Relationship of nitrate isotopic character to population density in the Loess Plateau of Northwest China

Applied Geochemistry 35 (2013) 110–119

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Relationship of nitrate isotopic character to population density in the Loess Plateau of Northwest China Meng Xing a,⇑, Weiguo Liu a,b,⇑, Zhoufeng Wang c, Jing Hu a a

State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China Xi’an Jiaotong University, School of Human Settlement and Civil Engineering, Xi’an 710049, China c Chang’an University, School of Environmental Science and Engineering, Xi’an 710064, China b

a r t i c l e

i n f o

Article history: Received 27 October 2012 Accepted 3 April 2013 Available online 18 April 2013 Editorial handling by S. Bottrell

a b s t r a c t Nitrate pollution of groundwater is an increasingly serious anthropogenic problem. In this study, the hydrogeochemistry of major ions and stable isotope ratios of NO 3 in groundwater were determined to identify the contamination sources and chemical transformation processes occurring in the shallow groundwater of Xi’an, the capital of Shaanxi province, NW China. Of a total of 32 groundwater samples, 1 . Most of 31% had NO 3 —N concentrations exceeding the accepted drinking water limit of 10 mg-N L these samples were from the urban center of the study area, while samples with <10 mg-N L1 were  mainly from suburban areas. Combined with information on NO 3 and Cl , the variation in isotopes of  in the groundwater suggest a mixing of multiple NO sources in areas on the urban/suburban borNO 3 3 der. By determining rainwater and river water NO 3 isotopic values, the groundwater recharge mode can be deduced for Xi’an city. Chemical fertilizers and nitrification of N-containing organic materials contrib ute NO 3 to suburban groundwater, while sewage effluent and nitrification dominate NO3 distribution in urban groundwater. Nitrification from organic soil N, manure and sewage was significant in some sampling areas, and NO 3 isotopic values from groundwater in Xi’an indicated that the effects of denitrification were not an obvious contributor. Thus, the d15 N—NO 3 enrichment process is mainly caused by the intense anthropogenic activity in the city center. From the urban center to suburban areas, the mean  d15 N—NO 3 values varied from +16.4‰ to +5.4‰, and the mean NO3 —N concentrations varied from  1 1 15 28.0 mg L to 4.0 mg L . In particular, the d N—NO3 value (r = 0.75, p < 0.01) correlated more significantly with distance from the urban center than did the NO 3 —N concentration data (r = 0.49, p < 0.01), which suggests that NO 3 isotopic values are an effective indicator of contamination sources. In addition, the d15 N—NO 3 values and population density show a significant logarithmic correlation in Xi’an city. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Groundwater is the source of water supply for many cities and for some the sole source of drinking water. With rapid economic growth and population increases in many metropolitan areas, especially industrial cities, increased loadings of N are now being input into aquatic systems via anthropogenic activities, including industrial wastewater discharge, agricultural N fertilizer application, urban domestic sewage, and human or animal waste (Aravena and Robertson, 1998; Fukada et al., 2003; Liu et al., 2006; Xue et al., 2009; Yue et al., 2010). In drinking water, NO 3 in excess of 10 mgN/L may be toxic to infants and may increase the risk of stomach cancer in others (Comely, 1945; Kamiyama et al., 1987).

⇑ Corresponding authors. Address: State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710075, China (W. Liu). Tel.: +86 29 88323495; fax: +86 29 88320456. E-mail addresses: [email protected] (M. Xing), [email protected] (W. Liu). 0883-2927/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apgeochem.2013.04.002

Increased NO 3 in shallow groundwater is likely caused by multiple non-point and point sources. Identification of contamination sources is the first step to improving groundwater quality, followed by removing the sources and controlling further increases  in NO 3 concentration. Traditional methods to determine NO3 contamination sources require investigation of land use and hydrological study of the contaminated regions (Chang et al., 2002). However, complex physical, chemical and biotransformation processes impact NO 3 concentration, and these methods cannot easily distinguish between NO 3 sources. Therefore, stable N isotopes (d15 N—NO 3 ) have been used extensively to identify the sources and fate of NO 3 in groundwater and stream water, which display characteristic isotopic ranges depending on their source (Böhlke and Denver, 1995; Mayer et al., 2002; Karr et al., 2003; Kellman and Hillaire-Marcel, 2003; Thorburn et al., 2003; Widory et al., 2004; Moore et al., 2006). Because d15 N—NO 3 values are often modified by isotopic fractionation through physical, chemical and microbial process such as volatilization, nitrification and denitrification, d15 N—NO 3 values alone often cannot conclusively identify

M. Xing et al. / Applied Geochemistry 35 (2013) 110–119

the origin of NO 3 in aquatic systems (Kaown et al., 2009). Recently, multiple-isotopic approaches using the N and O isotopes of NO 3 have proven successful methods for tracing the origin of NO 3 in contaminated groundwater (Aravena and Robertson, 1998; Rock and Mayer, 2002; Widory et al., 2005). This study used a dual-isotopic approach in combination with hydrochemical studies of the shallow groundwater, surface water, rainwater, and sewage was used to identify the sources of NO 3 and the particular N species, to understand their transformation, and to determine the relationship between NO 3 and other anthropogenic components in the groundwater of an urban site on the Loess Plateau. This research will inform future management decisions aimed at protecting groundwater quality. Xi’an, the capital city of Shaanxi province and the largest city in northwestern China, is located on the Guanzhong Plain on the southern edge of the Loess Plateau at an average elevation of 400 m. Xi’an is one of northwestern China’s key cities, a center for politics, the economy and culture. This region has distinct seasonal variations with rainy, humid summers and dry winters. The population of Xi’an is greater than 3.8 million. The city is situated in a semi-arid zone where the major sources of Asian dust are located, with a mean temperature of 13.0–13.4 °C and annual rainfall of 558–750 mm (Han et al., 2009). The prevailing winds are northeasterly, especially in winter. Xi’an has 13 administrative districts and counties. The most urbanized regions have a total area of 363.6 km2. Baqiao, Beilin, Lianhu, Weiyang, Xinchen, and Yanta represent the main urban area, around which lie Changan and Lintong, two vigorously developing regions with strong urban–rural interactions. The Gaoling, Huxian, Lantian, Yanliang, and Zhouzhi areas surrounding Xi’an are predominantly rural (He et al., 2008).

111

The region consists of 6 landscape types, including Qinling Mountain, Li Mountain, Loess hills, Loess tableland, alluvial plain and river flood plains. Groundwater is abundant in Xi’an, and the distribution and water storage capacity are controlled by lithology, landscape and hydrometeorology (Gan, 1987). Groundwater can be classified as derived from both unconsolidated porous-fissured aquifers and bedrock porous-fissured aquifers in this research area (Gan, 1987). The shallow groundwater is mainly located in Holocene alluvial aquifers, Holocene proluvial aquifers, Upper-Pleistocene alluvial aquifers and middle Pleistocene aquifers. The aquifer lithologies are comprised of fine sand, sandy gravel, sandy clay and loessal sandy clay (Gan, 1987). The groundwater flow system is mainly determined by the topographic relief effect, and flow is from south to north and from SE to NW (Gan, 1987). The age of the shallow groundwater in the aquifer is on the order of a few tens of years or less (Qin, 2005). The isotopic values of these samples should reflect the present land-use characteristics of the area. As shown in Fig. 1, Xi’an has inner and outer ring roads; the downtown area is enclosed by the inner ring. Prior to the 1950s, land outside the inner ring road was mainly agricultural, but since then, Xi’an has developed into an important industrial city. Due to extremely rapid urbanization over the last two decades, the previously agricultural areas outside the inner ring road have become densely populated residential and commercial areas (Han et al., 2008). 2. Materials and methods 2.1. Sample collection Groundwater samples were collected in April and July of 2010 from Xi’an and the surrounding areas (Wang et al., 2010). Fifteen

Fig. 1. Map showing the Xi’an sampling locations.

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shallow groundwater samples were collected from the north foot of Qinling Mountain to the south bank of the Weihe River along the groundwater flow direction during July of 2010 (Fig. 1). In April 2010, 17 shallow groundwater samples were gathered from the eastern Weihe River to the western Bahe River (Fig. 1). The groundwater sampling area was divided into 6 districts according to sampling location distance from the center of Xi’an. The diameter of these districts are as follows: A, 6 km; B, 12 km; C, 18 km; D, 24 km; E, 30 km; F, greater than 30 km. Groundwater samples were distributed according to their area (Fig. 1). At this time, 15 river samples, 1 sewage sample, 31 rainwater samples, 4 soil and 5 fertilizer samples were also collected for d15N (including total or þ 18 ganic N, NO 3 and NH4 ) and d O—NO3 determination (Fig. 1). The surface river waters were collected from the bank during base flow. Samples of rainwater were collected manually from the beginning of each precipitation event in a polyethylene collector (700 mm  450 mm  170 mm). The rainwater samples were collected over 4 months from April to July in 2010 on the rooftop of the Chinese Academy of Sciences’ Institute of Earth and Environment building, 10 m above ground level. The N sources can be recognized from the d15N, d18O values of these samples. 2.2. Analytical methods Sample conductivity and pH were measured soon after transport of the samples to the laboratory. Sodium and K were determined by flame atomic emission spectrophotometry. Calcium and Mg were determined by the EDTA titration method. Sulfate concentration was determined using the EDTA titration described by Howarth (1978). The Cl concentration was evaluated by using the AgNO3 titration method, and HCO 3 was also measured by acid titration. Nitrate concentration was determined using spectrophotometry with phenol disulfonic acid. Blanks, calibration check standards, and reference standards were analyzed with each sample set.  18 To determine the d15 N—NO 3 and d O—NO3 contents, groundwater samples were filtered through glass fiber filters (Whatman, GF/F, 47-mm diameter, 0.7-lm pore size) to remove large particles and then filtered through 0.40-lm polycarbonate membranes. Dissolved NO 3 was converted to solid AgNO3 using an ion exchange method (Silva et al., 2000; Xing and Liu, 2011). The AgNO3 was then analyzed for d15N using a CE FLASH 1112 elemental analyzer (EA) interfaced with a Delta-Plus continuous flow isotope ratio mass spectrometer and d18O was determined using the Delta-Plus instrument connected to a high-temperature conversion elemental analyzer. Dissolved organic matter and O-bearing ions other than NO 3 were carefully removed during the preparation of AgNO3 for d18O analysis. Two laboratory reference material KNO3 (d15N = +6.3‰, d18O = +15.6‰) and a type of cellulose (d18O = +29.0‰, not containing N), and the international isotopic reference material IAEA-N3 (d15N = +4.7‰, d18O = +23.0‰) were measured to monitor analytical accuracy. The standard deviation for duplicate analysis was less than 0.3‰ for d15N and less than 1.0‰ for d18O. 3. Results and discussion 3.1. Water chemistry The chemical composition of groundwater samples obtained in April and July of 2010 are summarized in Table 1. The groundwater was slightly alkaline; the pH ranged from 7.1 to 8.4, similar to the range (7.4–8.4) measured by Qin et al. (2005). At all sampling locations groundwater is very shallow and groundwater well connected with the surface environment (Table 1). Hence, the entire shallow groundwater system is an open, well-oxygenated system.

Calcium was the dominant cation and HCO 3 the dominant anion in the groundwater of area F. Groundwater can be characterized as Ca-HCO3 type using stoichiometric analysis in this area. As groundwater flows from the F to A area, it is gradually transformed into a Na-HCO3 type water. Panno et al. (2006a,b) estimated the background concentration 1 for Cl and NO for Cl and 3 in the Midwestern USA as 1–15 mg L 0–2.5 mg L1 for NO —N. Sun and Wu (2005) investigated the 3 groundwater contamination condition of Xi’an city, and recognized 1 the background concentration for Cl and NO for 3 as 10–32 mg L Cl and 0.6–8.6 mg L1 for NO —N. Examination of the present 3 data from area F reveals background concentration for these ions 1  (5–40 mg L for Cl and 2.4–6.5 mg L1 for NO 3 —N) similar to these values. The background samples contain pristine groundwater (e.g. F4 and F6), and groundwater with long standing agricultural impact before the most recent development (e.g. F1, F2, F3 and F5). By comparing the distance from the city center with the ion concentrations, it was found that both Cl and NO 3 are clearly elevated in most samples from the A through the E zones, and lower in the F zone, where some samples reach background levels (Fig. 2). In addition, the other ions, except for Ca+ an HCO 3 , have the same variation tendency (Table 1). The F sample sites were far from the city center; as expected with lower population density and little anthropogenic activity, ion concentrations in this areas were lower than in other areas (Table 1). These background values can then be used to determine which samples are affected by contaminants and which are near pristine. Higher ion concentrations in A and B than F were most likely due to the high concentration of downtown sites, sampled at 3 km and 6 km from the city center. These groundwater samples were possibly impacted by municipal sewage. In addition, the samples were defined as impacted by anthropogenic activities including the effect of agricultural activities. The C, D and E sample sites were distributed between urban and suburban areas, and thus their groundwater ion concentrations were more complex. Sulfate and K are contaminants that may be derived from fertilizers and sewage/manure. The extremely high K+ concentration in D2 and the very high Cl and HCO 3 concentrations and very low NO 3 concentration, may indicate that this well was contaminated by animal waste containing only NHþ 4 at the time of sampling. Alternatively, it could be contamination by KCl (Table 1). At site E4, the elevated SO2 4 concentration accompanied by high Na and low NO 3 concentrations suggests that this well was contaminated by (NH4)2SO4 fertilizer or by sewage containing Na2SO4 (Table 1). 1 The NO to 3 —N concentrations in Xi’an ranged from 0.04 mg L 1 44.2 mg L (Table 1). Of the 32 groundwater samples, 31% had NO 3 —N concentrations exceeding the acceptable drinking water limit of 10 mg-N L1. These samples were largely distributed in the urban center of the study area (A and B), while samples with less than 10 mg-N L1 were mainly distributed in the urban/suburban border regions and suburban areas (C, D, E, and F) (Table 1; Fig. 4).  3.2. NO 3 and Cl concentration variations

Concentrations of Cl in groundwater ranged from 5.0 to 241 mg L1, with a mean value of 89.9 mg L1 (Table 1). The distribution of natural solutes in surface and groundwater has traditionally been recognized as distinct from that derived from anthropogenic sources (Liu et al., 2006; Jia et al., 2009; Li et al., 2010). Potential sources for Cl include natural sources (dissolution of minerals), agricultural chemicals (KCl), animal waste, septic effluent and road salt (Liu et al., 2006). High Cl content was detected in municipal sewage samples and some contaminated groundwater, which may indicate that most of the Cl is of anthropogenic origin.

Table 1 Hydrogeochemical parameters and isotopic analysis of groundwater samples in Xi’an, China. Cl (mg L1)

SO2 4 (mg L1)

HCO 3 (mg L1)

NO 3 —N (mg L1)

d15 N—NO 3 (‰)

d18 O—NO 3 (‰)

Land use in vicinity

71.0 88.8 56.2 39.5 63.9 21.0

60.4 139 157 65.2 105 49.6

99.3 241 174 83.2 149 72.7

186 320 298 162 242 79.0

520 642 620 512 573 67.0

14.9 44.2 37.8 15.1 28.0 15.2

16.3 16.4 17.6 15.4 16.4 0.9

6.0 6.3 3.8 7.1 5.8 1.4

Residential Residential Residential Residential

119 115 114 188 134 36.3

65.1 45.4 25.7 39.5 43.9 16.4

31.7 87.9 81.4 151 87.9 48.8

56.9 86.1 43.8 190 94.2 66.3

112 184 137 233 166 53.1

454 475 500 679 527 103

6.00 5.41 16.2 42.8 17.6 17.5

12.1 13.0 11.9 11.1 12.0 0.8

nd 6.6 4.3 17.7 9.5 7.1

Residential Residential Residential Agricultural

0.6 2.2 2.3 1.1 1.8 3.4 5.2 2.4 1.5

89.7 129 133 38.6 70.7 133 132 104 37.9

40.7 39.5 22.7 31.6 51.3 126 138 64.3 47.3

10.1 76.0 121 65.8 103 44.9 33.5 65.0 39.2

35.0 89.1 108 27.7 78.8 123 65.7 75.3 35.3

27.2 131 164 73.5 114 195 313 145 92.3

312 506 589 379 543 487 444 466 95.7

0.04 6.00 5.81 18.0 4.58 15.4 5.60 7.91 6.37

nd 11.4 9.9 7.9 9.8 9.7 19.9 11.4 4.3

nd 5.4 5.9 4.9 3.2 4.0 10.5 5.7 2.6

Agricultural Residential Residential Residential Residential Residential Residential

20

2.7

338

59.1

69.3

180

289

685

0.07

nd

nd

Agricultural

7.6 8.4 7.4 7.6 8.1 7.8 0.4

30 10 30 14 24 21 8

40.3 2.5 2.3 1.3 2.3 8.6 15.5

196 108 73.8 89.8 77.2 147 104

90.8 29.5 106 65.1 43.4 65.6 28.6

139 29.1 60.7 104 62.8 77.5 38.4

193 26.3 100 73.0 61.3 105 66.9

354 112 66.9 139 89.3 175 118

716 305 543 620 447 553 156

0.04 2.03 26.9 13.2 8.83 8.51 10.4

nd 12.2 10.1 9.4 8.6 10.1 1.6

nd 8.5 6.4 5.0 0.9 5.2 3.2

Agricultural Agricultural Agricultural Residential Residential

13.7

7.5

35

5.8

125

148

102

149

309

592

0.07

nd

nd

Agricultural

E2 E3 E4 E5 Mean SD

13.4 14.7 14.0 13.7 13.9 0.5

7.3 7.4 7.2 7.5 7.4 0.1

12 10 30 5 20 15

3.5 0.4 3.0 1.5 2.8 2.0

163 16.5 208 16.0 106 86.8

135 52.0 167 68.3 114 50.7

119 82.9 96.3 16.4 83.3 39.6

218 60.0 164 30.0 124 77.2

241 18.9 486 60.3 223 190

740 457 691 210 538 213

0.10 6.11 0.05 7.84 2.83 3.83

nd 13.5 nd 6.4 10.0 5.0

nd 7.1 nd 9.1 8.1 1.4

Agricultural Agricultural Agricultural Agricultural

F1 F2 F3 F4 F5 F6 Mean SD

18.3 26.9 30.1 32.3 22.8 28.3 26.5 5.1

7.5 7.5 7.5 7.4 7.3 7.1 7.4 0.1

63 20 10 5 7 6 21 24

0.5 0.8 0.6 1.3 1.6 0.1 0.8 0.5

16.5 6.7 11.7 10.1 18.9 4.7 11.4 5.5

67.4 62.6 101 43.3 78.9 102 75.9 23.0

21.6 15.8 15.2 11.1 14.0 18.1 16.0 3.6

14.6 5.0 20.0 10.0 40.0 35.0 20.8 14.0

6.2 17.3 16.5 45.3 14.0 69.6 28.1 24.3

327 253 358 148 290 247 270 73.6

2.35 4.71 4.35 2.61 6.52 3.33 4.0 1.6

5.8 6.1 4.8 -0.1 10.4 5.2 5.4 3.4

4.0 5.9 7.6 7.2 11.6 1.4 6.3 3.5

Agricultural Agricultural Agricultural Undevelope Agricultural Agricultural

Well depth (m)

K+ (mg L1)

Na+ (mg L1)

2.5 1.0 1.4 2.4 1.8 0.7

7.6 7.6 7.6 7.8 7.6 0.1

10 8 6 6 8 2

3.1 16.9 16.7 2.2 9.7 8.2

160 204 145 163 168 25.4

B1 B2 B3 B4 Mean SD

4.9 4.2 3.3 4.3 4.2 0.7

8.0 7.7 7.8 7.6 7.8 0.2

12 35 6 33 22 15

7.1 1.8 1.9 1.3 3.0 2.7

C (6–9 km)

C1 C2 C3 C4 C5 C6 C7 Mean SD

7.3 7.9 7.0 6.0 6.7 7.5 8.8 7.3 0.9

8.0 7.9 7.5 7.6 7.7 7.1 7.4 7.6 0.3

55 25 19 33 22 23 9 27 14

D (9– 12 km)

D1

11.8

8.1

D2 D3 D4 D5 D6 Mean SD

10.1 10.1 9.1 10.1 10.4 10.3 0.9

E1

No.

A (0–3 km)

A1 A2 A3 A4 Mean SD

B (3–6 km)

E (12– 15 km)

F (>15 km)

Distance (km)

Ca2+ (mg L1)

M. Xing et al. / Applied Geochemistry 35 (2013) 110–119

Mg2+ (mg L1)

pH

Area

nd = Not detected 113

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M. Xing et al. / Applied Geochemistry 35 (2013) 110–119

 Fig. 2. Variation in the NO 3 concentration with Cl concentration in groundwater.

Chloride is a good indicator of sewage and manure contamination of groundwater because it is biologically and chemically stable and its concentration changes only by mixing with other aquatic  systems. Consequently, the NO concentration is considered 3 =Cl more accurate than any single concentration for the study of N dynamics (Koba et al., 1997, 2009) and sources (Liu et al., 2006).  Variation of NO 3 is plotted versus Cl concentration in Fig. 2. Samples from A, B, C, D and E were more concentrated in manure and sewage input, whereas the F samples were mainly contaminated with fertilizer input or were pristine. Sewage and livestock effluent generally have high Cl and SO2 concentrations (Krapac et al., 4  2002; Karr et al., 2003; Jia et al., 2009) and low NO ratios. 3 =Cl However, some samples from locations D and E were either from road salt or have NHþ 4 as their dominant N ion, as for the D2 and E4 sampling sites, which had extremely high K+ and SO2 4 concentrations, respectively. Therefore, the A, B, C, D and E samples were possibly influenced by anthropogenic activity, while the F samples were likely predominantly controlled by fertilizer use or by background condition (Fig. 2).

3.3. d15N and d18O values of nitrate Nitrate isotopic compositions of precipitation were generally within the range reported in previous studies (Elliott et al., 2007; Kendall et al., 2007), with a mean d15 N—NO 3 value ± standard deviation of +1.4 ± 2.4‰ and mean d18 O—NO 3 value ± standard deviation of +38.5 ± 13.4‰ (Table 2). In addition, the measured isotopic composition of rainwater NO 3 was consistent with nitrification of NH4-fertilizer (Fig. 3). The volume-weighted mean con1 centration (VWMC) of NO and the VWMC of 3 —N was 1.3 mg L 1 NHþ —N was 3.8 mg L (Table 2). Bai and Wang (2008) collected 4 35 precipitation samples in Xi’an in 2007 and 2008, and the 1 NO . They are 3 —N concentration ranged from 0.04 to 7.18 mg L comparable to the recently reported values (NHþ —N and NO 4 3 —N

15 18 Fig. 3. General NO 3 source fingerprints in a diagram of d N and d O, and characteristic NO 3 isotopic composition of groundwater and precipitation studied here.

range from 0.5 to 5.5 mg L1 and from 0.4 to 6.5 mg L1) observed in the Guangzhou region (Jia and Chen, 2010), another rapidly developing and polluted area located in SE China. Guanzhong Plain is the most important grain producing area, and the main application of chemical fertilizer is NH4-fertilizer. However, NH3 volatilization losses from urea and NH4HCO3 applied to farmland varies greatly, in the range of 1–40% (Cai, 1997). From the estimation by Xing and Zhu (2000) NH3 loss from farmland in China was 1.80 Tg N in 1990 and increased to 2.71 Tg N in 1995. In addition, the NH3 loss from animal excreta was estimated to be 3.58 Tg N in 1995. Thus, the higher concentration of N in rainwater may be caused by the agriculture/soil sources in the warm sampling season (April to July; Xing and Liu, 2012). In addition, the concentration of NO 3 and its isotopic composition in rainwater is similar to the composition of the upland recharge areas, suggesting that NO 3 from rain is the major source input into the upland groundwater. This result is consistent with most conclusions of research in this area (Jiang and Han, 2007; Duan et al., 2007). The isotopic composition of groundwater NO 3 can indicate major contaminant sources, because NO 3 from different sources has  18 characteristic d15 N—NO 3 and d O—NO3 values. Fig. 3 shows the typical range of the d15N and d18O values from various natural and anthropogenic NO 3 sources (Heaton, 1986; Kendall and Mcdonnell, 1998; Silva et al., 2002). Nitrate originating from pre18 cipitation and synthetic NO 3 fertilizer has much higher d O values than that from nitrification of reduced N sources, i.e., fertilizer apþ plied as NHþ 4 and urea, soil organic N, and NH4 from sewage and  manure. The NO3 from these types of reduced N sources can also be distinguished by their d15N values, although there is some overlap in terms of d15N abundance (Jia et al., 2009). Potential sources of NO 3 in the study area include chemical fertilizer (urea, NH4HCO3 and N/P/K fertilizer mix) used in the sub-

Table 2 Isotopic signature of N source materials in Xi’an. Sample Soil (n = 4) Inorganic fertilizer (n = 5) Sewage (n = 1) River water (n = 15) Rain water (n = 31)

Composition Total organic Ammonium bicarbonate Nitrate Nitrate Nitrate

VWMC = volume-weighted mean concentration.

1 NHþ 4 —N (mg L ) AVR ± SD

9.98 0.62 ± 0.54 3.8 (VWMC)

1 NO 3 —N (mg L )

d15N (‰)

AVR ± SD

AVR ± SD

d18 O—NO 3 (‰) AVR ± SD

0.28 2.29 ± 0.92 1.3 (VWMC)

3.3 ± 0.6 4.1 ± 1.2 11.5 5.9 ± 2.3 1.4 ± 2.4

38.5 ± 13.4

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urbs, soil organic N, septic-derived effluent, livestock waste, and wet deposition. Nitrogen isotope compositions from various sources are listed in Table 2. The isotopic signatures of these potential sources are similar to published data (Kendall and Mcdonnell, 1998; Liu et al., 2006; Xue et al., 2009). Generally, denitrification mainly occurs only where O2 concentrations are less than 20 lM (Kendall and Mcdonnell, 1998). The entire shallow groundwater system in the Xi’an area is an open, well-oxygenated system, hence denitrification is unlikely to occur, or will be insignificant in the study. The d15N values of NO 3 in the studied groundwater samples gradually decreased from the city center to the outer city. Proceeding from A to F, the d15 N—NO 3 mean value changed from +16.4‰ to +5.4‰ (Table 1). The d18 O—NO 3 values did not show such variation. In a plot of d15N versus d18O (Fig. 3), the 6 sample groups were distributed in the same sequence. The A and B samples were mainly affected by sewage. The NO 3 sources of the C, D and E samples were diffusely distributed, but the dominant materials were still sewage and manure. The groundwater NO 3 sources for F were complicated and included NHþ 4 fertilizer, soil organic N, sewage and manure. Although there were undoubtedly multiple NO 3 sources in all tested areas, the narrower ranges for d15N and d18O in A and B suggest a single dominant NO 3 source for A and B, while the greater scatter in the other areas indicates a mixture of sources. The result agrees with the population distribution of Xi’an. The market, restaurant, hospital, and other municipal facilities are mainly distributed in the city center (A and B areas), where the population density is higher. Large amounts of municipal sewage are likely produced in this area and may discharge into the groundwater via leaking sewers, leading to contamination. From the center stretching to the urban–rural area (e.g. the C, D, E, and F areas), municipal facilities and population density gradually decrease, and agricultural land area gradually increases. Water with fertilizer and soil organic matter input displays lower d15 N—NO 3 values than water affected by municipal wastewater and sewage; the lowest groundwater d15 N—NO 3 values were observed in F. The groundwater d15 N—NO values decrease with sampling distance from the ur3 ban center (Fig. 4) and reflect the urbanization of Xi’an, as NO 3 concentration is significantly correlated with sampling distance (y = 0.70x + 18.16, r = 0.49, p < 0.01). In addition, higher correlation coefficients between the d15 N—NO 3 values and sampling distance from the urban center (y = 0.36x + 14.67, r = 0.75, p < 0.01) indicate that NO 3 isotopic values are a useful tool for tracing changes in NO 3 source into city groundwater. Similar to this study, in Long Island, USA, Kreitler et al. (1978) found that the

mean d15 N—NO 3 value in the eastern part of the island which was largely agricultural, was relative low (+5.3‰), then values increased in the western part of the island (to +16.7‰) as the urban center was approached. However, Kreitler et al. (1978) did not measure the d18 O—NO 3 value of the groundwater, therefore, they could not conclude that the enrichment in the d15 N—NO 3 value was caused by a microorganism denitrification effect or by anthropogenic impact. The d15 N—NO 3 values for the Chanhe river water ranged from +1.6‰ to +9.2‰ (Xing and Liu, 2010), the Bahe river from +4.5‰ to +8.1‰, and the Fenghe river from +2.4‰ to +7.2‰. All river water d15 N—NO 3 values showed an increasing trend from upstream to downstream, in accordance with groundwater values from south to north. The F4 (0.1‰), R1 (+1.6‰) and R10 (+2.4‰) d15 N—NO 3 values were very close to the values for precipitation (+1.4 ± 2.4‰). This result is consistent with the land use pattern in the sampling locations and can be attributed to the neighboring Qinling Mountain area, which experiences less anthropogenic activity. Natural sources (precipitation and soil  NO 3 ) were likely the major NO3 input for groundwater and surface water. With groundwater and river water gradually flowing into the urban region, the proportion of agricultural and other anthropogenic sources of NO 3 in the river and groundwater gradually change (Fig. 1, Tables 1 and 3). When river samples were collected in 2008, an industrial sewage sample with a higher d15 N—NO 3 value (+11.5‰) than the other river samples was found. The sample’s sewage d15 N—NO 3 value was similar to the A and B samples taken from the city center. This result suggests that the NO 3 source of the A and B groundwater is mainly municipal sewage. In conclusion, the stream data which shows d15 N—NO 3 enrichment as the city center is approached, the similar to the groundwater d15N distribution, probably suggests that a large plume of contaminated groundwater and perhaps contaminated surface-water runoff are feeding the river given that these samples were taken during base flow.

3.4. Nitrification and denitrification Nitrification and denitrification are the two important biologically mediated reactions that fractionate N and O isotopes, leading to heavier observed isotopic values (Jia et al., 2009). The fractionation factors for d15N and d18O during denitrification vary with local conditions and reaction rates; the ratio of changes in d15N and d18O is typically close to 1:2 (Böttcher et al., 1990; Aravena and Robertson, 1998; Kendall and Mcdonnell, 1998). For the nitrification process, the discrepancy between the N isotopic compositions

Table 3 Nitrate isotopic values and ion concentrations in the river near Xi’an.

Fig. 4. The relationship between groundwater d15 N—NO 3 concentration and distance from urban center.

No.

NHþ 4 —N (mg L1)

NO 3 —N (mg L1)

d15 N—NO 3 (‰)

Land use in vicinity of river

R1 R2 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R13 R14 R15

0.18 0.18 0.24 0.28 0.22 0.37 1.60 1.62 1.62 0.23 0.73 0.78 0.41 0.47 0.42

2.55 2.15 2.66 3.38 2.49 2.79 4.69 2.62 2.10 1.04 1.09 1.74 1.66 1.80 1.66

1.6 3.6 6.8 9.0 9.2 4.5 6.4 6.9 8.1 2.4 3.6 6.6 7.2 7.2 5.7

Undevelope Agricultural Agricultural Residential Residential Agricultural Residential Residential Residential Undevelope Agricultural Agricultural Agricultural Agricultural Agricultural

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of atmospheric deposition, synthetic fertilizer, soil organic matter, and sewage/manure are subtle, making it difficult to differentiate among NO 3 —N sources based only on N isotope values. The d18 O—NO 3 data provide a further indication of when nitrification occurred (Kendall and Mcdonnell, 1998; Lee et al., 2008). 2= During nitrification, NO 3 generally 3 of the O utilized is from 1= the ambient water and 3 from atmospheric O2 (Böttcher et al., 1990; Kaown et al., 2009; Kendall and Mcdonnell, 1998). If there is no isotope fractionation during O incorporation, the d18 O—NO 3 value of the newly produced NO 3 can be calculated from known d18O values for atmospheric O2 (d18O–O2) and ambient water (d18O–H2O):

d18 O—NO3 ¼ 2=3ð18 O—H2 OÞ þ 1=3ð18 O—O2 Þ 18

ð1Þ

Atmospheric O2 is known to have a d O of +23.5‰ (Kroopnic and Craig, 1972), and ambient water values can be derived from previous studies. In Xi’an, Qin et al. (2005) measured groundwater water d18O values ranging from approximately 12‰ to 3‰. Inserting these values into Eq. (1), the expected d18 O—NO 3 value for groundwater NO derived from nitrification would be approx3 imately 0.2‰ to +5.8‰. This result is close to the observed  d18 O—NO3 values of the A3, B3, C4, C5, C6, D5, D6, F1 and F6 groundwater samples, which varied from +3.2‰ to +5‰, and lower than the other groundwater sample d18 O—NO 3 values. The samples from areas with suitable environmental conditions for nitrification (e.g. microorganisms, lithology and dissolved O2) are mainly distributed east of the city center; these sampling areas contain  NHþ 4 ions that are transformed into NO3 .  18 As shown in Fig. 3, d O—NO3 values for atmospheric and synthetic NO 3 fertilizer sources are higher; these two sources are not accounted for in the dominant groundwater NO 3 origins of Xi’an.  Instead, low d18 O—NO 3 values indicate that groundwater NO3 is derived from nitrification of organic soil N, manure and sewage (Fig. 3). The theoretical calculated range for d18 O—NO 3 values produced by nitrification is plotted in Fig. 3. Samples with higher  18 d18 O—NO 3 values than the theoretical range for NO3 —d O from nitrification were recorded in this study. Recent studies suggest that this theoretical range is most likely an oversimplification for predicting the d18O of NO 3 during nitrification (Mayer et al., 2001; Burns and Kendall, 2002; Pardo et al., 2004). In some cases, d18O is greater than expected based on incubation experiments and measurement of NO 3 from forest soil, suggesting that the contribution of atmospheric O is sometimes greater than 1=3 (Panno et al., 2006b). Based on an evaluation of many studies, Kendall and Mcdonnell (1998) concluded that +15‰ is a more appropriate  upper value for d18 O—NO 3 in microbial NO3 . If this factor was con-

sidered in the analysis, d18 O—NO 3 values produced by nitrification process would have a broader range (Fig. 4). Most of the groundwater d18 O—NO 3 values are most likely contained within this scope. Because mixing of sources alone cannot account for the overall variation in isotopic values, it appears that nitrification plays an important role. Microbial denitrification is another process that can significantly alter isotopic composition, which occurs when O is limited and organic C is available (Knowles, 1982):

4NO3 þ 5CH2 O þ 4Hþ ! 2N2 þ 5CO2 þ 7H2 O

ð2Þ

In addition, under some aquifer conditions, where organic matter is absent or unreactive, Fe sulfide (pyrite) can act as an electron donor for denitrification (Korom, 1992; Korom et al., 2005; Rivett et al., 2008; Baker et al., 2012): 2þ 5FeS2 þ 14NO3 þ 4Hþ ! 7N2 þ 10SO2 þ 2H2 O 4 þ 5Fe

ð3Þ

Kinetic isotope effects during microbial denitrification are responsible for preferentially converting the lighter isotopes 14N and 16O to N2 and N2O, enriching the heavy isotopes in the remaining NO 3 (Mayer et al., 2002; Fukada et al., 2003; Xue et al., 2009). Thus, the d15N and d18O values of the remaining NO 3 increase as the NO 3 concentration decreases. In this study, no obvious positive or negative correlations be 15 tween the NO and 3 —N concentrations and the d N—NO3 d18 O—NO values was found in the collected groundwater samples, 3 which suggests that denitrification was not the dominant mechanism controlling N and O isotope composition in Xi’an (Fig. 5a and b). This conclusion is in accordance with the sampling site conditions, which is an open, well-oxygenated system. In addition, the value of the d18 O—NO 3 data is that there is no concurrent increase in the city center as d15 N—NO 3 values increase. This demonstrates that the increase is not the result of denitrification, and source changes may be the main reason for the NO 3 isotopic values variation. 3.5. Relationship between population density and d15N and d18O values of nitrate According to the administrative plan, Xi’an is divided into 9 districts: Xincheng, Beilin, Lianhu, Baqiao, Weiyang, Yanta, Yanliang, Lintong and Chang’an. Groundwater samples in this study came from seven of the nine regions (Fig. 1, Table 4). Overall, the Lianhu district had a higher d15 N—NO 3 value (mean value = +14.9‰), and the Chang’an district had a lower d15 N—NO value (mean 3 value = +6.5‰), but there were no obvious differences in these

   18 Fig. 5. Cross-plot of (a) d15 N—NO 3 versus NO3 —N concentrations in groundwater samples and (b) d O—NO3 versus NO3 —N concentrations in groundwater samples.

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M. Xing et al. / Applied Geochemistry 35 (2013) 110–119 Table 4 Statistics on land area, population density, land for cultivation, crop yield and isotope levels of groundwater samples from Xi’an. Name of district

District area (km2)

Density of population (person/km2)

Land for cultivation (km2)

Crop yield (kg)

Mean d15 N—NO 3 (‰)

Mean d18 O—NO 3 (‰)

Groundwater sample number

No data No data No data 16.9 41.5

No data No data No data 0.2  107 2.9  107

13.5 13.0 14.9 11.2 10.6

6.0 4.0 6.2 7.5 5.7

118.2 465.0

7.2  107 41.6  107

8.2 6.5

6.4 6.4

A4, B2, B3 A3, C4, C5, C6, C7 A1, A2, B1 D3, D4 B4, C1, C2, C3, D1, D2, E1, E2, E4 D5, D6, E5 E3, F1, F2, F3, F4, F5, F6

Beilin Xincheng Lianhu Yanta Weiyang

24 30 43 149 262

34,479 21,477 17,340 7535 2340

Baqiao Chang’an

325 1590

1700 642

Fig. 6. The logarithmic relationship between groundwater d15 N—NO and 3 d18 O—NO 3 and human population density in Xi’an.

new d18 O—NO 3 values from each district. Population density, land for cultivation and crop yield data were also collected for Xi’an for 2009 from Xi’an Statistics (http://www.xatj.gov.cn/) (Table 4). A significant logarithmic correlation between d15 N—NO 3 values and population density (Fig. 6, R2 = 0.88, P < 0.01) was found but there was no correlation between d18 O—NO 3 values and population density (Fig. 6). This result demonstrates that anthropogenic activities have an important effect on groundwater d15 N—NO 3 values due to greater sewage input into the groundwater, whereas nitrification is most likely the predominant factor controlling d18 O—NO 3 levels in Xi’an. In addition, the relationship between d15 N—NO 3 values and land for cultivation and crop yield also support the conclusions. This finding is similar to the results of Cabana and Rasmussen (1996), who showed that the d15N signature of primary consumers was closely correlated with the human population density in their research watershed. As shown in Fig. 6, the d15 N—NO 3 values increased more gently after the population density exceeded 20,000 person/km2, which may indicate that a d15 N—NO 3 of 13.4‰ (calculated according to y = 1.88Ln(x)  5.21) was the typical value of anthropogenic activity sources. 4. Conclusions The hydrogeochemistry of major dissolved constituents and   15 18 stable isotopes of NO 3 (d N—NO3 , d O—NO3 ) were determined to identify contamination sources in a shallow groundwater system in Xi’an. The d15N and d18O values of NO 3 showed that the A and B zone samples, closest to the city center, were mostly distributed in the typical range for sewage indicating a strong dominance of sewage N inputs into urban groundwater. This accords with studies of NO 3 in other urban locations (e.g. Fukada et al., 2004;

Wakida and Lerner, 2005) and other solutes (Bottrell et al., 2008; Hosono et al., 2010). However, the dominant sewage input is localized in the city center (possibly due to a reliance on older infrastructure here, e.g. Bottrell et al., 2008) and more complex mixtures of sources are required to explain NO 3 isotopic compositions at locations further from the city, including NHþ 4 fertilizer, soil organic N, sewage and manure, though in the furthest zone analyzed (F) pollutant NO 3 loadings are low. By combining rainwater and river water NO 3 isotope compositions, this study found that the rainwater recharging the upland headwater and river water laterally feeding the groundwater along the stream were the main sources of groundwater in this region. The d18 O—NO 3 values from groundwater in Xi’an indicated that nitrification of organic soil N, manure and sewage was significant, but denitrification in this area was not significant. In other words, it is inferred that sewage or  manure NO 3 is a more likely source of isotopically enriched NO3 contaminant in the study area. The higher correlation coefficients between d15 N—NO 3 values and sampling distance (r = 0.57, p < 0.01), rather than between isotope values, and the concentration data (r = 0.49, p < 0.01) may indicate that NO 3 isotopes are a useful tool for tracing NO 3 source changes in groundwater. It was found that anthropogenic activity has already had an important effect on groundwater d15 N—NO 3 values. Groundwater contamination by NO 3 is strongly correlated with distance from the city center and population density in Xi’an. With increasing urbanization, widespread NO 3 contamination in Xi’an will continue unless land use management practices are dramatically changed. The findings suggest that attempts should be made to eliminate or minimize anthropogenic NO 3 input into the recharge area. Because of the apparent lack of bacterial denitrification, the only feasible form of NO 3 removal from the aquifer will be continued flushing by recharge, provided that NO 3 sources are eliminated. Isotopic analysis seems particularly well suited to forensic investigations of NO 3 sources in urban environments due to the distinctive isotopic signatures of urban NO 3 sources, namely, sewage and fertilizer. Acknowledgments This research was supported by the National Natural Sciences Foundation of China (No. 40673012), the Key Research Program of the Chinese Academy of Sciences (Grant KZZD-EW-04), the Knowledge Innovation Program of the Chinese Academy of Sciences (No. KZCX2-YW-149). We thank the editor, Simon Bottrell, and two anonymous reviewers for comments which improved the quality of the manuscript. References Aravena, R., Robertson, W.D., 1998. Use of multiple isotope tracers to evaluate denitrification in ground water: study of nitrate from a large-flux septic system plume. Ground Water 36, 975–982.

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