Methods of Assessing Water Quality in Terms of Public Health

Abstract

Nitrates (V) in drinking water are harmful to human health and are a probable carcinogen. Nitrates (V) in drinking water are responsible for the formation of methaemoglobin, which causes hypoxia, cyanosis, collapse and even death. Newborn babies and infants up to six months of age are most at risk. For this reason, a European Union directive was introduced in 1991 to protect waters from pollution caused by nitrogenous fertilisers of agricultural origin, restricting their use. The aim of the study was to analyse the content of nitrogen compounds (ammoniacal nitrogen, nitrates (III and V)) in water samples from selected rural waterworks in Tłokinia Wielka. Analyses were performed using spectrophotometric methods. The results showed that excessive levels were present in the tap water from public water supplies, which are regularly monitored by the State Sanitary Inspectorate and water supply companies.

1. Introduction

Water is one of the basic elements necessary for the functioning of cells of both plant and animal organisms. Its content in the human body varies depending on sex and age. Water quality is important for health. European Union (EU) [1] and Polish regulations [2] impose maximal values on physical, chemical and biological parameters of water for human consumption. Chemical compounds that are dangerous to human health in excessive concentrations include anthropogenic nitrogen compounds (ammonium ions as well as nitrates (III and V)), originating mainly from the excessive use of fertilisers in agriculture, which causes pollution of drinking water [3,4]. The maximum allowed concentrations of the chemical compounds in potable water in Poland are 50 mg NO₃/l for nitrates (V), 0.50 mg NO₂/l for nitrates (III) and 0.50 mg NH₄/L for ammonium ions. Food, apart from drinking water, is one of the main sources of human exposure to nitrates. Nitrates (V) are found in many food products [5,6] and are considered a probable carcinogen due to their potential for endogenous transformation into genotoxic nitroso compounds [4].

Nitrates (V) pose a health risk related to the formation of methemoglobin (Met Hb). This reaction occurs mainly in infants younger than 6 months who consume water containing nitrates (V), take medications with high NO₃ content, or undergo intestinal infections. When the methemoglobin level exceeds 10%, methemoglobinemia begins, which is a life-threatening condition [4]. Symptoms of methemoglobinemia include headaches, cyanosis, dementia, brain hypoxia, tachycardia, fainting, convulsions, comas and even lethargy. It can even lead to death [6]. Nutritionists recommend the consumption of spring water or natural mineral water, which is characterised by natural composition. The water available on the market includes bottled water recommended for infants and children up to three years of age and certified by the National Institute of Hygiene, the Institute of Mother and Child and the Children’s Memorial Health Institute.

The water supply network, which is subject to regular control by the State Sanitary Inspectorate [2], is the primary source of drinking water for residents in Poland. Data from the State Sanitary Inspectorate are communicated to the Central Statistical Office. As provided by Central Statistical Office, the length of the water distribution network is nearly 317,000 km, and over 6 million people use the water supply network [7]. Approximately 76.9% of the length of the water distribution network and 62.0% of its connections to residential buildings are located in rural areas, and about 98% of the urban population has access to a collective water supply. People, especially those from rural areas, also use private intakes (household wells) of water of unknown physicochemical and bacteriological composition [8,9,10].

The research aimed to analyse nitrogen compounds in the Public Waterworks of the Tłokinia Wielka in the district of Kalisz in the years 2020–2023 and assess health risks caused by their presence. Also, the amount of water consumed was determined, and household water intakes in the study area were investigated.

2. Materials and Methods

2.1. Geographical Characteristics of the Study Area

Kalisz is a district town located in the southeastern part of Greater Poland, on the Kalisz Upland, at the mouth of the Swędrnia into the Prosna River. The commune of Opatówek and the village of Tłokinia Wielka lie in the close vicinity of the city of Kalisz. Tłokinia Wielka, according to the physiographical division, is located within the South Greater Poland Lowland on the Turek Upland. However, according to the Greater Poland Lowland geomorphological division, this area belongs to the Kalisz Upland region and lies at the edge of the Kalisz Hills subregion. These are hummocky end-moraines, elevated to 140.0–173.0 m above sea level. The area discussed is located in the Warta River basin and drained by the Prosna and its tributaries. The hydrographic network is quite poorly developed. This region is drained by artificial channels and an unnamed water course leading to the Trojanówka (Cienia) River—a right tributary of the Prosna. The Swędrnia River flows 2.85 km southwest and the Trojanówka River 2.40 km southeast of the water intake of the waterworks.

The inhabitants of the Opatówek commune are provided with water by the Tłokinia Wielka waterworks. The system supplies water from two deep wells: the primary one (No. 1), 46.50 m deep, made in 1987, and the auxiliary one (No. 2), 48.50 m deep, made in 1995. Both the wells draw water from Quaternary (Pleistocene) aquifers. Presently, only well No. 1—the primary one located in the town of Tłokinia Wielka—and an auxiliary 130 m deep well (No. 3) located in the town of Opatówek, on Ludowa Street, drawing water from Mesozoic (Jurassic) formations, are being used. Well No. 2 is currently out of use due to a high nitrate content in the water [11].

2.2. Physicochemical Specification of the Water Available in the Area

Based on physicochemical analyses performed in 1987, it can be concluded that (raw) groundwater from the Pleistocene intake in Tłokinia Wielka has significant hardness. This is water of pH close to neutral, an oxidizability of 1.47–1.8 mg/L and a small content of ammonia, chlorides and sulfates. After contact with air, it turns slightly cloudy due to the precipitation of iron compounds. In order to protect consumers and maintain quality parameters, groundwater from the intake in Tłokinia Wielka is subject to a simple purification (water aeration, de-ironing and disinfection with UV lamps). The Tłokinia Wielka intake is located in the ecoregion of the Central Plain. The primary well No. 1, which draws water from the Quaternary (Pleistocene) aquifers, is supported by a deep well at Ludowa Street in Opatówek, taking water from Mesozoic (Jurassic) rock formations [11].

2.3. Water Intakes and Population Water Demand

Water from the intake in Tłokinia Wielka is used to meet the social and living needs of residents and entrepreneurs in the following localities: Tłokinia Wielka (426 people), Tłokinia Kościelna (630 people), Tłokinia Mała (145 people), Tłokinia Nowa (79 people), Nędzerzew (51 people), part of Zduny (300 people), Rożdżały (141 people), Kobierno (160 people), Borów (344 people), Zmyślanka (62 people), Sierzchów (462 people), Modła (44 people) and Warszew (114 people). This gives approximately 2958 people in total (Figure 1). The region covered by the rural water supply network in Tłokinia Wielka is an area where the leading activity is agricultural production, especially horticulture. Due to the above, water is also used for agricultural and horticultural purposes. The furthest point supplied from the Tłokinia Wielka intake is Modła in the Opatówek commune.

2.4. Principle of the Method

All analyses were performed by the Water and Soil Analysis Laboratory of the District Sanitary and Epidemiological Station (PSSE) in Kalisz (Poland). The laboratory is certified by the Polish Center for Accreditation (PCA) with the number PCA AB 578. Samples were taken in accordance with standard PN-ISO 5667-5:2017-10/Ap1:2019-07—Water quality. Sampling. Part 5: Guidance on sampling of drinking water from treatment works and piped distribution systems [12].

The uncertainty of the results took into account the sampling procedure. Water was collected in polypropylene containers in the amount of 1 L for the determination of nitrates (V) and determination of nitrates (III). The samples were collected in the course of surveillance conducted by PSSE in accordance with the Regulation of the Minister of Health of 7 December 2017 on the quality of water intended for human consumption (Journal of Laws of 2017, item 2294) [2]. Water was sampled from the Tłokinia Wielka waterworks in places permanently designated for this purpose and equipped with taps: a spigot in the filter hall of the water purification plant (SUW) in Tłokinia Wielka and faucets within the network in Tłokinia Wielka, Rożdżały and Borów.

2.5. Determinations of Nitrogen Compounds—Ammonium Ions, Nitrates (III and V)

Determinations of nitrogen compounds (ammonium ions, nitrates (III and V)) were performed with standardised UV-VIS spectrometry methods [11,13,14] using a Varian Carry-50 UV-VIS spectrophotometer (Mulgrave, Victoria, Australia). All referential materials used were certified, and the analytical reagents were of appropriate purity. Quartz cuvettes with optical path lengths of 10 mm (nitrates (V)) and 50 mm (ammonium and nitrite (III) ions) were used.

The method for determining the ammonium ion (nitrogen) involves measuring the absorbance of blue-coloured solutions at a wavelength of 655 nm. To perform the measurement, reagents were added to 40 mL of the analysed sample, mixed and topped up to 50 mL. The solution was placed in an incubator for 60 min at a temperature of 25 ± 1 °C. After the designated time, the absorbance was measured in a 50 mm quartz cuvette at a wavelength of 655 nm. A blank test was performed in the same way as with the test sample, but 40 mL of distilled water was taken instead [14].

The method for determining nitrates (III) involves the reaction of nitrites in the analysed sample with 4-aminobenzenesulfonamide in the presence of phosphoric acid at pH 1.9, which produces a diazonium salt that with N-(-1naphthyl)-1,2-diaminoethene dihydrochloride (added with the 4-aminobenzenesulfonamide reagent) forms a pink solution. Nitrates (III) content is determined photometrically at a wavelength of 540 nm. The measurement was made by taking 40 mL of the tested sample and then adding 1.0 mL of the colour-forming reagent with a pipette. After mixing, it was filled up to the full volume with water and then set aside for 20 min. As described above, a blank test was performed, replacing the test sample with distilled water. The nitrite content was read from the standard curve. The final result was the arithmetic mean of the results of two independent determinations [15].

The method of determining nitrates (V) is based on the reaction of nitrates (V) with sodium salicylate in concentrated sulfuric acid (VI). This reaction produces nitrosalicylic acid, which—after alkalinisation—turns into an ionised form with an intense yellow colour. The content of nitrates (V) is determined photometrically at a wavelength of 410 nm in a cuvette with an absorbing layer thickness of 10 mm [16]. The determination was made by evaporating 25 mL of the tested water sample and then adding 1 mL of concentrated sulfuric acid (VI) to the remaining sediment. The sulfuric acid was spread over the evaporator walls covered with the sediment. After 10 min, distilled water and an alkaline sodium–potassium tartrate solution were added. Then, the solution was quantitatively transferred to a 50 cm³ Nessler cylinder, topped up with distilled water to the mark and mixed. Absorbance was measured at a wavelength of 410 nm in a 10 mm cuvette. As a reference, a blank and a control sample were prepared simultaneously. The nitrate (V) content was read from the standard curve. The final result was the arithmetic mean of the results of two parallel determinations, differing by no more than 20% [16].

3. Results

As part of the monitoring, water samples were taken from the Tłokinia Wielka waterworks, and the following results were obtained (the outcomes presented only concern samples in which the presence of the tested nitrogen compounds was confirmed).

The presented results come from the laboratory in Kalisz and were performed as part of water supervision (

Table 1. Results of determinations of nitrogen compounds in drinking water from the Tłokinia Wielka waterworks in the Kalisz district, Opatówek commune.

Sampling Date	Sampling Place	Ammonium Ions Concentration [mg/L] with Calculated Uncertainty *	Nitrate (III) Concentration [mg/L] with Calculated Uncertainty *	Nitrate (V) Concentration [mg/L] with Calculated Uncertainty *
25 February 2020	Borów network	0.14 ± 0.02	0.06 ± 0.01	21.4 ± 3.0
23 June 2020	Suw	0.20 ± 0.15	0.06 ± 0.01	37.0 ± 5.0
04 August 2020	Tłokinia Wielka network	0.05 ± 0.01	0.08 ± 0.01	32.2 ± 4.6
16 February 2021	Rożdżały network	0.04 ± 0.01	0.19 ± 0.02	28.0 ± 3.3
25 February 2021	Suw	0.40 ± 0.06	0.06 ± 0.01	29.0 ± 4.0
20 July 2021	Tłokinia Wielka network	0.05 ± 0.02	0.33 ± 0.04	29.7 ± 3.5
17 February 2022	Suw	0.13 ± 0.02	<0.04 (0.04 ± 0.01)	51.0 ± 8.0
15 March 2022	Borów network	<0.04 (0.04 ± 0.01)	<0.04 (0.04 ± 0.01)	51.1 ± 7.5
19 July 2022	Tłokinia Wielka network	<0.04 (0.04 ± 0.01)	<0.04 (0.04 ± 0.01)	29.1 ± 4.3
14 February 2023	Rożdżały network	<0.04 (0.04 ± 0.01)	<0.04 (0.04 ± 0.01)	53.6 ± 7.5
17 February 2023	Suw	-	-	11.0 ± 1.5
17 February 2023	Rożdżały network	-	-	23.0 ± 3.2

Notes: * the uncertainty was calculated in the Water and Soil Analysis Laboratory of the District Sanitary and Epidemiological Station (PSSE) in Kalisz; “<”—the result below the limit of quantification; measurement uncertainty is expressed as the expanded uncertainty for a confidence level of 95% and coverage factor k = 2.

) in accordance with the Regulation of the Minister of Health of 7 December 2017 on the quality of water intended for human consumption [2].

The methods used to determine the composition of water are consistent with the RMZ guidelines and have been validated and verified in laboratory conditions at the PSSE in Kalisz. The results validity monitoring programme included annual verification of standard curves, correctness and repeatability, limits of detection and quantification, intermediate precision, repeatability of the actual drinking water sample and recovery for a given parameter. See

Table 2. List of validated methods for determining chemical compounds (NH₄₊, NO₂, NO₃).

Parameter	Criteria [1]	Ammonium (Nitrogen) Ions	Nitrates (III)	Nitrates (V)
Scope of method accreditation *		0.04–0.50 mg/L	0.04–0.50 mg/L	0.2–60.0 mg/L
Correctness	≤10%	0.04 mg/L—5.00% 0.06 mg/L—2.33% 0.12 mg/L—2.50% 0.25 mg/L—1.92% 0.40 mg/L—1.90% 0.60 mg/L—0.97%	0.040 mg/L—2.50% 0.050 mg/L—0.80% 0.060 mg/L—2.33% 0.080 mg/L—2.25% 0.125 mg/L—2.50% 0.150 mg/L—0.32% 0.200 mg/L—0.67% 0.250 mg/L—0.10% 0.500 mg/L—0.16%	0.2 mg/L—2.00% 0.5 mg/L—2.00% 1.0 mg/L—1.00% 2.0 mg/L—2.60% 4.0 mg/L—2.30% 6.0 mg/L—0.27% 8.0 mg/L—0.58% 20.0 mg/L—0.43% 50.0 mg/L—0.13%
Intra-laboratory reproducibility (precision under repeatability conditions)	≤10%	0.04 mg/L—6.37% 0.06 mg/L—4.25% 0.12 mg/L—2.37% 0.25 mg/L—0.82% 0.40 mg/L—1.82% 0.60 mg/L—2.43%	0.040 mg/L—4.88% 0.050 mg/L—3.60% 0.060 mg/L—2.29% 0.080 mg/L—1.37% 0.125 mg/L—0.44% 0.150 mg/L—1.73% 0.200 mg/L—0.82% 0.250 mg/L—0.33% 0.500 mg/L—0.46%	0.2 mg/L—6.45% 0.5 mg/L—4.79% 1.0 mg/L—1.32% 2.0 mg/L—1.93% 4.0 mg/L—2.63% 6.0 mg/L—0.80% 8.0 mg/L—0.99% 20.0 mg/L—1.57% 50.0 mg/L—0.29%
Intermediate precision	≤10%	0.05 mg/L—7.07% 0.50 mg/L—2.17%	0.050 mg/L—4.16% 0.250 mg/L—2.17% 0.500 mg/L—2.66%	0.3 mg/L—3.88% 6.25 mg/L—2.63% 20.0 mg/L—3.28% 50.0 mg/L—2.59%
Repeatability of drinking water sample	≤10%	4.67%	3.47%	1.94%
Limit of detection	-	0.010 mg/L	0.002 mg/L	0.025 mg/L
Limit of quantification	30% of the parametric value	0.020 mg/L	0.004 mg/L	0.050 mg/L
Coefficient of variation of the method		6.68%	5.41%	0.43%
Correlation coefficient of calibration curve	≥0.997	r2—0.998	r2—0.998	r2—0.999
Recovery rate	90–110%	0.05 mg/L—99.0% 0.25 mg/L—103.3% 0.45 mg/L—93.0%	0.050 mg/L—96.2% 0.125 mg/L—102.1% 0.220mg/L—99.2% 0.500 mg/L—100.3%	0.8 mg/L—96.8 4.0 mg/L—99.5 7.2 mg/L—99.4% 20.0 mg/L—99.5% 50.0 mg/L—100.1%
Expanded measurement uncertainty of the method without sampling	NH4+—40% NO2—20% NO3—15%	20.0%	9.8%	13.9%
Expanded measurement uncertainty of the method with sampling		20.4%	11.8%	14.1%
(Acceptable) parametric value [1].		0.50 mg/L	0.50 mg/L	50.0 mg/L

Notes: * The scope of the accreditation method is related to the use of the logo of the Polish Center for Accreditation (PCA). All results above the range are without the PCA logo but are still valuable for determining the exact value of a given parameter.

.

All estimated parameters included are consistent with the methodology guidelines of the Regulation of the Minister of Health of 7 December 2017 on the quality of water intended for human consumption [2].

On the basis of the results of the analyses performed for water samples from the Tłokinia Wielka waterworks (Table 1, Figure 2), it can be inferred that the concentration of nitrates (V) in the water supply network seasonally rises above the acceptable limit. The concentration was exceeded in the first quarter of 2022 in the network in the village of Borów and at the water purification plant in Tłokinia Wielka, and later in February 2023 in the network in the village of Rożdżały. In 2023, the test was repeated after 3 days, and the nitrate (V) concentration decreased to 23 mg/L in the village of Rożdżały and to 11 mg/L at the water purification plant. Water mixing was used to improve water quality so that the concentration of nitrates (V) did not exceed the recommended value. Water from well No. 1, containing large amounts of nitrates (V), was mixed with water from well No. 2 (which alone is unable to provide enough water for the entire supply network of the Tłokinia Wielka waterworks). In this way, water recipients from Tłokinia Wielka and the surrounding area had good-quality water. The research shows that a quarter of the collected and analysed samples had exceeded nitrate (V) concentrations.

As far as the concentration of ammonium ions and nitrates (III) (Table 1, Figure 3) is concerned, their values are lower than the limit of quantification in the months when the parametric value of nitrates (V) is exceeded. This means that nitrate (V) contamination is permanent. In February 2021, a sharp growth (0.40 mg/L) in the concentration of ammonium ions (nitrogen) was observed, as well as an increase (0.06 mg/L) in the value of nitrates (III); the concentration of nitrates (V) was within the normal limits and amounted to nearly 30 mg/L. A similar situation was observed in June 2020. The concentrations of ammonium ions and nitrates (III and V) were 0.20 mg/L, 0.06 mg/L and 37.0 mg/L, respectively. Such an increase in ammonium nitrogen concentration indicates fresh contamination and oxidation reactions of ammonium salts into stable nitrates.

As for the concentration of nitrates (V) in regard to the place of sampling (Table 1, Figure 4), varied values of these compounds at the water purification plant in Tłokinia Wielka are observed. They differed depending on the season and year of sample collection. However, the observed nitrate content ranged from 11 mg/L (the control sample taken after the limit was exceeded) to 51 mg/L in February 2022. In 2020, in the village of Borów, the nitrate (V) concentration was 21.4 mg/L, while two years later, in the same place, the parametric value was exceeded. In 2020 and 2022, water in the Tłokinia Wielka network met the requirements for potable water in accordance with the Regulation of the Minister of Health of 7 December 2017 ( [2]. In February 2023, in the village of Rożdżały, the value of nitrates (V) was exceeded and amounted to 53.6 mg/L. In the years 2020–2023, during the entire study period, the concentrations of ammonium ions and nitrates (III) did not exceed the recommended value (0.50 mg/L), but seasonally their values increased to maxima of 0.40 mg/L for ammonium ions (water purification plant in 2021) and 0.33mg/L for nitrates (III) (water supply network in Tłokinia Wielka; Figure 5 and Figure 6).

Well No. 1 has been in use for 36 years. Its depth (45 m) is small for a well feeding the water supply network, and the water composition periodically exceeds the permissible values. As the responsible authority, Opatówek Town and Municipal Office has already taken steps to improve the quality of water supplied by the Tłokinia Wielka waterworks. A new well was drilled to a depth of 110 m. Preliminary analyses of the quality of water from this well were carried out. Raw water tests showed the absence of nitrogen compounds. According to optimistic forecasts, well No. 1 in the village of Tłokinia Wielka will be excluded from exploitation, and the quality of water will be ensured by well No. 3 and the new well.

In 2022, water extraction from well No. 1 in Tłokinia Wielka and from well No. 3 in Opatówek amounted to 277,390 m³ (

Table 3. Monthly water extraction in wells No. 3 (Opatówek, Ludowa Street) and No. 1 (Tłokinia Wielka) in 2022 [15].

Month	Extraction in Well No. 3—Opatówek Ludowa Street [m3]	Extraction in Well No. 1—Tłokinia Wielka [m3]
January	6900	6240
February	9086	4084
March	16,938	916
April	19,358	1796
May	24,878	8055
June	24,848	11,762
July	24,482	13,477
August	17,051	14,720
September	10,954	11,362
October	9617	9208
November	8314	7632
December	8068	7644
Sum	180,494	96,896
Total extraction in 2022	277,390

) [17]. Well No. 1 recorded the highest water consumption in July and August, reaching 13,477 and 14,720 m³/month, respectively. However, well No. 3 had the highest extraction between May and July, amounting to over 24,000 m³ each month [17]. Consequently, the annual water consumption per inhabitant was 93.78 m³, whereas the monthly use was 7.81 m³. Data from the Central Statistical Office show that the average annual water consumption by households in 2021 was 33.7 m³ per inhabitant, 35.3 m³ in cities and 31.2 m³ in rural areas [17]. In Greater Poland, water consumption per capita, ranging from 35.1 m³ to 39.2 m³ and being higher than the national average, was one of the highest in the country.

It should be emphasised that the area supplied by the Tłokinia Wielka water distribution network is an area with predominantly agricultural production, especially horticulture. High water consumption in March–August, with the maximum in May, is related to the irrigation of agricultural fields and the use of water for agri- and horticultural production. According to information provided by the Opatówek Municipal Office, water consumption in 2023 was expected to be lower. The decrease in water consumption is related to the fact that in the area covered by the Tłokinia Wielka water supply system, additional wells were being drilled to provide water for crop fields.

4. Discussion

The results of water quality analyses conducted in the Kalisz district reveal contamination with nitrogen compounds, especially nitrates (V). Nitrates (V) occur in piped water. Tap water is supplied by municipal enterprises that collect, purify and deliver it to consumers. Ensuring a constant supply of good-quality water is required by law. However, as the above research shows, providing water supplies of satisfactory quality is becoming increasingly difficult. The quality of municipal water is influenced by factors such as the source of water intake, methods of obtaining and purification, as well as the condition of water intakes, distribution pipes, connections and internal home installations [18]. The primary source of potable water for residents in Poland is water from the supply network, which is subject to continuous control and quality assessment by the State Sanitary Inspectorate [2]. The quality of drinking water should meet the requirements specified in the Regulation of the Minister of Health of 7 December 2017 on the quality of water for human consumption [2]. Water is safe for human consumption when it is free from pathogenic microorganisms and parasites, as well as from other substances, e.g., uranium [19] and nitrates, in amounts that pose a potential health hazard [20]. The main anthropogenic source of nitrogen in the environment is nitrogen fertilisers used in agriculture. About half of runoff from fields pollutes surface and groundwater, increasing the concentration of nitrates (V) in our aquatic resources [21]. The maximum nitrate (V) concentration level in municipal water sources in the United States is 10 mg/L of nitrogen (NO₃-N), that is 44.3 mg/L of nitrate (V) (NO₃), which is roughly equivalent to the recommendations of the World Health Organization of 50 mg/L [22].

The present study revealed that water samples from 25% of tap water intakes contained nitrates (V) and did not meet the requirements of the Regulation of the Minister of Health [2].

The Regulation of the Minister of the Environment on the criteria for assessing the state of groundwater of 23 July 2008 specifies the natural content of constituents in groundwater in Poland (as a hydrogeochemical background). For nitrogen compounds, it amounts to 0–1 mg NH₄/L for ammonium ions, 0–0.03 mg NO₂/L for nitrates (III) and 0–5 mg NO₃/L for nitrates (V) [23]. The water available on the market includes bottled water recommended for infants and children up to three years of age and certified by the National Institute of Hygiene, the Institute of Mother and Child and the Children’s Memorial Health Institute. The most significant health risk associated with the exposure of newborns to methemoglobinemia may occur when drinking water from dug wells. It decreases with the age of the infant.

In order to identify the chronic effects of human exposure to nitrogen compounds contained in water, long-term population studies should be conducted. No such studies have been conducted in Poland. Studies similar to ours were conducted in the United States in 1988–2015. On their basis, M.H. Ward et al. [6] drew the following conclusions: 98% of the tested wells supplying water to households contained nitrates (V). We reached similar conclusions in our studies—the tested well-contained nitrates (V). Ward et al. also showed that water in municipal deep wells exceeded the parametric values of nitrates (in the USA, the permissible value is 1 mg/L NO₃-N, i.e., 4.43 mg/L NO₃). The threshold value was exceeded in 2% of public wells. Similar studies were also conducted in European countries—Greece, Finland and Norway. The detected concentration of nitrates (V) was 29.2 mg/L, 4.43 mg/L and 0.89 mg/L, respectively [6]. Not only Europe and the USA but also countries such as Morocco, Nigeria, Japan, Lebanon and Senegal have problems with the content of nitrates, which is too high in drinking water (average value 190 mg/L NO3). Extreme accumulation of nitrates (V) in water was recorded in the Gaza Strip, where the concentration reached about 500 mg/L NO₃. These studies show that Poland is not the only country struggling with the problem of increased amounts of nitrogen compounds in drinking water [24]. Monitoring and testing of water is crucial for human health. Both the regulations of the European Union and Polish legal acts oblige the responsible bodies to test drinking water distributed by water supply companies. The State Sanitary Inspectorate supervises these studies. However, there is no supervision over self-dug wells, and no records are kept of them.

It is necessary to raise public awareness of the quality of the water we consume, especially from private water sources. The accumulation of nitrates in global water bodies is likely to increase in the future due to population growth, increased use of nitrogen fertilisers and higher concentrations and animal populations in agriculture. Even with higher expenditures, reducing nitrate (V) concentrations in water resources will only be possible through local, national and global efforts. The latest examples are the International Nitrogen Initiative [25] and the EU Nitrates Directive [26,27], which aim to quantify the impact of the nitrogen cycle on the human body, as well as to approve and promote sustainable nitrogen management methods. Following the implementation of the EU Directive, which includes the identification of sensitive areas, the establishment of codes of good agricultural practice and the introduction of national monitoring and reporting, downward trends in nitrate concentrations in groundwater have been observed in some European countries [5]. In the United States, nitrogen fertilisation of cropland is not regulated, and efforts to reduce runoff remain voluntary. Nitrate (V) concentrations in groundwater and surface water continue to increase in most areas of the United States [28]. Climatic factors that affect the presence of nitrate in groundwater include the amount, intensity and timing of precipitation. Increasing precipitation intensity, especially in winter and spring, can lead to increased nitrate (V) runoff from cropland and its infiltration into groundwater.

5. Conclusions

Based on the research completed, the following conclusions were formulated:

• The acceptable values of nitrates (V) were exceeded in 25% of tested samples of tap water from the Tłokinia Wielka water supply network located in the Opatówek commune, Kalisz district, in the southeastern part of Greater Poland;
• Water from the Tłokinia Wielka water supply network periodically does not meet the requirements set forth by the legal regulations. The Municipal Office of Opatówek, which manages the waterworks, took actions to improve the water quality for residents of the areas supplied by the Tłokinia Wielka waterworks; a new deep well was drilled, and preliminary analyses of raw water were carried out. The results are satisfactory—the water does not contain nitrogen compounds.