Concentrations of trace elements in water results from both natural and anthropogenic conditions. Natural conditions include physico-chemical properties such as water solubility, pH, redox potential, and the capability of forming soluble complexes (McComb et al. 2014). Anthropogenic conditions include advances in civilization and industrial development, inter alia, in the mining and tanning industries, metallurgy, the fertilizer industry, pesticide production, ore refineries, and the pulp and paper industry, all of which, by producing wastewaters rich in trace elements, significantly contribute to the accumulation of these compounds in aquatic environments, and they are often difficult to remove with routine treatment methods (Janyasuthiwong et al. 2015).
Some trace elements are essential microelements to the human body such as copper, manganese, iron, and zinc (Szczuko et al. 2019). The nutrient reference values (NRV) for them indicate the quantities of which they should be found in the diet in the foods and fluids consumed (Table 7). A substantial group, however, comprises toxic elements that have the tendency to bioaccumulate (Al-Fartusie and Mohssan 2017). Organizations such as the WHO, the United States Environmental Protection Agency (USEPA), and the EU all strive to effectively reduce the emission of trace elements into the environment by formulating strict regulations on the quality of wastewaters discharged by industry. Unfortunately, values differ among the organizations, which renders it difficult to mitigate effectively the devastation of the natural environment. For example, the WHO set the maximum permissible concentrations of Cu and Pb in mining and galvanizing wastewaters at 2.0 and 0.01 mg/l, respectively (WHO 2004), while the USEPA levels are 1.3 and 0.015 mg/l, respectively (USEPA 2006).
In Poland, regulations in force that govern the MAC values for trace elements in surface waters, drinking water, and treated wastewater discharged into waters, respectively, are found in the following issues of the Polish Journal of Laws: item. 1747 (2019), item. 2294 (2017), item. 1311 (2019). No values exceeding the allowable limits were noted in any of the samples tested (Table 2).
These results indicated that the average adult ingested with drinking water barely 0.03 to 0.16 NRV, while water treatment decreased the NRV ingested by 27–36.4%.
In the study presented in this paper, the water abstracted from Lake Miedwie and subjected to multi-stage treatment, was characterized by safe quantities of all the elements analyzed that were below the MAC (Journal of Laws item. 1747 2019, Table 8). The tests on drinking water performed for the present study, similarly to those performed in 2005–2006 in Poland on the concentrations of metals (manganese, copper, lead, cadmium, nickel, chromium, arsenic, aluminium), indicated that concentrations of these elements were below approximately 10% of the maximum acceptable concentrations (Herasimowicz-Bąk and Brzeski 2009). Only the iron content in the samples from 16 years ago slightly exceeded the MAC, which could have stemmed from the older water and wastewater infrastructure in operation in Szczecin at the time.
Many correlations among elements were noted in the present study; however, no information regarding this was found in the available literature. Only Rahman (2021) reported similar findings in the strong positive correlation of As with Fe in water in southwestern Bangladesh.
The concentrations of elements (As, Pb, Ni, Mn, Fe, Cu, Zn) confirmed in raw water samples were below the MACs (Table 2, Table 8). In comparison to the studies of many other authors, the quality of water of Lake Miedwie was characterized by high quality parameters. However, despite the modern solutions applied at the water and wastewater treatment plants, unnecessary trace element were not completely removed. Water treatment reduced the concentrations of specific trace elements within a wide range from 48.5 (As, Mn) to 97% (Pb); however, wastewater was less effective as it reduced trace elements by 28.6 (Ni, Mn) to 60.8% (Fe). Drinking water from Lake Miedwie did not exceed standards in any of the study periods. A comparative analysis of drinking water from various European countries indicated that MACs (4.63%) were exceeded (Birke et al. 2010; Janyasuthiwong et al. 2015). Elevated trace element contents in water and wastewater were confirmed in the period from spring to early autumn. The higher trace element contents in raw water and drinking water during this period could have been linked with the higher flow of elements from sediments to the water that occurs as temperatures increase (Richir and Gobert 2016). Presumably, the use of coagulants in the treatment plants significantly effected the reduction of trace elements. PAX-1905, a high basicity coagulant, was used at the Żelewo Water Production Plant. Zinc occurring in water in dissolved form is a component of enzymes and is a catalyst in many reactions. The content of this element in water is highly variable and depends on geological formations and pollutants from many sectors including pigment production, battery construction, and ammunition manufacture (Al-Fartusie and Mohssan 2017). The zinc content in potable groundwater from different parts of the world fluctuates within a range of 15–80 µg/l. It was determined that in the 2017–2019 period it was 0.007±0.004 mg/l. The USEPA determined that the permissible zinc content in treated wastewater was 2 mg/l (USEPA 2006), which is four-fold higher than that in the treated wastewater tested in the present study (0.54 mg/l).
The effectiveness of filtration through filtration beds greatly affects the content of trace elements in treated water. While trace elements are, in fact, retained in beds, they are not permanently bound to them. Filtration bed contamination is one of the reasons there are trace elements in drinking water (Richr and Gobert 2016). Nickel occurs in water mainly as [Ni(H2O)6]2+ ions and is part of the active sites of many enzymes (Poonkothai and Vijayavathi 2012). Because of the potentially high toxicity of this element, the recommended dietary allowance (RDA) of this element has not been determined. Nevertheless, many studies confirm that the estimated daily consumption of nickel in food and water globally is 80–130 µg/day (Plum et al. 2010). Water samples collected from the surface waters of Woji Creek, Rivers State, Nigeria in 2019, were confirmed to have a mean concentration of Ni of 0.3545 ± 0.1652 mg/l, (Ibezim-Ezeani and Ihunwo 2020). These values were significantly higher than those obtained in the present study.
Manganese and iron are among the most common trace elements in aquatic environments. A large percentage of the population of the Baltic states is at risk of potential exposure to elevated levels of manganese and iron in drinking water since approximately 30% of groundwater samples collected exceeded the standards for these elements set forth in European Union Council Directive 98/83/EC on the quality of water intended for human consumption. Although these are essential nutrients, when they occur in high concentrations in drinking water they are linked with various health problems. Iron that occurs in water as Fe+2 i Fe+3 ions is responsible for tissue respiration (Al-Fartusie and Mohssan 2017). As is the case with other elements, the acceptable iron concentration in drinking water varies in many guidelines. For example, Turkish drinking water standards (TDWS 2005) permit 200 μg/l iron, while the USEPA (2006) limit is 300 μg/l. The limit for manganese is 50 μg/L, which is the same as that in the TDWS (2005) and of the USEPA (2006). Manganese plays defense roles in cells, provides protection against reactive oxygen species, and also regulates the urea cycle and proper dopamine production (Al-Fartusie and Mohssan 2017). Tap water sampled from Eskisehir Province in the Central Anatolian Region of Turkey in 2013 had an iron level of 110 µg/l and a manganese level of 104 µg/l (Yuce and Alptekin 2013). These levels were extremely high in comparison to the drinking water analyzed in the current study, in which iron did not exceed 10 µg/l or manganese 7 µg/l.
Copper occurs in water as Cu+ ions, and it participates, inter alia, in the formation of crosslinks in collagen, elastin, and melanin and in maintaining keratin structure. The toxicity of copper in the aquatic environment depends primarily on the alkalinity of the water and also on its hardness. Copper is less toxic in more alkaline, harder water as it is less available because of the formation of copper carbonate complexes. This is why the toxicity of copper increases with decreasing water alkalinity and hardness, pH, dissolved oxygen concentration, chelating agents, humic acid content, and suspended matter content (Rio and Martin 2012).
The occurrence of lead in drinking water is undesirable as it provides no known health benefits, while the negative effects from it are many, the most important of which is lead poisoning. Lead can affect nearly all the organs and systems in the human body, and it can cause serious damage to the brain, kidneys, nervous system, and reproductive system (Ibezim-Ezeani and Ihunwo 2020)
Due to their low alkalinity and buffering capacity, soft waters are more dangerous because of the greater mobility of lead in the form of soluble salts (Janyasuthiwong et al. 2015), while water that is hard and highly alkaline (and also with higher pH values) contains sparingly or practically insoluble lead salts, such as phosphate, sulphate, hydroxide, carbonate, and basic carbonate (white lead). The permissible lead content in drinking water in Poland is 0.001 mg/l and is in line with WHO recommendations. In the current study, Pb was detected in only 8% of drinking water samples, and it did not exceed 0.001 mg/l. In raw wastewater, however, the levels detected did not exceed 0.056 mg/l. Samples collected from surface waters in Woji Creek, Rivers State, in Nigeria in 2019 had confirmed mean concentrations of Pb of 1.316 ± 0.620 mg/l (Ibezim-Ezeani and Ihunwo 2020), and these values substantially exceeded those of the current study. Etxabe et al. (2010) and Haider et al. (2002) observed in Spain and Austria, respectively, that lead concentrations in drinking water were higher than those in water sampled at treatment plants. These authors concluded that the poor condition of the water supply network could have resulted in lead leaching from the pipes into the water. High arsenic concentrations in natural water the world over are a significant problem and pose risks because of the toxic properties of this element. Removing arsenic can be done through oxidation, precipitation, coagulation, membrane filtration, and adsorption (Mohan and Pittman 2007). The arsenic limit in drinking water set by the WHO is10 μg/l. Kelepertsis et al. (2006) reported higher arsenic concentrations (125 μg/l) in drinking water in eastern Thessaly in Greece, while Jovanovic et al. (2011) confirmed that 63% of all water samples exceeded Serbian and European standards for arsenic content in drinking water.
Cavar et al. (2005) reported that the mean arsenic concentrations in drinking water samples from three towns in eastern Croatia were 38, 172, and 619 μg/l, which posed serious health risks to approximately 3% of the Croatian population. Research by Tamasi and Cini (2004) indicated that arsenic concentrations in drinking water from southern Tuscany in Italy were higher than those in samples collected at treatment plants. These authors concluded that the poor condition of the water supply network could have caused arsenic to leach from the pipes into the water. Although it has been many years since legal regulations throughout the world were tightened, including in the European Union, trace elements occur in the environment and can still pose real risks. The current study confirmed the necessity of continuing research on the effectiveness of various water treatment methods and filtration beds and also of considering drinking water along with the food humans consume when estimating intake sources of trace elements.