1. Introduction
Nitrogen pollution in groundwater, especially in areas of intensive agriculture, has aroused widespread concern throughout the world [
1]. It has been reported that the excess of the main inorganic nitrogen compounds in drinking water (NO
3−, NH
4+, NO
2−) are detrimental to human health [
2,
3,
4].
The mechanism of the formation process of inorganic nitrogen compounds in groundwater is complex due to the variety of nitrogen sources and the intricacy of influencing factors in the environment. Numerous studies have reported that the inorganic nitrogen compounds in groundwater could originate from nitrogen-based fertilizer, manure, as well as domestic and industrial pollution. Besides, the discharge from septic tanks, leaking sewers, and eutrophic surface water can all contribute [
5]. In some cases, the atmospheric nitrogen deposition [
6] and soil organic nitrogen mineralization [
7] can also play important roles in nitrogen pollution in groundwater. In addition, some environmental factors such as pH, oxidation-reduction potential (Eh), soil organic matter, and bacterial activity [
8,
9,
10,
11] can greatly affect the species and amount of nitrogen in groundwater.
In previous research, several methods have been developed to identify nitrogen sources, among which the chemical analysis and stable isotope methods are the most traditional ones. Moreover, some new tracing methods and comprehensive methods have also been developed. As for chemical analysis, it has been widely used as an auxiliary identifying method because some characteristic ions (e.g., Ca
2+, Mg
2+, and SO
42−) in groundwater can carry information about their origins [
12], and some halides (Cl
−, Br
− and I
−) usually have something to do with anthropogenic activities and remain relatively conserved in the subsurface environment [
13,
14,
15]. The ratios of NO
3−/Cl
−, Cl
−/Br
−, and I
−/Na
+ in groundwater are also key identifiers of the origin, by which Katz [
16], Panno [
17], and Pastén-Zapata [
13] successfully identified the sources of nitrate coming from wastewater. Nevertheless, the types and concentrations of characteristic components vary widely in different pollution sources and change greatly during the physical and chemical reactions occurring in the subsurface environment, limiting the ability of chemical analyses to achieve accurate pollution source results.
In a similar manner, the stable isotope ratios of nitrogen (δ15N, δ18O) and boron (δ11B) are effective indicators of the pollution sources since different sources of nitrogen often share distinct isotopic compositions [
18]. However, the use of single isotope tracers often cannot discriminate the sources correctly. This is because isotope ratios between sources have overlap values and the nitrification, denitrification, and other reactions that nitrogen may experience in the subsurface environment could cause the isotope values to deviate from theoretical ones, impacting the accuracy of the results. Hence, the nitrate-nitrogen and nitrate-oxygen dual-isotope methods have become a powerful tool in nitrate source identification since Kendall [
19] reviewed the distribution of δ15N and δ18O values of various sources. The dual-isotope method can not only improve the accuracy of source differentiation but also make it possible to quantify the contribution of different sources to the pollution [
20,
21,
22].
In recent years, some new types of tracers have been applied to identify sources of nitrogen pollution in groundwater. Nakagawa et al. [
23] used coprostanol, which is produced by bacterial reduction of cholesterol in the gut of higher animals, as an indicator to investigate nitrate sources of pollution for an aquifer in Shimabara, Nagasaki, Japan, and verified that coprostanol had the potential for nitrate source identification by comparing the results with those obtained by the dual-isotope method. This indicates that it is essential to develop and adopt some new types of tracers as additional tools to support the dual-isotope method, which will make the identification process more efficient and accurate.
The single identification methods inevitably have some limitations in application due to the complexity of nitrogen formation processes, and some comprehensive methods have shown advantages in recent years, namely the adoption of multitracers and the integration of source appointment with the analysis of relevant factors (e.g., land use types). Moreover, some researchers have used geospatial-based assessment [
24,
25], groundwater age interpretation [
26], and microbial community analysis [
27] as auxiliaries to enhance the investigation of nitrogen sources. Multivariate statistics [
28,
29] is a powerful tool to integrate all the identification methods and relevant studies together, and thus the factor analysis [
30], principal component analysis [
31,
32], clustering analysis [
13,
30], and factorial correspondence analysis [
14] have been widely applied in hydrogeological research. Nevertheless, such studies have either focused on the origin and fate of the pollutants or the reactions experienced by nitrogen before or after leaching into groundwater, but both lines of study have seldom been combined in an analysis of the whole formation process of pollution systematically. As such deep insight into the essential nature of the pollution process is lacking.
There is no doubt that agricultural nonpoint source nitrogen pollution in groundwater is a serious worldwide problem since the pollution behaviors are intricate and disordered. Thus, to make clear the sources and pollution process of this kind of nitrogen pollution, it is necessary to investigate the potential pollution sources and relevant information such as the population density and amounts of fertilizer application at the early stage of source identification. Meanwhile, statistical analysis methods like multivariate statistics and geospatial statistics should be utilized to explore the intrinsic connection between nitrogen concentration and other factors that may contribute to pollution. Moreover, the tracing methods such as chemical analysis and stable isotope methods can provide more direct and powerful support to the analysis process.
The Sanjiang Plain in Northeast China (
Figure 1), which is one of the most important national food production bases, has been intensively developed for agriculture since the 1950s. It has experienced four instances of large-scale reclamation, during which the large areas of wetland were adapted into paddy fields, and large amounts of fertilizers were applied to the soil every year. At the same time, the deterioration of groundwater quality in this area had become the key factor that limited the sustainable development of local water supply and agricultural planting. Recent studies reported that the Sanjiang Plain has faced the risk of serious inorganic nitrogen pollution of groundwater in some regions [
33,
34]. However, the information about nitrogen sources and behavior in this area is limited, and the formation process of nitrogen pollution in regional groundwater remains unclear, which will undoubtedly constrain the progress of its control and impact the large-scale plan of agricultural development.
In this study, the survey was conducted in a typical paddy irrigation area of Songhua River watershed on the distribution of the potential nitrogen sources, groundwater inorganic nitrogen compounds (nitrate, ammonia, and nitrite), and topsoil total nitrogen concentration. Then, multivariate statistics and the geospatial-based assessment were combined to identify the nitrogen sources and the governing factors affecting pollution. After describing the methods and detailing the results, this paper discusses the formation process of inorganic nitrogen in groundwater. The results of this study provide a scientific basis for pollution control in the irrigation area and promote the future development of the agricultural security of the Sanjiang Plain.
3. Materials and Methods
3.1. Pollution Source Investigation
Due to the major activity of the area being rice farming and industrial pollution being absent, a field survey that focused on the domestic and agricultural situations was conducted to investigate the potential nitrogen sources in the Puyang irrigation area. The amount and distribution of towns, villages, population, livestock, fertilization, sewage systems, and landfills were all included in the survey.
3.2. Sampling and Analysis
A total of 78 groundwater samples and 19 soil samples were collected in August 2017. Most of the groundwater sampling points were evenly distributed within the domain of Puyang irrigation area, whereas some densely populated villages and previously monitored pollution areas were sampled much more intensively. Some outside wells adjacent to the irrigation area were also sampled to properly map the distribution of nitrogen pollution intensity. The types of sampling wells mainly consisted of domestic wells and irrigation wells. Because the depths of the two types of wells were significantly different (the depths of the domestic wells were generally less than 20 m, while those of the irrigation wells were about 30 m), the adjacent domestic and irrigation wells were both sampled in order to better understand how the well depth affected the nitrogen pollution of the groundwater. The pH, electrical conductivity (EC), dissolved oxygen (DO), and oxidation-reduction potential (Eh) of the groundwater were measured in situ using a portable multiparameter meter (HQ40d, Hach, Loveland, CO, USA), which was previously calibrated. All water samples for chemical analysis were filtered with a 0.45-μm filter before laboratory analysis. The inorganic nitrogen, including NH4+, NO3−, NO2−, was analyzed by ion chromatography. In addition, the use type of the land where the sampling wells were located, the well depth, and the potential pollution source conditions were recorded, and the water depth of the sampling wells was measured in situ.
Topsoil samples were collected to analyze the concentrations of the total organic and inorganic nitrogen (TN). The position of sampling points was close to the water sampling well, covering various land use types and geomorphic units. TN was analyzed by Jilin University Testing Center.
3.3. Statistical Methods and Graphical Representation
The average value, median value, and standard deviation of the pH, EC, DO, and Eh were evaluated to depict the results of the chemical analysis of groundwater. Nonparametric testing was used because inorganic nitrogen data in the study were not normally distributed. The concentrations of NH
4+ and NO
3− of water samples in this study were grouped by well depth and land use types. Mann–Whitney U test was used to determine whether there was a significant difference (α = 0.05) between every two groups (
n > 10) of data [
35], which made the basis of the regrouping of the data.
To understand the spatial distribution characteristics of nitrogen pollution in groundwater, the pollution intensity of NH4+ and NO3− of the study area were mapped according to the ordinary kriging interpolation in ArcGIS (ESRI, Redlands, CA, USA) software.
3.4. Statistical Methods and Graphical Representation
Some indicators of nitrogen pollution sources in groundwater such as land use type and Eh, which also reflect the formation process that the nitrogen load experiences in the subsurface environment, are highly correlated among themselves. Multivariate statistical analysis could provide insight into the relationship between variables. In this study, factor analysis (FA) was conducted to highlight the main factors that determined the nitrogen concentration in groundwater. Then, the correspondence analysis (CA) was combined to further analyze and the conclusions drawn from FA. Both the FA and CA were performed in SPSS 20.0 software.
FA is widely applied for data reduction in hydrochemical and hydrogeological studies [
31] and has also been used in nitrogen source appointment in recent years [
14]. The main process of FA includes establishing an orthogonal factor model, selecting common factors, and performing factor rotation. In this study, eight quantitative variables (nitrogen concentration, Eh, EC, well depth, DO, water depth) and three qualitative variables (land use type, water richness of aquifer, landform pattern) were selected. Prior to FA, the qualitative variables were transformed into ordinal ones based on the practical situation and the regrouped results from the significance test, and all the variables were standardized to eliminate the effects of dimension.
CA is a multivariate analysis method that can reduce the original variables to a small number of orthogonal factors that by definition are independent [
36]. It can not only study variables and samples simultaneously but also study both the qualitative and quantitative variables by dividing them into classes [
36]. The correlation of variables can be depicted by a correspondence analysis plot, of which the vicinity of points could reflect the close level between variables. Due to the weakness of the analysis of the qualitative variables by FA, which could only distinguish between the effects of the two land use type groups on the nitrogen pollution in this study, CA was proposed to determine the qualitative variables, especially the land use type’s effect on nitrogen pollution in the study area, and further analyze the correlation between the main influencing factors and identify the sources of nitrogen pollution. The variables of CA in this study include the concentrations of NH
4+ and NO
3−, land use type, well depth, and Eh. Among them, the concentrations of NH
4+ and NO
3− were divided into three classes, while Eh and the well depth were divided into two groups.
5. Discussion
5.1. Nitrogen Source Appointments
According to the results of the potential nitrogen source investigation in the study area, the dominating N sources include the excess N of fertilization (mainly composed of urea and ammonia), domestic sewage, and manure. Regarding the different land use types of paddy field, concentrated residential land, decentralized residential land, warehouse, and livestock farm, the N fertilization mainly contributed to the pollution in paddy fields and the land surrounded by paddy fields, such as decentralized residential land and warehouse, and the domestic sewage and manure pollution mainly occurred in residential land, vegetable field, and livestock farm, also occurring in decentralized residential land.
Figure 10 shows that the moderate to high concentrations of NO
3−-N in groundwater (>1.0 mg/L) appeared to be relevant to the TN value of nearby topsoil, proving that the NO
3− pollution in groundwater was a result of the surface nitrogen infiltration. The FA results indicate that greater NO
3− pollution risks exist for groundwater in concentrated residential land, warehouse, and vegetation field than groundwater in other land use types. It is expressed in detail by CA that the vegetation field and the concentrated residential land are most likely to result in NO
3− pollution in groundwater. This indicates that the NO
3− in groundwater of the irrigation area originated from domestic sewage and manure. This conclusion can be supported by FA results suggesting that the EC, which is regarded as an indicative index of wastewater, rises together with NO
3−. Considering the spatial distribution of NO
3− concentration in groundwater, the areas of excessive NO
3− are mostly distributed around the densely populated town and villages or located in vegetation fields, which demonstrates again that it is the domestic sewage and manure that generate NO
3− pollution in groundwater.
The CA results also indicate that paddy field has the greatest potential to impose NH4+ pollution, from which it can be concluded that the NH4+ in groundwater mainly came from the fertilizer N excess. Concerning the spatial distribution of NH4+ in groundwater, the highest concentration of NH4+ occurred in the northwest of the irrigation area. According to the fertilizer application investigation, the fertilizer rate in the north part of the irrigation area was larger than that in the south, and the northwest was the intensive agricultural district of the area; these findings are in accordance with the spatial distribution characteristics of NH4+ in groundwater and support the conclusion that NH4+ in groundwater originated from fertilizer. Meanwhile, the highest value of TN in topsoil also occurred in the northwest of the area, which was in the vicinity of the well with the highest concentration of NH4+. It is further suggesting that fertilizer was the main contributor to both the soil N and the NH4+ in groundwater.
5.2. Governing Factors Determining the Nitrogen Distribution in Groundwater
The nitrogen components in groundwater are a result of nitrogen emission and a series of physical, chemical, and biochemical reactions in the surface and subsurface environment. Besides the nitrogen sources, the specific characteristics of aquifers and vadose zones such as their permeability and thickness, the soil medium, and the environmental factors (e.g., dissolved oxygen and reducing matter, temperature, and soil water content) can all influence the distribution of nitrogen in groundwater. In this study, some comprehensive and accessible indexes (land use type, water richness of the aquifer, landform pattern, Eh, EC, DO, well depth, and water depth) were selected to facilitate the analysis of the influential elements and determine the governing factors. Among the selected indexes, the land use type and EC were indicative of the pollution sources; water richness of the aquifer, water depth, and landform pattern can represent the specific characteristics of aquifers and vadose zones; and the Eh, DO, and well depth reflect the oxidizing and reducing matter in the environment.
The FA results have highlighted the main factors that determine the nitrogen concentration in groundwater. The results show that the NO3− pollution influencing index (PC1) has a strong to moderate positive correlation with land use type, NO2− concentration, Eh, and EC and a negative correlation with the well depth, indicating that both the nitrogen sources and the redox environment are important for the development of NO3− pollution in groundwater of the study area. NO3− is stable in the oxidizing environment, but denitrification (the reduction of NO3− to N2 and NO2−) happens as a result of microbial action when groundwater conditions become reducing; as such, Eh has a positive correlation with NO3− distribution. Moreover, in the results of CA, the high to moderate concentrations and low concentrations of NO3− belong to oxidizing and reducing environments, respectively, again proving that the redox environment is one of the main factors affecting NO3− distribution. The groundwater in shallow wells has a higher potential to accumulate NO3−. This is due to the mixing of the groundwater from different depths, which dilutes the polluting shallow water, and it is also due to the reducing matter in deeper aquifers making the denitrification possible and attenuating NO3−, which has been verified in the results of FA.
The results of FA also suggest that the DO and Eh are the most important factors that determine the NH4+ concentration. In an aerobic and oxidizing environment, NH4+ is easily oxidized, thus making it difficult to keep the NH4+ loading stable in the groundwater. Besides, the dilution effect is another factor affecting NH4+ distribution by diminishing the concentration of it. For the irrigation area, the NH4+ sources (mainly from fertilizer) are not highly variable in spatial distribution, and thus the variety of pollutants on the surface contribute little to the NH4+ difference in groundwater. The anoxic and reducing environment is the dominant factor that determines whether NH4+ can exist in a stable state and the concentration at which it exists in groundwater.
5.3. Formation Process of Inorganic Nitrogen in Groundwater
The inorganic nitrogen in groundwater is a result of surface nitrogen emission and the physical, chemical, and biochemical reactions that the nitrogen load experiences in the subsurface environment. According to the above-mentioned analysis, the nitrogen speciation and concentration are greatly affected by the redox environment of the aquifer, which is represented by the combination of Eh, DO, and well depth. When the groundwater was in the oxidizing condition, the NH4+ concentration was low, and the NO3− concentration was determined by nitrogen loading. When the groundwater was in a reducing environment, the NO3− concentration was fairly low, and the NH4+ concentration was determined by the amount of fertilizer application. The high levels of NO2− were accompanied by high concentrations of NO3−, as an immediate product of nitrification. The formation process of inorganic nitrogen pollution in groundwater can be summarized as follows:
(1) NH
4+-N pollution: The paddy field, of which the soil was generally in a reducing environment due to the standing water, was mainly treated with ammonium fertilizer and urea, which easily transforms into ammonium; thus, the nitrogen loading was mostly in the form of NH
4+-N. Previous studies have mentioned that NH
4+ is apt to be assimilated by vegetation [
37] and volatilization [
38], and the excess NH
4+ would be absorbed by soil materials to a great extent [
39]. This greatly attenuates NH
4+-N content before leaching into the groundwater. The NH
4+-N that leaches into groundwater has two different fates: one is to remain stable as NH
4+-N if the groundwater is in a reducing environment, while the other is to be transformed into NO
3− or NO
2− if the groundwater is in an oxidizing aquifer. The threshold concentration of NO
3−-N in groundwater is much greater than that of NH
4+-N, so there will not be enough oxidized NO
3− to lead to pollution, but the NH
4+-N in a reducing environment has a large potential risk.
(2) NO3−-N and NO2−-N pollution: The soil of residential land is commonly in an oxidizing environment, and thus the nitrogen emission from manure and sewage water is mainly in the form of organic N and NO3−-N. These kinds of nitrogen are not lessened as much as NH4+-N in the vadose zone, and most of them will leach into groundwater. Afterward, if the groundwater is in an oxidizing environment, NO3−-N will remain stable, and organic nitrogen will be transformed into NO3−-N or NO2−-N by mineralization and nitrification with microorganisms, causing the NO3−-N and NO2−-N pollution of groundwater. If the groundwater is in a reducing environment, NO3−-N will be transformed into N2O and N2 and attenuated to a large extent.
6. Conclusions
Groundwater inorganic nitrogen and topsoil total nitrogen were analyzed in the Puyang irrigation area of Sanjiang Plain, and a pollution source investigation was conducted to identify the sources, influencing factors, and formation process of inorganic nitrogen pollution in regional shallow groundwater. In the study area, the potential nitrogen sources are fertilizer, manure, rural domestic waste, and septic system leakage, while atmospheric nitrogen deposition was not considered in this study. For all of the land use types evaluated, the land use types could be reclassified into two groups, with one including concentrated residential land, warehouse, and vegetation field and the other including paddy field, decentralized residential land, and livestock farm. These groups were determined by the distribution characteristic of inorganic nitrogen, where the former might have higher NO3− and lower NH4+ concentration than the latter. As for the well depth, the concentration of NH4+ in WO20 was found to significantly higher than that in WU20 by Mann-Whitney U test. The opposite relationship was found for NO3−.
The results of multivariate statistical analysis showed that the land use type, well depth, NO2− concentration, Eh, and EC were highly related to the NO3− pollution, and the high concentration of NO3− was likely to be found in vegetation field and concentrated residential land and was associated with an oxidizing environment; the NH4+ pollution had the strongest correlation with DO and Eh, and the reducing environment, decentralized residential land, and paddy field had more potential to impose NH4+ pollution. These results highlight that the nitrogen sources and the redox environment determine the distribution of NO3− and the redox environment governs the distribution of NH4+ in the shallow groundwater of the irrigation area.
The NH4+ pollution area was mainly distributed in the northwest of the area, where the fertilizer application rate was much higher and the highest value of topsoil TN was found, supporting the conclusion drawn from multivariate statistical analysis that the NH4+ in groundwater originated from fertilizer. As for the high concentration of NO3− in groundwater, which was mainly situated around the densely populated villages and towns and was relevant to the TN value of nearby topsoil, this was thought to come from manure and domestic waste.
The formation process of inorganic nitrogen pollution in shallow groundwater of the area can be summarized as follows: (1) the NH4+ from fertilizer was greatly attenuated by volatilization, plant uptake, and soil matter absorption and then accumulated in a reducing aquifer or was transformed into NO3− and NO2− by nitrification in an oxidizing aquifer with microorganisms; (2) the organic nitrogen and NO3− in manure and domestic waste were leached, losing little on the surface, to the vadose zone, where they remained steady as NO3−-N in the oxidizing groundwater or were attenuated by microorganism-caused denitrification in the reducing groundwater.