (^{Bradl, 2005}).

The use of wastewater in agriculture can increase income MO and nutrients to cultivated soils, contributing to maintaining and increasing soil fertility, but can also bring harmful environmental effects that impair soil quality. That is, the dynamics of organic matter in soil is important because decomposition influences the release of organic and inorganic molecules linked to it (^{Zamora et al., 2008}). Therefore, the input sewage sludge for several years may modify the chemical features and soil fertility (^{Rovira et al., 2002}).

Chemical determinations in soil

The measured pH is moderately alkaline to alkaline, according to (NOM-021-RECNAT, 2000). The values for samples collected are: Jagüey del Cerro, 7.62, and 8.20 in Tulancingo Santa Ana. The sample obtained in the first no salinity problems (0.79 S m-1). Meanwhile in Santa Ana if any (4.86 S m-1).

As for the permanence of organic matter in the soil, it is transient and continuously renewed by the addition of plant residues or other (^{Ramos, 2000}). The Jagüey del Cerro the content is very low (0.27%) and Santa Ana sample is low (1.34%). Inorganic nitrogen concentrations were low in both Jagüey of hill (13.8 mg kg-1) and Santa Ana (18.4 mg kg-1).

The phosphorus in the Jagüey del Cerro of hill, where irrigation is pumped classified as moderately low (12.05 mg kg-1) it is, unlike Santa Ana where it is very high (125.86 mg kg-1). With regard to the concentrations of potassium in the soil, it was observed that are considerably high in the two samples.

The calcium levels in the soil fluctuate medium (1 807 mg kg-1) in Jagüey of hill and high (4321 mg kg-1) in Santa Ana. In the two samples concentrations ranging from moderately high to high with presents Fe (22.4 to 28.7 mg kg-1) and Mn (27.18 to 26.6 mg kg-1); similarly it was obtained for Cu (0.9 to 2.12 mg kg-1) and Zn (1.6 to 4.86 mg kg-1) medium to high concentrations occur. The boron is needed in small quantities for crop development it promotes the germination of pollen grains, pollen tube growth and seed formation and cell walls. It is the most common micronutrient that appears as limiting for plants and is the most used as fertilizer.

The concentration of boron in Jagüey del Cerro is found at high levels (1.49 mg kg-1) and in Santa Ana at levels classified as very high (3.05 mg kg-1), indicating toxicity to sensitive crops are established in these sites. According to ^{Vázquez (1997)} general requirements for crops boron present in the area covered. Regarding the texture, the results of Jagüey of hill correspond to a loam and one in Santa Ana clay loam.

Potentially toxic elements (EPT): Pb, Cd, and Cr

The solubility of several elements, including Pb, Cd and Cr, is inversely related to the pH (^{Mc Bride et al., 1997}). The soils of Jagüey del Cerro and Santa Ana had low concentrations: for Pb, 1.1 and 1.06 mg kg-1, respectively, and 1.62 and 0.23 Cd mg kg-1. The samples are below the values considered critical; according to the established by the US-EPA (1993) they are not contaminated soils. Similarly, subject to the NOM-021-RECNAT-2000, the concentrations of these elements are not present a problem of toxicity to plants.

^{Ramos et al. (2001)} concluded that the content of heavy metals in soils irrigated with wastewater will depend on the concentration containing these waters at the time of irrigation and time you have to use them, this relates to the results obtained in this work and the low concentration of EPT can be directly related to the water used for irrigation. Despite not finding problems with EPT continuous monitoring of soils it is required to detect any problem that comes to file in relation to them.

Establishment of kochia-mesquite system

In the area of Jagüey del Cerro design agroforestry system (kochia-mesquite) had as a basis for settlement the problems expressed by the producers involved in the project during the workshops: given the topological location of the land, irrigation is too expensive and that must be pumped and thus costs increase.

The establishment of species agricultural kochia and forestry mesquite was made at the start of rainy season, with plant quality, strains 40 cm deep and 40 cm in diameter dug with a shovel. Before planting was placed in the bottom of the compost strain, with a design of timber boundaries. Figure 2 shows the spatial and temporal arrangement of the species in the agroforestry system; This only covers the management of pruning’s whose purpose is the formation of a thick trunk as possible and right by removing branches that are born at the base of the plant.

Figure 2 Spatial design of agroforestry plantation according to tree species and agricultural crops in the DR 028 Tulancingo, Hidalgo. 

This system attempts a holistic management of natural resources, associating in the same spot without planned herbaceous vegetation for cattle feed, Shrub and/or vegetation tree that can provide positive impacts on the environment and satisfactions that generate additional income for rural producers, such as wood, wood, resins, fruits, etc (^{Musálem, 2001}).

Cost of establishment of agroforestry system kochia-mezquite

The current value of the initial investment to establish one hectare of agroforestry system kochia-mesquite was $5 890.00. Without considering the acquisition of the cart and the horse to transport the harvested forage at an estimated cost of $7 000.00 which, when added, gives an initial investment of $12 890.00 (Table 1).

Table 1 Cost of establishing a hectare of kochia-mesquite agroforestry system in the DR 028 Tulancingo, Hidalgo. 

It does not include the subsoiling and some other activities to be less frequent. In contrast, the cost of establishing a hectare of alfalfa, dominant crop in the area, is $12 058.00, this according to ^{Pérez (2011)}, who conducted an economic assessment of the use of wastewater in the module II District 028 irrigation Tulancingo, Hidalgo.

The yield of one hectare of kochia was 46.7 t. The Instituto de Tecnología del Agua (ICTA, 2002) states that kochia (Kochia scoparia) has shown a high forage potential, and its production of green matter of 56 t ha-1. It is a plant that adapts to a wide variety of climates it is tolerant to saline soils and low temperatures (^{Anaya, 2004}). It is high palatability, especially its palatability, good digestibility and low in fiber. It has a percentage of crude protein of 18 to 28%.

Comparatively, according to ^{Pérez (2011)}, one hectare of alfalfa, dominant crop in the area, has a production ranging between 25.85 and 34.5 t spring in summer; added to this the kochia requires four to six times less water than alfalfa, and has higher yields than this. Thus, more than obvious, the feasibility of kochia as a forage crop in the area (^{Gutiérrez, 2000}).

As mesquite, which has a 100% survival, ^{Franco et al. (2006)} note that due to its low water requirement of great part of arid regions and is set to soils with high salt content, but also develops in clay soils and provides wood and fodder.

Place mesquite on the edge of the land brings benefits CATIE (1998), achieved a clear and safe boundaries of the property, avoiding possible legal conf licts with neighboring; produce timber without interfering with the harvest, act as barriers windbreak protection, increase the scenic beauty of the farm and its fruit is also consumed as food; foliage, like pods, feeding livestock, especially during drought or other fodder shortages.

^{Ramírez (2002)} mentions some of the most common mistakes made by incorporating timber to the boundaries of the land, which directly affect both economic and ecological gains. These are not performing work reseeding, do pruning, not perform thinning, repruning (maul or desmochar), “anillar” trees, not wait shifts adequate short to harvest them, and place the wires fences (barbed wire) directly on the trunks.