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Geofluids
Volume 2019, Article ID 8964182, 12 pages
https://doi.org/10.1155/2019/8964182
Research Article

Tracing the Origin of Groundwater Nitrate in an Area Affected by Acid Rain Using Dual Isotopic Composition of Nitrate

1Key Laboratory of Shale Gas and Geoengineering, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2College of Earth and Planetary Sciences, University of Chinese Academy of Sciences, Beijing 100049, China
3Innovation Academy for Earth Science, Chinese Academy of Sciences, Beijing 100029, China
4Department of Geoscience, University of Calgary, Calgary, Alberta, Canada T2N 1N4
5College of Geoscience and Surveying Engineering, China University of Mining & Technology, Beijing 100083, China

Correspondence should be addressed to Tianming Huang; nc.ca.sacggi.liam@gnauhmt

Received 7 September 2019; Revised 8 November 2019; Accepted 15 November 2019; Published 29 November 2019

Academic Editor: Jean-Luc Michelot

Copyright © 2019 Tianming Huang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Acid rain with a relatively high concentration of ammonium and nitrate can accelerate rock weathering. However, its impact on groundwater nitrate is uncertain. This study evaluated the dual isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) from precipitation to groundwater in a rural mountainous area affected by acid rain. The average concentration for NH4+ is 1.25 mg/L and NO3- is 2.59 mg/L of acid rain. Groundwater NO3- concentrations ranged from <0.05 to 11.8 mg/L (baseline), and NH4+ concentrations ranged from 0.06 to 0.28 mg/L. The results show that groundwater δ18O-NO3- values (-4.7‰ to +4.2‰) were lower than the values of rainfall δ18O-NO3- (+24.9‰ to +67.3‰), suggesting that rainfall NO3- contributes little to groundwater NO3-. Groundwater δ15N-NO3- values (+0.1‰ to +7.5‰) were higher than the values of δ15N-NO3- derived from the nitrification of rainfall NH4+ (less than -4.7‰ in the study area), suggesting that nitrification of rainfall NH4+ also contributes little to groundwater NO3-. This implies that rainfall NO3- and NH4+ have been utilized. The dual isotopic composition of nitrate shows that baseline groundwater NO3- is derived mainly from nitrification of soil nitrogen. The denitrification process is limited in the groundwater system. This study shows that the rainfall NO3- and NH4+ contribute little to groundwater NO3-, improving the understanding of the nitrogen cycle in areas with a high concentration of NH4+ and NO3- in rainfall.

1. Introduction

Since the beginning of the twentieth century, the atmospheric concentration of acidic gases, such as SO2, NOx, and NH3 which are mainly a result of industrial activity and coal burning has increased steadily [14]. Consequently, there were large depositions of atmospheric acid in parts of Europe, North America, and SW China. Sulfur and nitrogen concentrations and deposition in North America and Europe have declined significantly due to emission reduction policies [5]. In the last 40 years, the southwest and southeast parts of China have witnessed varying degrees of acid rain (rainfall with pH value lower than 5.6 is defined as acid rain) [69]. Based on the “China Environmental Status Bulletin in 2014”, released by the Ministry of Environmental Protection [8], 44% of 470 cities witnessed acid rainfall and rainfall in 30% of cities had an annual average pH lower than 5.6.

Acid rain has a high concentration of NH4+ and NO3- [8, 9]. During recharge to a river or groundwater system, NH4+ is nitrified into NO3- under oxidative conditions [10]. As two protons worth of acidity are produced for every NH4+ to form NO3- during nitrification, this process tends to make the environment more acidic [1113]. An acidic environment accelerates rock weathering, irrespective of the source of NH4+ either nitrogen fertilizers [14] or rainfall [15].

Increased inputs of anthropogenic nitrogen could lead to excess nitrogen in ecosystems [16]. The development of new agricultural practices to satisfy a growing global demand for food has led to extensive eutrophication of freshwaters and coastal zones [17]. In order to prevent methemoglobinemia, the maximum contaminant level for nitrate in drinking water has been set at 50 mg/L as NO3- by the World Health Organization [18] and 10 mg/L as NO3--N by the United States Environmental Protection Agency and National Health Commission of the People’s Republic of China (in this study, the concentration of nitrate is expressed as NO3-). As nitrogen is a reactive element in ecosystems [1], a series of reactions and processes control nitrogen dynamics in the soil and groundwater. These processes, which include assimilation, nitrification, denitrification, volatilization, sorption/desorption, and consumption by plants, significantly influence groundwater nitrate [19, 20].

Previous studies focused mainly on the sources of river/groundwater nitrate using multiple environmental tracers, such as the dual isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) and the nitrogen isotope of ammonium (δ15N-NH4+). For example, the ranges of isotopic compositions of nitrate suggested that the major sources of nitrate in the large Changjiang (Yangtze) River come from modified fertilizer and urban sewage effluent [21]. Wastewater was found to be the main source of nitrate contamination in urban areas [22]. The nitrate input from rainfall to groundwater is often neglected, but nitrification is generally regarded as a prerequisite for nitrate leaching [23]. However, few studies on estimating groundwater nitrate concentrations accounted for atmospheric deposition of nitrogen [2426]. Furthermore, the mixing model of the dual isotopic composition of nitrate (δ15N-NO3- and δ18O-NO3-) revealed that the nitrate from atmospheric deposition contributed 3% of the river nitrate in a river subbasin in Mecklenburg-Vorpommern (Germany) [27]. This proportion was found to be 30% for direct NO3- input to spring water from rainfall in northeast Bavaria (Germany) without any microbial interaction [23]; here, the δ18O-NO3- values for spring water range from +11‰ to +33‰. Therefore, the impact of NH4+ and NO3- in rainfall on groundwater systems remains uncertain.

This study aims to assess the potential input of nitrate from acid rain using chemical and isotopic data of rainfall and groundwater in a rural mountainous area in SW China. The results would have important implications for the groundwater nitrogen cycle in areas with a high concentration of rain ammonium and assessment of the baseline level of nitrate related to nitrate contamination due to anthropogenic activities.

2. Study Area

The study area is located in the northern Xishui County (25°0635–28°5015N and 105°5020–106°4430E), Zunyi, Guizhou Province, SW China. It has an average annual temperature of 13.1°C and average annual precipitation of 1138 mm. The study area is a mountainous area with elevation ranging from 750 m to 1700 m a.s.l. (Figure 1). The Xishui River flows into the Yangtze River. The land use/land cover (LULC) in Xishui County is mainly forest (62%) and farmland (31%) [28]. The study area has different lithologies from carbonates to silicates, and therefore, it is an ideal site to assess the impact of lithologies on nitrate concentration.

Figure 1: Study area. (a) Topography with the lowest elevation of 750 m a.s.l. and the highest elevation of 1700 m a.s.l.; (b) aquifer types (the introduction can be found in the text), groundwater and surface water flow direction, and spring sampling sites. The bigger dots indicate the spring samples with higher discharges.

The study area tectonically belongs to the transitional zone between the northern part of the Central Guizhou Uplift and the southeastern part of the Sichuan Basin [29]. The sampling site is in the northwestern limb of the Sangmuchang anticline (Figure 1). The oldest stratum exposed at the core of the anticline is the Proterozoic dolomite rock. The stratum layers from old to new (from the core to northwestern) are Cambrian, Ordovician, Silurian, Permian, Triassic, Jurassic, and Cretaceous [30]. Shallow groundwater aquifers in the study area can be divided into four types (Figure 1) based on lithology and water abundance [31]: (I) fracture-cave water occurring mainly in carbonate rock (limestone and dolomite), (II) cave-fracture water generally occurring in shale with little limestone (discharge of springs of 0.1–10 L/s), (III) pore-fracture water only occurring in the Triassic sandstone with a relatively rich yield of 25–125 m3 per day per meter for a well, and (IV) fracture water occurring in sandstone and mudstone with a groundwater runoff modulus of less than 2 L s-1 km-2. The varying lithology and strong anisotropy of shallow aquifers means that shallow groundwater in the study area generally emerges in the form of springs and shows heterogeneity due to the mixing of various geochemical components.

The population is relatively sparse in the area, and people mainly live in towns and lowland areas. In the mountainous areas (main sampling area), the population is even more sparely distributed. There are almost no wells in the study area; therefore, spring waters are sampled at different stratum. Some of these springs are used for distributed domestic water supply (one or more families use one spring).

3. Sampling and Analysis

A total of 23 samples were collected in 2018 from spring flowing from different types of aquifers. The locations of the sampling points are shown in Figure 1. Physical and chemical parameters such as electrical conductivity (EC), pH, oxidation-reduction potential (ORP), and temperature were measured in situ using a multiparameter device (Hach HQ40d). The titration of HCO3- and CO32- was conducted on site using a 16900 Digital Titrator (Hach) with 0.8 mol/L sulfuric acid for titration and phenolphthalein and methyl orange as indicators. Samples used for cation (Na+, K+, Ca2+, Mg2+, and Sr2+) analysis were immediately acidified with distilled HNO3 (1 mol/L) to a pH of less than 2. The anions, Cl-, SO42-, NO3-, and F-, were analyzed by ion chromatography (ICS-1100), and the cations, Na+, K+, Ca2+, and Mg2+, were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) at the Beijing Research Institute of Uranium Geology (BRIUG). The trace element Sr2+ was analyzed using inductively coupled plasma mass spectrometry (ICP-MS). NH4+ and NO2- were measured using ultraviolet-visible spectroscopy, based on the methods of the GB/T 5750.6 standard, with a detection limit of 0.02 mg/L and 0.002 mg/L. The charge balance errors for all groundwater samples ranged from -1.1% to 4.8% and were within ±5%.

The water stable isotopes were measured at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGG-CAS) using a Picarro L1102-i isotopic water liquid analyzer. The results were reported in the form of δ2H and δ18O () using the Vienna standard mean ocean water (VSMOW) as the reference. The analytical precision was 0.5‰ for δ2H and 0.1‰ for δ18O. The 87Sr/86Sr ratio was measured at BRIUG using a Finnigan MAT 261 multiple collector thermal ionization mass spectrometer (MC-TIMS). Analysis of the NIST NBS 987 standard resulted in a ratio of . The groundwater N and O isotopes in NO3- were measured via the denitrifier method [32, 33] at the Isotope Bioscience Laboratory, Faculty of Bioscience Engineering, Ghent University, Belgium, where NO3- was converted to N2O by denitrifying bacteria Pseudomonas aureofaciens. The results were reported in the form of δ15N-NO3- and δ18O-NO3- using air (atmospheric nitrogen) and VSMOW as the standards. The analytical precisions for both δ15N-NO3- and δ18O-NO3- were both better than 0.5‰.

4. Results and Discussion

4.1. Geochemical and Isotopic Composition of Rainfall

No systematic observation data is available for rainfall in the study area. The Jinyunshan precipitation observation station managed by the Acid Deposition Monitoring Network in East Asia has long-term observation data. The station (106°22E and 29°49N) is located in Beibei District, Chongqing [9], about 40 km from the urban area of Chongqing. Jinyunshan forms one of the Natural Protection Region of Chongqing. The altitude of the Jinyunshan Mountain is about 700–900 m a.s.l., and the summit is at 952 m a.s.l. The physiognomy is typical low mountains. The climate in Jinyunshan is suitable for the growth of diversified plants due to high precipitation and high humidity of air and soil. The dominant plants consist of evergreen vegetation with a stable structure. As a Natural Protection Region, there are no industries, and only a few farming houses. Since this station, which is about 150 km far away from the study area, is situated in a rural site, it represents a rural district that is less impacted by anthropogenic activities than urban areas [9], similar to the study area. These areas experience acid rain with a similar pattern [8]. Therefore, the chemical composition of rainfall in Jinyunshan was used to represent the chemical characteristics of precipitation in the study area. The chemical compositions of rainfall over whole regions affected by acid rain in SW China are all similar, such as Zunyi [34, 35], Guiyang [36], and Guilin [37, 38] (Table 1). Unfortunately, there is no data on dry deposition. Therefore, only wet deposition was considered in this study.

Table 1: Chemical composition of rainfall for Jinyunshan (JYS) from 2002 to 2017 [9], Zunyi (ZY) from 2002 to 2006 [34, 35], Guilin (GL) in 2010 [38], and Guiyang (GY) from 2003 to 2009 [36, 37, 40]. The unit for precipitation (Preci) is mm/yr and the unit for concentration is mg/L.

As the water cycle is relatively short for spring water, rainfall data from 2013 to 2017 was used. Systematic observations of rainfall chemical composition show that the pH values ranged from 4.20 to 5.15 from 2013 to 2017 (Table 1) with a weighted average of 4.55, indicating the presence of acid rain. The main anions (in meq/L) are SO42- (69%) and NO3- (27%), and the main cations are Ca2+ (39%), NH4+ (37%), and H+ (16%) (Figure 2). The NO3- and NH4+ concentrations showed limited variation, with averages of 2.59 mg/L and 1.25 mg/L, respectively. The total inorganic nitrogen from atmospheric wet deposition is expected to be 17.7 kg N ha-1 yr-1, similar to the value of 18.4 kg N ha-1 yr-1 from southern Ontario, Canada, from 1980 to 1985 [24].

Figure 2: Precipitation ion compositions (% in meq/L). The ion compositions are the weighted average values from Jinyunshan, Beibei District, Chongqing, from 2013 to 2017. SO42-, NO3-, Ca2+, NH4+, and H+ are the main ions in precipitation (>15%). The data are from [9].

Previous studies have shown that the δ15N-NO3- of rainfall in Guiyang (about 190 km from the study area) ranges from -12.7‰ to +15.8‰ [3942] with averages value of -1.9‰ [39], +1.5‰ [40], +2.3‰ [41], and +3.1‰ [42] during different sampling periods. The δ18O-NO3- of rainfall ranges from +25.2‰ to +40.1‰ with an average of +34.2‰ [40]. In the Shapingba District, Chongqing (about 140 km from the study area, and close to the Jinyunshan station), δ15N-NO3- values range from -1.0‰ to +6.9‰ with an average of +1.5‰ and those of δ18O-NO3- ranged from +24.9‰ to +67.3‰ with an average of +43.6‰ (unpublished data from Dr. Pingheng Yang from Southwest University, China).

The δ15N-NH4+ of rainfall in Guiyang ranged from -28.7‰ to +7.8‰ [39, 40] with average values of -10.6‰ and -4.7‰ during different observation periods. In Chongqing, the value ranged from -8.6‰ to +1.3‰ [43] with an average of -6.7‰.

The rainfall around the study area is similar to other global studies with respect to δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+ [44, 45], and the average values are within the typical range, suggesting that they are important end-members with which to trace the origin of NO3- in springs in the study area.

4.2. Water Chemistry and Isotopic Composition of Springs

The data for major ions, pH, and stable isotopic composition for spring water are shown in Tables 2 and 3. The pH values of spring water ranged from 6.39 to 8.29, with an average of 7.61, which is mostly neutral to slightly alkaline. The total dissolved solids (TDS) ranged from 53 mg/L to 368 mg/L (XS21 of 16 mg/L is not included), with an average value of 210 mg/L. The Na+ (0.1–6.8 mg/L) and Cl- (0.4–7.6 mg/L) concentrations were extremely low, with median values of 1.4 and 1.3 mg/L, respectably. The Piper plot (Figure 3) shows that the main water types were HCO3--Ca2+, HCO3--Ca2+·Mg2+, and HCO3-·SO42--Ca2+·Mg2+ ( is named). Some of the water types were HCO3-·SO42--Ca2+·Mg2+·Na+ (XS21 and XS22), and the corresponding strata are the Triassic and Jurassic sandstone, siltstone, mudstone, and shale. The TDS were mainly composed of HCO3- ( of 0.84 for TDS and HCO3-) and Ca2++Mg2+ ( of 0.98 for TDS and Ca2++Mg2+).

Table 2: Geochemical composition of spring water (mg/L).
Table 3: Strontium and nitrate concentration and isotopic compositions for spring waters.
Figure 3: Piper diagram of spring water samples. The HCO3--Ca2+ and HCO3--Ca2+·Mg2+ types were found to be dominant.

The spring water in the study area corresponds to the local meteoric water line (LMWL) of for Zunyi, obtained from the GNIP database [46], compared with the global meteoric water line (GMWL) of [47]. The difference in water stable isotopic composition for springs was mainly caused by recharge altitude. At higher altitudes, where the average temperatures are lower, precipitation will be isotopically depleted and for δ18O, the depletion varies between about -0.15‰ and -0.5‰ per 100 m rise in altitude [48]. The outcrops of springs with most depleted isotopic values (δ18O less than -8‰, XS2, 3, and 5) are located in the Cambrian carbonates with the highest altitudes within all samples, and the outcrop of spring with most enriched isotopic value (δ18O of -6.1‰) was located in lower altitudes (Figures 1 and 4).

Figure 4: Stable isotopic composition of springs. All springs lie around the LMWL, suggesting limited evaporation during recharge. LMWL for Zunyi: [46]; GMWL: [47].

However, in the higher altitudes, the groundwater samples were found to deviate from the LMWL (Figure 4). The deuterium excess value () [49] was around 18‰, as compared to the value of 12‰–15‰ in the lower altitudes (Table 3). In general, evaporation decreases and moisture recycling increases the deuterium excess [50, 51]. In mountainous areas, moisture recycling could contribute to additional moisture with high deuterium excess value for precipitation [50, 52], resulting in high deuterium excess in spring water at higher altitudes.

In most cases, carbonate rocks have higher contents of strontium but lower 87Sr/86Sr as compared to strontium derived from silicate rocks with lower contents of strontium and higher 87Sr/86Sr [53]. Marine carbonate commonly has an 87Sr/86Sr value of less than 0.710. Groundwater flowing through carbonate aquifers will more readily reach high concentration levels of Sr2+, resulting in fluids with low 87Sr/86Sr [53]. This contrasts with silicate groundwaters that tend to have higher 87Sr/86Sr and lower concentrations of Sr2+. In Guizhou, the 87Sr/86Sr ratio for carbonate rocks (including calcite and dolomite) could range from 0.7075 to 0.7100 [54]. Meanwhile, the results of previous studies show that Sr2+ originating from weathering of silicate rocks commonly has an 87Sr/86Sr ratio higher than 0.7150 [55, 56]. The concentrations of Sr2+ in shallow groundwater ranged from 0.01 mg/L to 1.97 mg/L, with an average of 0.37 mg/L (Table 3). Figure 5 shows the relationship between 1/Sr2+ and 87Sr/86Sr (Figure 5(a)) and the relationship between Mg2+/Ca2+ and 87Sr/86Sr (Figure 5(b)) for spring water. Generally, when Sr2+ concentration decreased (i.e., 1/Sr2+ increased), the 87Sr/86Sr value increased. When the end-members for limestone, dolomite, and silicate in Guizhou [54] were used, silicate weathering contributed partially to the input of Sr2+. However, no relationship was found between NO3- concentration and Sr2+ concentration (), Mg2+/Ca2+ ratio (), and 87Sr/86Sr ratio () for spring waters, suggesting that the impact of lithology on NO3- concentration is limited.

Figure 5: Relationship between 87Sr/86Sr and 1/Sr2+ (a) and between 87Sr/86Sr and Mg2+/Ca2+ (b). The three end-members of limestone, dolomite, and silicate are modified from [54].

Nitrate concentrations for spring samples ranged from undetected (<0.05 mg/L) to 63.9 mg/L with a median value of 3.2 mg/L. NH4+ concentrations for all spring water samples ranged from 0.06 mg/L to 0.28 mg/L with a median value of 0.08 mg/L. The NH4+ concentrations were significantly lower than those from rainfall (1.25 mg/L). Nitrite (NO2-) was not detected in any spring water sample. The δ15N-NO3- for all spring water samples ranged from +0.1‰ to +7.5‰, while the δ18O-NO3- ranged from -4.7‰ to +4.2‰ (Table 3).

4.3. Origin of Nitrate for Spring Water

According to the definition from Shand and Edmunds [57], the groundwater baseline quality is “The range of concentrations of a given element, isotope or chemical compound in solution, derived entirely from natural, geological, biological or atmospheric sources, under condition not perturbed by anthropogenic activity”. In terms of NO3-, the baseline value can be determined using spring water samples from the upper stream, which are not influenced at all by anthropogenic activity (no. 1 to no. 15, Table 2). The median (50%) and upper values for NO3- baseline level, calculated using SigmaPlot (version 10.0), were found to be 2.9 mg/L and 11.0 mg/L. These values are consistent with the baseline value for NO3- (median value of 2–9 mg/L and upper value of 8.2–14.4 mg/L) in arid-semiarid northern China [58]. The natural baseline for nitrate is unlikely to be more than 9–13 mg/L in most temperate regions covered by forest or grassland [57, 59]. Based on the baseline level, NO3- concentrations in other springs were within the baseline level (from <0.05 to 7.5 mg/L), except for spring sample XS17, which had an NO3- concentration of 63.9 mg/L; it was affected by anthropogenic activity.

Potential origins of NO3-, with NO3- concentrations less than the upper baseline level in the study area, are (1) direct NO3- input from rainfall, because rainfall has an average concentration of 2.59 mg/L; (2) NH4+ from rainfall, because it has an average concentration of 1.25 mg/L NH4+ while concentrations of the latter in groundwater were less than 0.28 mg/L, suggesting that the nitrification of rainfall NH4+ could potentially contribute NO3- to groundwater; and (3) NH4+ from soil nitrogen, which is a common source of groundwater NO3- [19, 21]. Denitrification, however, is a multistep process involving various nitrogen oxides (e.g., N2O, NO) as intermediate compounds resulting from a biologically mediated reduction of nitrate to N2 [19]. This process would decrease NO3- concentration and increase δ15N and δ18O in the residual NO3- by a factor of 2 : 1 [19, 48, 60].

When rainfall recharges springs, the enrichment factor () (evapotranspiration from rainfall to the spring) is difficult to determine in a complicated hydrogeological system. However, the range of 2–5 is used based on different sites with similar hydrogeological conditions in SW China [15, 61, 62]. As the NO3- concentration in rainfall was 2.59 mg/L, spring water should have NO3- concentrations of 5.2–13.0 mg/L if there is no loss during recharge. The value overlaps the spring NO3- value (from <0.05 to 11.8 mg/L). However, plot of δ15N-NO3- and δ18O-NO3- showed that the groundwater NO3- concentrations were similar to those of rainfall NO3- (Figure 6). The latter were less than +4.2‰ (Table 3) while the former were larger than +24.9‰ and up to +67.3‰ in Guiyang and Chongqing. Besides, groundwater NO3- concentrations were lower compared with those in rain after during evapotranspiration (Figure 7(a)). Furthermore, groundwater NO3- enrichment was not caused by evapotranspiration (Figure 7(b)), because the NO3-/Cl- ratio would remain constant when NO3- increased. NO3- is the most usable form of nitrogen for plants [25]. Nitrate from atmospheric deposition is intensively cycled through the organic nitrogen pool in all watersheds [63] and can be taken up by plants [64]. The vegetation is dense in the study area. Therefore, NO3- in spring water is not derived from rainfall NO3-.

Figure 6: The relationship between δ15N-NO3- and δ18O-NO3- for spring water. The range for δ15N-NO3- and δ18O-NO3- and average range for δ15N-NH4+ for precipitation is from Guiyang and Chongqing. The schematic of typical ranges for other sources is from [44, 45].
Figure 7: The relationship between NO3- and Cl- for spring and evapotranspiration line for rainfall (a) and between NO3- and NO3-/Cl- (b). During evapotranspiration, NO3-/Cl- would remain constant.

The ammonium (NH4+) concentration in spring water (0.06–0.28 mg/L) is significantly lower than that in rainfall (1.25 mg/L), suggesting that most NH4+ in rainfall has been nitrified to NO3- during the recharge process [11, 13, 15, 65]: where 1.00 mg/L NH4+ can produce 3.44 mg/L NO3-. When NH4+ concentration in rainfall is 1.25 mg/L and the enrichment factor is 2–5, NH4+ concentration for recharging spring would reach 2.50–6.25 mg/L, potentially resulting in 8.6–21.5 mg/L NO3- in spring water. This value is similar to or higher than the NO3- concentration in spring water (from <0.05 to 11.8 mg/L). Therefore, nitrification of NH4+ in rainfall may be a potential source for spring NO3-. The average values of δ15N-NH4+ for rainfall in adjacent areas (Guiyang and Chongqing) ranged from -10.6‰ to -4.7‰ [39, 41, 43]. There is isotopic fractionation for nitrogen, commonly a few per mill lighter [19] and up to -35‰ [66, 67] with respect to the ammonium source during nitrification of NH4+. However, it is difficult to accurately predict δ15N-NO3- from simple measurement of δ15N-NH4+ due to complicated transformation processes and transformation rate [10, 44, 68]. If spring NO3- is derived from nitrification of rainfall NH4+, the δ15N-NO3- of spring would have an average value of less than -4.7‰. When the average value of δ15N-NO3- for spring water is +4.4‰, significantly larger than that from nitrification of rainfall NH4+ (less than -4.7‰), it can be concluded that spring NO3- is not derived from rainfall NH4+ through nitrification. Therefore, rainfall NH4+ can be said to have been consumed and contributes little to spring NH4+ and NO3-.

Nitrification of soil nitrogen could be an important source for the natural groundwater system [69]. The δ15N values of total soil N vary from -10‰ to +15‰, but typically range from +2‰ to +9‰ [19, 45, 70]. During mineralization of soil N to NH4+ (sometimes it is called ammonification), there is very little isotopic fractionation for δ15N [19]. However, the further nitrification process will result in a slightly lighter (few per mill) δ15N for nitrate in N-limited systems [19]. The δ15N-NO3- for spring water was within the range of NO3- derived from soil N nitrification (Figure 6). In addition, nitrate derived from nitrification should have δ18O-NO3- between -9‰ and +11‰ as the O in NO3- is derived from H2O (2/3) with δ18O values in the normal range of -25‰ to +4‰ and from O2 (1/3) with δ18O of +23.5‰ [71, 72]. However, these values can vary due to changes in ammonium abundance and nitrification rates [10]. The δ15N-NO3- for spring water ranged from +0.1‰ to +7.5‰, while the δ18O-NO3- ranged from -4.7‰ to +4.2‰, all within the acceptable range for nitrification of soil nitrogen (Figure 6). Therefore, nitrate from nitrification of soil nitrogen is a potential source for spring nitrate.

A weakly positive relationship was found between NO3- concentration and δ15N-NO3- (, ), suggesting that the increase in δ15N-NO3- is not related to denitrification, which would result in a decrease in NO3- concentration. In addition, the ORP values ranged from -57.6 to +49.9 mV (Table 1) with an average value of -16 mV. The absence of correlation for ORP and NO3- suggests that denitrification process is limited in the study area.

Based on the aforementioned results, NO3- with concentration less than 12 mg/L in spring waters in the study area mainly originated from the nitrification of soil N, rather than direct input of rainfall NO3- or nitrification of rain NH4+ (Figure 8). Although the latter two processes can potentially contribute up to 13.8–34.5 mg/L NO3- in a spring, NO3- and NH4+ transformation could affect the fate of nitrate in this mountainous area affected by regional acid rain. In similar study areas in Guizhou Province, Li et al. [69] attributed the origin of NO3- with similar concentration and similar isotopic composition (15N-NO3- and δ18O-NO3-) to nitrification of soil N. This study has provided isotopic evidence that rainfall input has contributed little to NO3- in spring waters.

Figure 8: Conceptual model of the nitrogen cycle in the study area. Note: FE = enrichment factor.

For spring water XS17, which has been affected by anthropogenic activities, the NO3- concentration was 63.9 mg/L with δ15N-NO3- value of +2.2‰ and δ18O-NO3- value of +1.1‰ (Figure 6). There are two potential sources: (1) nitrification of ammonium from fertilizer and (2) manure and septic wastewater. Based on field surveys, this spring water was found to be affected by distributed croplands. The infiltrating water or/and overflow may flow into the recharge area of the spring XS17. The high NO3- concentration is related to the use of ammonium fertilizers. NO3- from manure and septic waster would have higher δ15N-NO3- (typically +7‰–+25‰) value [27, 44, 45, 73]. The δ15N-NO3- value of +2.2‰ is significantly lower than that from manure and septic wastewater, suggesting that the spring XS17 with NO3- concentration of 63.9 mg/L was derived from nitrification of ammonium fertilizers (Figure 6).

5. Conclusions

While some studies have considered the atmospheric input of nitrate to groundwater systems, this study shows that the direct input of rainfall nitrate and input through nitrification of rainfall ammonium contributes little to groundwater nitrate, even when nitrate and ammonium concentrations in rainfall are high. The inorganic nitrogen from rainfall is consumed, and groundwater nitrate is mainly derived from nitrification of soil nitrogen at the natural groundwater system. In the study area, the moisture recycling in higher altitudes could contribute to additional moisture, resulting in high deuterium excess in spring samples at higher altitudes. The 87Sr/86Sr ratios show the chemical types of spring water are controlled by lithology. However, the impact of lithology and evapotranspiration on NO3- concentration is limited in the study area. This study helps understand the nitrogen cycle and trace groundwater nitrate sources.

Data Availability

The data used to support the findings of this study are included within the article.

Conflicts of Interest

The authors declare that there is no conflict of interest regarding the publication of this paper.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grants 41672254 and 41877207), the Youth Innovation Promotion Association CAS (Grant 2018087), and a CAS scholarship to visit the University of Calgary (Grant 201825).

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