Major Ion Chemistry of Waters and Possible Controls under Winter Irrigation in the Saline Land of Arid Regions

Abstract

To reduce downstream ecological damage, it is crucial to analyze water and salt sources in saline–alkali farmland drainage and optimize soil salt discharge. This study employs statistical, hydrochemical, and isotope methods to identify controlling factors and characteristics in water bodies during winter irrigation. The results show average TDS values of 0.59, 6.40, and 4.14 g/L for irrigation, phreatic, and drainage water. Irrigation and phreatic water mainly belong to the HCO₃-Ca·Mg·Na and Cl·SO₄-Na·Mg types. Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻) values suggest the rock–water interaction and evaporation influences in irrigation water, while evaporation primarily controls phreatic water. The main salt types include NaCl, MgSO₄, Na₂CO₃, and CaCO₃ for irrigation water and NaCl, MgSO₄, Na₂SO₄, and CaCO₃ for phreatic water. These findings lay a research basis for analyzing water and salt sources in farmland drainage during winter irrigation in saline–alkali land.

1. Introduction

Farmland drainage in saline–alkali soil is an important strategy to prevent soil salinization and optimize crop yield [1,2]. The study of the characteristics of water and salt discharge is key to judging whether soil is in a state of salt accumulation or desalinization, and it is also an important basis for the analysis of the soil’s water–salt balance in saline–alkali soil [3]. With the deepening of study on leaching and desalination of saline–alkali land, the characteristics of farmland drainage water in saline–alkali land have been paid more and more attention [4,5,6]. Farmland drainage flows and salinity data during a drainage test in the Murrumbidgee Irrigation Area were monitored by Hornbuckle et al. [7], and they found that the drainage flow and salinity of multi-stage drainage systems were higher than those of single-stage drainage systems. Comparing the data of farmland drainage flow and selenium loads forming shallow drains and deep drains, Deverel et al. [8] found that the drainage selenium loads form shallow drains less often than they form deep drains. Feng et al. [9] conducted an experimental study on the effects of different irrigation water salinities and the buried depth of subsurface pipes on soil salinity and summer maize yield in Nanjing, and the results showed that the amount of salt discharge from farmland increased with increases in the buried depth and irrigation water salinity. Bassil et al. [10] analyzed the drainage test data of shaft drainage at different locations, and they found that the lower the terrain, the greater the drainage volume and the higher the drainage salinity. Bahceci et al. [11] used the SaltMod model to simulate the drainage volume and soil salt content in the root zone and groundwater table at different drainage depths. The results showed that when the drainage depth increased from 1.2 m to 1.6 m, there was no significant change in soil salt content in the root zone, but the drainage volume increased significantly. Parsons et al. [12] used the Watrcom model to simulate and analyze the drainage volume, relative yield, and irrigation water requirement of maize under conditions of conventional drainage, controlled drainage, and subirrigation. The results showed that the yield of maize in dry years of subirrigation treatment was higher than that of controlled drainage treatment and higher than that of conventional drainage treatment, and controlled drainage treatment could significantly reduce the drainage volume compared with conventional drainage treatment. The above analysis on the characteristics of farmland drainage in saline–alkali land mainly covers the following aspects: (1) In terms of farmland drainage methods, the influence of different drainage modes or drainage parameters on the drainage characteristics of farmland was studied. (2) In terms of controlled drainage, the influence of different drainage depths on the drainage characteristics of farmland was studied. (3) In terms of research methods, indoor experiments, field experiments, or numerical simulations were carried out to ascertain the characteristics of farmland drainage. (4) In terms of the characteristics of farmland drainage, the changes in farmland drainage flows, drainage volume, drainage salinity, drainage pH, and ions in drainage were studied. Although the research angles were different, there was a common point: previous studies have mainly focused on the monitoring of drainage volumes and drainage quality, and there have been fewer studies related to the traceability of water and salt in winter irrigation and farmland drainage [5]. However, analyzing the source structure of water and salt in farmland drainage is particularly important, and it is the key to optimizing the effects of soil salt discharge from farmland and protecting the downstream ecological environment [13].

Surface water or groundwater has a series of water–rock interactions with its surrounding environment through the process of surface runoff and groundwater seepage, and the hydrochemical characteristics of the water body show a series of changes [14,15]. In addition, the types of hydrochemicals, main ion sources, evolution mechanism, and influencing factors can be analyzed according to the hydrochemical characteristics of various water bodies [16,17,18]. At the same time, the recharge sources and water quality evolution process of various water bodies can also effectively be captured using hydrogen and oxygen stable isotope technology [19,20]. Geochemical modeling can effectively simulate water–rock interactions; for example, Apollaro et al. [21] simulated water–rock interactions in near-surface steam-heated zones by employing reaction path modeling in kinetic mode, focusing on the primary chemical components. Their study successfully reproduced the observed alteration of solid phases in the “Cave di Caolino” area on Lipari Island. Minerals in rocks and soils are one of the main factors determining water chemistry [22]. Apollaro et al. [23] conducted geochemical research on the soils, stream sediments, and rocks in the Fiume Grande watershed, revealing the roles of natural weathering and anthropogenic pollution in determining the distribution of major oxides and various trace elements in the examined geological environmental matrix. Based on the study of the hydrochemical characteristics of fissure or karst water in Yucatan, Mexico, and Florida, United States of America, Hanshaw et al. [24] defined the evolution law of groundwater in a carbonate environment. The discovery of this law has greatly promoted the development of a hydrochemical evolution law for karst in a carbonate zone. Lawrence et al. [25] analyzed the hydrochemical characteristics of groundwater bodies in Suwannee County and established recharge relationships for each water body. Rao et al. [26] analyzed the hydrochemical characteristics and quality of groundwater in the Varaha River Basin and proposed management measures to improve local water quality. Kim et al. [27] and Abou et al. [28] used the hydrochemical method to analyze the chemical characteristics of nitrate-contaminated groundwater in a Hongseong agricultural area and a Damascus oasis area, respectively. In terms of hydrogen and oxygen stable isotopes, Lambs [29] studied the recharge sources of river water and groundwater using hydrogen and oxygen stable isotope technology. Chiogna et al. [30] studied the hydrogen and oxygen stable isotope characteristics of surface water and groundwater within the Vermigliana catchment, clarifying the main recharge sources and the hydrological cycle of the Vermigliana catchment. Mccarthy et al. [31] studied the dynamic relationship between the Columbia River and groundwater using the hydrogen and oxygen stable isotope technique and determined the contribution of river water to groundwater. Qiu et al. [32] conducted research on the hydrogen and oxygen stable isotope characteristics of different water bodies in the Qilian Mountains, in the northwestern Tibetan Plateau. The results showed that the variations in δ¹⁸O values in river water and groundwater were similar, and the differences in deuterium excess were small, suggesting obvious hydraulic connections between surface water and groundwater. Taniguchi et al. [33] analyzed groundwater sources in the Heihe region using hydrogen and oxygen stable isotopes. Additionally, they determined the relationship between the electrical conductivity and isotopic components of groundwater. It is found that the above studies on hydrochemistry and hydrogen and oxygen stable isotopes were mostly focused on large-scale surface water and groundwater. Their primary focus was on the recharge relationship of different water bodies and the evolution and transport of their chemical composites. However, there is a gap in research on small-scale farmlands, particularly in winter irrigation and drainage in saline–alkali farmland. Clarifying the hydrochemical characteristics and controlling factors of various water bodies under winter irrigation is an important prerequisite for analyzing the source structure of water and salt in winter irrigation farmland drainage.

Therefore, the primary aim of this study is to characterize hydrochemical features through the analysis of hydrogen and oxygen stable isotopes and major ions in diverse water bodies. It specifically examines hydrochemical types and factors influencing dissolved solutes while delving into the origins of major ions across different water bodies. To achieve these goals and classify water bodies, this study employs classification methods, ion ratios, the Piper trilinear diagram, and the Gibbs diagram.

2. Materials and Methods

2.1. Site Description

The experiment was conducted in the second company (40°53′03″ N, 86°56′58″ E) of No.31 regiment, Yuli County, Korla City, Xinjiang, China (Figure 1). The study area experiences an annual average temperature of 10.5 °C and a mean annual total sunshine duration of 2941.8 h. It features a warm temperate continental desert climate with an annual precipitation of 34.1 mm and a mean annual free surface evaporation of 2417 mm. The groundwater level fluctuates between 1.5 and 3 m below the soil surface, and the shallow aquifer above is saline-water-bearing. The bedrock is sedimentary rock, and the predominant soil types are wind-blown sandy soil and brown desert soil. Additionally, the climate and special geographical conditions of this region make it prone to salt accumulation on the soil surface. The soil salt content in the cultivated layer ranges from 0.2% to 1.45%. The soil texture in the area is mainly composed of silt and fine sand, with sandy silt following closely. There is a small amount of locally present silt. In the 0-50 cm soil layer, the predominant texture is sandy loam, while the 50-100 cm layer consists mainly of sandy soil with strong water permeability.

2.2. Sample Collection

Water samples were collected during the winter irrigation salt leaching experiment conducted from November to December 2019. The samples comprised irrigation water, phreatic water, initial farmland drainage water, medium-term farmland drainage water, and final farmland drainage water. A total of 23 water samples were collected, and the sampling locations are shown in Figure 2, including 5 samples in field 1 (I1, GW1, and D1-D3 in Figure 2), 5 samples in field 2 (I2, GW2, and D4-D6 in Figure 2), and 13 samples in field 3 (I3, GW3, GW4, GW5, and D7-D15 in Figure 2). Field 1 and field 2 were subsurface-pipe drainage fields, and field 3 was a ditch drainage field. In subsurface-pipe drainage fields, the irrigation water sampling points (I1 and I2) were located at the end of the agricultural canal, and the phreatic water sampling points (GW1 and GW2) and farmland drainage water sampling points (D1-D3 and D4-D6) were located in the collecting wells. In the ditch drainage field, the irrigation water sampling point (I3) was located at the end of the agricultural canal. The phreatic water sampling points (GW3-GW5) and farmland drainage water sampling points were located in the upstream (D7, D10, and D13), midstream (D8, D11, and D14), and downstream (D9, D12, and D15) sections of the drainage ditch. In summary, phreatic water samples were taken from collecting wells or ditches before irrigation, while farmland drainage water samples were taken from collecting wells or ditches after drainage. Pre-washed polyethylene bottles were used to collect samples and were subsequently rinsed with the obtained samples. The collection details are shown in

Table 1. Collection information of various water samples in each field.

Field	Type	Sample No.	δD (‰)	δ18O (‰)	Sampling Date	Note
Field 1	Irrigation water	I1	−47.1	−6.5	21 November 2019
	Phreatic water	GW1	−43.2	−5.7	20 November 2019	*
	Initial farmland drainage water	D1	−44.2	−5.9	27 November 2019
	Medium-term farmland drainage	D2	−42.2	−5.5	1 December 2019
	Final farmland drainage water	D3	−41.1	−5.3	4 December 2019
Field 2	Irrigation water	I2	−47.0	−6.5	19 November 2019
	Phreatic water	GW2	−40.7	−4.9	17 November 2019
	Initial farmland drainage water	D4	−47.4	−6.5	20 November 2019	*
	Medium-term farmland drainage	D5	−44.8	−6.0	26 November 2019
	Final farmland drainage water	D6	−44.1	−5.8	4 December 2019
Field 3	Irrigation water	I3	−45.5	−6.0	23 November 2019
	Upstream phreatic water	GW3	−38.8	−4.1	20 November 2019
	Midstream phreatic water	GW4	−42.8	−4.7	20 November 2019	*
	Downstream phreatic water	GW5	−43.3	−5.0	20 November 2019	*
	Initial upstream farmland drainage water	D7	−40.5	−4.7	26 November 2019
	Initial midstream farmland drainage water	D8	−41.6	−4.7	26 November 2019
	Initial downstream farmland drainage water	D9	−42.5	−4.9	26 November 2019
	Medium-term upstream farmland drainage water	D10	−40.8	−4.7	30 November 2019
	Medium-term midstream farmland drainage water	D11	−40.9	−4.7	30 November 2019
	Medium-term downstream farmland drainage water	D12	−42.4	−4.9	30 November 2019
	Final upstream farmland drainage water	D13	−40.8	−4.9	3 December 2019
	Final midstream farmland drainage water	D14	−41.8	−4.8	3 December 2019
	Final downstream farmland drainage water	D15	−42.3	−4.9	3 December 2019

Note: * indicates that the sample cannot represent the actual leaching situation; see Section 3.1 for specific reasons.

. In situ measurements of water samples included electrical conductivity (EC) and pH, conducted using a DDS-307A conductivity meter and phs-25 pH meter, respectively. Prior to use, the DDS-307A conductivity meter (Changzhou Zhongjie Experimental Instrument Co. Ltd., Changzhou, China) and phs-25 pH meter (Shanghai Yonggui Analytical Instrument Co. Ltd., Shanghai, China) must be calibrated. For the DDS-307A conductivity meter, a 100.0 μS/cm concentration of calibration solution was prepared, and then the DDS-307A conductivity meter was used to measure the solution’s conductivity with the selector switch knob set to “check”, the “constant” compensation adjusting knob aligned with the “1” scale line, and the “temperature” compensation adjusting knob set to the “25” degree line. The “Calibration” knob was adjusted until the conductivity meter displayed 100.0 μS/cm, completing the calibration. The phs-25 pH meter calibration involved preparing a calibration solution with a pH value of 6.86. The pH meter was then used to measure the solution’s pH, with adjustments made to the temperature regulator to match the solution’s temperature. After stabilization of the reading, it represented the pH value of the calibration solution. Any discrepancies were addressed by adjusting the positioning regulator. Hydrogen and oxygen isotopes were tested at the State Key Laboratory of Biogeology and Environmental Geology, China University of Geosciences (Wuhan). The samples were filtered with a needle filter (filter membrane: 0.22 microns) and tested using the MAT253 Stable Isotope Mass Spectrometer (Thermo Fisher Scientific (Bremen) GmbH, Bremen, Germany) before being placed on the machine. To obtain the final results, each measured sample and standard was subjected to six injection measurements, with one standard sample per three samples. It must be noted that the obtained results indicate that the δD error is ±0.6‰ and the δ¹⁸O error is ±0.2‰. All samples used for the determination of hydrogen and oxygen isotopes and water chemical ions were stored in well-sealed plastic bottles of 500 mL in size and refrigerated at 4 °C until analysis.

2.3. Sample Measurements

2.3.1. Hydrogen and Oxygen Isotopes

Hydrogen and oxygen isotopes were measured using the MAT253 Stable Isotope Mass Spectrometer, manufactured by Thermo Fisher Scientific (Bremen) GmbH, Bremen, Germany. All isotopic ratio results of samples were reported in δ-notation (‰) relative to the international Vienna Standard Mean Ocean Water (VSMOW) standard, and the isotope compositions (δD or δ¹⁸O) were calculated as follows:

$$\delta =\frac{(R_{sample}-R_{standard})}{R_{standard}}\times 1000\mathrm{‰}$$

(1)

where δ represents δD or δ¹⁸O, and R_sample and R_standard are the D/H (or ¹⁸O/¹⁶O) molar ratios of the sample water and standard water (VSMOW), respectively. The analytical precision was ±0.6‰ for δD and ±0.2‰ for δ¹⁸O.

2.3.2. Major Ions

The cations (K⁺, Ca²⁺, Na⁺, and Mg²⁺) of samples were measured using an iCAP-6300 inductively coupled plasma atomic emission spectrometer (Thermo Fisher Scientific, Waltham, MA, USA), anions such as Cl⁻, NO₃⁻ and SO₄²⁻ were measured by an ICS-2100 ion chromatography (Thermo Fisher Scientific, Waltham, USA), and HCO₃⁻ was measured by acid–base titration. Chemical analyses were only accepted when the precision was greater than ±5% for major ions.

3. Results and Discussion

3.1. Isotopic Characteristics

According to data from the China Meteorological Data Service Center “https://data.cma.cn/ (accessed on 8 January 2020)”, Korla Meteorological Station recorded a precipitation of 65.6 mm in 2019 with no rainfall in November and December. Therefore, evaporation during the winter irrigation salt leaching experiment can be considered negligible. Farmland drainage water was mainly a mixture of irrigation water and phreatic water. The δD and δ¹⁸O in farmland drainage water were mainly affected by δD and δ¹⁸O in irrigation water and phreatic water. Consequently, the sample points of irrigation water, phreatic water, and farmland drainage water in the same field should exhibit two properties on the δD-δ¹⁸O diagram: (1) the sample points of irrigation water, phreatic water, and farmland drainage water being distributed along a straight line; (2) the sample points of farmland drainage water being located between irrigation water and phreatic water.

The sample data of each field in Table 1 were plotted on the δD-δ¹⁸O diagram (Figure 3). It is evident from Figure 3 that irrigation water, phreatic water, and farmland drainage water generally formed a straight line (property (1)). However, some sample points did not meet property (2) (the sample points of farmland drainage water being located between irrigation water and phreatic water). For example, consider the phreatic water sample of field 1, the initial farmland drainage water sample of field 2, and the midstream and downstream phreatic water samples of field 3. According to the field records of the winter irrigation salt leaching experiment, it was discovered that the leaching of field 1 was carried out after the leaching in the second field on the east side. Additionally, a collecting pipe was shared between field 1 and the second field on the east side of field 1. When obtaining phreatic water samples from the collecting well in field 1, a significant amount of farmland drainage water from the second field on the east side of field 1 was inevitably mixed. Therefore, it was reasonable to conclude that the phreatic water sample points in Figure 3a were problematic. In field 2, a portion of irrigation water overflowed into the collecting well during the initial stage of farmland drainage. Furthermore, the δD and δ¹⁸O content of irrigation water in field 2 was similar to that in fields 1 and 3. Thus, it was reasonable to conclude that the irrigation water sample points were unproblematic, while the initial farmland drainage water sample points shown in Figure 3b were problematic. Before the phreatic sampling in field 3, external water flowed from the branch ditch into the downstream and midstream of the farm ditch due to backwater. Therefore, it was reasonable to conclude that the midstream and downstream phreatic water sample points in Figure 3d,e were problematic.

After excluding problematic data from Table 1, we obtained the statistical characteristics of δD, δ¹⁸O, and deuterium excess (d) of various water samples in the study area (

Table 2. Statistical characteristics of δD, δ¹⁸O, and d of various water samples in the study area.

	Type	Max. (‰)	Min. (‰)	Mean (‰)	SD (‰)
δD	Irrigation water	−45.50	−47.10	−46.53	0.73
	Phreatic water	−38.80	−40.70	−39.75	0.95
	Farmland drainage water	−40.50	−44.80	−42.14	1.33
δ18O	Irrigation water	−6.00	−6.50	−6.33	0.24
	Phreatic water	−4.10	−4.90	−4.50	0.40
	Farmland drainage water	−4.70	−6.00	−5.12	0.46
d	Irrigation water	5.00	2.50	4.13	1.42
	Phreatic water	−1.50	−6.00	−3.75	3.18
	Farmland drainage water	3.20	−4.00	−1.17	2.78

Note: d is deuterium excess, d = δD − 8δ¹⁸O.

). The δD values of irrigation water, phreatic water, and farmland drainage water range from −47.10‰ to −45.50‰, from −40.70‰ to −38.80‰, and from −44.80‰ to −40.50‰, respectively, and the mean values are −46.53‰, −39.75‰, and −42.14‰, respectively, with standard deviation ranging from 0.73‰ to 1.33‰. The δ¹⁸O values of irrigation water, phreatic water, and farmland drainage water range from −6.50‰ to −6.00‰, −4.90‰ to −4.10‰, and −6.00‰ to −4.70‰, respectively, and the mean values are −6.33‰, −4.50‰, and −5.12‰, respectively, with standard deviation ranging from 0.24‰ to 0.46‰. The d values of irrigation water, phreatic water, and farmland drainage range from 2.50‰ to 5.00‰, −6.00‰ to −1.50‰, and −4.00‰ to −3.20‰, respectively, and the mean values are 4.13‰, −3.75‰, and −1.17‰, respectively, with standard deviation ranging from 1.42‰ to 3.18‰. Water-evaporation lines were compared within the same type of water body, and the water-evaporation lines of irrigation water, phreatic water, and farmland drainage water were δD = 3.10δ¹⁸O − 26.90, δD = 2.38δ¹⁸O − 29.06, and δD = 2.43δ¹⁸O − 29.70, respectively. Notably, the slopes of various water-evaporation lines were consistently proved to be lower than those of local atmospheric precipitation lines (δD = 7.17δ¹⁸O − 2.98). Furthermore, the water-evaporation line slope of the water body decreased in the following order: irrigation water, farmland drainage water, and phreatic water. This observation indicates that irrigation water and phreatic water underwent significant evaporation before winter irrigation, resulting in substantial isotope enrichment, Moreover, the impact of evaporation on phreatic water was greater than that on irrigation water [34].

3.2. Hydrochemical Characteristics

The hydrochemical characteristics of various water samples in the study area, excluding data with issues, are presented in

Table 3. Hydrochemical characteristics of various water samples in the study area.

Type	Eigenvalue	pH	TDSEC (g/L)	K+ (mg/L)	Ca2+ (mg/L)	Na+ (mg/L)	Mg2+ (mg/L)	Cl− (mg/L)	NO3− (mg/L)	SO42− (mg/L)	HCO3− (mg/L)
Irrigation water	Maximum	8.58	0.66	9.07	75.40	85.57	42.49	97.35	0.71	160.91	507.18
	Minimum	8.05	0.52	7.59	73.75	71.88	40.23	78.77	0.50	128.68	220.52
	Average	8.31	0.59	8.49	74.60	76.75	41.02	85.62	0.61	139.53	325.87
Phreatic water	Maximum	8.06	8.61	104.37	354.61	2099.03	560.45	2275.33	370.11	2434.25	823.26
	Minimum	7.48	4.20	53.40	214.39	928.05	287.98	1196.08	68.60	1304.23	294.02
	Average	7.77	6.40	78.88	284.50	1513.54	424.22	1735.70	219.36	1869.24	558.64
Farmland drainage water	Maximum	8.28	6.22	69.41	253.58	1381.48	400.94	1673.45	185.75	1986.77	757.10
	Minimum	7.24	1.17	28.15	94.14	203.82	76.48	284.27	19.40	282.14	371.20
	Average	7.90	4.14	56.14	204.97	860.17	263.85	1105.37	96.77	1249.03	606.15

Note: TDS in the table represents total dissolved solids.

. According to the pH (7.24~8.58) of irrigation water, phreatic water, and farmland drainage water, it was found that all types of water were weakly alkaline. The average of total dissolved solids (TDS_EC) in irrigation water, phreatic water, and farmland drainage water was 0.59, 6.40, and 4.14 g/L, respectively. The experimental area was located in the arid region of northwest China, with an annual precipitation of 34.1 mm. In addition, during our experimental period, no rainfall occurred, allowing us to disregard the contribution of marine aerosols typically present in rainfall [35]. This indicated that phreatic water contained a large amount of salt compared to irrigation water, and the salt in soil mainly comes from the evaporation of phreatic water [36]. The dominant ions in irrigation water were Na⁺, Ca²⁺, HCO₃⁻, and SO₄²⁻, accounting for 38.2%, 37.1%, 59.1%, and 25.3% of the total cations or anions, respectively. Phreatic water and farmland drainage water exhibited dominance in Na⁺, SO₄²⁻, and Cl⁻. In phreatic water, Na⁺ accounted for 65.8% of the total cations, and SO₄²⁻ and Cl⁻ accounted for 42.6% and 39.6% of the total anions, respectively. Similarly, in farmland drainage water, Na⁺ accounted for 62.1% of the total cations, and SO₄²⁻ and Cl⁻ accounted for 40.9% and 36.2% of the total anions, respectively.

3.3. Hydrochemical Type

As a method of hydrochemical analysis, the Piper trilinear diagram was first proposed by Piper in 1944 [37]. The diagram consists of two equilateral triangles and a diamond, and the percentage of milligram equivalent per liter of the main anions (Cl⁻, SO₄²⁻, and HCO₃⁻+CO₃²⁻) and cations (Na⁺+K⁺, Ca²⁺, and Mg²⁺) is the concentration unit. It is mainly used to reflect the hydrochemical types of various water samples, to analyze their possible hydrochemical evolution laws, and to judge whether there was mixing between different water bodies [38,39,40]. In this study, the Schukalev classification method [41] and the Piper trilinear diagram (Figure 4) were used to define the hydrochemical types of various water samples. From the data obtained using the Piper trilinear diagram, it is obvious that irrigation water belongs to the HCO₃·SO₄·Cl-Ca·Mg·Na water type in field 1 and field 3. In field 1, initial farmland drainage water, medium-term farmland drainage water, and final farmland drainage water belong to the Cl·SO₄-Na·Mg water type. In field 3, phreatic water, initial farmland drainage water, medium-term farmland drainage water, and final farmland drainage water belong to the Cl·SO₄-Na·Mg water type. In field 2, irrigation water belongs to the HCO₃-Ca·Mg·Na water type, phreatic water and medium-term farmland drainage water belong to the Cl·SO₄-Na·Mg water type, and final farmland drainage water belongs to the Cl·HCO₃·SO₄-Na·Mg water type.

3.4. Factors Controlling Dissolved Solutes

According to the correlation between total dissolved salt and Na⁺/(Na⁺ + Ca²⁺) and the correlation between total dissolved salt (TDS) and Cl⁻/(Cl⁻ + HCO₃⁻), the Gibbs diagram can qualitatively judge the relative importance of natural factors (evaporation dominance, rock–water interaction dominance and precipitation dominance) controlling the hydrochemical characteristics [42] and is suitable for surface water and groundwater [43,44]. The Gibbs diagram shows that when the TDS exceeds 1000 mg/L and the value of Na⁺/(Na⁺ + Ca²⁺) or Cl⁻/(Cl⁻ + HCO₃⁻) approaches 1, the water sample chemistry suggests evaporation dominance [45]; when the TDS is in the range of 70~300 mg/L and the value of Na⁺/(Na⁺ + Ca²⁺) or Cl⁻/(Cl⁻ + HCO₃⁻) is close to 0.5, the chemistry of water samples points to rock–water interaction dominance [46]. Conversely, when TDS is less than 10 mg/L and the value of Na⁺/(Na⁺ + Ca²⁺) or Cl⁻/(Cl⁻ + HCO₃⁻) is near 1, the chemistry of water samples indicates precipitation dominance [47].

According to Figure 5, the TDS of irrigation water in each field varies between 631.33 mg/L and 914.79 mg/L, with the value of Na⁺/(Na⁺ + Ca²⁺) or Cl⁻/(Cl⁻ + HCO₃⁻) ranging from 0.1 to 0.6. This indicates that the chemistry of irrigation water samples falls under rock–water interaction dominance and evaporation dominance. In contrast, the TDS of phreatic water in each field was more than 1000 mg/L, with the value of Na⁺/(Na⁺ + Ca²⁺) or Cl⁻/(Cl⁻ + HCO₃⁻) mostly around 0.8, This suggests that the chemistry of phreatic water samples is characterized by evaporation dominance. Since farmland drainage water samples in each field were a mixture of irrigation water and phreatic water, they align with both rock–water interaction dominance and evaporation dominance.

3.5. Origins of Major Ions

To identify salt types in each water sample, this study analyzed the sources of major ions in each sample. Throughout the hydrochemical evolution, various factors influence the major ions in water, with leaching, mixing, evaporation, and cation alternating adsorption being crucial mechanisms [48,49,50,51]. In this experiment, irrigation water and phreatic water were primarily influenced by long-term evaporation. Farmland drainage, a mixture of irrigation water and phreatic water, primarily sourced ions from these two water types, with a minor contribution from soil leaching.

The salt composition of the soil in the study area is shown in

Table 4. Salt composition of the soil in the study area; unit: meq/kg.

	CO32−	HCO3−	Cl−	SO42−	Ca2+	Mg2+	Na+ + K+
Maximum	8.00	16.92	77.02	128.88	100.38	37.84	85.37
Minimum	0.00	2.59	3.10	1.92	2.02	1.06	7.55
Average	0.24	5.30	11.31	19.76	7.24	6.20	18.25

; the result shows that the cations were mainly Na⁺ + K⁺, accounting for 57.6% of the total cations, with an average of 18.25 meq/kg. The anions were mainly Cl⁻ and SO₄²⁻, accounting for 30.9% and 54.0% of the total anions, respectively, and their average values were 11.31 meq/kg and 19.76 meq/kg, respectively.

Analyzing the salt composition of the soil (Table 4) and referencing the classification standard of soil salinization types (

Table 5. The division standard of soil salinization types.

Cl−/SO42−	Salinization Type	(Na+ + K+)/(Ca2+ + Mg2+)	Mg2+/Ca2+	Salinization Type
>2	Chloride	>2		Sodium
1~2	Sulfate-chloride	1~2	>1	Magnesium-sodium
0.2~1	Chloride-sulfate	1~2	<1	Calcium-sodium
>0.2	Sulfate	<1	>1	Calcium-magnesium

Note: the ratios in the table are equivalent ratios.

) [52], we determined that the Cl⁻/SO₄²⁻ equivalent ratio was 0.57, categorizing the soil as the chloride-sulfate type. Additionally, the (Na⁺ + K⁺)/(Ca²⁺ + Mg²⁺) equivalent ratio and Mg²⁺/Ca²⁺ ratio were 1.36 and 0.86, respectively, classifying the soil as the calcium-sodium type.

Based on the soil salinization type and the major ion ratio relationship diagram (Figure 6) for various water samples in the study area, an analysis was conducted on the major ion sources and hydrochemical evolution process. If all Na⁺ and K⁺ ions in water samples come from the dissolution of NaCl, the equivalent ratio of Na⁺ + K⁺ to Cl⁻ should be 1:1 [53]. As shown in Figure 6a, both irrigation water samples and phreatic water samples were located outside the line of Cl⁻=Na⁺ and tended towards the side of Na⁺. This suggests that Na⁺ and Cl⁻ of irrigation water and phreatic water mainly come from not only the dissolution of NaCl but also are affected by other sodium salts.

In previous studies, we established that Cl⁻ in phreatic water mainly comes from NaCl. According to Table 4, there is a minor presence of Mg²⁺ in the soil, and the salt in the soil mainly comes from the salt in phreatic water [54]. This implies the presence of Mg²⁺ in phreatic water. Figure 6b shows phreatic water samples were located outside the line of SO₄²⁻ = Mg²⁺ and tended towards the side of SO₄²⁻. This suggests that Mg²⁺ in phreatic water mainly comes from the dissolution of MgSO₄, while SO₄²⁻ mainly comes from the dissolution of MgSO₄, with potential additional sulfate sources. The irrigation water was located on the SO₄²⁻ = Mg²⁺ line, indicating that the SO₄²⁻ and Mg²⁺ in irrigation water mainly come from the dissolution of MgSO₄.

As shown in Figure 6c, irrigation water samples were located near the Cl⁻ + SO₄²⁻ = Na⁺ + Mg²⁺ line and tended towards the side of Na⁺ + Mg²⁺. Combined with Figure 6a,b, it can be seen that besides the dissolution of NaCl and MgSO₄, irrigation water samples were also affected by other sodium salts. Phreatic water samples were located near the Cl⁻ + SO₄²⁻ = Na⁺ + Mg²⁺ line. By integrating information from Figure 6a–c, it can be seen that Na⁺, Mg²⁺, Cl⁻, and SO₄²⁻ in phreatic water samples mainly come from the dissolution of NaCl, Na₂SO₄, and MgSO₄.

According to Figure 6d–f, the result shows that the irrigation water samples were located on the Ca²⁺ side near the SO₄²⁻ = Ca²⁺ line, on the HCO₃⁻ side near the HCO₃⁻ = Ca²⁺ line, and on the HCO₃⁻ + SO₄²⁻ side near the HCO₃⁻ + SO₄²⁻ = Ca²⁺ + Mg²⁺ line, indicating that the Ca²⁺ in the irrigation water mainly comes from the dissolution of CaCO₃, and the HCO₃⁻ not only comes from the dissolution of CaCO₃, but also has the influence of other carbonates. According to Figure 6g, the irrigation water samples were located near the Cl⁻ + HCO₃⁻ = Na⁺ + Ca²⁺ line, indicating that there was still dissolution of Na₂CO₃ in the irrigation water samples. The phreatic water samples were on the SO₄²⁻ side of the SO₄²⁻ and Ca²⁺ diagram (Figure 6d), near the HCO₃⁻ = Ca²⁺ line (Figure 6e), and on or near the HCO₃⁻ + SO₄²⁻ = Ca²⁺ + Mg²⁺ line (Figure 6d), indicating that the Ca²⁺ in phreatic water mainly arises from the dissolution of CaCO₃.

According to the above analysis, the salt in irrigation water mainly comes from rock salt dissolution, calcite dissolution, and gypsum dissolution. The salt in phreatic water mainly comes from rock salt dissolution and calcite dissolution.

4. Conclusions

It is of immense importance to elucidate the hydrochemical characteristics and potential influences on different water bodies during winter irrigation in saline lands. This understanding serves as a crucial prerequisite for analyzing the source composition of water and salt in farmland drainage, optimizing the efficiency of farmland salt discharge, and safeguarding the ecological environment in downstream salt-discharge regions. This study analyzed the hydrogen and oxygen stable isotopes as well as hydrochemical features of various water bodies during winter irrigation, leading to the following key conclusions:

In this study, the average d values in irrigation water and phreatic water were 4.13‰ and −3.75‰, respectively, both lower than the global atmospheric waterline (10‰), indicating significant hydrogen stable isotope enrichment due to evaporation. Notably, phreatic water exhibited greater enrichment than irrigation water. The dominant ions in irrigation water, including Na⁺, Ca²⁺, HCO₃⁻, and SO₄²⁻, classified its hydrochemical type as HCO₃⁻Ca·Mg·Na. Conversely, phreatic water and farmland drainage water, dominated by Na⁺, SO₄²⁻, and Cl⁻, fell into the Cl·SO₄-Na·Mg hydrochemical type.

The TDS of irrigation water ranged from 631.33 to 914.79 mg/L, with Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻) ratios between 0.1 and 0.6, indicating rock–water interaction and evaporation dominance. Phreatic water, with TDS exceeding 1000 mg/L and Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻) ratios around 0.8, demonstrated a dominance in evaporation. The study area’s soil salinity was classified as the chloride-sulfate type based on the chloride-to-sulfate equivalent ratio and the calcium-sodium type according to the cation equivalent ratio. The main sources of salt in irrigation water are rock salt dissolution, calcite dissolution, and gypsum dissolution. Phreatic water salt primarily derives from the dissolution of rock salt and calcite.