Hydrochemical Characteristics and Suitability Assessment of Groundwater Quality for Irrigation

Abstract

The hydrochemical properties of groundwater play a crucial role in crop growth and soil health, as well as the maintenance of irrigation equipment. To ensure suitable water quality for irrigation, the present study was conducted to evaluate the hydrochemical properties of irrigation groundwater in a typical agricultural region of Serbia. Groundwater was sampled at three monitoring locations in the Srem region in Republic of Serbia between 2011 and 2020. Examined parameters included electrical conductivity (EC), total dissolved salts (TDS), pH values, and the concentrations of cations (sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺) and potassium (K⁺)) and anions (bicarbonate (HCO₃⁻), chloride (Cl⁻), sulfate (SO₄²⁻), and nitrate (NO₃⁻)). Further assessment was done using three classification systems; Nejgebauer’s, the US USSL, and the FAO classification. To obtain a more detailed assessment, additional indices were included, such as Soluble Sodium Percentage (SSP), Sodium Adsorption Ratio (SAR), Residual Sodium Carbonate (RSC), Permeability Index (PI), Kelly’s Ratio (KR), Magnesium Adsorption Ratio (MAR), and Total Hardness (TH). The PCA analysis detected that the concentration of K⁺, Ca²⁺, and Na⁺ in the irrigation water were lower in recent years, while the concentration of Cl⁻ was higher. The cluster analysis grouped the parameters into three clusters; I—electroconductivity, II—dry residue and HCO₃⁻, and III—other water parameters for all three monitoring localities. According to the applied classifications, the majority of underground water samples were suitable for irrigation. However, some samples fell out of the range of the first class, indicating the need for regular water quality monitoring. In light of climate changes that influence water deficiency, the urgent need for wise and sustainable water use implies the application of a comprehensive approach to irrigation water quality assessment, as shown in this study.

1. Introduction

Irrigation takes a special place in the water resource management system, given that globally, agricultural water use accounts for 70% of the total amount of freshwater used [1]. Approximately 20% of arable land worldwide is irrigated, and in the European Union, it averages around 6% [2]. The use of unsuitable water quality has led to soil salinity, alkalinization, and overwetting, impacting 20% of global irrigated land [3]. Specific challenges in irrigation, such as soil salinization, soil pollution, water and air pollution, high energy costs, and social and sanitary impacts indicate the need to monitor the quality of irrigation water and its impact on soil resources [4]. In the face of climate change predictions, elevating temperatures and decreasing precipitation in northern Serbia highlight the necessity of irrigation. According to the projected climate scenarios obtained using different climate models, by the year 2080, the average annual air temperature will increase approximately 2.3 to 2.6 °C and the trend of decreasing total annual precipitation will occur in the entire area of northern Serbia [5]. Such predictions elevate irrigation to the status of a necessary measure, without which intensive and profitable agricultural production will most likely not be possible. The quality of irrigation water profoundly affects crop yields, soil properties, and poses challenges like salinization and pollution. Problems that can occur in the soil such as salinity, sodicity, limited infiltration, contamination, etc. are caused by the use of low-quality water for irrigation [6]. Groundwater resources are under numerous natural (geological-pedological, climatic, and topographic characteristics) and anthropogenic (settlements, industry, and agriculture) influences [7,8]. A crucial resource, such as groundwater, faces influences from both natural and anthropogenic factors in the northern part of Serbia, where it serves both drinking and irrigation purposes. Research on water quality for irrigation in this area has a long tradition, therefore some classifications and guidelines of local character have been developed, for example, Nejgebauer’s classification.

Previous research in this region, like Nejgebauer’s classification, indicates significant variability in groundwater quality for irrigation [9,10,11]. The picture of water quality consists of an analysis of various physical, chemical, and biological parameters. Based on the results of the hydrochemical parameters, it is possible to obtain relevant data on water classification. From the aspect of irrigation and its negative influences on soil, water quality is viewed through the content of major cations and anions, salinity indicators (electrical conductivity, EC and total dissolved salts, TDS) and specific indices. These indices, such as Sodium Adsorption Ratio (SAR), Residual Sodium Carbonate (RSC), total hardness (TH), etc. represent relative relationships between ions, and are important due to the fact that ion exchange is a geochemical process that plays an important role in the formation of groundwater chemistry. Guided by this approach, researchers in different parts of the world assess the quality of groundwater for irrigation purposes [12,13,14,15].

This study aims to contribute broader insights into the suitability of groundwater for irrigation, in a typical agricultural region, in the Republic of Serbia, considering its potential impact on regional agriculture and development plans.

2. Materials and Methods

2.1. Study Area

The Srem region is located in the Vojvodina province, northern Serbia, in the southern part of the Pannonian-Carpathian Basin, around 45° northern latitude and between 19° and 20° eastern longitude. It encompasses around 3486 square kilometers, with 66% of land use devoted to agriculture. Several different types of soil are present, but predominant soil type is chernozem. Vojvodina’s geomorphological units, formed in the Pleistocene and Holocene epochs, include high areas, sandstones, loess plains, loess terraces, alluvial terraces, and alluvial plains. Agriculture in this region, focusing on cereals, fruits, and grapes, heavily relies on groundwater, constituting 49% of total water use. According to [16] in the Srem region, the total irrigated area is 3655 ha, of which plough land and gardens constitute 2484 ha, orchards 1152 ha, vineyards 11 ha, meadows and pastures 3 ha, and other perennial plantations 5 ha. The terrain is slightly inclined, and the representative wells on which the monitoring of water quality was carried out are located near the settlements of Nikinci, Laćarak, and Šid. The wells, at depths of 20–30 m, primarily consist of fluvial and alluvial sediments from the youngest Quaternary period [17]. Figure 1 shows the location of Serbia, the Srem region, and groundwater monitoring wells of shallow aquifers.

2.2. Collection of Water Samples and Data Analysis

Data on physical and chemical parameters were obtained from the Hydrological Yearbook of water quality by the Serbian Environmental Protection Agency (SEPA) [18] for the period from 2011–2020. According to this publication, sampling and testing of groundwater quality, is performed throughout each year. The existing network of monitoring locations was used, as previously defined by SEPA. The study area and the existing three monitoring locations were selected because of their importance in future projects that plan to expand irrigation in this region.

The basic parameters of water quality such as total dissolved salts, electrical conductivity, as well as concentrations of cations and anions were the basis for statistical analysis of the physical and chemical parameters that were performed.

In order to apply certain classifications for assessing the quality of groundwater for irrigation, it is necessary to carry out a detailed analysis of the required parameters. Parameters included in this research are electrical conductivity (EC), total dissolved salts (TDS), pH values, concentrations of cations–sodium (Na⁺), calcium (Ca²⁺), magnesium (Mg²⁺) and potassium (K⁺) and anions–bicarbonate (HCO₃⁻), chloride (Cl⁻), sulfate (SO₄²⁻), and nitrate (NO₃⁻). To determine the reliability of the major cations and anions, it is usual to calculate the ion balance error (IBE). It is proposed that this value should be less than ±5% to consider an analysis valid [19]. The standard methods used are shown in

Table 1. Analyzed parameters, determination methods, and levels of quantification (SEPA) [18].

Parameter	Determination Method	Level of Quantification
TDS (mg/L)	US EPA 160.1	5
EC (μS/cm)	US EPA 120.1	5
pH	SRPS H.Z1.111: 1987	-
Ca2+ (mg/L)	ISO 6058	4
Mg2+ (mg/L)	ISO 6059	4
Na+ (mg/L)	APHA AWWA WEF 3111 B	0.2
K+ (mg/L)	APHA AWWA WEF 3111 B	0.1
HCO3− (mg/L)	SRPS EN ISO 9963-1	6
Cl− (mg/L)	SRPS ISO 9297	5
SO42− (mg/L)	ASTM D516-90	4
NO3−	UP 1.98/PC 12	0.2

.

Differences between parameters of irrigation groundwater, locality, years, and their mutual interactions were determined using Factorial ANOVA and t-test. After that, the main components that separate the examined years in relation to the water parameters were further separated by PCA analysis. In order to determine the connection between the parameters of irrigation groundwater, a cluster analysis was performed. All statistical analyses were performed using STATISTICA 13.2 software (Dell™ Statistica™ 13.2 University License).

2.3. Irrigation Water Quality Indices

Irrigation water suitability was assessed through indices like sodium percentage (SSP), sodium absorption ratio (SAR), residual sodium carbonate (RSC), permeability index (PI), Kelly’s ratio (KR), magnesium adsorption ratio (MAR), and total hardness (TH), each providing insights into different aspects of water quality. These indices collectively help in evaluating potential risks associated with sodium, magnesium, and overall water hardness.

Table 2. Formulas of irrigation water quality indices.

	Criteria	Formulas
	Sodium hazards
(1)	Sodium percentage (SSP)	$\mathrm{SSP}=\frac{{\mathrm{Na}}^{+}+{\mathrm{K}}^{+}}{{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}+{\mathrm{Na}}^{+}+{\mathrm{K}}^{+}}\times 100$
(2)	Sodium absorption ratio (SAR)	$\mathrm{SAR}=\frac{{\mathrm{Na}}^{+}}{\sqrt{\frac{\left({\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right)}{2}}}$
(3)	Residual sodium carbonate (RSC)	$\mathrm{RSC}=\left({\mathrm{HCO}}_3^{-}+{\mathrm{CO}}_3^{2-}\right)-\left({\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}\right)$
(4)	Permeability index (PI)	$\mathrm{PI}=\frac{{\mathrm{Na}}^{+}+\sqrt{{\mathrm{HCO}}_3^{-}}}{{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}+{\mathrm{Na}}^{+}}\times 100$
(5)	Kelly’s ratio (KR)	$\mathrm{KR}=\frac{{\mathrm{Na}}^{+}}{{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}}$
	Magnesium hazards
(6)	Magnesium adsorption ratio (MAR)	$\mathrm{MAR}=\frac{{\mathrm{Mg}}^{2+}}{{\mathrm{Ca}}^{2+}+{\mathrm{Mg}}^{2+}}$
(7)	Total hardness	$\mathrm{TH}=2.5\times {\mathrm{Ca}}^{2+}+4.1\times {\mathrm{Mg}}^{2+}$

shows the standard formulas for indices of the irrigation groundwater quality.

2.4. Classifications for Irrigation Water Quality

In this study, three classifications were applied: Nejgebauer’s, US USSL, and FAO. The domestic classification is Nejgebauer’s classification, which is adapted to the natural conditions of the Vojvodina province [20]. This classification considers dry residue and the ratio of Ca²⁺ and Mg²⁺ to Na⁺ (

Table 3. The Nejgebauer’s classification for irrigation purposes.

Class	Subclass	Conditions	Suitability of Water for Irrigation
I	Ia	TDS < 700 mg/L, (Ca+Mg):(Na+K) > 3	Suitable water
	Ib	TDS < 700 mg/L, (Ca+Mg):Na > 3
II		TDS < 700 mg/L, (Ca+Mg):Na > 1	Good water
III	IIIa	TDS = 700–3000 mg/L, (Ca+Mg):Na > 3	Waters that need to be tested
	IIIb	TDS = 700–3000 mg/L, (Ca+Mg):Na > 1
IV	IVa	TDS < 700 mg/L, (Ca+Mg):Na < 1	Unsuitable water
	IVb	TDS = 700–3000 mg/L, (Ca+Mg):Na < 1
	IVc	TDS > 3000 mg/L, (Ca+Mg):Na > 3
	IVd	TDS > 3000 mg/L

).

The other two classifications that were carried out in this study, which are widely accepted, are the US Salinity Laboratory classification (USSL) and the FAO classification.

The USSL classification delves into the relationship between salinity and alkalinity, while the FAO provides detailed analyses, considering salinity, infiltration, and toxicity of specific ions. The classifications aid in categorizing water for irrigation based on potential salinization and alkalization risks [21].

Table 4. The USSL diagram classification.

CS Class	S1	S2	S3	S4
C1	Suitable/good water	Moderate to good	Moderate	Moderate to poor
C2	Moderate to good	Moderate	Moderate to poor	Poor
C3	Moderate	Moderate to poor	Poor	Very poor
C4	Moderate to poor	Poor	Very poor	Unsuitable water

explains how classes are defined for the assessment of usability.

The basis of these classifications is the analysis of potential problems of salinization and alkalization, i.e., analysis of the concentration of total salt in water and of sodium, or its relation to divalent cations (Ca²⁺ and Mg²⁺).The FAO classification provides more detailed analyses. The FAO classification looks at salinity that affects crop water availability, infiltration that affects the infiltration rate of water into the soil, and the toxicity of certain ions such as Na⁺ and Cl⁻. This classification has an additional advantage because it also includes a list of plants that are sensitive, semi-sensitive, or tolerant to the presence of the mentioned specific ions. In the study area, some of the cultivated plants are: beans and corn as sensitive plants; tomato, carrot, wheat, and rye as semi-sensitive; sugar beet and barley as tolerant plants [22].

3. Results and Discussion

The main problems resulting in the inadequacy of irrigation could be: salinization, alkalization, waterlogging, or soil acidification. In the framework of the occurrence of possible unwanted consequences, it is necessary to monitor the hydrochemical characteristics and perform water classification in order to determine whether it is possible or with what restrictions to use water for irrigation.

Hydrochemical Characteristics

According to the ion balance error (IBE), all groundwater samples in the study area are within the limits of 5%, indicating a valid water quality analysis. According to average concentrations, ion compositions were as follows: at Nikinci and Šid locations Ca²⁺ > Mg²⁺ > Na⁺ > K⁺, at the Laćarak location Ca²⁺ > Na⁺ > Mg²⁺ > K⁺; and at Nikinci and Laćarak locations HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻ > CO₃⁻, while at the Šid location HCO₃⁻ > Cl⁻ > SO₄²⁻ > NO₃⁻ > CO₃⁻. Geochemical classification of groundwater was done using the trilinear Piper diagram [23], as previously presented in similar studies [24,25]. This diagram is a common method that has distinct zones that describe the dominant cations and anions that influence the hydrochemistry of groundwater. As can be seen in Figure 2, in the cation triangle, most of the samples from the three monitoring locations belong to the Ca and no dominant water type, while in the anion triangle, all of samples belong to HCO₃ water type, except one sample from the Šid monitoring location. Observing the diamond field of the diagram, it can be concluded that all three locations can be classified as a Ca·Mg–HCO₃ hydrochemical type (only one sample from the Šid monitoring location belonged to Na-Cl type), where the alkaline earth metals Ca²⁺ and Mg²⁺ exceed alkali metals and the weak acid HCO₃⁻ exceeds the strong acids Cl⁻ and SO₄²⁻ [26]. This hydrochemical type of groundwater can be related to carbonate-rich minerals in the aquifers [27].

Groundwater chemical components are characterized by water-rock interactions and chemical processes [28]. Gibbs (1970), [29] proposed two diagrams to understand the hydrogeochemical procedures with reference to atmospheric precipitation, rock–water interaction, and evaporation. In other words, these diagrams illustrate the relationship between groundwater chemistry and aquifer lithology. As shown in Figure 3, groundwater samples from the analyzed area were plotted in the zone of evaporation dominance, close to the rock-dominant zone. Similar results were presented by [28], suggesting that evaporation-sedimentation is the main factor affecting the chemical composition of this groundwater. Additionally, the movement toward the evaporation zone can be attributed to the lack of precipitation during the summer season, which has increased the intensity of evaporation [27].

Physical parameters such as total dissolved salts (TDS), electrical conductivity (EC) and pH value (pH) are presented for each measured well, for the period from 2011 to 2020 in

Table 5. Analyzed parameters on the monitoring locations in period 2011–2020.

	Nikinci				Šid				Laćarak
Parameter	Min	Max	Mean	SD	Min	Max	Mean	SD	Min	Max	Mean	SD
Ca2+ (mg/L)	70	96	89.51	7.29	86	123	107.75	13.73	68	111.2	82.84	13.96
Mg2+ (mg/L)	59.7	84	75.62	7.92	25.3	39	31.86	4.40	18.4	80.2	47.61	16.74
Na+ (mg/L)	13.5	39.8	30.63	8.58	12	26.7	17.32	5.20	10.9	82	57.7	21.33
K+ (mg/L)	0.6	25.5	4.64	7.61	0.7	4.08	1.48	0.97	0.5	6.24	1.56	1.69
HCO3− (mg/L)	505	773	653	74.22	378	460	404.8	28.92	564	700	591.7	38.68
Cl− (mg/L)	9.5	40.2	30.01	8.29	13	38	26.91	7.63	6.3	13.3	9.27	2.29
SO42− (mg/L)	5	107	41.5	27.73	5	27	16.5	6.29	2	48	24.8	12.80
NO3-N (mg/L)	0.2	12.24	2.56	3.84	4.4	27.2	14.36	7.53	0.03	5.8	0.747	1.79
EC (µS/cm)	975	1153	1043.8	49.69	667	931	793.2	88.19	676	902	820.9	60.02
TDS (mg/L)	549	683	628.8	46.92	406	512	457.9	37.72	394	559	498	45.95
pH value	7.11	7.7	7.52	0.18	6.95	7.63	7.41	0.20	7.01	7.68	7.421	0.23

. Total dissolved salts are one of the most important parameters for irrigation because they significantly impact the growth of plants and the quantity and quality of the yield. The values at all three monitoring locations do not have a wide range. Also, the amount of accumulated salt in the soil is directly dependent on the salinity of the irrigation water. Electrical conductivity is directly related to the sum of cations or anions and is strongly correlated with the dry residue, and that is the reason for the same narrow range of EC values [30].

Concentrations of main cations and anions are also shown in Table 5. Calcium and magnesium contribute to water hardness and in larger quantities can have a negative effect, not only on crops, but also on irrigation equipment. Values of Ca²⁺ at all three monitoring locations were between 68 and 123 mg/L. Magnesium had a wider range, from 18.4 to 84 mg/L. Sodium as a cation, is singled out because of its effect on the soil (it causes unfavorable physical and chemical changes), especially on the soil structure when it is present in the soil in an adsorbed form. Sodium affects the dispersion of the soil, which further affects the reduction of water and air penetration into the soil. Irrigation water can be a source of excess sodium. Values of Na⁺ were in the range of 10.9 to 82 mg/L. In the group of anions, chlorides, bicarbonates, and sulfates play dominant roles.

The presence of chloride is a common phenomenon whose toxic effect is easily recognized on irrigated plants in the form of burns on the leaves or death of the tissue of the leaf itself. This ion ranged from 6.3 to 40.2 mg/L in the irrigation water. Bicarbonates had a range from 378 to 773 mg/L. Sulfates are more soluble and less likely to form residue or clog irrigation systems at lower concentrations. Values of this ion ranged from 2 to 107 mg/L. Concentrations of NO₃⁻ were in the range of 0.2 to 12.24 mg/L, which can be considered acceptable. Ayers and Westcott (1985), [31] state that the ideal amount of nitrogen in irrigation water should be below 5 mg/L, so that there are no side effects for sensitive crops, while for most other crops the tolerant amount of nitrogen ranges up to 30 mg/L.

The analyzed hydrochemical characteristics and their values of minimum, maximum, means and standard deviations (SD) are given in Table 5 and in Appendix A.

Starting from the correlation matrix of the values of 11 analyzed irrigation groundwater parameters for the 10 observed years, the initial set of variables was reduced to two main components, which explains 50.31% of the variability of the initial set of variables (Figure 4), by applying the principal component analysis (PCA) method. When looking at the years in the coordinate system determined by the main components, it can be seen that the PC1 axis separated the years according to the lower values of K⁺ (−0.84) and Ca²⁺ (−0.84) in water. Specifically, it separated the years 2017 and 2020, which are characterized by higher values of these elements, from the other observed years. In relation to the PC1 axis, the years 2012 and 2014 were the most similar. The second axis of PC2 separated the years according to higher values of Cl⁻ (0.94) and lower values of Na⁺ (−0.79) in water, i.e., 2012, 2015, 2018, and 2020 from the other observed years. In relation to PC2, the years 2015 and 2020 were the most similar.

The cluster analysis grouped the water parameters in all three locations into 3 clusters, where the first cluster is electrical conductivity, the second cluster is dry residue and HCO₃, while all other investigated water parameters are grouped into the 3rd cluster (Figure 5). At the Nikinci site, the most similar were NO₃ and pH, followed by K and Ca, and Mg, Na, and Cl (Figure 5a). At the Šid locality, K and pH were the most similar, then Mg and Cl, and Na, SO_4, and NO₃ (Figure 5b), while at the Laćarak locality, K and NO₃, and Cl and pH were the most similar (Figure 5c).

Indices such as soluble sodium percentage (SSP), sodium absorption ratio (SAR), residual sodium carbonate (RSC), permeability index (PI), Kelly’s ratio (KR), magnesium adsorption ratio (MAR), and total hardness (TH) provided a detailed assessment of the groundwater’s impact on soil sodicity and magnesium hazard as well as total hardness (

Table 6. Descriptive statistics of irrigation water quality indices at three monitoring locations.

	Nikinci				Šid				Laćarak
Irrigation Water Quality Indices	Min	Max	Average	SD	Min	Max	Average	SD	Min	Max	Average	SD
SSP	6.54	17.83	11.91	3.59	6.31	13.24	8.90	2.31	4.77	31.87	24.17	8.89
SAR	0.25	0.83	0.58	0.18	0.26	0.58	0.37	0.11	0.21	1.82	1.28	0.51
RSC	−3.37	2.01	−0.07	1.51	−2.43	−0.51	−1.41	0.62	−0.73	2.52	1.59	0.99
PI	29.54	47.11	38.15	4.79	33.58	42.79	37.96	3.01	33.06	61.00	53.27	9.56
KR	0.05	0.20	0.13	0.04	0.06	0.15	0.09	0.03	0.05	0.47	0.33	0.14
MAR	52.63	60.51	58.41	2.27	26.66	38.92	33.13	4.56	21.62	61.98	48.03	11.08
TH	252	355	320	33	111	168	138	18	82	338	203	70

). While most samples showed suitability for irrigation, occasional deviations emphasized the need for continuous monitoring.

The formula for sodium percentage (SSP) is given in the Table 2 (Formula (1), and the values ranged from 6.54–17.83 (Nikinci), 6.31–13.24 (Šid), and 4.77–31.87 (Laćarak), indicating high variations (Table 6). The average values were 11.91% at Nikinci, 8.90% at Šid, and 24.17% at Laćarak. These values classify the water at Nikinci and Šid as excellent (SSP < 20%) and at Laćarak as good (SSP 20–40%).

Residual sodium carbonate (SAR) is a well-known value used for irrigation water quality and it is calculated according to Formula (2) [32], given in the Table 2. In this study area, results indicated generally low SAR values, with averages of 0.58 (Nikinci), 0.37 (Šid) and 1.59 (Laćarak) (Table 6), which indicates that the irrigation water quality, according to this index, falls into the category of good quality (SAR < 3).

The value of RSC was calculated according to Formula (3) as shown in Table 2 [33]. As indicated in Table 6, at the measuring point at Šid, there was no deviation from the first class (RSC < 1.25). At the Nikinci point, the second class (RSC 1.25–2.5) was observed during the measurement in two years, and at the measuring point at Laćarak, the first class occurred during measurements in two years, the third unsuitable class (RSC > 2.5) occurred during only one year, and the other samples belonged to the second doubtful class. Soil irrigated with water having a high RSC and presumably high pH can become infertile due to NaCO₃ deposition and reduced internal drainage [34]. Such results indicate the necessary monitoring of water quality.

The permeability index (PI) was calculated based on Equation (4) from Table 2, which is given by Doneen (1964) [35]. With a range of between 25 to 75%, all analyzed samples at all monitoring locations belong to the second group, indicating good quality. These results indicate that water in this area is suitable for irrigation.

Kelly’s ratio (KR) is given by Formula (5) in Table 2 [36]. It is a very important indexe that reveals the level of sodium relative to calcium and magnesium ion concentrations in irrigation water. All analyzed samples at all monitoring locations of irrigation water were suitable.

The magnesium adsorption ratio (MAR) [37,38] is a crucial index that determines irrigation water quality in terms of magnesium hazard. Higher values of magnesium pose a risk to agricultural yields after releasing sodium from the soil [39]. High magnesium values are a warning against using such water because long-term usage negatively influences the soil’s chemical structure and ultimately affects the crop yield. At the measuring point at Nikinci, all analyzed samples were unsuitable (MAR > 50%), while at the measuring point at Šid, all analyzed samples were suitable for irrigation purpose (MAR < 50%). At the measuring point at Laćarak, 50% of samples were suitable and 50% were unsuitable for irrigation. These results suggest that water quality must be monitored due to potential harm to soil infiltration properties, growing plants, and irrigation equipment.

Total hardness (TH) was calculated based on Equation (7) from Table 2. Average values were 138 at the measuring point at Šid, 203 at the measuring point at Laćarak, and 320 at the measuring point at Nikinci (Table 6). These results indicate that irrigation water is hard (TH 150–300 mg/L) to very hard (TH > 300 mg/L) and do not control corrosion of equipment. TH values around 100 mg/L provide corrosion control and are considered an acceptable limit [40]. Lower water hardness usually causes less loss of pipe mass and less pronounced corrosion.

Nejgebauer’s classification indicated good to excellent water quality throughout the observation period. USSL classification highlighted variations in salinity, indicating the need for moderate leaching measures. FAO classification further supported the need for monitoring, with occasional shifts between classes.

Nejgebauer’s classification indicated good to excellent water quality throughout the observation period. At the measuring location at Nikinci, there was fluctuation in the water quality from Ia to Ib class of water for irrigation. At the measuring location at Šid, water quality was uniform through entire analyzed period and it belong to Ia class, while at the measuring point at Laćarak, both the Ia and II class of irrigation water were observed. The percentage representation of certain classes during the analyzed period according to the Nejgebauer’s classification on all monitoring locations are shown in Figure 6.

USSL classification highlighted variations in salinity, indicating the need for moderate leaching measures. Water samples at the measuring point at Nikinci were uniform throughout the entire analyzed period and belonged to the C3-S1 class. At the measuring point at Šid, water quality was in the C2-S1 class from 2011 to 2014. After that, until 2020, the remaining 60% of the analyzed samples belonged to the C3-S1 class. The measuring point at Laćarak was uniform in the C3-S1 class except during 2017, when it belonged to the C2-S1 class (Figure 7).

Class C2 “medium salty water”, indicates that it can be used for irrigation with medium intensity leaching. Plants of medium salt tolerance can be grown in most cases without special measures to combat salinity. “Salty” water–C3 class, indicates that water cannot be used on poorly drained soils. Even with sufficient drainage, special measures are needed to prevent salinization and the selection of plants with high salt tolerance is also required. Due to the appearance of class C3 in the irrigation season, regular controls and measures must be in line with the assessment of groundwater quality. Authors of classifications such as FAO and USSL indicate the need for different intensity of soil leaching, depending on the type of soil and climatic conditions. This measure includes applying irrigation water in excess of crop requirements for leaching salt from the soil and ensuring the drainage of salt water through well-designed drainage systems [41]. Class S1 indicates that water can be used for irrigation of most soils with little risk of harmful levels of adsorbed Na. However, plants sensitive to Na (some fruits) can accumulate significant amounts, and therefore, this must be monitored.

The FAO classification further supported the need for monitoring, with occasional shifts between classes. Results during the observed period are presented in

Table 7. The FAO classification-percentage representation of classes.

	Nikinci		Šid		Laćarak
Potential Problems	Class (%)		Class (%)		Class (%)
Salinization	II	100	I	40	I	10
			II	60	II	90
Infiltration	I	100	I	90	I	90
			II	10	II	10
Toxicity of Na+	I	100	I	100	I	100
Toxicity of Cl−	I	100	I	100	I	100

. At the measuring point at Nikinci, the water quality in terms of salinization did not change from second class, which has moderate limitations for use in irrigation. At the measuring point at Šid, 40% of the results were in first class, and the other samples were in the second class, indicating that moderate measures are necessary when using water for irrigation. The situation is similar at the measuring point at Laćarak, with the fact that only 10% belonged to the first class and the rest of the measured samples to the second class.

In the part of the FAO classification related to water infiltration through the soil profile, at the measuring point at Nikinci, all samples were in first class. At the other two monitoring locations, Šid, and Laćarak, only 10% of the samples belonged to the first class, while the majority of the analyzed samples, 90%, belonged to the second class, indicating moderate restrictions in use.

In terms of the potential effects of Na⁺ and Cl⁻, at all monitoring locations, the concentration fell within the first class, indicating no limitations in using water for irrigation in 100% of analyzed samples.

4. Conclusions

The identification and continuous monitoring of irrigation groundwater quality are important for maintaining the freshwater resources in areas where agricultural measures such as irrigation are necessary for stable and profitable plant production. The hydrochemical characteristics and suitability assessment of irrigation groundwater quality in the study area were performed based on the assessment of results through both domestic and worldwide classification and irrigation water quality indices.

Hydrochemical classification of water showed that almost all samples during the monitoring period belonged to Ca·Mg–HCO₃ water type. Additionally, it was found that the groundwater chemistry of the region was dominantly influenced by evaporation-sedimentation processes.

The PCA analysis separated sample years according to lower values of K⁺, Ca²⁺, and Na⁺, and higher values of Cl⁻ in the irrigation water. The cluster analysis grouped parameters into 3 clusters (I-electroconductivity, II-dry residue and HCO₃⁻, III-other water parameters) for all three localities. According to the applied classifications, the majority of underground water samples were suitable for irrigation.

The majority of the samples at the monitoring locations were suitable for irrigation purposes, with occasional occurrence of deviations that suggest caution in using the water for irrigation purposes. In this context, at the monitoring locations at Šid and Laćarak, there was a shift from medium to high risk of soil salinization (C2-C3 classes according to USSL), while at Nikinci, the risk was high throughout the entire period (C3 class). According to Nejgebauer´s classification, water quality was assessed as good and excellent throughout the entire observation period. The FAO classification indicates a moderate restriction in use at the Nikinci location (second class), with shifts between first classes and second classes at the two other monitoring locations.

Specific indices suggest that the groundwater in this area is generally of good quality for irrigation purposes. The problem of potential NaCO₃ deposition occurred at the Laćarak location in 2016. A potential magnesium hazard was observed at the Nikinci and Laćarak locations. High water hardness was also observed at all monitoring locations.

The study presents a comprehensive assessment of groundwater quality for irrigation in the Srem region, in the Republic of Serbia. While the majority of samples meet irrigation water quality standards, occasional deviations underscore the need for regular monitoring, especially in light of climate changes impacting water availability and its usability. The results contribute valuable insights for policymakers and decision-makers in planning sustainable irrigation practices. Continuous monitoring is essential for ensuring stable crop production while safeguarding soil resources and irrigation equipment.