Hydrogeochemical Characterization, Processes, and Water Quality Assessment of Groundwater in an Agricultural Reclamation Area of the Sanjiang Plain, China

Abstract

Understanding groundwater quality and its controlling mechanisms is vital for the sustainable use of water resources in agriculturally intensive regions. This study evaluates the hydrochemical characteristics, controlling geochemical processes, and overall water quality of 226 groundwater samples collected from a typical agricultural reclamation area in the Sanjiang Plain, northeastern China. Major ion compositions indicate that groundwater is predominantly of the Ca–HCO₃ type, with bicarbonate, calcium, and magnesium as the dominant constituents. Spatial and statistical analyses reveal that rock weathering—particularly the dissolution of carbonates and silicates—is the primary natural process influencing groundwater chemistry, while cation exchange contributes moderately. Anthropogenic inputs, especially from fertilizers, livestock waste, and wastewater discharge, were found to elevate concentrations of NO₃⁻, Cl⁻, and SO₄²⁻ in localized zones. The entropy-weighted water quality index (EWQI) was applied to assess overall groundwater suitability. Results show that 89.8% of samples fall into “excellent” or “good” categories, though 6.6% of samples indicate poor to very poor water quality. This study identified the hydrochemical characteristics, sources of substances, and water quality of groundwater in the reclamation area, providing a basis for scientific prevention and control, rational utilization, and protection of groundwater resources.

1. Introduction

Groundwater, as the largest reservoir of freshwater on Earth, serves as a critical resource supporting human survival, agricultural productivity, economic development, and ecosystem stability globally [1,2,3,4,5]. With accelerating population growth and intensified industrial and agricultural activities worldwide, groundwater resources have increasingly faced severe environmental pressures, manifesting as declining water quality and heightened contamination risks. Consequently, the effective assessment, protection, and sustainable management of groundwater resources have become critical issues drawing considerable global attention [6,7,8].

Groundwater quality degradation primarily arises from both natural processes, including geochemical interactions between water and geological formations, and anthropogenic activities, notably excessive application of fertilizers, domestic sewage discharge, and livestock farming practices [9,10,11]. These activities contribute significantly to elevated concentrations of nitrates, ammonium, and other contaminants in groundwater systems, posing potential health hazards to human populations [12,13]. Previous studies indicate nitrate and ammonium contamination as prominent water quality concerns, with significant adverse health effects reported in various populations, especially vulnerable groups like infants and children [11,14,15].

The Sanjiang Plain, located in northeastern China, is an ecologically sensitive region and a crucial grain production base. This region has experienced significant environmental impacts associated with intensive agricultural practices, which potentially affect groundwater quality through fertilizer overuse and livestock farming activities. Previous research conducted in portions of the Sanjiang Plain indicated substantial nitrate and ammonium contamination, resulting in groundwater quality frequently falling within Classes IV and V, representing significant degradation and potential health risks [16].

However, despite its strategic ecological and agricultural significance, the detailed linkage between groundwater quality variations and agricultural practices in the Sanjiang Plain remains inadequately explored. Specifically, comprehensive spatial assessments and robust evaluations of health risks associated with groundwater contamination are lacking [17,18,19,20,21]. Addressing these research gaps, this study systematically examines groundwater chemical characteristics and spatial variations within a representative agricultural reclamation area in the Sanjiang Plain. By employing integrated analytical approaches—including ion ratio analysis, correlation analysis, principal component analysis, geographic information systems (GIS), and entropy-weighted water quality indexing (EWQI)—this research aims to clearly identify primary contamination sources, characterize the spatial distribution of groundwater quality, and evaluate associated human health risks. The research results provide data support for the prevention and control of groundwater pollution in agricultural reclamation areas.

2. Materials and Methods

2.1. Study Area

The Sanjiang Plain, located in northeastern Heilongjiang Province, China (Figure 1a), is formed by the alluvial and diluvial deposits of the Heilongjiang, Ussuri, and Songhua rivers. It represents China’s largest marshland ecosystem, covering approximately 10.89 × 10⁴ km². Renowned for its fertile soils, the region yields an annual grain production of about 15 million tons, making it a critical grain-producing base in China. The specific research area of this study is situated in the eastern Sanjiang Plain (Figure 1b). Covering an area of approximately 7800 km² and with a population of around 70,000, this region is predominantly agricultural, focusing heavily on grain cultivation. Land-use patterns are primarily composed of paddy fields (over 60%), supplemented by dryland farming, forestry, and grasslands [22]. Livestock farming of cattle, sheep, and pigs also occurs near wetland and forest areas, with animal waste commonly disposed of around these localities [23]. The region has a humid to semi-humid continental monsoon climate, with annual precipitation of 500–550 mm and a mean annual temperature of 1.3–3.9 °C—values at the cold limit of the temperate zone. Ground freezing typically lasts around 190 days per year, with freeze depths ranging from 1.4 to 2.0 m [24]. Recharge of groundwater predominantly originates from atmospheric precipitation infiltration, supplemented by irrigation seepage and lateral recharge [25,26]. The primary discharge pathways are anthropogenic extraction, lateral runoff, and minor evaporative losses. During flood seasons, localized recharge from river surface water can occur, while groundwater discharges into rivers during dry periods [27,28].

Topographically, the area inclines gradually from west to east, featuring alluvial plains, river terraces, and expansive marshlands along river floodplains. Bedrock outcrops, primarily sandstone, limestone, basalt, and granite, are exposed in the eastern and northwestern upland regions. In the study area, no salt rock and sulfide ore spots are exposed. Groundwater from Quaternary aquifers serves as the primary source of drinking and irrigation water (Figure 1c). The aquifer systems are mainly characterized by pre-Quaternary bedrock single-layer structures and clay-dominated dual-layer porous structures of the low plains. The Quaternary strata predominantly comprise fluvial alluvium overlain extensively by silty clay, cohesive soil, and gravelly soils. The porous groundwater in unconsolidated sediments represents the most extensive groundwater type within the region, while bedrock fissure water distribution is limited and typically less productive. The abundant groundwater resources, hosted predominantly in silty sands and fine sands with aquifer thicknesses ranging from 50 to 250 m, provide favorable natural conditions for sustainable groundwater extraction and utilization (Figure 1c).

2.2. Sample Collection and Analysis

A total of 226 groundwater samples were collected during three sampling campaigns conducted in September 2022, July 2023, and July 2024. The spatial distribution of sampling points is illustrated in Figure 1. Groundwater samples were obtained from depths ranging between 2.1 and 27.8 m, predominantly sourced from Quaternary unconsolidated porous aquifers. Prior to sampling, groundwater was pumped for at least 10 min to ensure representative samples. In situ measurements, including pH, Eh, and total dissolved solids (TDSs), were performed using a portable multiparameter water quality analyzer (HQ2200, HACH Company, Loveland, CO, USA). Samples intended for cation analysis were acidified using nitric acid to achieve a pH below 2. All groundwater samples were strictly collected, preserved, and transported in accordance with the Technical Specifications for Groundwater Environmental Monitoring (HJ164—2020) [29] and analyzed following the Standard Examination Methods for Drinking Water (GB5750—2006) [30]. The water chemistry tests were performed by the Hydrogeology and Environmental Geology Detection Center of Ministry of Natural Resources. Cation concentrations (Na⁺, K⁺, Ca²⁺, Mg²⁺) were measured using a flame atomic absorption spectrometer (ContrAA300, Analytik Jena AG, Jena, Germany). Anions including Cl⁻, NO₃⁻, and SO₄²⁻ were determined using ion chromatography (883i, Metrohm AG, Herisau, Swiss). Concentrations of HCO₃⁻ and CO₃²⁻ were obtained through titration with hydrochloric acid. NH₃-N was quantified using colorimetry (TU/1901 Beijing Purkinje General Instrument Co., Ltd., Beijing, China). Total hardness (TH) was calculated via titration using disodium ethylenediaminetetraacetic acid (EDTA). All measurements maintained a relative standard deviation of less than 10%, ensuring data reliability and precision.

2.3. Entropy-Weighted Water Quality Index (EWQI)

The entropy-weighted water quality index (EWQI) is an improved evaluation method that objectively assesses water quality by reducing subjective bias in determining the significance of individual water quality parameters. Unlike traditional indices, EWQI uses information entropy to quantify the disorder or uncertainty inherent in parameter distributions, thereby assigning unbiased weights [31,32]. The WQI was calculated using the following Equation (1). There, w_i is the weight of each parameter, while C_i and S_i denote the measured concentration and the standard reference value of each parameter, respectively. The estimated EWQI values were used to categorize groundwater quality into five levels: ‘Excellent’ for values under 50, ‘Good’ between 50 and 100, ‘Moderate’ from 100 to 150, ‘Poor’ between 150 and 200, and ‘Extremely Poor’ for values exceeding 200. Groundwater samples categorized as “Excellent” or “Good” are deemed suitable for drinking purposes.

$$WQI={\displaystyle \sum _{i=1}^n\frac{w_i}{{\displaystyle \sum _i^n{w}_i}}} \times {\displaystyle \frac{{\mathrm{C}}_{\mathrm{i}}}{{\mathrm{S}}_{\mathrm{i}}}} \times 100$$

(1)

3. Results and Discussion

3.1. General Hydrogeochemical Characteristics

3.1.1. Chemical Composition Characteristics of Groundwater

A total of 226 groundwater samples were analyzed to assess the hydrogeochemical characteristics of the Sanjiang Plain. The groundwater pH ranged from 5.54 to 8.71 (mean: 6.80), indicating mildly acidic to weakly alkaline conditions (

Table 1. Statistical analysis of hydrochemical parameters of groundwater samples (units of all parameters are mg/L, except pH).

	Eh	pH	TDS	TH	K+	Na+	Ca2+	Mg2+	HCO3−	SO42−	Cl−	NO3−	NH3-N	F−
Average	−17.8	6.80	224.1	139.0	2.4	16.7	36.3	11.6	190.5	25.6	3.8	3.3	0.7	0.4
Median	−29.1	6.67	206.5	123.8	2.2	15.2	33.0	10.5	184.3	7.4	1.0	0.4	0.6	0.3
SD	96.3	0.54	82.1	54.6	1.0	7.3	15.6	4.6	72.3	52.1	7.9	11.3	0.8	0.1
Min	−231.9	5.54	37.3	55.2	0.6	5.3	14.6	3.0	17.7	0.05	0.2	0.01	0.02	0.1
Max	292.8	8.71	770.0	403.2	8.3	71.7	112.0	33.1	555.9	450.9	53.7	114.6	5.1	0.7
CV	−5.42	0.08	0.37	0.39	0.43	0.44	0.43	0.39	0.38	2.04	2.07	3.40	1.10	0.37
WHO (2011) [34]		6.5–8.5	1000	450	-	200	200	150	-	250	250	45	0.5	1.5

and Figure 2). The redox potential (Eh) values varied from −231.9 mV to 292.8 mV, with an average of −17.8 mV, indicating predominantly reducing to mildly oxidizing environments. Total dissolved solids (TDSs) ranged between 37.3 and 770.0 mg/L, with a mean of 224.1 mg/L. While none of the samples exceeded the WHO guideline of 1000 mg/L, relatively elevated TDS levels were noted in the central and southern parts of the study area (Figure 3). Total hardness (TH) ranged from 55.2 to 403.2 mg/L (mean: 139.0 mg/L). Among the samples, 12 fell into the soft category (0–75 mg/L), 148 were classified as moderate (75–150 mg/L), 61 were hard (150–300 mg/L), and 5 were very hard (>300 mg/L) [33], classifying most samples as “moderate” to “hard” water.

The average concentration sequence of major cations was Ca²⁺ > Na⁺ > Mg²⁺ > K⁺, and for anions, it was HCO₃⁻ > SO₄²⁻ > Cl⁻ > NO₃⁻. Calcium ranged from 14.6 to 112.0 mg/L and magnesium from 3.0 to 33.1 mg/L, with high values observed in the central–eastern and southern regions. These patterns align well with those of SO₄²⁻ and HCO₃⁻, suggesting consistent geochemical processes. Based on the geological background of the research area, there are no exposed strata rich in gypsum or other sulfur-containing minerals, and sulfate ions should mainly come from the application of pesticides and fertilizers. Sodium concentrations (5.3–71.7 mg/L) showed enrichment in the southwestern region, while potassium (0.6–8.3 mg/L) was locally elevated in the eastern and central zones (Figure 3).

Bicarbonate (HCO₃⁻) was the dominant anion (17.7–555.9 mg/L), with the highest values recorded in the southern part of the study area. Sulfate (SO₄²⁻) showed substantial variation (0.05–450.9 mg/L), with its highest concentrations found in the central–eastern areas. Chloride (Cl⁻) concentrations ranged from 0.2 to 53.7 mg/L, with elevated values localized in the eastern and central regions. Nitrate (NO₃⁻) concentrations spanned from 0.01 to 114.6 mg/L (mean: 3.3 mg/L), with five samples exceeding the WHO limit of 45 mg/L. High NO₃⁻ levels were mainly concentrated in the eastern and central zones, suggesting a strong link to agricultural and human activities [35]. Ammonia nitrogen (NH₃-N) concentrations ranged from 0.02 to 5.1 mg/L (mean: 0.7 mg/L), with 127 samples (56%) surpassing the 0.5 mg/L threshold. The highest NH₃-N values were observed in the northern region, with a distinct arc-shaped distribution in the south. These patterns likely reflect contamination from livestock and fertilizer inputs. These ions, particularly SO₄²⁻, Cl⁻, and NO₃⁻, exhibited high coefficients of variation (204% to 340%), indicating significant spatial heterogeneity and potential anthropogenic influence.

3.1.2. Hydrochemical Type

To further understand the groundwater chemistry, according to the content of anions and cations in water, ions with a percentage greater than 25% milligram equivalent were classified and named [36,37]. The results showed that the hydrochemical types in the study area were mainly Ca·Mg-HCO3, Ca-HCO3, and Ca·Mg·Na-HCO3, with sample sizes of 107, 47, and 17, respectively, accounting for approximately 46.90%, 20.80%, and 7.52% of the total sample size, respectively. Other types were only sporadically visible, and the hydrochemical types and distribution are shown in Figure 4. Points with sulfate ions, chloride ions, and nitrate ions as the main components of anions, with quantities of 39, 6, and 1, respectively, were mainly located in the central and northern parts of the working area, and the surrounding areas of the well points were mainly paddy fields. There is no exposure to gypsum or salt rock in the research area, and its sulfate ions, chloride ions, and nitrate ions may come from excessive use of pesticides and fertilizers or leaching and infiltration of feces, organic fertilizers, or domestic pollution.

The dominance of Ca²⁺, Mg²⁺, HCO₃⁻, and SO₄²⁻ in the groundwater reflects the dissolution of carbonate and gypsum-bearing formations that are widely distributed in the study area [38]. However, the observed spatial heterogeneity and exceedance of nitrate and ammonia in specific areas underscore the influence of anthropogenic sources, particularly agricultural practices and livestock activities.

3.1.3. Relationship Between Chemical Indexes

Correlation analysis was used to identify the origins and interactions of ions in groundwater. Components originating from the same source tend to show strong correlations, whereas those from different sources usually correlate poorly [39]. In this study, significant positive correlations were observed between TDS and Ca²⁺ (r = 0.820), Mg²⁺ (r = 0.81), and SO₄²⁻ (r = 0.71) (Figure 5), indicating that these three ions are the primary contributors to total dissolved solids in the groundwater of the Sanjiang Plain. Additionally, F⁻, NO₃⁻, and K⁺ exhibited significant mutual correlations, suggesting a shared origin. This common source is likely associated with the dissolution of evaporite minerals such as halite or potentially anthropogenic inputs such as fertilizers. Given that there is no exposed salt rock in the research area, their source may be human activities. These correlation patterns support the hypothesis that both geogenic and anthropogenic factors are influencing the chemical composition of groundwater in the study area.

3.2. Control Factors of Groundwater Hydrochemistry

3.2.1. Natural Factors

To investigate the geochemical processes influencing groundwater composition, Gibbs diagrams were employed to determine the primary mechanisms governing solute concentrations [15,40,41,42]. These diagrams distinguish between three end-member processes: atmospheric precipitation, rock weathering, and evaporation–crystallization. As shown in Figure 6, most groundwater samples in the Sanjiang Plain plot within the rock-weathering dominance zone, characterized by TDS values ranging from 70 to 300 mg/L, Cl⁻/(Cl⁻ + HCO₃⁻) and Na⁺/(Na⁺ + Ca²⁺) ratios below 0.5. The mean values of Na⁺/(Na⁺ + Ca²⁺) and Cl⁻/(Cl⁻ + HCO₃⁻) were 0.325 and 0.025, respectively, indicating that groundwater chemistry is primarily controlled by water–rock interactions, with minimal influence from precipitation or evaporation processes. These results are consistent with the Piper diagram findings.

Further insights into mineral weathering processes were derived using ionic ratio plots (Figure 6) [43,44]. The relationships between Ca²⁺/Na⁺ vs. Mg²⁺/Na⁺ and Ca²⁺/Na⁺ vs. HCO₃⁻/Na⁺ reveal that most groundwater samples plot between the carbonate and silicate weathering domains (Figure 7), suggesting that both types of rocks contribute to groundwater chemistry, with silicate weathering being more dominant. This agrees with the regional lithological setting, where sandstones and granites prevail, and carbonates are locally distributed.

Cation exchange processes were assessed using the diagram of (Na⁺ + K⁺ − Cl⁻) vs. [(2SO₄²⁻ + HCO₃⁻) − 2(Ca²⁺ + Mg²⁺)] (Figure 8a). The majority of groundwater samples deviate from the 1:1 line, indicating that cation exchange plays only a minor role in the area. The Schoeller indices CAI-I (=(Cl⁻ − (Na⁺ + K⁺))/Cl⁻) and CAI-II (=(Cl⁻ − (Na⁺ + K⁺))/(HCO₃⁻ + SO₄²⁻ + CO₃²⁻ + NO₃⁻)) are commonly used to identify cation exchange processes [45,46]. CAI1 values ranged from −104.12 to 0.58 (mean: −29.06 ± 24.22), and CAI2 values ranged from −0.38 to 0.22 (mean: −0.19 ± 0.08). Both indices were predominantly negative, suggesting weak reverse cation exchange, where Na⁺ and K⁺ in groundwater are substituted by Ca²⁺ and Mg²⁺ from the aquifer matrix (Figure 8b).

Additional ion ratio analyses support the above conclusions. In Figure 9a, most Cl⁻ vs. Na⁺ plots fall below the 1:1 line, ruling out halite dissolution as a major Na⁺ source. Excess Na⁺ likely originates from silicate weathering or ion exchange. The SO₄²⁻ vs. Ca²⁺ relationship (Figure 9b) shows that most samples lie above the 1:1 line, indicating SO₄²⁻ levels exceed those expected from gypsum dissolution alone. The HCO₃⁻ vs. Ca²⁺ plot (Figure 9c) displays most samples above both the y = x and y = 2x lines, suggesting that calcite and dolomite dissolution contribute significantly to groundwater chemistry [47,48]. Additionally, in the (HCO₃⁻ + SO₄²⁻) vs. (Ca²⁺ + Mg²⁺) plot (Figure 9d), data cluster near the 1:1 line, supporting the occurrence of mineral dissolution with partial influence from cation exchange. Saturation indices (SI), calculated using PHREEQC 3.0, provide insight into mineral equilibrium conditions [49,50]. The SI values for calcite, dolomite, gypsum, and halite are shown in Figure 9e,f. Most samples have SI values below zero, indicating undersaturation and potential for mineral dissolution. A few samples exhibit supersaturation (SI > 0) for calcite and dolomite, suggesting localized precipitation or limited dissolution equilibrium.

Therefore, the research area is mainly dominated by the weathering and dissolution of carbonates (limestone and dolomite), followed by silicates, and the main processes are as follows:

CaCO₃ (calcite) → Ca²⁺ + CO₃²⁻,

CaMg(CO₃)₂ (dolomite) → Ca²⁺ + Mg²⁺ + 2CO₃²⁺.

CaAl₂Si₂O₈ (anorthite) → Ca²⁺ + 2AlO₂⁻ + 2SiO₂

NaAlSi₃O₈ (albite) → 2Na⁺ +AlO₂ + 3SiO₂

3.2.2. Human Activity Input

Anthropogenic activities significantly alter groundwater recharge conditions and influence chemical composition. Among common indicators, Cl⁻ and NO₃⁻ are less affected by geochemical reactions and more reflective of human impacts such as fertilizer application, wastewater discharge, and livestock waste [51,52,53]. The ratios of Cl⁻/Na⁺ and NO₃⁻/Na⁺ are commonly used to identify such influences. As shown in Figure 10, groundwater samples in the Sanjiang Plain exhibit elevated Cl⁻/Na⁺ and NO₃⁻/Na⁺ ratios, indicating substantial anthropogenic input. These elevated levels are consistent with the region’s intensive agricultural practices and urban wastewater discharge. This is consistent with the high level of sulfate ions in the research area, confirming that human activities play a critical role in shaping groundwater chemistry in certain areas of the study region.

3.3. Groundwater Quality

The EWQI was adopted to assess the overall groundwater quality in the Sanjiang Plain, as it effectively integrates the relative significance of various hydrochemical parameters [21,54]. As shown in Figure 11a, EWQI values across 226 groundwater samples ranged from 1.31 to 298.32, with an average of 33.21, indicating that most samples exhibit excellent water quality. Specifically, 180 samples (approximately 79.6%) were classified as “excellent,” while 23 samples (10.2%) fell into the “good” category. These two classes collectively account for 89.8% of the total samples, suggesting that the majority of the groundwater is suitable for various uses, including drinking and irrigation. A smaller subset, consisting of 8 samples (3.5%), was categorized as “moderate,” which is still within acceptable limits for drinking water. However, 2 samples (0.9%) and 13 samples (5.7%) were categorized as “poor” and “very poor,” respectively, indicating localized areas of concern.

Further analysis revealed that groundwater samples with higher concentrations of Cl⁻, NO₃⁻, and SO₄²⁻ tended to have elevated EWQI values, identifying these anions as the primary contributors to water quality deterioration. Spatial distribution mapping of EWQI (Figure 11b) showed that samples with poorer water quality were predominantly located near densely populated residential areas and livestock farms, particularly those adjacent to wetlands. These locations are associated with anthropogenic activities such as intensive agriculture, fertilizer and pesticide use, and livestock waste discharge. The observed spatial patterns suggest that NO₃⁻, Cl⁻, and SO₄²⁻ pollution is largely attributable to agricultural runoff and livestock operations. Therefore, future efforts to safeguard groundwater quality in this region should prioritize improved nutrient management practices and effective treatment and disposal of animal waste in agricultural settings.

4. Conclusions

This study provides a comprehensive assessment of groundwater hydrochemistry, controlling mechanisms, and overall quality in a typical agricultural reclamation area of the Sanjiang Plain. Based on the analysis of 226 groundwater samples, the results highlight the following key findings: (1) Groundwater in the study area is primarily characterized by a Ca–HCO₃ type with soft to moderately hard water. Bicarbonate, calcium, magnesium, and sulfate dominate the ion composition, reflecting influence from carbonate and silicate mineral dissolution. (2) The Gibbs and ion ratio diagrams demonstrate that rock weathering—particularly silicate and carbonate dissolution—is the dominant geochemical process shaping groundwater chemistry. Cation exchange plays a secondary but detectable role, as confirmed by ion balance diagrams and chlor-alkali indices. (3) Elevated levels of NO₃⁻, Cl⁻, and SO₄²⁻ in certain areas—especially near residential zones and livestock farms—point to significant human impacts. These contaminants are largely attributed to fertilizer application, pesticide usage, and manure discharge from agricultural and livestock activities. (4) According to the entropy-weighted water quality index (EWQI), approximately 90% of groundwater samples fall within the “excellent” to “good” quality range, suggesting broad suitability for drinking and irrigation. However, localized zones of poor and very poor water quality necessitate attention and remediation.

Overall, this research underscores the dual influence of natural weathering and anthropogenic activities on groundwater chemistry in the Sanjiang Plain. To maintain and improve groundwater quality, future management should emphasize integrated land-use planning, sustainable agricultural practices, and enhanced wastewater treatment, particularly in areas with intensive human activity.