**2. Heavy metal contamination of soils, plants, and drinking water**

The excess of heavy metals in the soil inhibits the development of microorganisms and disrupts processes related to the transformation of organic matter. It also causes the accumulation of toxic elements in plant tissues, leading to disturbances in plant reproduction and thus lowering their nutritional value [13–15]. Excessive accumulation of the mentioned elements in the soil, however, is harmful to plants in particular [3, 7, 16, 17].

In soil, heavy metals can occur in different forms—dissolved in soil solution, exchangeable in organic and inorganic components, being structural components of soil grids, and as insoluble sediments with other soil components. The first two forms are the most available to plants. The concentration of elements in the soil depends on the pH of the soil—the higher (to slightly alkaline) the higher the immobilization of elements. The mobility of heavy metals in the soil varies. In acidic soils, Cd, Ni, and Zn are particularly mobile, Cr is moderately mobile and Cu and Pb are immobile. In neutral and alkaline soils, Cr is highly mobile, Cd and Zn are moderately mobile, and Ni is immobile. Other factors, such as cation exchange capacity, redox potential, organic matter content, type and amount of clay minerals, and oxide content of antagonistic elements Fe, Al, and Mn, also determine the increase of heavy metals in the soil and thus their availability to plants [18].

The natural cadmium content in soils is 0.2–1.05 mg kg−1 d.m. It is a highly mobile, active element, and easily assimilated by plants because of the available form of Cd2+ ion. The environmental hazard is related to the fact that Cd is one of the most toxic metals that show adverse effects on soil biological activity, plant metabolism, human health, and the animal kingdom. Excess cadmium in the plant manifests itself by twisting of leaves and the appearance of brown spots on the leaves [7].

Another toxic element is lead. Its natural content in soil is strongly related to the composition of the rock substrate. It is characterized by the lowest mobility among heavy metals. The highest Pb content in soil is found in highly industrialized areas. Lead can enter the body from two sources—the food chain and through inhalation of soil dust. It is a very dangerous metal with negative effects on humans, animals, and plants. Excess lead leads to reduced yields and dark green or red spots on leaves. Lead content in soil exceeding 500 mg kg−1 is a toxic value. A characteristic feature of this heavy metal is its accumulation in the human body, as it does not disintegrate in this environment. Getting into the human body a dose of about 20–50 g leads to death [7, 19–23].

In the case of copper, its excess causes tissue damage and elongation of root cells, alteration of membrane permeability and leakage of ions (e.g., K) and solutes from roots, peroxidation of chloroplast membrane lipids and inhibition of photosynthetic electron transport, immobilization of Cu in cell walls, in cell vacuoles and nondispersive Cu-protein complexes, as well as DNA damage and consequently inhibition of photosynthetic processes. Manganese, on the other hand, has a significant effect on some soil properties, particularly raising pH. Mn compounds are known for their rapid oxidation and reduction under varying soil conditions, thus oxidizing [7].

Nickel has become a major pollutant that is released during emissions from metal processing and increasing coal and oil burning, sludge application. Some phosphate fertilizers may also be an important source of Ni [3, 7].

In Poland, the permissible content of heavy metals (so-called risk-causing substances) on agricultural land is defined by the Regulation of the Minister of Environment of September 1, 2016, on the manner of conducting the assessment of land surface pollution (Annex No. 1 to the Regulation) [24]. According to the Ordinance, risk-causing substances that are particularly important for the protection of the earth surface and the permissible contents of these substances in the soil and the permissible contents of these substances in the soil [mg kg−1 dry mass of the earthy parts of the soil (<2 mm)] are defined (**Table 2**)—for depths 0–0.25 m ppt and more than 0.25 m ppt, with the division taking into account the soil groups and separated based on their use, the soil subgroups separated based on the soil properties (defined for the soil group II) and the soil and groundwater permeability.

Koncewicz-Baran and Gondek [25] investigated the content of general forms and bioavailable elements (Cd, Cr, Ni, Cu, Pb, Zn, and Mn) in agriculturally used soils showed that among the investigated soils, the natural content of Ni, Cu, and Pb was the highest. In other soils, increased content of Cd and Zn was determined.

Li et al. [26] pointed out the problem of soil contamination in China due to increased industrial development and urbanization. In Hunan Province, Central China, a study was conducted on the content of heavy metals in soil (Pb, Zn, Cu, Cd, As, Hg, Cr, and Ni). The results showed that the content of each heavy metal in the soil varied spatially. The highest accumulation was shown for Cd, followed by Pb, Zn, As, and Hg.

Plants can accumulate toxic metals from the soil over a very wide range. This depends on its temperature, reaction, water capacity, and potential. Under conditions of high immission, plants take up heavy metals from the air through the leaf blades. Strong accumulation of heavy metals in the root system and the aboveground parts of plants is a result of a poorly developed mechanism of chemical homeostasis in plants, which leads to non-selective absorption of elements and creates a high risk of including heavy metals in the food chain system. Changes in toxic metal concentrations in fodder plants may occur as a result of drying, ensiling, and granulation processes. In


#### **Table 2.**

*Permissible content of selected elements causing the risk.*

*Influence of Heavy Metals on Quality of Raw Materials, Animal Products, and Human… DOI: http://dx.doi.org/10.5772/intechopen.102497*

addition, heavy metals can be introduced into animal feed rations through enrichment with yeast, meat, bone, and fish meals and inorganic mineral additives, such as phosphate, dolomite, and chalk [3].

Karimi et al. [27] investigated the levels of toxic heavy metals (As, Cd, Hg, Pb) in agricultural products, such as legumes, wheat, and potatoes, in Markazi Province, Iran. Markazi Province is the most industrialized region in the country. Lead mines and other industrial activities are located there, which carries the presence of heavy metals in the soil. The results showed that among the samples analyzed, the carcinogenic risk index was within the acceptable level. However, in the case of wheat, it was found to be the most important source of toxic metal exposure due to its high consumption compared to the other crops, i.e., earthlings and legumes.

Studies conducted in Nigeria, where soils are contaminated with oil, showed that among the heavy metals tested (Pb, Cd, Cr, Mn, Fe, and Zn), for crops in the test samples, the elements Pb and Cr exceeded the limits set by WHO [28]. In Turkey of 12 districts of Sakarya city where cucurbit crops are grown, organochlorine pesticides have been applied to fields for more than 30 years. Studies of heavy metals (As, Cd, Cu, Cr, Ni, Pb, Zn) showed that the concentrations of Cu, Ni, and Cr were at 108.2 mg kg−1, 219.9 mg kg−1, and 173.1 mg kg−1, respectively, and were the highest, i.e., 2–7 times higher than the limits given in the Turkish Soil Pollution Control Regulation [29].

Exposure of animals to heavy metals from drinking water is a very big problem. It is difficult to estimate, especially when animals use random intakes and when they are kept in an extensive (poultry) or grazing (sheep, cattle) way. Naveedullah et al. [11] conducted a study on the distribution of selected metals (Zn, Cu, Mn, Fe, Cr, Cd, and Pb) in soils in the Siling reservoir watershed in China, the various ecological and health risks associated with selected metals to the inhabitants. Protection of soil quality in the reservoir watershed is of great importance to preserve water quality, which is a source of drinking water. The study revealed seasonal variations of selected heavy metal content in soil samples. In addition, the multivariate analysis conducted showed significant anthropogenic, point, and non-point pollution of selected metals in the Siling reservoir watershed. Through the use of enrichment factor, geoaccumulation index and contamination factor, moderate to high contamination was found in soil samples during the summer and winter seasons. Low soil pH and high organic matter content increase the leaching of some elements from the soil into the aquifer formations and increase the toxic metal content in the water sources. According to the authors, the quality and quantity of fertilizers used were important causes leading to the accumulation of heavy metals in soils depending on land use.
