Agriculture, in Brazil and worldwide, is the activity that consumes the most water in comparison to the consumption of human, herds and industrial (Oliveira et al., 2013; Holanda et al., 2016). This peculiarity, in part, is due to insufficient and irregular rainfall, high temperature air and soil, low humidity, high evaporation rates and evapotranspiration. Beyond the quantitative aspects (regular   rainfall     and    volume    of    water   available), specifically in agriculture, the world also needs chemical quality water that allows the use in the irrigation without causing high edaphic degradation and high cultures production losses (Cavalcante et al., 2012; Ganiyu et al. 2018). In addition to these inconveniences, the surface and groundwater salt content in semi-arid areas, which is generally above the limits tolerated by plants to human food   and    animals   importance,   results   in  a  serious irrigated agriculture obstacle (Andrade Júnior et al., 2006; Ammari et al., 2013; Koff et al., 2017). When considering the limitations of water resources, in quantity and quality, and the growing demand for food in the world for nearly fifty years, have awakened as an alternative the use of restrictive water due to the excess of salts for irrigation of plants of economic importance (Costa; 1982; Silva Júnior et al., 1999; Rhoades et al., 2000; Ben-Asher et al., 2006; Holanda et al., 2016; Vieira et al., 2016; Murad et al., 2018). However, even assuming this need not all high waters salt concentration, such as sea water or equivalent concentration, should be used for irrigation. This assertion is based on Ayers and Westcot (1999), Cavalcante et al. (2012) and Holland et al. (2016), when they affirm that plants with food and commercial importance are, in great majority, low or medium tolerance to the water salinity.

The use of water with restrictions due to excess salts in irrigated agriculture, irrespective of the source (surface or underground source), requires the monitoring of the electrical conductivity, cation and dissolved concentrations (Medeiros et al., 2003; Barroso et al., 2011; Jang et al., 2012; Oliveira et al., 2013; Lira et al., 2015; Chu et al., 2016; Rassol et al., 2017; Ganiyu et al., 2018). This monitoring, if possible, should be at various times of the year, considering one or more sources of property, municipality or watershed, making possible a specific panorama and the elaboration of strategies of use of this water resource.

The salinity studies the total concentration of salt mixture effects, or saline complex, dissolved in the irrigation water - CS (g L^-1); the value can be estimated by the product between the water electrical conductivity- ECw (dS m^-1) and the 0.64 multiplication factor waters with up to 5 dS m^-1 or 0.80 for salinity water higher than 5 dS m^-1 (Rhoades et al., 2000). The total salt concentration can also be obtained analytically by the calcium (Ca²⁺), magnesium (Mg²⁺), potassium (K⁺) and sodium (Na⁺) quantification in equivalence with chloride (Cl^-), bicarbonate (HCO₃^-), carbonate (CO₃^-) and sulfate (SO₄^2-) (Richards, 1954). Toxicity refers to the specific effects of some ions such as sodium, chloride, magnesium, boron and the ratio of sodium to total cations dissolved in water. Sodium is evaluated by the sodium adsorption ratio - SAR (Ayers and Westcot, 1999), by the value of residual sodium carbonate (RSC) which is the difference between the sum of carbonate plus bicarbonate plus calcium plus magnesium RSC = (CO₃^2-+HCO₃^-) - (Ca²⁺ + Mg²⁺) (Richards, 1954; Almeida, 2010; Maskooni et al., 2017). The aim of this study was to monitor monthly the electrical conductivity and the cationic and anionic contents in amazon well water.

The work was carried out in experimental area of the Center for Human   and   Agrarian   Sciences,   State   University   of  Paraíba, Campus - IV, municipally Catolé do Rocha, State of Paraíba, Brazil. Water samples were collected monthly, from January to December 2017, from an Amazon well (6° 21’ 11”S, 37° 43’ 21” W and 244 m above sea level) with a diameter of 4 m, depth of 11 m and an annual mean daily flow of 2.2 m³ h^-1. The region climate is BSwh’, according to Köppen classification (Alvares et al., 2013), defined as hot semi-arid. The municipality rainfall is less than 800 mm and are irregularly distributed in the rainy season, with more than 65% precipitated from February to April month; the annual averages temperature and relative humidity are, respectively, 27°C and below 50%. The soil of the area, according to the criteria of the Brazilian Soil Classification System (Embrapa, 2013) was classified as Eutrophic Entisol. The physical and chemical properties (fertility and salinity) in a sample of 0-0.2 m layer of this soil were analyzed (Embrapa, 2017) with their values given in Table 1.

The electrical conductivity of the water was determined by direct reading in conductivity. Ca²⁺ + Mg²⁺ was analyzed by complexometry with disodium EDTA, using Eriochrome-Black as an indicator. Ca²⁺ determined by complexiometry in the presence of carbonic acid indicator calcone. Na⁺ and K⁺ were determined flame spectrometer. For CO₃^2- and HCO₃^- the titration was with H₂SO₄ using the respective indicators, phenolphthalein and methyl orange. After determination of carbonate and bicarbonate was added K₂CrO₄ and titrated with AgNO₃ for determination of Cl^-. SO₄^2- was determined in photocolorimeter after the addition of HCl and BaCl₂.2H₂O (Richards, 1954). The average of monthly data of electrical conductivity and ion concentration were related to each other by a regression test. For each ionic component was calculated the minimum, average and maximum (Silva Júnior et al., 1999). The average of the mean electrical conductivity and ion concentration of each component was used to simulate the irrigation full blade 400 mm, according to the following equation (Richards, 1954; Rhoades et al., 2000):

Sad = (Ecw × 0.64 × Iwd) / 1,000;

were: Sad = total salts added by water (kg ha^-1); Ecw = Electrical  conductivity water (dS m^-1); Iwd = Irrigation water depth (mm), 1 mm = 10 m³ ha^-1 = 10,000 L ha^-1, thus 400 mm is equivalent to 4,000,000 L ha^-1. The quantification of each sais added was calculated from the total volume applied and their respective participation in the water.

The electrical conductivity in the period from January to December/2017 increased from 0.89 to 1.16 dS m^-1 resulting in a saline increase of 30.3%; this value is considered high and already uses moderate restraint (Ayers and Westcot, 1999; Medeiros et al., 2003; Almeida, 2010, Cavalcante et al., 2012, Holanda et al., 2016) the great importance economic cultivated plants. The relation between the electrical conductivity and the total dissolved salts (TDS) in water (Figure 1) was 64%. This level expresses high dispersion, but the increasing factor between both is consistent with Richards (1954) and shows the dependence between the electrical conductivity and the total dissolved salts, with 80% correlation contents salts water dissolved, due to the increase in electrical conductivity of the water. In this way, it is verified that although the electrical conductivity correlates with the content of dissolved salts, it does not faithfully represent the  concentration  of  salts  but  by  its value the total solubilized in water (Richards, 1954; Rhoades et al., 2000; Holanda et al., 2016) and in the soil is estimated (Cavalcante et al., 2012; Ribeiro et al., 2016).

In Figure 2, it shows the mean, average and maximum values of cations, anions and electrical conductivity throughout the year 2017. The order of the mean cation contents was 7.06 (Na⁺) > 1.36 (Ca²⁺) > 0.83 (Mg²⁺) > 0.32 (K⁺) mmol_c L^-1, for the anions of 7.45 (Cl^-) > 1.87 (HCO₃^-) > 0.42 (SO₄^2-) > 0.15 (CO₃^2-) mmol_c L^-1 and, the electrical conductivity of water amplitude was 0.89 to 1.16 dS m^-1 with 1.01 dS m^-1 mean (Figure 2). These values fluctuate with the mineralogical nature, with the intensity of the transformation of the source material and with the solubility of the primary components of the soils, especially the sodium, calcium and magnesium contents (Ribeiro et al., 2016).

Among the cations the predominance is sodium and anions the chloride, with concentrations in general similar, but with chloride (7.45 mmol_c L^-1) supremacy in relation to sodium (7.06 mmol_c L^-1) (Figure 3), which is why most scientific investigations refer to both species as expressed the literature on salinity. This superiority, compared to the other components of each species, is not only due to the aggressive action of Na⁺ in the soils physical attributes depletion, but also in the structure loss and pore space reduction to the root growth, nutritional and plant toxicity (Richards, 954; Ayers and Westcot, 1999; Almeida, 2010).

Despite the higher molar concentration of chloride compared to sodium, the percentage share of sodium exceeds chloride within valence, obeying the order 76.6 (Na⁺) > 12.8 (Ca²⁺) > 5.8 (K⁺) > 4.8 (Mg²⁺) for cations and 65.4 (Cl^-) > 28.5 (HCO₃^-) > 5.0 (SO₄^2-) > 1.1 (CO₃^2-) for ions (Figure 3B). These orders differ from those obtained for the concentrations (Figure 2) because they are calculated based on the concentration in mg L^-1 and vary according to the equivalent each species weight (Cavalcante et al., 2012). This situation resembles the sequences presented by Silva Júnior et  al. (1999), Costa (1982), Medeiros et al. (2003), Andrade Júnior et al. (2006), Leal et al. (2009), Barroso et al. (2011), Oliveira et al. (2013) and Holanda et al. (2016), respectively for crystalline Brazilian Northeast waters in the Paraíba, Piauí, Rio Grande do Norte and Ceará, States all of which are inserted in Brazil semi-arid region.

The reported situation resembles the sequences recorded by Jang et al. (2012) in Taiwan, Hassanli and Ebrahimian (2016) in Karaj, Maskooni et al. (2017) in Iran, Rassol et al. (2016) in Pakistan, Koffi et al. (2017) in Ghana, Gill and Terry (2016) in Australia, Ganiyu et al. (2018) in Nigeria, among many others. The results also show that composition terms and salt content the semi-arid world present situation, even considering the differences in soil source materials, is very similar between the West and the East lands. Waters with electrical conductivity of up to 1.16 dS m^-1, even if they had moderate  restrictions  on plants and soil, if the world had enough water for irrigation to meet the world food and livestock requirements, it would suggest less employment saline waters in agriculture. This statement seems consistent since soil irrigation with depth water 400 mm (1.16 dS m^-1) during drought 120 days in the Catolé do Rocha municipality, Paraíba State, Brazil, with Ca²⁺, Mg²⁺, Na⁺ and K⁺, concentrations respectively, 2.59, 1.29, 6.75 and 0.54 mmol_c L^-1, would add to the soil a total of 2,970 kg ha^-1 which salts, 1,892, 633, 258 and 187 kg ha^-1 are salts of sodium, calcium, potassium and magnesium, respectively.

The Amazon well water electrical conductivity during the 2017 year increased from 0.89 to 1.16 dS m^-1. The salts total  added to the soil with an irrigation depth of 400 mm  was 2,970 kg ha^-1, of the 63.7; 21.3; 8.7 and 6.3% corresponded to salts of sodium, calcium, potassium and magnesium. The amplitude of the electrical conductivity and the addition of salts of the soil by a water depth not high, justify the monitoring of the quality of irrigation water in the semiarid region from anywhere in the world.

The authors have not declared any conflict of interests.

The authors thank the National Council for Technological Development Scientific (CNPq), the Higher Education Personnel Coordination (CAPES) and the Science Technology in Salinity National Institute (INCTSal), respectively, for the awarding of scientific initiation scholarships in undergraduate, postgraduate Master's degree, PhD and research productivity and financial aid to carry out the project activities.