Water is one of the most vital and perhaps one of the most precious assets of the world’s population, but its global situation is worrying. Its quality has been affected mainly due to the contamination by anthropic activities, which has been aggravated in the last years. Water is a fundamental means of life and indispensable to a wide range of human activities, such as public and industrial supplies, agricultural irrigation, electric energy production and leisure and recreation, as well as the preservation of aquatic life  .
Some heavy metals are highly toxic substances and are not compatible with most biological treatment of existing effluents. Toxic heavy metals occur in the aquatic environment in various forms: in ionic solution or in the form of organic or inorganic soluble complexes  .
Recently, there has been a major concern in food associated with health risks arising from the release of toxic contaminants. Among the main contaminants are the toxic metals, because they are extremely persistent in the environment and accumulate in plant cultures  .
Heavy toxic metals in excess can cause many diseases and serious physiological problems, since they are cumulative in the human body. The residues containing cadmium and chromium have high contamination power, and it is easy to reach the water table  .
Heavy metals are chemical elements (metals and some semimetals) that have a density greater than 5 g/cm3. They are generally toxic to living organisms and are therefore considered pollutants  .
The toxicity of heavy metals dissolved to plants and animals is entirely dependent on the concentration of metals in the free form (dissolved in the water column)  .
The parameters analyzed for an adequate interpretation of water quality for irrigation should be related to their effects in each crop, as well as in soil and irrigation management, thus avoiding problems with public health  .
Monitoring of toxic wastes in fruits and vegetables is essential to protect consumers, and to verify compliance for good agricultural practices and to ensure trade activities. Good agricultural practice can be understood as the correct and effective use of agrochemicals and fertilizers and water for irrigation, considering the possible toxicological risks involved in their application, so that the residues are the smallest possible and toxicologically acceptable  .
Toxic heavy metals
Cadmium (Cd) has a dermal, respiratory and gastrointestinal absorption. It has been found in various organs such as the pancreas, testis, thyroid, salivary glands and heart, among others. The kidney is the organ in which a critical concentration of cadmium is first reached as a consequence of the accumulation of this metal. The renal cortex is the region where cadmium is most concentrated. Cadmium intoxication is characterized by renal damage with proteinuria. Cadmium also has an effect on the nervous system, having repercussions on the visual system, on the smell, or provoking polyneuropathies and diverse neurological alterations. Neuropsychological effects are also attributed to exposure to cadmium, such as changes in memory, cognitive and psychomotor velocity, among others  .
Chromium (Cr), is also a heavy metal of high hardness, much used in the field of metallurgy to increase resistance to corrosive agents. The most common oxidation states of chromium are: Cr+3, Cr+6. They are more stable in tri and hexavalent forms, besides the elemental form. Most of the toxic effects induced by chromium occur in the respiratory tract, when the route of introduction is pulmonary. Some systemic effects are reported in very high concentrations, but usually of lower prevalence. Toxic effects in occupationally exposed individuals at high concentrations of chromium, particularly chromium (VI), include nasal septum ulceration and perforation, respiratory tract irritation, possible cardiovascular, gastrointestinal, haematological, hepatic and renal effects, as well as elevated risk of cancer pulmonary function  .
Lead toxicity (Pb) generates from clear or clinical effects to subtle or biochemical effects. The latter involve various organ systems and biochemical activities. In children, the critical effects affect the nervous system, whereas in adults, with excessive or even accidental occupational exposure, the care is with peripheral neuropathy and chronic nephropathy. In rare situations, the effects on heme synthesis provide indicators of exposure to lead in the absence of chemically perceptible consequences. Also the gastrointestinal and reproductive systems are the target of lead intoxication  .
2. Materials and Methods
The present research was carried out with an investigative character, to obtain information about the presence of toxic heavy metals Cd, Cr and Pb. These metals are not essential to the growth of vegetables, but can cause serious problems for the environment, as well as for the organism human.
The research was carried out in the UEMG Experimental Farm lake, Passos-MG Academic Unit (Figure 1). The lake was divided into ten strategic points for sampling. For pH analysis, plastic flasks were used, and 200 mL of water were collected for triplicate measurements. PH measurements before and after filtration were performed in the “in natura” water without pretreatment.
For analysis of toxic heavy metals, the collection was carried out in polypropylene flasks with a lid, and 1000 mL of water was withdrawn per flask. For determination of Cd, Cr, and Pb, the samples were treated with acid digestion. The equipment used was a Perkin Elmer Atomic Absorption Spectrophotometer-model PINAACLE 900 T.
At the sampling points, the flasks were opened for collection of water, avoiding the contact of the mouth of the bottle with the hands or any metal object that
Source: Google Earth.
Figure 1. Approximate area of the experimental farm lake: 40,600 m2.
causes problems of contamination, avoiding air space over the liquid. The flasks were closed immediately after sample collection and shaken for complete homogenization of the preservative, as three bottles of water were collected for each sampling point. 0.5 mL of 40% HNO3 (nitric acid) was added to 100 mL of sample.
2.2. Reagents and Solutions
All reagents used in this work have analytical purity (P.A.), without any purification.
3. Analytical System
The pH measurements in the water samples were performed in natura, without any previous treatment. A pH meter brand LOGEN mod. LS300.
Analysis of Toxic Heavy Metals
The concentration of the metals Cadmium (Cd), Chromium (Cr) and Lead (Pb) were determined with acid digestion of the samples, and the solutions obtained were analyzed in a Perkin Elmer Atomic Absorption Spectrophotometer-PINAACLE 900 T model, at Institute of Chemistry of São Carlos-IQSC, University of São Paulo, Brazil, in the Central of Instrumental Chemical Analyzes (CAQI).
4. Results and Discussion
The values of toxic heavy metals found were: Cadmium: 0.727 mg/L at 0.754 mg/L; Chromium: 0.177 mg/L at 0.256 mg/L and Lead: 0.023 mg/L at 0.081 mg/L. Significant amounts, however, are above the limit established by CONAMA 357/2005 as shown in Table 1 & Table 2. The hazards of contamination of water by toxic heavy metals are not restricted only to the direct consumption of this water, but also to the consequences in the chain when we refer to bioaccumulative toxics (Figures 2-4).
The mean pH values for water at all collection points were classified as low (pH < 7.0), indicating that there was no risk of obstruction to the localized irrigation system. Very low pH values can cause corrosion and high values can cause scale problems in pipes. Regarding the electrical conductivity, there was a low risk of water obstruction at all points of collection  .
Table 2 Concentrations of toxic heavy metals (Cd, Cr, Pb) from samples of irrigation water collected in the lake.
Table 3 Concentrations of toxic heavy metals (Cd, Cr, Pb) from samples of irrigation water collected in the lake. Compared to the specification required by Conama 357/2005.
Table 1. Average values and pH standard deviation of irrigation water.
Table 2. Average values and standard deviation of toxic heavy metals.
Table 3. The results of heavy metals presents in irrigation water.
Figure 2. pH values found in the samples of irrigation water prior to filtration.
Figure 3. pH values of samples at irrigation water after filtration.
Figure 4. Heavy metals obtained at water samples.
It is concluded that the values of toxic heavy metals are above the limit established by CONAMA 357/2005, being of extreme importance the analysis of these heavy metals also present in plants and soil, irrigated by this same water. Therefore, we suggest the construction of a treatment unit near the lake in the municipality of Passos-MG, using polyurethane foams derived from vegetable oil subjected to acid treatment for the adsorption of these toxic heavy metals, which may be bioaccumulative, causing damage to the human health and damaging the environment.
To Universidade do Estado de Minas Gerais (UEMG)-Unidade Passos e à Fundação de Amparo à Pesquisa do Estado de Minas Gerais (FAPEMIG) for financial support.
 Pimentel, C.A., Camelo, F.L., Silva, da.O.de.D., Oliveira, de.A.B., Gusmao, S.da.A. and Alves, M.A. (2015) Avaliacao ecotoxicologica da qualidade das aguas do rio Catarino, Re-alengo, RJ. In: XII Congresso Nacional de Meio Ambiente de Pocos de Caldas, 7, 2015, Congresso, Pocos de Caldas, 1-7.
 Novaes, C.A. and Aguiar, de.P.M.R.M. (2002) Remocao de metais pesados de efluentes industriais por aluminossilicatos. Química Nova, 25, 1145-1154.
 Valentim, E., Hazin, A.C., Nascimento, F.A., Silveira, B.P., Almeida, M.A. and Silva, S.C.F. (2013) Determinacao de metais pesados em amostras de agua usada para irrigacao de hortalicas cultivadas em beira de estrada. Scientia Plena, 9, 2-3.
 Carvalho, A.W., Bosco, D.M.S. and Jimenez, S.R. (2004) Remocao de metais pesados de efluentes aquosos pela zeolita natural escolecita-influência da temperatura e do pH da adsorcao em sistemas monoelementares. Química Nova, 27, 734-738. http://quimicanova.sbq.org.br/detalhe_artigo.asp?id=4010
 Souza, de.C.L., Lima, S.de.F.R. and Guedes, A.de.J. (2005) Metais pesados em agua do rio Jundiai-Macaiba/RN. Revista de Geologia, 18, 131-142.
 Silva, da.C.B.L. (2008) Avaliacao espaco temporal de metais pesados no rio Paranaiba do Sul e rio Imbe por meio de plantas de Eichhornia crassipes (Mart.) Solms (AGUAPE), seston e sedimento. Campos do Goytacazes-RJ, 2008. 99f (tese de mestrado em biociencias e biotecnologia) Universidade Estadual do Norte Fluminense, RJ.
 Minghini, I. (2007) Avaliacao qualitativa da agua residuaria de abatedouro de aves para fins de reuso em irrigacao, Botucatu, SP: UNESP, 2007. 66f (Tese de mestrado em irrigacao e drenagem) Faculdade de Ciências Agron?micas, Botucatu.