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

Huang, Tianming; Li, Zhenbin; Ma, Baoqiang; Long, Yin

doi:https://doi.org/10.1155/2019/8964182

Geofluids

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Abstract Introduction Study Area Sampling and Analysis Results and Discussion Conclusions Data Availability Conflicts of Interest Acknowledgments References Copyright Related Articles

Research Article | Open Access

Volume 2019 | Article ID 8964182 | https://doi.org/10.1155/2019/8964182

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

Tianming Huang,^1,2,3,4Zhenbin Li,^1,2,3Baoqiang Ma,^1,5and Yin Long^1,2,3

Academic Editor: Jean-Luc Michelot

Received07 Sept 2019

Revised08 Nov 2019

Accepted15 Nov 2019

Published29 Nov 2019

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, NO_x, and NH₃ which are mainly a result of industrial activity and coal burning has increased steadily [1–4]. 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) [6–9]. 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 [11–13]. 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 [24–26]. 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.

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 CO₃^2- 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^-, SO₄^2-, 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.

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 SO₄^2- (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].

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‰ [39–42] 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^-·SO₄^2--Ca²⁺·Mg²⁺ ( is named). Some of the water types were HCO₃^-·SO₄^2--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⁺).

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).

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 Mg^2+/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.

(a)

(b)

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^-.

(a)

(b)

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.

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).

References

J. N. Galloway and E. B. Cowling, “Reactive nitrogen and the world: 200 years of change,” Ambio, vol. 31, no. 2, pp. 64–71, 2002.
View at: Publisher Site | Google Scholar
D. Moncoulon, A. Probst, and J. P. Party, “Alteration, depots atmospheriques et prelevement par la vegetation : role dans la sensibilite des ecosystemes aux depots acides et charges critiques.,” Comptes Rendus Geoscience, vol. 336, no. 16, pp. 1417–1426, 2004.
View at: Publisher Site | Google Scholar
V. Majer, P. Kram, and J. B. Shanley, “Rapid regional recovery from sulfate and nitrate pollution in streams of the western Czech Republic - comparison to other recovering areas,” Environmental Pollution, vol. 135, no. 1, pp. 17–28, 2005.
View at: Publisher Site | Google Scholar
F. Oulehle, K. Jiri, C. Tomas et al., “Predicting sulphur and nitrogen deposition using a simple statistical method,” Atmospheric Environment, vol. 140, pp. 456–468, 2016.
View at: Publisher Site | Google Scholar
R. Vet, R. S. Artz, S. Carou et al., “A global assessment of precipitation chemistry and deposition of sulfur, nitrogen, sea salt, base cations, organic acids, acidity and pH, and phosphorus,” Atmospheric Environment, vol. 93, pp. 3–100, 2014.
View at: Publisher Site | Google Scholar
W. Wang, W. Zhang, X. Hong, and Q. Shi, “Study on factors related to acidity of rain water in China,” China Environmental Science, vol. 13, no. 1, pp. 401–407, 1993.
View at: Google Scholar
Z. Li, “Relation between ion concentration and pH in precipitation in number of Chinese cities,” Acta Scientiae Circumstantiae, vol. 19, no. 3, pp. 303–306, 1999.
View at: Google Scholar
Ministry of Environmental Protection of the People’s Republic of China, China Environmental Bulletin in 2014, Beijing, China, 2015, http://www.mee.gov.cn/hjzl/zghjzkgb/lnzghjzkgb/.
EANET–Acid Deposition Monitoring Network in East Asia, “EANET data on the acid deposition in the East Asian region,” 2019, https://www.eanet.asia.
View at: Google Scholar
B. Mayer, S. M. Bollwerk, T. Mansfeldt, B. Hütter, and J. Veizer, “The oxygen isotope composition of nitrate generated by nitrification in acid forest floors,” Geochimica et Cosmochimica Acta, vol. 65, no. 16, pp. 2743–2756, 2001.
View at: Publisher Site | Google Scholar
N. van Breemen, P. A. Burrough, E. J. Velthorst et al., “Soil acidification from atmospheric ammonium sulphate in forest canopy throughfall,” Nature, vol. 299, no. 5883, pp. 548–550, 1982.
View at: Publisher Site | Google Scholar
R. W. Howarth, “The nitrogen cycle,” in Encyclopedia of Global Environmental Change, The Earth System: Biological and Ecological Dimensions of Global Environmental Change, H. A. Mooney and J. G. Canadell, Eds., vol. 2, pp. 429–435, Wiley, Chichester, UK, 2002.
View at: Google Scholar
C. A. J. Appelo and D. Postma, Geochemistry, Groundwater and Pollution (2nd Edition), A.A. Balkema Publishers, Amsterdam, the Netherlands, 2005.
K. Semhi, P. A. Suchet, N. Clauer, and J. L. Probst, “Impact of nitrogen fertilizers on the natural weathering-erosion processes and fluvial transport in the Garonne basin,” Applied Geochemistry, vol. 15, no. 6, pp. 865–878, 2000.
View at: Publisher Site | Google Scholar
T. Huang, Y. Fan, Y. Long, and Z. Pang, “Quantitative calculation for the contribution of acid rain to carbonate weathering,” Journal of Hydrology, vol. 568, pp. 360–371, 2019.
View at: Publisher Site | Google Scholar
P. Matson, K. A. Lohse, and S. J. Hall, “The globalization of nitrogen deposition: consequences for terrestrial ecosystems,” Ambio, vol. 31, no. 2, pp. 113–119, 2002.
View at: Publisher Site | Google Scholar
D. E. Canfield, A. N. Glazer, and P. G. Falkowski, “The evolution and future of Earth’s nitrogen cycle,” Science, vol. 330, no. 6001, pp. 192–196, 2010.
View at: Publisher Site | Google Scholar
World Health Organization, Guidelines for Drinking-Water Quality (Fourth Edition), WHO Press, Geneva, Switzerland, 2011.
C. Kendall and R. Aravena, “Nitrate isotopes in groundwater systems,” in Environmental Tracers in Subsurface Hydrology, P. G. Cook and A. L. Herczeg, Eds., Kluwer Academic Publishers, Boston, 2000.
View at: Google Scholar
L. Yuan, Z. Pang, and T. Huang, “Integrated assessment on groundwater nitrate by unsaturated zone probing and aquifer sampling with environmental tracers,” Environmental Pollution, vol. 171, pp. 226–233, 2012.
View at: Publisher Site | Google Scholar
S. L. Li, C. Q. Liu, J. Li et al., “Assessment of the sources of nitrate in the Changjiang River, China using a nitrogen and oxygen isotopic approach,” Environmental Science & Technology, vol. 44, no. 5, pp. 1573–1578, 2010.
View at: Publisher Site | Google Scholar
Z. Kovač, Z. Nakić, J. Barešić, and J. Parlov, “Nitrate origin in the Zagreb aquifer system,” Geofluids, vol. 2018, Article ID 2789691, 15 pages, 2018.
View at: Publisher Site | Google Scholar
W. Durka, E. D. Schulze, G. Gebauer, and S. Voerkelius, “Effects of forest decline on uptake and leaching of deposited nitrate determined from 15N and 18O measurements,” Nature, vol. 372, no. 6508, pp. 765–767, 1994.
View at: Publisher Site | Google Scholar
D. A. J. Barry, D. Goorahoo, and M. J. Goss, “Estimation of nitrate concentrations in groundwater using a whole farm nitrogen budget,” Journal of Environmental Quality, vol. 22, no. 4, pp. 767–775, 1993.
View at: Publisher Site | Google Scholar
F. T. Wakida and D. N. Lerner, “Non-agricultural sources of groundwater nitrate: a review and case study,” Water Research, vol. 39, no. 1, pp. 3–16, 2005.
View at: Publisher Site | Google Scholar
M. N. Almasri and J. J. Kaluarachchi, “Modeling nitrate contamination of groundwater in agricultural watersheds,” Journal of Hydrology, vol. 343, no. 3-4, pp. 211–229, 2007.
View at: Publisher Site | Google Scholar
B. Deutsch, M. Mewes, I. Liskow, and M. Voss, “Quantification of diffuse nitrate inputs into a small river system using stable isotopes of oxygen and nitrogen in nitrate,” Organic Geochemistry, vol. 37, no. 10, pp. 1333–1342, 2006.
View at: Publisher Site | Google Scholar
F. Liu, S. Xie, P. Xie, and W. Zhang, “Analysis on characteristics of landscape pattern of land use in Xishui County of Guizhou Province,” Tianjin Agricultural Sciences, vol. 21, no. 12, pp. 84–88, 2015.
View at: Google Scholar
D. Yang, S. Liu, Y. Shan et al., “Fracture characteristics of shale in Upper Ordovician-Lower Silurian in Xishui area, southeast of Sichuan Basin, China,” Journal of Chengdu University of Technology, vol. 40, pp. 543–553, 2013.
View at: Google Scholar
Guizhou Geological Bureau, The Geology Map of Tongzi in Guizhou (1:200,000), China Geology Map Press, Beijing, China, 1978.
PLA’s 00932 Unit, The Integration Hydrogeology Map of Tongzi in Guizhou (1:200,000), China Geology Map Press, Beijing, China, 1978.
D. M. Sigman, K. L. Casciotti, M. Andreani, C. Barford, M. Galanter, and J. K. Böhlke, “A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater,” Analytical Chemistry, vol. 73, no. 17, pp. 4145–4153, 2001.
View at: Publisher Site | Google Scholar
K. L. Casciotti, D. M. Sigman, M. Galanter, J. Hastings, K. Böhlke, and A. Hilkert, “Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method,” Analytical Chemistry, vol. 74, no. 19, pp. 4905–4912, 2002.
View at: Publisher Site | Google Scholar
L. Li, “Pollution characteristics and origin of acid rain in Zunyi,” Guizhou Environmental Science & Technology, vol. 11, no. 4, pp. 6–9, 2005.
View at: Google Scholar
C. Zhao, J. Yu, X. Li, and S. Shuai, “The characteristics of acid rain in Zunyi,” Journal of Guizhou Meteorology, vol. 33, no. 3, pp. 16–19, 2009.
View at: Google Scholar
N. Luo, L. Zeng, Y. Wu, and H. Lyu, “The characteristics of acid rain in Guiyang,” Journal of Guizhou Meteorology, vol. 37, no. 4, pp. 18–23, 2013.
View at: Google Scholar
H. Xiao, H. Xiao, and Y. Wang, “Chemical characteristics and source apportionment of precipitation in Guiyang,” China Environmental Science, vol. 30, pp. 1590–1596, 2010.
View at: Google Scholar
H. Zhang, S. Yu, S. He, Q. Liu, and Y. Li, “Analysis on the chemical characteristics of the atmospheric precipitation in Guilin,” Carsologica Sinica, vol. 31, pp. 289–295, 2012.
View at: Google Scholar
X. Liu, H. Xiao, H. Xiao et al., “Stable isotope analyses of precipitation nitrogen sources in Guiyang, southwestern China,” Environmental Pollution, vol. 230, pp. 486–494, 2017.
View at: Publisher Site | Google Scholar
S. L. Li, C. Q. Liu, J. Hu, J. Li, and N. An, “The characteristics of N and O isotopes for inorganic N from rainfall in Guiyang,” Bulletin of Mineralogy, Petrology and Geochemistry, vol. 25, pp. 57–59, 2006.
View at: Google Scholar
H. Xiao, H. Xiao, A. Long, and Y. Wang, “Who controls the monthly variations of NH₄⁺ nitrogen isotope composition in precipitation?” Atmospheric Environment, vol. 54, pp. 201–206, 2012.
View at: Publisher Site | Google Scholar
H. Xiao and C. Q. Liu, “Sources of nitrogen and sulfur in wet deposition at Guiyang, Southwest China,” Atmospheric Environment, vol. 36, no. 33, pp. 5121–5130, 2002.
View at: Publisher Site | Google Scholar
X. Li, H. Masuda, K. Koba, and H. Zeng, “Nitrogen isotope study on nitrate-contaminated groundwater in the Sichuan Basin, China,” Water, Air, & Soil Pollution, vol. 178, no. 1-4, pp. 145–156, 2007.
View at: Publisher Site | Google Scholar
C. Kendall, “Tracing sources and cycling of nitrate in catchments,” in Isotope Tracers in Catchment Hydrology, C. Kendall and J. J. McDonnell, Eds., pp. 519–576, Elsevier, Amsterdam, Netherlands, 1998.
View at: Google Scholar
D. Xue, J. Botte, B. De Baets et al., “Present limitations and future prospects of stable isotope methods for nitrate source identification in surface- and groundwater,” Water Research, vol. 43, no. 5, pp. 1159–1170, 2009.
View at: Publisher Site | Google Scholar
IAEA/WMO, Global Network of Isotopes in Precipitation, 2019, The GNIP Database. http://www.iaea.org/water.
H. Craig, “Isotopic variations in meteoric waters,” Science, vol. 133, no. 3465, pp. 1702-1703, 1961.
View at: Publisher Site | Google Scholar
I. D. Clark and P. Fritz, Environmental Isotopes in Hydrogeology, Lewis, Boca Raton, Florida, 1997.
W. Dansgaard, “Stable isotopes in precipitation,” Tellus, vol. 16, no. 4, pp. 436–468, 1964.
View at: Publisher Site | Google Scholar
K. Froehlich, M. Kralik, W. Papesch, D. Rank, and H. Scheifinger, “Deuterium excess in precipitation of Alpine regions – moisture recycling,” Isotopes in Environmental and Health Studies, vol. 44, no. 1, pp. 61–70, 2008.
View at: Publisher Site | Google Scholar
T. Huang and Z. Pang, “The role of deuterium excess in determining the water salinisation mechanism: a case study of the arid Tarim River Basin, NW China,” Applied Geochemistry, vol. 27, no. 12, pp. 2382–2388, 2012.
View at: Publisher Site | Google Scholar
Z. Pang, Y. Kong, K. Froehlich et al., “Processes affecting isotopes in precipitation of an arid region,” Tellus B: Chemical and Physical Meteorology, vol. 63, no. 3, pp. 352–359, 2011.
View at: Publisher Site | Google Scholar
R. H. McNutt, “Strontium isotopes,” in Environmental Tracers in Subsurface Hydrology, P. G. Cook and A. L. Herczeg, Eds., pp. 233–260, Kluwer, Boston, 2000.
View at: Google Scholar
G. Han and C. Q. Liu, “Water geochemistry controlled by carbonate dissolution: a study of the river waters draining karst-dominated terrain, Guizhou Province, China,” Chemical Geology, vol. 204, no. 1-2, pp. 1–21, 2004.
View at: Publisher Site | Google Scholar
P. Negrel, C. J. Allegre, B. Dupre, and E. Lewin, “Erosion sources determined by inversion of major and trace element ratios and strontium isotopic ratios in river water: the Congo Basin case,” Earth and Planetary Science Letters, vol. 120, no. 1-2, pp. 59–76, 1993.
View at: Publisher Site | Google Scholar
S. J. Goldstein and S. B. Jacobsen, “The Nd and Sr isotopic systematics of river-water dissolved material: Implications for the sources of Nd and Sr in seawater,” Chemical Geology, vol. 66, no. 3-4, pp. 245–272, 1987.
View at: Publisher Site | Google Scholar
P. Shand and W. M. Edmunds, “The baseline inorganic chemistry of European groundwaters,” in Natural Groundwater Quality, W. M. Edmunds and P. Shand, Eds., pp. 22–58, Blackwell, New York, NY, USA, 2008.
View at: Publisher Site | Google Scholar
T. Huang, Z. Pang, and L. Yuan, “Nitrate in groundwater and the unsaturated zone in (semi)arid northern China: baseline and factors controlling its transport and fate,” Environmental Earth Sciences, vol. 70, no. 1, pp. 145–156, 2013.
View at: Publisher Site | Google Scholar
D. K. Mueller and D. R. Helsel, “Nutrients in the Nation's Waters--Too Much of a Good Thing?” Circular, vol. 1136, no. 24, 1996.
View at: Google Scholar
R. Aravena, M. L. Evans, and J. A. Cherry, “Stable isotopes of oxygen and nitrogen in source identification of nitrate from septic systems,” Ground Water, vol. 31, no. 2, pp. 180–186, 1993.
View at: Publisher Site | Google Scholar
S. L. Li, D. Calmels, G. Han, J. Gaillardet, and C. Q. Liu, “Sulfuric acid as an agent of carbonate weathering constrained by δ¹³CDIC: Examples from Southwest China,” Earth and Planetary Science Letters, vol. 270, no. 3-4, pp. 189–199, 2008.
View at: Publisher Site | Google Scholar
C. Q. Liu, Y. K. Jiang, F. X. Tao, Y. C. Lang, and S. L. Li, “Chemical weathering of carbonate rocks by sulfuric acid and the carbon cycling in Southwest China,” Geochimica, vol. 37, pp. 404–414, 2008.
View at: Google Scholar
B. Mayer, E. W. Boyer, C. Goodale et al., “Sources of nitrate in rivers draining sixteen watersheds in the northeastern U.S.: Isotopic constraints,” Biogeochemistry, vol. 57, no. 1, pp. 171–197, 2002.
View at: Publisher Site | Google Scholar
R. F. Spalding and M. E. Exner, “Occurrence of nitrate in groundwater-a review,” Journal of Environmental Quality, vol. 22, no. 3, pp. 392–402, 1993.
View at: Publisher Site | Google Scholar
Z. Shen, Y. Zhu, and Y. Zhong, Hydrogeochemistry, Geological Publishing House, Beijing, 1993.
C. C. Delwiche and P. L. Steyn, “Nitrogen isotope fractionation in soils and microbial reactions,” Environmental Science & Technology, vol. 4, no. 11, pp. 929–935, 1970.
View at: Publisher Site | Google Scholar
A. Mariotti, F. Mariotti, N. Amarger et al., “Fractionnements isotopique de I’azote atmospheric par les plantes,” Physiology Vegetation, vol. 18, pp. 163–181, 1980.
View at: Google Scholar
Z. Pang, L. Yuan, T. Huang, Y. Kong, J. Liu, and Y. Li, “Impacts of human activities on the occurrence of groundwater nitrate in an alluvial plain: a multiple isotopic tracers approach,” Journal of Earth Science, vol. 24, no. 1, pp. 111–124, 2013.
View at: Publisher Site | Google Scholar
S. L. Li, C. Q. Liu, J. Li et al., “Evaluation of nitrate source in surface water of southwestern China based on stable isotopes,” Environmental Earth Sciences, vol. 68, no. 1, pp. 219–228, 2013.
View at: Publisher Site | Google Scholar
R. Amundson, A. T. Austin, E. A. G. Schuur et al., “Global patterns of the isotopic composition of soil and plant nitrogen,” Global Biogeochemical Cycles, vol. 17, no. 1, p. 1031, 2003.
View at: Publisher Site | Google Scholar
P. Kroopnick and H. Craig, “Atmospheric oxygen: isotopic composition and solubility fractionation,” Science, vol. 175, no. 4017, pp. 54-55, 1972.
View at: Publisher Site | Google Scholar
T. C. Hollocher, “Source of the oxygen atoms of nitrate in the oxidation of nitrite by Nitrobacter agilis and evidence against a P-O-N anhydride mechanism in oxidative phosphorylation,” Archives of Biochemistry and Biophysics, vol. 233, no. 2, pp. 721–727, 1984.
View at: Publisher Site | Google Scholar
P. Yang, Y. Li, C. Groves, and A. Hong, “Coupled hydrogeochemical evaluation of a vulnerable karst aquifer impacted by septic effluent in a protected natural area,” Science of the Total Environment, vol. 658, pp. 1475–1484, 2019.
View at: Publisher Site | Google Scholar

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