Assessment of Groundwater Quality in Relation to Organic versus Mineral Fertilization

Abstract

Groundwater pollution is a threat to the environment and human health because it is an important source of drinking water. Groundwater is used to supply water to communities and pollution occurs when harmful substances and contaminants infiltrate into the groundwater. Through excessive use of fertilizers, agriculture is a major contributor to groundwater pollution. This study tests the impact of organic and mineral fertilization on the groundwater physiochemical parameters and explores the potential consequences of using manure as fertilizer on groundwater nitrate pollution. The experiment was carried out in Satu Mare County, Romania, where both organic (manure) and mineral fertilizers were applied to potato cultures to test their impact on 18 physiochemical parameters of groundwater quality. Basic Statistics, Nitrate Pollution Index (NPI), and Principal Components Analysis were used for emphasizing the impact of mineral and organic fertilization on groundwater quality and relationships between analyzed groundwater parameters. The results show that groundwater corresponding to the site where the higher dose of organic fertilization was applied is characterized by nitrate concentrations (64.92 mg/L) and pH values (6.3 pH units) beyond the allowed limits. Based on the calculated NPI (2.21), it falls within the significant pollution category. Two principal factors were identified as having an impact on groundwater quality: fertilizer type and administered dose, respectively.

1. Introduction

Agriculture is an important factor in the development of the economy, with the aim of providing food for the population [1]. Food requirements have not only increased over the last century, but interest in food quality has also developed; consumers not only want to benefit from food safety but also want to know in detail the activities that contribute to obtaining high-quality food [2,3]. In order to meet the need for food, farmers resorted to the expansion of irrigation and the increasingly intensive use of fertilizers and pesticides. The effect of these measures led to an increase in the degree of water pollution from agriculture, with a high potential impact on human health [4,5]. One of the challenges of the 21st century is the development of sustainable agriculture. According to the FAO definition in the Water Strategy for the Development of a Sustainable Agriculture from 1990, sustainable development involves the responsible handling of natural resources and the adaptation of technologies and institutions to guarantee the fulfilment of present and future generations’ needs while safeguarding the environment. Soil–water symbiosis is a principle on which agriculture is based, thus water is both a “victim” and a “participant” in agricultural pollution [1]. Groundwater represents 0.5% of the total water resources on earth, serving as the main source of drinking water worldwide and meeting the water needs of 2.5 billion people. As a result, managing and monitoring groundwater, as well as its quantity and quality, is essential. In addition, treatment of contaminated groundwater is difficult and expensive [6,7,8].

Due to population growth, the safe use of groundwater has become more difficult [9]. Groundwater pollution from agricultural activities occurs due to increased concentrations of nutrients (nitrogen N and phosphorus P compounds) resulting from the excessive use of fertilizers, animal husbandry, and incorrect storage of agricultural inputs [10,11]. Nitrate (NO₃⁻) is one of the most common inorganic pollutants [6], because in the last decades, the overconsumption of both synthetic and organic nitrogen fertilizers has resulted in soil and groundwater contamination. Due to this reality, nitrogen groundwater pollution became one of the most challenging environmental concerns [12,13,14]. Infiltration from landfills, mining activities, and wastewater discharges lead to pollution with other inorganic substances, toxic substances, and compounds that can be associated with water salinity due to high concentrations of Ca²⁺, Mg²⁺, Na⁺, Cl⁻, and F⁻ [7,15,16]. Thanks to the soil–water symbiosis, both nutrients and toxic substances are absorbed at the plant level.

For obtaining general information concerning groundwater contamination with nitrate, the calculation of the nitrate pollution index became a common tool [17]. To analyze the potential pollution of groundwater resulting from organic and inorganic pollutants, a statistical approach is used. Research concerning the analysis of anions and cations content, whether through an integrated approach or the assessment of the sewerage network construction impact on groundwater quality, uses statistical tools, such as Principal Components Analysis, Discriminant Analysis, or Cluster Analysis [18,19,20,21].

Despite a significant reduction in the cultivated area of potatoes (Solanum tuberosum L.) in recent years, due to the multidirectional use of tubers, the potato is still one of the most important crops [22], providing major contributions to human nutrition, animal feed, places of work, and income [7]. In addition to minerals, potato tubers contain unwanted substances called antinutritive or toxic. Potassium (K) is the basic mineral in potatoes. Phosphorus (P) and magnesium (Mg) are present in potato tubers in moderate amounts, while calcium (Ca) is present in small amounts [23]. The main anti-nutrients contained in potato tubers are nitrates. Tubers also contain small amounts of toxic nitrites. The presence of nitrates in excessive amounts is dangerous because they are precursors to the highly toxic nitrites that cause methemoglobinemia or vitamin A deficiency [24]. According to Commission Regulation (EC) No. 1822/2005 of 8 November 2005, the maximum limit of nitrates in potatoes must not exceed 200 mg kg⁻¹ fresh weight of the tubers (food). Nitrogen absorbed from the soil in the form of nitrate and ammonium is used by plants to synthesize amino acids, proteins, chlorophyll, and other substances. However, when there is an excessive amount of nitrogen, mainly from nitrogen fertilization, plants are not able to metabolize it all and may accumulate it as nitrates [21,23]. In order to reduce nitrate pollution due to mineral and organic nitrogen supply, a series of sustainable fertilization strategies are tested [25,26,27].

The specific agricultural practices in North West Romania involve the extensive use of organic fertilizers, but research concerning their impact on groundwater quality has not been conducted. For this reason, this study aims to identify the impact of using organic fertilizers compared to mineral fertilization on the physiochemical parameters of groundwater quality and test the potential groundwater pollution by nitrates resulting from the use of manure as a fertilizer. This research is of great importance, being the first complex study conducted in the area.

The use of organic fertilizers, such as manure, in agricultural practices is presumed to have a significant impact on the physiochemical parameters of groundwater quality compared to mineral fertilization. Specifically, in this study, we adopt the hypothesis that the use of organic fertilizers will lead to higher concentrations of nitrates in groundwater, indicating a potential risk of groundwater pollution.

2. Materials and Methods

2.1. Experimental Area and Treatments Description

The experiment was carried out in Apa commune in Satu Mare county 47°45′48.6″ N 23°11′53.2″ E, Romania. Satu Mare county is located in the extreme North–West of Romania, bordering Hungary to the west and Ukraine to the north, and in the interior, it borders the counties of Maramures, Salaj, and Bihor. The Apa commune is located in the eastern part of Satu Mare county in the Someș Plain (Figure 1).

The experimental sites were located in four private vegetal farms. Four plots (one by each farm) of 100 m² were organized. Cambisol was the type of soil identified in all experimental sites. Potato culture was selected for this study because it is one of the most spread cultures in the area. It was installed on the same day at each of the four experimental sites, 15 March 2022, respectively, and harvested during 5-7.06.6.2022.

Nitrogen mineral and organic fertilizations were applied to potato cultures (

Table 1. The fertilization pattern.

Plot Number as Shown in Figure 1	Type of Fertilization	Administered Doses	Date of Administration
1	Organic	39.52 t/Ha	10 February 2023
2	Organic	79.04 t/Ha	10 February 2023
3	Organic	118.56 t/Ha	10 February 2023
4	Mineral	NPK: 14:7:21	18 April 2022

). NPK fertilizer was applied for mineral fertilization. In the early phase of potato growing, after plantation, only nitrogen was applied (N₃₄:P₀:K₀) as ammonium nitrate, and afterwards, the doses were adjusted to N₁₄:P₇:K₂₁. The organic fertilization was performed using sheep manure in three doses. The sheep manure content in NPK and dry matter (DM) was as follows: 0.85% N, 0.19% P, 0.76% K, and 29.83% DM [28]. It was applied on the field four weeks prior to the installation of the potato culture. The groundwater sampling was performed after potato harvesting (10 June 2022) and laboratory analysis in July 2022.

The samples were taken from underground waters, located in proximity to the experimental fields from Apa commune, where potato crops were installed (site 1—47°45′42.29″ N 23°11′23.74″ E, site 2—47°45′36.11″ N 23°12′42.56″ E, site 3—47°45′48.81″ N 23°12′13.4″ E, site 4—47°46′9.19″ N 23°12′13.21″ E). Ten water samples were collected during potato cultivation development as follows: once in March 2022, three times in April 2022, four times in May 2022, and twice in June. Before analysis, the water samples were mixed to obtain one sample at each experimental site. From each of the 10 samples collected by the experimental site, 18 groundwater quality parameters were quantified. The collection of samples was carried out following the sampling protocols [29], avoiding sample contamination during sampling and transport. Groundwater samples were taken from a depth of 20 m by drilling.

2.2. Methodology

For a more accurate picture of the groundwater quality, in addition to the nutrient values, the pH and turbidity values were also analyzed, according to standardized methodology (

Table 2. The standardized methodology applied for groundwater quality assessment.

Parameter	Measure Unit	Limit According to the Law 458/2002 Modified by the Law 311/2004	Method
			Standard/Method	Internal Laboratory Procedure (PLST)
Turbidity	NTU	≤5	SR EN ISO 7027-1:2016 [30]	PSLT—02 [31]
pH	unit. pH	≥6.5, ≤9.5	SR EN ISO 10523:2012 [32]	PSLT—07 [31]
Electric conductivity at 20 °C	µS/cm	<2500	SR EN 27888:1997 [33]	PSLT—13 [31]
Chlorides	mg/L	250	SR ISO 9297:2001 [34]	PSLT—06 [31]
Permanganate index	mg O2/L	5	SR EN ISO 8467:2001 [35]	PSLT—11 [31]
Ammonium	mg/L	0.5	SR ISO 7150-1:2001 [36]	PSLT—10 [31]
Nitrites	mg/L	0.5	SR EN 26777:2002/C91:2006 [37]	PSLT—09 [31]
Nitrates	mg/L	50	SR ISO 7890-3:2000 [38]	PSLT—08 [31]
Total hardness	°G	≥5	SR ISO 6059:2008 [39]	PSLT—05 [31]
Calcium	mg/L	-	SR ISO 6058:2008 [40]	PSLT—03 [31]
Magnesium	mg/L	-	SR ISO 6059:2008 [39]	PSLT—05 [31]
Sulfates	mg/L	250	SR ISO 6059:2008 [39]	PSLT—05 [31]
Fluorides	mg/L	1.2	Method 8029 HACH [41]	PSLT—27 [31]
Sulfides	µg/L	100	Method 8131 HACH [42]	PSLT—26 [31]
Alkalinity	mL HCl 0.1 N	-	SR ISO 9963-1:2002 [43]	PSLT—17 [31]
Bicarbonates	mg/L	-	SR ISO 9963-1:2002 [43]	PSLT—17 [31]
Iron	µg/L	200	Method LCK521 HACH [44]	PSLT—22 [31]
Phosphates	mg/L	-	SR EN ISO 6878:2008 [45]	PSLT—16 [31]

).

Turbidity was determined using a HACH 2100P Turbidimeter, according to European Standard EU ISO 7027 [30]. The pH measurements were performed using an Inolab 740 multimeter WTW and the determination was performed according to the International Standard ISO 10523/2012 [30]. The determination of ammonium, nitrites and nitrates, was performed using a LAMBDA Spectrophotometer UV-VIS BIO 40 from PERKIN ELMER [35,36,37].

The determination of nitrogen content was carried out in accordance with ISO 7890 for Water Quality, Determination of nitrogen content part 3—Spectrometric method with sulfosalicylic acid. It was performed by the spectrometric measurement of the absorbance of the yellow compound obtained by the reaction of sulfosalicylic acid (formed by adding sodium salicylate and sulfuric acid to the sample) with nitrogen, followed by treatment with an alkaline solution. The disodium salt of ethylenediaminetetraacetic acid (EDTANa₂) was added to the alkaline solution to prevent the precipitation of calcium and magnesium salts. Sodium azide is added to remove interference with nitrites. During the analysis, only reagents of recognized analytical quality and distilled water or equivalent purity are used. Reagents and doses: 1. sulfuric acid c(H₂SO₄) = 18 mol/L, ρ = 1.84 g/mL; 2. glacial acetic acid, c(CH₃COOH) = 17 mol/l, ρ = 1.05 g/mL; 3. alkaline solution ρNAOH = 200 g/L, ρ [CH₂-N(CH₂COOH)CH₂-COONa]*2H₂O = 50 g/L; 4. sodium azide, ρNaN₃ = 0.5 g/L—is a very toxic solution. As an alternative, the sulfamic acid solution can be used with ρNH₂-SO₃H = 0.75 g/L; 5. sodium salicylate, ρHO-C₅H₄-COONa = 10 g/L; 6. nitrate, basic standard solution ρ = 1000 mg/L or ρ = 1 mg/L. Cuvettes with an optical path of 40 mm and a sample volume of 25 mL were used. The detection limit is in the range ρ_N = 0.003 mg/L to 0.013 mg/L. The equipment used is the specific one of an accredited water analysis laboratory and includes common materials and a spectrometer LAMBDA Spectrophotometer UV-VIS BIO 40 from PERKIN ELMER [37].

For the determination of chemical compounds such as chloride, calcium, and magnesium, the volumetric titration method was used [33,38,39].

To identify the potential nitrate groundwater contamination, the nitrate pollution index was calculated [16], according to the formula:

$$NPI=\frac{\left(C_s-HAV\right)}{HAV}$$

where NPI is the contamination pollution index, HAV is the threshold value for the nitrates of anthropogenic origin (20 mg/L), and C_s is the nitrate concentration of each sample [37]. According to Iqbal et al. (2023) and Ramalingam et al. (2022), based on NPI values, water quality can be classified into five classes as follows: 1—unpolluted (NPI < 0), 2—lightly polluted (NPI = 0–1), 3—moderately polluted (NPI = 1–2), 2—significantly polluted (NPI = 2–3), and 5—very significantly polluted (NPI > 3) [16,26].

2.3. Statistics

The statistical analysis was performed using the XLSTAT program. Basic statistics were implemented for the calculation of the means and dispersion parameters of the analyzed groundwater quality parameters. One-step ANOVA was used to emphasize the significance of differences between the studied parameters based on fertilization type and doses, at a threshold of 95%. The Pearson correlations between groundwater parameters were calculated after testing the normal distributions using Skewness and Kurtosis values (K-test, S-test), which were verified by histograms. The significance of the Pearson correlation coefficient was determined at 95% and 99% confidence intervals. The ”factor” analysis by its component Principal Components Analysis (PCA) was performed to summarize results from the multitude of physiochemical variables characterizing the groundwater quality. According to statistical methodology, at least 5 cases are recommended for performing the basic statistics, while in order to conduct PCA, a minimum sample size of 150 cases (5 to 10 cases/variable) has been recommended. In our case, we have 18 variables; thus, the analysis was conducted on 180 cases, with 10 cases per variable. The validity of implementing PCA was tested using the Kaiser–Maier–Olkin (KMO) test for sampling adequacy and Bartlett probes. If p < 0.05 for the Bartlett test and the KMO value > 0.5, then the conditions for applying PCA are met [46].

3. Results and Discussions

3.1. Physiochemical Characterization of the Groundwater Quality

The results of the statistical study of the evolution of the 18 physiochemical parameters analyzed in 40 water samples collected from the four experimental sites are presented in Table 2. The pH values and nitrate content in groundwater samples collected from experimental site 3, where potato culture was fertilized with sheep manure at doses of 118.56 t/Ha, exceed the allowed limits mentioned by the Romanian standards (Table 1). A lower mean pH is reported in above mentioned site (pH = 6.37) compared to allowed limits (≥6.5 ≤9.5) and higher mean nitrate content (NO₃⁻ = 64.29 mg/L) compared to the allowed limit of 50 mg/L. These means differ significantly from those reported for the groundwater samples corresponding to the other experimental sites. Concerning the groundwater of all experimental sites, the nitrate (NO₃⁻) concentrations over 20 mg/L identified in all samples allow their framing within the category of samples with high nitrate (NO₃⁻) content [47]. Also, higher mean concentrations of conductivity, permanganate index, alkalinity, and bicarbonates (HCO₃⁻) are observed in groundwater samples collected from experimental site 3, even though they do not exceed the allowed limits. Concerning most physiochemical parameters, no significant differences are reported between samples corresponding to all experimental sites (

Table 3. Descriptive statistics for the characterization of groundwater from experimental sites with organic and mineral fertilization.

Issue	N *	Site 1-Sheep Manure, 39.52 t/ha			Site 2-Sheep Manure, 79.04 t/Ha			Site 3-Sheep Manure, 118.56 t/Ha			Site 4-Mineral Fertilization N14:P7:K21
		Mean	s	CV, %	Mean	s	CV, %	Mean	s	CV, %	Mean	s	CV, %
Turbidity, NTU	10	0.22a	0.01	4.54	0.20a	0.01	5.00	0.23a	0.06	26.08	0.24a	0.01	4.16
pH	10	7.68a	0.77	10.02	7.08a	0.69	9.74	6.37b	0.72	11.30	7.57a	0.49	6.47
Conductivity, µS/cm	10	415.49a	7.13	1.71	421.24a	6.98	1.65	438.00b	4.59	1.47	429.44a	7.56	1.76
Chlorides, mg/L	10	30.44a	1.61	5.29	31.03a	1.35	4.35	31.88a	0.85	2.66	31.49a	1.49	4.73
Permanganate index, mg O2/L	10	0.25a	0.02	8.00	0.26a	0.02	7.69	0.32b	0.06	19.76	0.24a	0.01	4.16
Ammonium, mg/L	10	0.009a	0.002	22.23	0.010a	0.002	20.00	0.012a	0.001	10.39	0.013a	0.002	15.38
Nitrites, mg/L	10	0.004a	0.001	25.00	0.005a	0.001	20.00	0.008a	0.002	22.82	0.007a	0.002	28.57
Nitrates, mg/L	10	44.69a	2.66	5.96	43.58b	1.58	3.62	64.29b	1.63	2.54	44.68a	2.12	4.74
Total hardness, °G	10	10.53a	0.96	9.11	10.45a	1.10	10.52	10.60a	1.23	11.63	10.66a	1.35	12.67
Calcium, mg/L	10	51.28a	1.28	2.49	52.08a	1.14	2.20	52.70a	1.48	2.81	52.56a	0.98	1.87
Magnesium, mg/L	10	13.48a	1.26	9.36	13.76a	1.08	7.85	13.97a	1.39	9.96	13.99a	0.96	6.87
Sulfates, mg/L	10	61.60a	2.23	3.62	62.60a	1.96	3.13	68.19a	1.43	2.10	63.20a	1.88	2.97
Fluoride, mg/L	10	0.15a	0.01	8.89	0.16a	0.01	7.99	0.18a	0.01	4.54	0.17a	0.01	6.55
Sulfides, µg/L	10	3.27a	0.35	10.70	3.35a	0.42	12.53	3.40a	0.52	15.19	3.29a	0.44	13.37
Alkalinity, mL HCl 0.1 N	10	1.89a	0.31	16.40	1.72a	0.34	19.76	1.40b	0.27	19.28	1.68a	0.30	17.85
Bicarbonates, mg/L	10	92.36a	1.25	1.35	89.94a	1.15	1.27	85.40b	1.01	1.19	89.44a	1.28	1.43
Iron, µg/L	10	34.54a	0.50	1.44	34.74a	0.64	1.84	35.00a	1.39	3.98	34.92a	0.69	1.96
Phosphates, mg/L	10	0.041a	0.001	3.19	0.042a	0.001	3.12	0.043a	0.001	0.29	0.052a	0.003	5.76

Note(s): *—10 samples were taken from each of four experimental sites; s—standard deviation; CV—coefficient of variation, %. The same letter signifies no differences between means at a significance threshold of 5% (p > 0.05%).

). Li et al. (2022) reported similar limits and content hierarchy in groundwater cations (Ca, Mg), iron, and major anions (HCO₃⁻, SO₄²⁻, Cl⁻, NO₃⁻), but higher contents in F⁻ and NH₄⁺ in studies performed to emphasize the quality parameters of the groundwater collected from different sites [48]. Suthar et al. (2009) reported greater groundwater contamination in rural areas of India due to mineral and organic fertilization practices, compared to our results, concerning nitrates(NO₃⁻), chlorine (Cl⁻), and sulfates (SO₄²⁻) [49]. Studies on groundwaters quality emphasize that often their quality is depreciated mainly by NO₃⁻ pollution from anthropogenic sources in rural areas, where issues concerning communal wastewater occur, which are the result of the improper disposal of wastewater (discharged untreated onto agricultural lands) and may contain high quantities of NO₂⁻ and NO₃⁻. This is the result of failing septic systems and inadequate sanitation practices. The main reason for the incidence of these issues in rural areas is the lack of adequate infrastructure and proper wastewater treatment facilities [18,50,51,52].

3.2. Groundwater Pollution with Nitrates

According to NPI values (

Table 4. Descriptive statistics for nitrate pollution index (NPI) of groundwater collected from the experimental sites.

Issue	Mean	S 1	CV 2, %
Site 1-Sheep manure, 39.52 t/ha	1.23	0.04	3.25
Site 2-Sheep manure, 79.04 t/Ha	1.17	0.03	2.56
Site 3-Sheep manure, 118.56 t/Ha	2.21	0.07	3.16
Site 4-Mineral fertilization N14:P7:K21	1.24	0.04	3.23

Note(s): ¹ s—standard deviation; ² CV—coefficient of variation, %.

), the groundwater analyzed from three locations corresponding to mineral fertilization and organic fertilization with sheep manure falls within the moderate pollution category, while the groundwater collected from experimental site 3, where potato culture was fertilized with sheep manure at doses of 118.56 t/Ha, falls within the significant pollution category.

We consider that the reason for the highest concentration of NO₃⁻ and the highest NPI value reported at the experimental site fertilized with the highest quantity of organic fertilizer is not only due to the high nitrogen input from the fertilizer but also due to the contribution of other factors. Thus, some factors that could contribute may be the historical accumulations from infiltrations of untreated sewage and from uninsulated septic tanks [16] and also the high water permeability of the Cambisol structure. Another factor may be the higher biodisponibility of NO₃⁻ from the organic fertilizer compared to mineral fertilization, or even inappropriate management of fertilization practices [5]. The NPI is widely used to classify water quality for different purposes [53,54,55]. In a study conducted to assess NO₃⁻ groundwater pollution in 10 different sites, the mean NPI values ranged from 2.32 to 15.7, indicating a very significant polluted sampling area [56,57].

3.3. The Study of the Simple Correlations

Pearson’s simple correlation matrices between water quality parameters for each type of fertilization and experimental site were calculated to emphasize their relationships, and the opportunity of performing the PCA, which correlates the relationships among multiple parameters of groundwater quality. The simple Pearson correlations have values between −1 and +1. The negative correlation signifies an inverse evolution of the analyzed traits (the increase of one parameter is accompanied by the decrease of the other, in a manner described by the intensity of correlation, which increases in intensity as close as to 1 the correlation coefficient value) while the positive one signifies similar evolutions [51,52]. In all experimental sites, both positive and negative correlations between the same analyzed groundwater parameters are observed. Regardless of the type of fertilizer and doses administered, the most significant correlations are reported for pH (

Table 5. The simple correlations for Site 1-Sheep manure, administered at doses of 39.52 t/Ha.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
1		0.35	−0.57	0.20	0.52	0.22	−0.09	0.13	−0.29	0.02	−0.12	−0.08	−0.10	−0.13	0.16	−0.15	−0.17	−0.32
2	0.35		0.65	−0.04	−0.09	−0.31	−0.64	−0.68	0.25	0.19	0.07	0.15	0.62	−0.14	−0.18	0.01	0.10	0.77
3	−0.57	0.65		0.20	−0.19	−0.58	−0.36	−0.25	−0.21	0.08	0.14	0.17	0.15	0.23	−0.16	−0.04	0.24	0.17
4	0.20	−0.04	0.20		−0.22	−0.11	0.68	0.02	−0.07	0.18	0.17	0.19	0.23	−0.14	−0.04	0.12	0.15	0.16
5	0.52	−0.09	−0.19	−0.22		0.56	−0.18	−0.13	−0.15	0.16	−0.17	−0.06	−0.11	0.13	−0.23	0.02	−0.24	0.25
6	0.22	−0.31	−0.58	−0.11	0.56		0.15	−0.13	−0.13	−0.03	0.16	0.14	0.18	−0.14	−0.17	0.16	0.20	−0.53
7	−0.09	−0.64	−0.36	0.68	−0.18	0.15		−0.61	0.14	−0.05	0.60	−0.05	0.17	0.11	−0.19	0.14	0.13	0.14
8	0.13	−0.68	−0.25	0.02	−0.13	−0.33	−0.61		−0.02	0.10	−0.25	−0.15	−0.13	0.01	−0.17	−0.03	0.05	−0.01
9	−0.29	0.25	−0.21	−0.07	−0.15	−0.13	0.14	−0.02		0.05	0.11	−0.03	−0.02	0.21	0.21	0.61	0.16	−0.13
10	0.02	0.19	0.08	0.18	0.16	−0.03	−0.05	0.10	0.05		0.26	0.22	−0.14	0.12	0.25	0.15	−0.58	0.15
11	−0.12	0.07	0.14	0.17	−017	0.16	0.60	−0.25	0.11	0.26		−0.18	0.26	0.01	−0.24	0.55	−0.18	−0.28
12	−0.08	0.15	0.17	0.19	−0.06	0.14	−0.05	−0.15	−0.03	0.22	−0.18		0.21	0.22	0.14	−0.17	−0.06	−0.24
13	−0.10	0.62	0.15	0.23	−0.11	0.18	0.17	−0.13	0.02	−0.14	0.26	0.21		0.19	−0.16	0.09	−0.19	0.18
14	−0.13	−0.14	0.23	−0.14	0.13	−0.14	0.11	0.01	0.21	0.12	0.01	0.22	0.19		0.02	0.19	−0.11	−0.30
15	0.16	−0.18	−0.16	−0.04	−0.23	−0.17	−0.19	−0.17	0.21	0.25	−0.24	0.14	−0.16	0.02		0.17	0.53	−0.28
16	−0.15	0.01	−0.04	0.12	0.02	0.16	0.14	−0.03	0.61	0.15	0.55	−0.17	0.09	0.19	0.17		−0.54	−0.14
17	−0.17	0.10	0.24	0.15	−0.24	0.20	0.13	0.05	0.16	−0.58	−0.18	−0.06	−0.19	−0.11	0.53	−0.54		−0.19
18	−0.32	0.77	0.17	0.16	−0.53	0.25	0.14	−0.01	−0.13	0.15	−0.28	−0.24	0.18	−0.30	−0.28	−0.14	−0.19

Note(s): 1—turbidity, NTU; 2—pH; 3—conductivity, µS/cm; 4—chlorides, mg/L; 5—permanganate index, mg O₂/L; 6—ammonium, mg/L; 7—nitrites, mg/L; 8—nitrates, mg/L; 9—total hardness, °G; 10—calcium, mg/L; 11—magnesium, mg/L; 12—sulfates, mg/L; 13—fluoride, mg/L; 14—sulfides, µg/L; 15—alkalinity, mL HCl 0.1 N; 16—bicarbonates, mg/L; 17—iron, µg/L; 18—phosphates, mg/L.

,

Table 6. The simple correlations for Site 2-Sheep manure, administered at doses of 79.04 t/Ha.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
1		0.35	−0.57	0.20	0.52	0.22	−0.09	0.13	−0.29	0.02	−0.12	−0.08	−0.10	−0.13	0.16	−0.15	−0.17	−0.32
2	0.35		0.65	−0.04	−0.09	−0.31	−0.64	−0.68	0.25	0.19	0.07	0.15	0.62	−0.14	−0.18	0.01	0.10	0.77
3	−0.57	0.65		0.20	−0.19	−0.58	−0.36	−0.25	−0.21	0.08	0.14	0.17	0.15	0.23	−0.16	−0.04	0.24	0.17
4	0.20	−0.04	0.20		−0.22	−0.11	0.68	0.02	−0.07	0.18	0.17	0.19	0.23	−0.14	−0.04	0.12	0.15	0.16
5	0.52	−0.09	−0.19	−0.22		0.56	−0.18	−0.13	−0.15	0.16	−0.17	−0.06	−0.11	0.13	−0.23	0.02	−0.24	0.25
6	0.22	−0.31	−0.58	−0.11	0.56		0.15	−0.13	−0.13	−0.03	0.16	0.14	0.18	−0.14	−0.17	0.16	0.20	−0.53
7	−0.09	−0.64	−0.36	0.68	−0.18	0.15		−0.61	0.14	−0.05	0.60	−0.05	0.17	0.11	−0.19	0.14	0.13	0.14
8	0.13	−0.68	−0.25	0.02	−0.13	−0.33	−0.61		−0.02	0.10	−0.25	−0.15	−0.13	0.01	−0.17	−0.03	0.05	−0.01
9	−0.29	0.25	−0.21	−0.07	−0.15	−0.13	0.14	−0.02		0.05	0.11	−0.03	−0.02	0.21	0.21	0.61	0.16	−0.13
10	0.02	0.19	0.08	0.18	0.16	−0.03	−0.05	0.10	0.05		0.26	0.22	−0.14	0.12	0.25	0.15	−0.58	0.15
11	−0.12	0.07	0.14	0.17	−017	0.16	0.60	−0.25	0.11	0.26		−0.18	0.26	0.01	−0.24	0.55	−0.18	−0.28
12	−0.08	0.15	0.17	0.19	−0.06	0.14	−0.05	−0.15	−0.03	0.22	−0.18		0.21	0.22	0.14	−0.17	−0.06	−0.24
13	−0.10	0.62	0.15	0.23	−0.11	0.18	0.17	−0.13	0.02	−0.14	0.26	0.21		0.19	−0.16	0.09	−0.19	0.18
14	−0.13	−0.14	0.23	−0.14	0.13	−0.14	0.11	0.01	0.21	0.12	0.01	0.22	0.19		0.02	0.19	−0.11	−0.30
15	0.16	−0.18	−0.16	−0.04	−0.23	−0.17	−0.19	−0.17	0.21	0.25	−0.24	0.14	−0.16	0.02		0.17	0.53	−0.28
16	−0.15	0.01	−0.04	0.12	0.02	0.16	0.14	−0.03	0.61	0.15	0.55	−0.17	0.09	0.19	0.17		−0.54	−0.14
17	−0.17	0.10	0.24	0.15	−0.24	0.20	0.13	0.05	0.16	−0.58	−0.18	−0.06	−0.19	−0.11	0.53	−0.54		−0.19
18	−0.32	0.77	0.17	0.16	−0.53	0.25	0.14	−0.01	−0.13	0.15	−0.28	−0.24	0.18	−0.30	−0.28	−0.14	−0.19

Note(s): 1—turbidity, NTU; 2—pH; 3—conductivity, µS/cm; 4—chlorides, mg/L; 5—permanganate index, mg O₂/L; 6—ammonium, mg/L; 7—nitrites, mg/L; 8—nitrates, mg/L; 9—total hardness, °G; 10—calcium, mg/L; 11—magnesium, mg/L; 12—sulfates, mg/L; 13—fluoride, mg/L; 14—sulfides, µg/L; 15—alkalinity, mL HCl 0.1 N; 16—bicarbonates, mg/L; 17—iron, µg/L; 18—phosphates, mg/L.

,

Table 7. The simple correlations for Site 3-Sheep manure, administered at doses of 118.56 t/Ha.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
1		0.35	−0.57	0.20	0.52	0.22	−0.09	0.13	−0.29	0.02	−0.12	−0.08	−0.10	−0.13	0.16	−0.15	−0.17	−0.32
2	0.35		0.65	−0.04	−0.09	−0.31	−0.64	−0.68	0.25	0.19	0.07	0.15	0.62	−0.14	−0.18	0.01	0.10	0.77
3	−0.57	0.65		0.20	−0.19	−0.58	−0.36	−0.25	−0.21	0.08	0.14	0.17	0.15	0.23	−0.16	−0.04	0.24	0.17
4	0.20	−0.04	0.20		−0.22	−0.11	0.68	0.02	−0.07	0.18	0.17	0.19	0.23	−0.14	−0.04	0.12	0.15	0.16
5	0.52	−0.09	−0.19	−0.22		0.56	−0.18	−0.13	−0.15	0.16	−0.17	−0.06	−0.11	0.13	−0.23	0.02	−0.24	0.25
6	0.22	−0.31	−0.58	−0.11	0.56		0.15	−0.13	−0.13	−0.03	0.16	0.14	0.18	−0.14	−0.17	0.16	0.20	−0.53
7	−0.09	−0.64	−0.36	0.68	−0.18	0.15		−0.61	0.14	−0.05	0.60	−0.05	0.17	0.11	−0.19	0.14	0.13	0.14
8	0.13	−0.68	−0.25	0.02	−0.13	−0.33	−0.61		−0.02	0.10	−0.25	−0.15	−0.13	0.01	−0.17	−0.03	0.05	−0.01
9	−0.29	0.25	−0.21	−0.07	−0.15	−0.13	0.14	−0.02		0.05	0.11	−0.03	−0.02	0.21	0.21	0.61	0.16	−0.13
10	0.02	0.19	0.08	0.18	0.16	−0.03	−0.05	0.10	0.05		0.26	0.22	−0.14	0.12	0.25	0.15	−0.58	0.15
11	−0.12	0.07	0.14	0.17	−017	0.16	0.60	−0.25	0.11	0.26		−0.18	0.26	0.01	−0.24	0.55	−0.18	−0.28
12	−0.08	0.15	0.17	0.19	−0.06	0.14	−0.05	−0.15	−0.03	0.22	−0.18		0.21	0.22	0.14	−0.17	−0.06	−0.24
13	−0.10	0.62	0.15	0.23	−0.11	0.18	0.17	−0.13	0.02	−0.14	0.26	0.21		0.19	−0.16	0.09	−0.19	0.18
14	−0.13	−0.14	0.23	−0.14	0.13	−0.14	0.11	0.01	0.21	0.12	0.01	0.22	0.19		0.02	0.19	−0.11	−0.30
15	0.16	−0.18	−0.16	−0.04	−0.23	−0.17	−0.19	−0.17	0.21	0.25	−0.24	0.14	−0.16	0.02		0.17	0.53	−0.28
16	−0.15	0.01	−0.04	0.12	0.02	0.16	0.14	−0.03	0.61	0.15	0.55	−0.17	0.09	0.19	0.17		−0.54	−0.14
17	−0.17	0.10	0.24	0.15	−0.24	0.20	0.13	0.05	0.16	−0.58	−0.18	−0.06	−0.19	−0.11	0.53	−0.54		−0.19
18	−0.32	0.77	0.17	0.16	−0.53	0.25	0.14	−0.01	−0.13	0.15	−0.28	−0.24	0.18	−0.30	−0.28	−0.14	−0.19

Note(s): 1—turbidity, NTU; 2—pH; 3—conductivity, µS/cm; 4—chlorides, mg/L; 5—permanganate index, mg O₂/L; 6—ammonium, mg/L; 7—nitrites, mg/L; 8—nitrates, mg/L; 9—total hardness, °G; 10—calcium, mg/L; 11—magnesium, mg/L; 12—sulfates, mg/L; 13—fluoride, mg/L; 14—sulfides, µg/L; 15—alkalinity, mL HCl 0.1 N; 16—bicarbonates, mg/L; 17—iron, µg/L; 18—phosphates, mg/L.

and

Table 8. The simple correlations for Site 4-Mineral fertilization N₁₄:P₇:K_21.

	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16	17	18
1		0.35	−0.57	0.20	0.52	0.22	−0.09	0.13	−0.29	0.02	−0.12	−0.08	−0.10	−0.13	0.16	−0.15	−0.17	−0.32
2	0.35		0.65	−0.04	−0.09	−0.31	−0.64	−0.68	0.25	0.19	0.07	0.15	0.62	−0.14	−0.18	0.01	0.10	0.77
3	−0.57	0.65		0.20	−0.19	−0.58	−0.36	−0.25	−0.19	0.08	0.14	0.17	0.15	0.23	−0.16	−0.04	0.24	0.17
4	0.20	−0.04	0.20		−0.22	−0.11	0.68	0.02	−0.07	0.18	0.17	0.19	0.23	−0.14	−0.04	0.12	0.15	0.16
5	0.52	−0.09	−0.19	−0.22		0.56	−0.18	−0.13	−0.15	0.16	−0.17	−0.06	−0.11	0.13	−0.23	0.02	−0.24	0.25
6	0.22	−0.31	−0.58	−0.11	0.56		0.15	−0.13	−0.13	−0.03	0.16	0.14	0.18	−0.14	−0.17	0.16	0.20	−0.53
7	−0.09	−0.64	−0.36	0.68	−0.18	0.15		−0.61	0.14	−0.05	0.60	−0.05	0.17	0.11	−0.19	0.14	0.13	0.14
8	0.13	−0.68	−0.25	0.02	−0.13	−0.33	−0.61		−0.02	0.10	−0.25	−0.15	−0.13	0.01	−0.17	−0.03	0.05	−0.01
9	−0.29	0.25	−0.19	−0.07	−0.15	−0.13	0.14	−0.02		0.05	0.11	−0.03	−0.02	0.21	0.21	0.61	0.16	−0.13
10	0.02	0.19	0.08	0.18	0.16	−0.03	−0.05	0.10	0.05		0.26	0.22	−0.14	0.12	0.25	0.15	−0.58	0.15
11	−0.12	0.07	0.14	0.17	−017	0.16	0.60	−0.25	0.11	0.26		−0.18	0.26	0.01	−0.24	0.55	−0.18	−0.23
12	−0.08	0.15	0.17	0.19	−0.06	0.14	−0.05	−0.15	−0.03	0.22	−0.18		0.21	0.22	0.14	−0.17	−0.06	−0.24
13	−0.10	0.62	0.15	0.23	−0.11	0.18	0.17	−0.13	0.02	−0.14	0.26	0.21		0.19	−0.16	0.09	−0.19	0.18
14	−0.13	−0.14	0.23	−0.14	0.13	−0.14	0.11	0.01	0.21	0.12	0.01	0.22	0.19		0.02	0.19	−0.11	−0.30
15	0.16	−0.18	−0.16	−0.04	−0.23	−0.17	−0.19	−0.17	0.21	0.25	−0.24	0.14	−0.16	0.02		0.17	0.53	−0.28
16	−0.15	0.01	−0.04	0.12	0.02	0.16	0.14	−0.03	0.61	0.15	0.55	−0.17	0.09	0.19	0.17		−0.54	−0.14
17	−0.17	0.10	0.24	0.15	−0.24	0.20	0.13	0.05	0.16	−0.58	−0.18	−0.06	−0.19	−0.11	0.53	−0.54		−0.19
18	−0.32	0.77	0.17	0.16	−0.53	0.25	0.14	−0.01	−0.13	0.15	−0.23	−0.24	0.18	−0.30	−0.28	−0.14	−0.19

Note(s): 1—turbidity, NTU; 2—pH; 3—conductivity, µS/cm; 4—chlorides, mg/L; 5—permanganate index, mg O₂/L; 6—ammonium, mg/L; 7—nitrites, mg/L; 8—nitrates, mg/L; 9—total hardness, °G; 10—calcium, mg/L; 11—magnesium, mg/L; 12—sulfates, mg/L; 13—fluoride, mg/L; 14—sulfides, µg/L; 15—alkalinity, mL HCl 0.1 N; 16—bicarbonates, mg/L; 17—iron, µg/L; 18—phosphates, mg/L.

). The simple Pearson correlations are considered moderate when values of the correlation coefficients are ranging between r = 0.51–0.69 at a significance threshold of 5% (p < 0.05%) and strong when the correlation coefficients are over r = 0.77 at a significance threshold of 1%. Turbidity is moderately and significantly (p < 0.05%) correlated with conductivity (with values of r = −0.57 corresponding to sites 1 and 3, and r = −0.58 corresponding to sites 2 and 4) and permanganate index (with values ranging between r = 0.51 corresponding to site 1 and r = 0.55 corresponding to site 2). pH is moderately and significantly (p < 0.05%) correlated with conductivity (with values ranging between r = 0.65 corresponding to sites 1 and 3 and r = 0.69 corresponding to site 2), nitrites NO₂⁻ (with values ranging between r = −0.63 corresponding to site 4, and r = −0.65 corresponding to sites 2 and 3), nitrates (NO₃⁻) for groundwater located in sites 1 and 4 (r = −0.68), sulfates SO₄²⁻ (with values ranging between r = 0.62 corresponding to sites 1 and 4, and r = 0.65 corresponding to sites 2, and 3), and phosphates PO₄³⁻ in case of groundwater located in site 4 that was minerally fertilized (r = 0.69). pH is strongly and significantly (p < 0.01%) correlated with nitrates (NO₃⁻) for groundwater located in site 3 (r = −0.73) and phosphates PO₄³⁻ (with values ranging between r = 0.77 corresponding to site 1 and r = 0.82 corresponding to site 3). Except for pH, all other correlations between parameters of groundwater are moderate and significant (p < 0.05%). Conductivity is correlated with ammonium NH₄⁺, with correlation coefficients ranging between r = 0.55 corresponding to site 3 and r = 0.59 corresponding to site 4 (Table 5, Table 6, Table 7 and Table 8).

Chlorides (Cl⁻) are correlated with nitrites, with correlation coefficients ranging between r = 0.68 corresponding to sites 1 and 3 and r = 0.69 corresponding to sites 2 and 4. The permanganate index is correlated with ammonium (NH₄⁺), with values ranging between r = 0.52 corresponding to site 3 and r = 0.58 corresponding to site 2. Ammonium (NH₄⁺) is correlated with phosphates PO₄³⁻, with correlation coefficients ranging between r = −0.53 corresponding to sites 1 and 3 and r = −0.56 corresponding to sites 2 and 4. Nitrites (NO₂⁻) are correlated with nitrates NO₃⁻ (with correlation coefficients ranging between r = 0.61 corresponding to sites 1 and 2, and r = 0.64 corresponding to site 3) and magnesium Mg (with correlation coefficients ranging between r = 0.60 corresponding to sites 1 and 2 and r = 0.64 corresponding to site 4). Nitrates (NO₃⁻) are also negatively correlated with ammonium NH₄⁺ (with correlation coefficients ranging between r = −0.63 corresponding to site 1 and r = −0.64 corresponding to sites 2–4). Total hardness is positively correlated with bicarbonates (HCO₃⁻) content in groundwater with correlation coefficients ranging between r = 0.61 and r = 0.64 corresponding to sites 1 and 2, respectively. Calcium (Ca) is correlated with iron with correlation coefficients ranging between r = 0.58 corresponding to sites 1 and 4, respectively, and r = −0.61 corresponding to site 3. Magnesium (mg) is correlated with bicarbonate content in groundwater with correlation coefficients ranging between r = 0.53 and r = 0.57 corresponding to sites 4 and 2, respectively. Iron is correlated with alkalinity (with correlation coefficients ranging between r = 0.52 corresponding to sites 3 and r = 0.55 corresponding to sites 2 and 4, respectively) and bicarbonate (HCO₃⁻) contents in groundwater with correlation coefficients ranging between r = −0.52 corresponding to sites 3 and r = −0.59 corresponding to sites 3 and 2, respectively (Table 5, Table 6, Table 7 and Table 8).

The significant correlations identified in groundwater are similar for all analyzed sites, regardless of fertilization type and administered doses. Differences are reported only concerning the intensity of correlations. Between pH and phosphates (PO₄³⁻), strong significant positive correlations at the significance threshold of 1% (p< 0.01%) are reported when organic fertilization is applied, while moderate correlation is observed between pH and phosphates (PO₄³⁻) in the groundwater when mineral fertilization was administered. This suggests the stronger effect of phosphates (PO₄³⁻) on groundwater acidity and/or alkalinity when organic fertilization is used, compared with mineral fertilization. The strong correlation between pH and nitrates (NO₃⁻) recorded in site 3, where exceeding of nitrates (NO₃⁻) allowed limit is reported, emphasizes that the increase in nitrates (NO₃⁻) in groundwater influences to a great extent the pH, which reaches values not considered within allowed limits.

Unlike results reported by Tanwer et al. (2023) where alkalinity is significantly correlated with total hardness and nitrates (NO₃⁻) [20], our study does not demonstrate strong or significant correlations between above-mentioned parameters. Similar to our results, Suthar et al. (2009) also reported strong correlation between pH and NO₃⁻ in groundwater samples collected from mineral and organic fertilized areas [49], which indicates the contribution of nitrates (NO₃⁻) to the increase of the groundwater acidity. Alramthi et al. (2022) identified stronger and statistically significant correlations between chloride (Cl⁻) and nitrites (NO₂⁻), compared to those reported in the present study [6]. Li et al. (2020) reported weaker correlations (r = −0.277 and −0.302) between NO₃⁻ and NH4⁺ compared to those obtained in our study, which indicates the intensified transformation processes (nitrification versus denitrification) in experimental sites analyzed in our study [52].

3.4. Principal Components Analysis (PCA)

For a better understanding of the relationships between the main groundwater quality parameters when different fertilization strategies are applied, a PCA analysis was applied. The suitability of using PCA was demonstrated by KMO values above 0.500 and p < 0.01 for Bartlett test. In all experimental sites, two main factors are identified: F1, the dose of nitrogen administered and F2, the type of fertilization, while F1 accounts for a larger share of variance compared to F1 (Figure 1,

Table 9. The factor loadings of the groundwater parameters.

Issues	Site 1-Sheep Manure, 39.52 t/ha		Site 2-Sheep Manure, 79.04 t/Ha		Site 3-Sheep Manure, 118.56 t/Ha		Site 4-Mineral Fertilization N14:P7:K21
	Component		Component		Component		Component
	PC1	PC2	PC1	PC2	PC1	PC2	PC1	PC2
Turbidity, NTU	0.750	−0.042	−0.773	−0.386	−0.733	0.043	−0.108	0.087
pH	−0.263	0.850	0.389	−0.749	0.413	0.688	0.424	0.124
Conductivity, µS/cm	−0.527	0.022	−0.433	−0.370	0.016	−0.785	−0.034	−0.633
Chlorides, mg/L	−0.702	−0.032	0.162	−0.553	−0.242	0.289	0.531	0.187
Permanganate index, mg O2/L	−0.150	0.733	−0.501	−0.194	0.423	0.162	0.289	0.561
Ammonium, mg/L	0.159	0.152	0.444	−0.314	−0.626	0.403	0.390	−0.246
Nitrites, mg/L	−0.701	0.482	0.114	0.795	−0.259	0.938	−0.792	0.316
Nitrates, mg/L	−0.873	0.017	−0.054	0.589	0.368	−0.532	0.897	0.076
Total hardness, °G	−0.381	0.586	−0.074	0.776	0.598	0.301	−0.261	−0.535
Calcium, mg/L	−0.666	−0.424	0.071	−0.247	0.525	0.022	0.143	−0.862
Magnesium, mg/L	0.550	0.004	−0.700	0.430	0.088	0.679	−0.808	−0.256
Sulfates, mg/L	−0.766	0.101	0.854	0.296	−0.292	−0.239	−0.591	0.420
Fluoride, mg/L	0.093	0.543	−0.402	0.727	−0.509	0.531	0.221	−0.731
Sulfides, µg/L	0.212	0.391	0.747	0.053	0.649	0.307	0.593	0.507
Alkalinity, mL HCl 0.1 N	0.466	0.649	−0.303	−0.033	0.322	−0.376	0.397	−0.718
Bicarbonates, mg/L	0.485	0.407	0.714	−0.022	0.531	0.420	0.627	−0.319
Iron, µg/L	0.134	−0.253	0.453	0.150	−0.762	−0.073	−0.039	0.422
Phosphates, mg/L	−0.258	−0.391	0.709	0.308	0.323	0.137	−0.639	−0.531

).

PC1 and PC2 explain different percentages of variation across the experimental sites: PC1 explains 65.25% of variance, and PC2 explains 34.75% of variance corresponding to Site 1-Sheep manure, 39.52 t/ha; PC1 explains 64.92% of variance, and PC2 explains 35.75% of variance corresponding to Site 2-Sheep manure, 79.04 t/ha; PC1 explains 67.83% of variance, and PC2 explains 32.17% of variance corresponding to Site 3-Sheep manure 118.56 t/ha; PC1 explains 61.22% of variance, and PC2 explains 38.78% of variance corresponding to Site 4-mineral fertilization N₁₄:P₇:K₂₁. According to the biplot plan of the PCA (Figure 1), results show that the dose of nitrogen administered influences in a different manner the evolution of the groundwater parameters. The lowest doses of nitrogen administered by organic fertilization and the dose of nitrogen administered by mineral fertilization mainly led to increases in the salts (nitrites NO₂⁻, nitrates NO₃⁻, phosphates PO₄³⁻, sulfates SO₄²⁻), calcium Ca, ammonium NH₄⁺, and total hardness of the groundwater (Figure 2a,b,d, Table 9), while the biggest dose of nitrogen administered by organic fertilization increases the values of the physical parameters (turbidity, conductivity), alkalinity, sulfate SO₄²⁻, nitrates NO₃⁻, and iron Fe concentrations (Figure 2c, Table 9). These results may be attributed not only to the consequence of nitrogen source or administered dose, but also to the physiochemical and biological soil processes influencing soil solution and groundwater quality, and consequently, by nitrogen form release (NO₃⁻, and or NH₄⁺) [16,57,58].

The common groundwater parameters positively influenced by both mineral and organic fertilization are turbidity, pH, permanganate, index, bicarbonate (HCO₃⁻), chlorine (Cl⁻), nitrites (NO₃⁻), iron (Fe) content, and total hardness. Differences between the influences of the type of fertilization on groundwater parameters concern the phosphate (PO₄³⁻) content increased only with mineral fertilization and fluoride (F), magnesium (Mg), calcium (Ca), ammonium (NH₄⁺), alkalinity, and sulfides (SO₃²⁻), increased only with organic fertilization (Figure 2d, Table 9). Unlike the results of our study, where we identified two principal factors, in studies conducted on groundwater quality from a western Saudi Arabia area, Alshehri and Abdelrahman (2023) identified three principal factors, while Li et al. (2020) in southwestern China five principal factors, both accounting for low variances [18,52]. Both principal factors identified in our study strongly weighed nitrites (NO₃⁻), nitrates (NO₂⁻), sulfides (SO₃²⁻), or phosphates (PO₄³⁻), showing the importance of fertilizers inputs regardless of their origin [58,59,60,61].

4. Conclusions

The impact of organic (manure) fertilization, based on the administered dose, and mineral fertilization on physiochemical parameters of groundwater quality, was assessed. The administration of higher doses of organic fertilizer leads to the exceeding of allowed limits for nitrates (NO₃⁻) occurrence and groundwater acidifying. Few positive and negative significant correlations are identified between the same analyzed groundwater parameters in experimental sites, and this indicates similar interactions between fertilizers inputs and groundwater. The doses of nitrogen administered through organic and mineral fertilization have different influences on the groundwater parameters. The lowest doses of nitrogen administered by organic fertilization and mineral fertilization mainly led to increases in the salts, calcium (Ca), ammonium (NH₄⁺), and total hardness in groundwater and the biggest dose of nitrogen administered through organic fertilization increases the values of the groundwater physical parameters (turbidity, conductivity), alkalinity, sulfates (SO₄²⁻), nitrates (NO₃⁻), and iron (Fe) concentrations. The type of fertilization (organic versus mineral) has different influences on groundwater quality parameters. The phosphate content increased only when mineral fertilization is applied, while fluoride (F), magnesium (Mg), calcium (Ca), ammonium (NH₄⁺), alkalinity, and sulfides (SO₃²⁻) increased only when organic fertilization was used, regardless of administered dose. The administration of high organic fertilizer (manure) doses, which is a very common practice in rural areas, has a high harmful potential because wells that are in proximity to agricultural fields are still used as sources of drinking water. Further research needs to be conducted to obtain a clear picture of groundwater pollution due to anthropic contributions, in rural areas of Romania, including methodologies involving isotopic measurements.