Sources and biogeochemical behavior of nitrate and sulfate in an alluvial aquifer: Hydrochemical and stable isotope approaches

Applied Geochemistry 26 (2011) 1249–1260

Contents lists available at ScienceDirect

Applied Geochemistry journal homepage: www.elsevier.com/locate/apgeochem

Sources and biogeochemical behavior of nitrate and sulfate in an alluvial aquifer: Hydrochemical and stable isotope approaches Byoung-Young Choi a, Seong-Taek Yun a,b,⇑, Bernhard Mayer b, Kyoung-Ho Kim a a b

Department of Earth and Environmental Sciences and the Environmental Geosphere Research Lab (EGRL), Korea University, Seoul 136-701, South Korea Department of Geoscience, University of Calgary, Alberta, Canada T2N 1N4

a r t i c l e

i n f o

Article history: Received 15 August 2008 Accepted 14 April 2011 Available online 21 April 2011 Editorial handling by W.B. Lyons

a b s t r a c t 2 34 Based on hydrochemical and environmental isotope data (d15N and d18O of NO 3 , and d S of SO4 ) of depth-specific groundwater samples from multi-level samplers, the source(s) and biogeochemical behav2 ior of NO 3 and SO4 in a shallow (<25 m below ground level) sandy alluvial aquifer underneath a riverside agricultural area in South Korea were evaluated. The groundwater in the study area was characterized by 2 a large variability in the concentrations of NO 3 (0.02 to 35 mg/L NO3 AN) and SO4 (0.14 to 130 mg/L). A distinct vertical redox zoning was observed sub-dividing an oxic groundwater at shallow depths (<8– 10 m below ground surface) from sub-oxic groundwater at greater depths. The d15N and d18O values indi cated that elevated NO 3 concentrations in the oxic groundwater are due to manure-derived NO3 and nitrification of urea- and ammonia-containing fertilizers used on agricultural fields. Chemical and isotopic data also revealed that groundwater NO 3 concentrations significantly decrease due to denitrification in the lower oxic and sub-oxic groundwater. The d34Ssulfate values of the oxic groundwater ranged from concentrations with depth 14.4‰ to +2.4‰. The relationship between d34Ssulfate values and SO2 4 showed that increasing SO2 4 concentrations were caused by S-bearing fertilizers, not pyrite oxidation. 2 Bacterial (dissimilatory) SO4 reduction occurred locally in the sub-oxic groundwater, as indicated by increasing d34Ssulfate values (up to 64.1‰) with concomitant decreases of SO2 4 concentrations. This study shows that isotope data are very effective for discriminating different sources for the waters with high  SO2 4 and low NO3 concentrations in the lower oxic zone. It is also suggested that the use of N- and S-concontamination of shallow taining fertilizers should be better controlled to limit nitrate and SO2 4 groundwater. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Alluvial groundwater has been widely used in many countries for irrigation and drinking water purposes because it is highly productive and extractable (Doussan et al., 1997). Throughout the last few decades, alluvial groundwater worldwide has been progressively degraded in quality, largely due to the infiltration of agricultural contaminants derived from synthetic fertilizers and manure. High concentrations of NO 3 in alluvial groundwater, caused by the overuse of organic and inorganic fertilizers, has led to adverse effects on human health where NO 3 in drinking water exceeds recommended limits (mostly, 10 mg/L NO3 AN; US EPA, 1995) (Jacobs and Gilliam, 1985; Rajagopal and Tobin, 1989; Cooper, 1993; Chapelle, 2001; Powlson et al., 2008). Sulfate forms another major component of fertilizers (Vitòria et al., 2004; Otero et al., 2005) and

⇑ Corresponding author at: Department of Earth and Environmental Sciences and the Environmental Geosphere Research Lab (EGRL), Korea University, Seoul 136701, South Korea. Tel.: +82 2 3290 3176; fax: +82 2 3290 3189. E-mail address: [email protected] (S.-T. Yun). 0883-2927/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.apgeochem.2011.04.015

may affect the quality of drinking water in terms of hardness and taste (Trettin et al., 2002). 2 It is well known that NO 3 and SO4 in aquatic systems serve as important electron acceptors in anoxic environments. Hence, their occurrence in groundwater is often controlled by bacterially-mediated redox reactions (e.g. denitrification and bacterial SO2 4 reduction), as these reactions change the redox environment in an 2 aquifer and govern the natural attenuation of NO 3 and SO4 . Com pared to NO3 -free (uncontaminated) groundwater, NO 3 -rich groundwater can accelerate the oxidation of pyrite in aquifers and thus can produce elevated SO2 concentrations and acidity. 4 This process is known as denitrification using pyrite (as an electron donor) or pyrite oxidation by NO 3 (Postma et al., 1991; Engesgaard and Kipp, 1992; Moncaster et al., 2000; Pauwels et al., 2000; Schwientek et al., 2008). An improved understanding of the source(s) 2 and biogeochemical cycles of NO 3 and SO4 in aquifers forms an important basis for the management of groundwater quality as well as for the evaluation of anthropogenic impacts on groundwater quality. 2 The N, S and O isotope ratios of NO 3 and SO4 have been successfully used to trace the sources and transformations of N and

1250

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

S compounds in various ecosystems (Böttcher et al., 1990; Wassenaar, 1995; Robinson and Bottrell, 1997; Aravena and Robertson, 1998; Cey et al., 1999; Moncaster et al., 2000; Mayer et al., 2002; Massmann et al., 2003; Campbell et al., 2006; Taylor et al., 2006; Brenot et al., 2007; Balci et al., 2007; Spoelstra et al., 2007; Kaown 2 et al., 2009). In this study, the sources and the fate of NO 3 and SO4 in a riverside alluvial aquifer underneath an agricultural area in Korea are examined using chemical and environmental isotope techniques. To obtain vertical profiles of hydrochemical and isotopic compositions in relation to aquifer geology and depth, multi-level samplers (MLSs) were installed in the study area. The MLSs have the potential to yield better insights into the depth-depen2 dent behavior of NO 3 and SO4 by minimizing the mixing of waters of different compositions from various depths (Taylor et al., 2006; Meredith et al., 2007). The objectives of this study were (1) to elucidate the sources 2 and behavior of NO 3 and SO4 in a shallow (<25 m below ground level (mbgl)) riverside alluvial aquifer using hydrochemical and environmental isotopic data for groundwater obtained from MLSs and (2) to evaluate the change of groundwater quality induced by the inflow of severely NO 3 -contaminated water from an agricultural area resulting from overuse of synthetic fertilizers and manure. It was anticipated that the results of this work will provide useful information for the management of alluvial aquifers (and associated surface water systems) underneath agricultural fields that are common in South Korea and elsewhere.

2. Materials and methods 2.1. Study site The study area is located in the Keum River Watershed of South Korea (Fig. 1). According to KMA (2002), the climate of the study area is temperate with four distinct seasons with monthly mean temperatures ranging from 25 °C in August to 2 °C in January. During the last decade, average annual temperature and precipitation were 12.8 °C and 1300 mm, respectively. About 70% of the annual precipitation occurs in the monsoonal period from June to September. The study area is located on a wide flood plain alongside the strongly meandering Keum River (Fig. 1), underlain by up to 25 m of alluvium. The alluvium deposits were formed by channel relocation and point-bar accretion due to the meandering river (Park et al., 2007). The thickness of the alluvium in the study area generally increases with distance away from the river. The soils in the study area are intensively cultivated with crops including vegetables, fruit and rice, dependent on texture of the soils (Chae et al., 2004). Sandy soils (medium to coarse-grained) without silt occur as a band following the river’s course and are intensively cultivated for vegetables, while silty soils (corresponding to flood plain sediments) cover most of the study area and are typically used for rice paddy fields (Fig. 1). Large quantities of commercial fertilizers, including urea (CO(NH2)2) and N–P–K fertilizers with minor pro-

Fig. 1. Topographic map of the study area in the Keum River watershed, South Korea. Locations of the multi-level samplers (B1, B2, and B3) are also shown.

1251

2.4 1.8 6.1 10.6 43.2 29.0 54.4 0.1 0.2 13.4

0.2 2.5 5.6 ND 30.0 58.2 18.7 13.5

71.8 132.9 12.1 14.6

SO4 (mg/L)

May 2004

d34S (‰)

portions of lime, ammonium sulfate, K and Mg sulfate and KCl, and manure are extensively applied on agricultural fields year-round and especially during the growing season (spring to fall). This results in significant contamination of alluvial groundwater by NO 3 (Chae et al., 2009). Shallow wells (mostly, <8–10 mbgl) are abundant in the study area extracting shallow groundwater for irrigation from sandy aquifers.

13.4 1.4 52.1 64.1

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

15.0 n.a. 6.8 3.4 0.1 0.1 <0.1 4.3 6.3 11.3 14.3 17.8 Sub-oxic zone

B3-1 B3-2 B3-5 B3-7 B3-9 B3 Oxic zone

4.0 32.7 17.9 0.6 0.4 5.5 7.5 11 14.1 17.7 B2-2 B2-3 B2-5 B2-6 B2-8 Sub-oxic zone

Sub-oxic zone

B2 Oxic zone

28.2 18.6 0.2 0.2 6 8 10 12 B1-2 B1-4 B1-5 B1-6 B1 Oxic zone

Sampling (well) depth reported as meters below ground level (mbgl). n.a. = failed to obtain reliable data due to negligible concentrations of extracted compounds.

17.5 45.7 5.1 6.5 1.7 3.4 n.a. n.a. n.a. n.a. 6.4 15.8 n.a. n.a. n.a. 12.8 1.2 <0.1 <0.1 <0.1 8.5 n.a.

2.0 6.0 7.3 11.1

d N (‰)

NO3–N (mg/L)

0.2

0.4 0.2 0.5 4.8 ND 35.6 17.8 0.1 0.2

n.a. n.a.

7.7 11.5 n.a. n.a.

3.1 6.4 n.a. n.a.

n.a. n.a.

18.5 46.5 55.3 23.6 9.8

65.4

14.4

59.2 104.9 12.4 11.4 5.7 8.1

d N (‰) NO3–N (mg/L) d O (‰)

27.5 11.5 0.2 0.2

d O (‰)

SO4 (mg/L)

34

July 2003

18 15

May 2004

18 15

July 2003 Depth (mbgl) Sample no.

Groundwater level measurements in 33 irrigation wells in the study area indicated the general flow of alluvial groundwater toward the Keum River (Chae et al., 2004). Choi et al. (2010) studied the characteristics of recharge and subsurface flow of groundwater in the study area using the stable isotopic composition of water (d2H and d18O) and 3H data, as shown in a schematic diagram in Fig. 2. These authors found that the alluvial aquifer is sub-divided into two distinct groundwater compartments with different redox conditions: oxic groundwater in the upper part of the aquifer (above depths of 8–12 mbgl) and sub-oxic groundwater at lower levels. The boundaries between the oxic groundwater and the suboxic groundwater are shown in Figs. 3–5 as dashed lines and were in good agreement with the depths of a notable geologic change from upper permeable sand-rich layers to a lower silty layer with a mud cap. The deeper sub-oxic groundwater had 1.6‰ higher d18O and 9.0‰ higher d2H values than oxic groundwater and thus this water has a different origin or history (Hendry and Wassenaar, 2000; Hendry et al., 2000; Bartlett et al., 2010). Hence, Choi et al. (2010) suggested that shallow groundwater in the oxic zone predominantly recharges via direct infiltration of rain and irrigation water through permeable sandy soils preferentially on vegetable fields, while deeper sub-oxic groundwater mainly recharges

Borehole and zone

2.3. Hydrogeologic setting

Table 1 Summary of the environmental isotope data of dissolved nitrate and sulfate in alluvial groundwater in the study area.

For this study, 3 sets of multi-level samplers (MLSs) were installed at sites B1, B2 and B3 (Fig. 1) during September and October of 2002. The sites were chosen based on examination of the geology and previous chemical data for groundwater obtained from existing wells (Chae et al., 2004). Wells B1 and B3 were located adjacent to or within rice paddy fields on silty soils, whereas well B2 was completed within vegetable fields on sandy soils (Fig. 1). To ensure accessibility and safety during drilling, borehole B3 was installed at a location covered by a thin layer of fine sandy soil near a greenhouse and a narrow non-paved road across flooded rice paddies. Borehole B1 was constructed at the flank of a rice paddy. Therefore, a thin (0.8 m) cover of fine sand was locally present above the typical silty sediments of flooded paddies. Core drilling was conducted to depths of 15–19 m below ground surface reaching the boundary between alluvium and weathered bedrock. Detailed geologic logging of the extracted cores of alluvial sediments was performed (Choi et al., 2010). The installed MLSs were constructed with PVC pipes (2.5 cm diameter) and polyethylene (PE) tubes (0.5 cm diameter) with variable lengths with the PE tubes tightly attached to the PVC pipes. Tips of the tubes and slotted (15 cm long) pipes were wrapped with a stainless steel screen to allow groundwater inflow. Seven, 8 and 10 depth-specific samplers were installed in B1, B2 and B3, respectively. The depths of the respective samplers within each MLS are summarized in Table 1. Immediately after well installation, the samplers were purged with groundwater using a peristaltic pump (Cole-Palmer Inc.). The purge rates varied with sampling site (e.g., only about 20– 30 mL/min in B1 located in a dominantly silty alluvium, while about 300–500 mL/min in B2 completed in a sand-rich alluvium).

d S (‰)

2.2. Installation of multi-level samplers

1252

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

Fig. 2. A conceptual model showing the recharge and flow patterns of alluvial groundwater in the study area (modified after Choi et al., 2010). The vertical scale is exaggerated. The locations of three multi-level samplers (B1, B2, and B3) are also shown.

through less permeable silty soils on rice paddy fields after moderate (20–30%) evaporation (Fig. 2). Groundwater levels measured in the 3 MLSs varied more or less sensitively with precipitation events and also revealed the existence of distinct flow patterns (Choi et al., 2010). Negligible differences of hydraulic heads with depth in shallow oxic groundwater at B2 and B3 indicated a predominantly unconfined (and thus vertically highly connected) aquifer around the two sites. In contrast, there was a large head difference within borehole B1: the equipotential surface measured at 3 mbgl within a fine sandy silt layer was 2 m higher than that at 7 mbgl just below a 1.5 m thick, impermeable silty layer, suggesting that the aquifer around B1 has two different water masses with low connectivity. 2.4. Sample collection and analysis A total of 45 samples of alluvial groundwater were collected for this study from the 3 MLSs with a peristaltic pump during two sampling campaigns in July 2003 and May 2004. The sampling times were chosen to investigate the effect of fertilizer application occurring in early spring on groundwater quality. Prior to sampling, groundwater was purged until temperature and electrical conductivity (EC) stabilized. Temperature, pH, Eh, EC and dissolved O2 (DO) of water samples were measured in the field using electrodes using a specially designed flow chamber to minimize contact with air. To obtain accurate data, calibration of electrodes was carried out in the field before field measurements. The pH electrode was calibrated using buffer solutions with pH 4, 7 and 10. The Eh, EC, and DO electrodes were calibrated using a 420 mV standard solution (Orion 967901), 1414 lS/cm standard solution (Orion 011007), and air-saturated water, respectively. Alkalinity (as HCO 3 ) was determined in the field by volumetric titration using 0.05 N HNO3. The collected groundwater samples were immediately passed through a 0.45 lm membrane filter and were transferred to pre-acid (dilute HCl) washed 60 mL polyethylene bottles. Bottles for cation analysis were acidified by adding a few drops of concentrated HNO3. All samples were preserved at 4 °C in a cooler or refrigerator until analysis. Chemical analysis was performed at the Center for Mineral Resources Research (CMR) at Korea University. Major cations (Na+, K+, Ca2+ and Mg2+) and concentrations of total dissolved Fe and Mn were measured by ICP-AES (Perkin-Elmer Optima 3000XL) and an-

2 ions (Cl, NO 3 and SO4 ) by ion chromatograph (Dionex 120). Careful quality control on hydrochemistry data was conducted by routinely measuring blanks, duplicates, and standards. The charge balances were generally below ±5%. For a total of 16 groundwater samples, N and O isotope analyses on NO 3 were performed. Four samples (4–8 L) collected in July 2003 were acidified using HCl in the field and were immediately sent to the Environmental Isotope Laboratory of University of Waterloo for N and O isotope analyses. The sample preparation and analysis of N isotope ratios was conducted according to Silva et al. (2000). For O isotope analysis of NO 3 , nitrate oxygen was converted to CO2 by combustion of pure AgNO3 using excess graphite. Twelve samples collected in May 2004 were passed through prefilled poly-prepÒ columns with 2 mL AG 50W-X8 cation exchange resin (Bio-Rad Inc.) and subsequently through a pre-filled polyprepÒ column with 2 mL AG 1-X8 anion exchange resin (Silva et al., 2000; Mayer et al., 2001). Nitrate collected on anion exchange resins was eluted, converted to AgNO3 and subsequently N and O isotope ratios were determined at the Isotope Science Laboratory of the University of Calgary using continuous flow isotope ratio mass spectrometry (IRMS) (Silva et al., 2000). Nitrogen isotope ratios were determined using N2 generated in an elemental analyzer coupled to an IRMS and O isotope ratios were determined using CO generated in a TC/EA glassy carbon reactor at 1350 °C followed by IRMS. A total of 19 groundwater samples were collected for S isotope analysis on SO2 4 . The samples (about 2–4 L) were acidified (pH 2– 3) and then SO2 4 was precipitated as BaSO4 by addition of BaCl2. Seven samples collected in July 2003 were analyzed for S isotope compositions at the Korea Basic Science Institute, following the procedures described in Yanagisawa and Sakai (1983). Twelve samples collected in May 2004 were analyzed at the Isotope Science Laboratory of the University of Calgary, using elemental analyzer continuous flow isotope ratio mass spectrometry with SO2 as the measuring gas (e.g. Giesemann et al., 1994; Shanley et al., 2005). Stable isotope data of samples are reported in the d notation (‰) relative to international standards:

dsample ð‰Þ ¼ ½ðRsample =Rstandard Þ  1  1000 where R is the abundance ratio (i.e., 15N/14N, 18O/16O and 34S/32S). The d15N values are reported with respect to air N2 (AIR), d18O val-

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

1253

Fig. 3. Vertical profiles of measured physicochemical parameters observed in a multi-level sampler at site B1 near a rice paddy field. The dotted horizontal lines denote the depth of a marked hydrochemical change.

ues relative to Vienna Standard Mean Ocean Water (V-SMOW), and d34S values relative to the Canyon Diablo Troilite (V-CDT). Normalization of d15N and d18O values of NO 3 was accomplished using internal laboratory standards that were calibrated against the international reference materials IAEA N-1 and IAEA N-2 with assigned d15N values of 0.4‰ and +20.3‰, respectively, and IAEA NO3, USGS 34 and USGS 35 with assigned d18O values of +25.6‰, 27.9‰ and +57.5‰, respectively. Normalization of d34S measurements was achieved using internal laboratory standards that were calibrated against the international reference materials IAEA S-1 and IAEA S2 with assigned d34S values of 0.3‰ and +22.7‰, respectively. Precision and accuracy (as 1 sigma) of the reported values are ±0.2, ±0.2 and ±0.3‰ for N, O and S isotope values, respectively.

3. Results and discussion 3.1. Hydrochemical data and vertical zoning of groundwater The hydrochemical data for groundwater obtained from the MLSs are summarized in Figs. 3–5. In all three sets of MLSs, pH and Eh values and DO, alkalinity, total Fe, NO3 AN, and SO2 4 concentrations show an abrupt change at the boundaries between the shallow oxic groundwater and the deeper sub-oxic groundwater (horizontal dashed lines in Figs. 3–5). It is also noteworthy that all groundwater samples in the study area displayed a large variation in the concentrations of NO 3 (0.02 to 35 mg/L NO3 AN) and SO2 4 (0.14 to 130 mg/L).

1254

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

Fig. 4. Vertical profiles of measured physicochemical parameters observed in multi-level sampler at site B2 in a vegetable field adjacent to the Keum River. The dotted horizontal lines denote the depth of a marked hydrochemical change.

3.1.1. Shallow oxic groundwater At site B1 located adjacent to a rice paddy on silty soils, the oxic groundwater between 3 and 8 mbgl had higher Eh values and 2 higher concentrations of NO than deeper sub-oxic 3 and SO4 groundwater below 10 mbgl (Fig. 3). The elevated NO3 AN concentrations (up to 30 mg/L) in the upper part of the aquifer indicate that the oxic groundwater near site B1 is significantly contaminated by NO 3 potentially due to the overuse of synthetic fertilizers and/or manure on agricultural fields (Böhlke, 2002; Min et al., 2003; Chae et al., 2004). In the oxic groundwater, NO 3 concentrations decreased with depth, while SO2 concentrations increased 4 with depth with a peak SO2 concentration of 130 mg/L at a 4 depth of 8 mbgl (corresponding to sampling port B1-4). The profiles of SO2 4 concentrations in the oxic groundwater were similar to those of Cl concentrations at sites B1, B2, and B3 (Figs. 3–5).

It is also noteworthy that a significant decrease of NO 3 concentrations occurred in the deeper part of the oxic zone where dissolved O2 (DO) was less than 2 mg/L (Figs. 3–5), a threshold previously reported for denitrification in other studies (e.g., Mariotti et al., 1988; Gillham, 1991; Cey et al., 1999). In agricultural areas, high concentrations of, SO2 and Cl in 4 groundwater can result from the excessive application of fertilizers (Wassenaar, 1995; Böhlke, 2002; Chae et al., 2009; Kaown et al., 2009). Thus, the similar profiles of SO2 and Cl concentrations 4 in the oxic groundwater suggest that those ions originate from fertilizers and the distribution of concentrations with depth possibly reflects seasonal water quality variations in relation to groundwater recharge (Pauwels et al., 2001; Deutsch et al., 2005; Jones and Smart, 2005; Mantovi et al., 2006). For example, the low concentrations of SO2 and Cl in the upper part of the oxic groundwater 4

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

1255

Fig. 5. Vertical profiles of measured physicochemical parameters observed in a multi-level sampler at site B3 in a rice paddy field. The dotted horizontal lines denote the depth of a marked hydrochemical change.

may be due to infiltration of rain or irrigation water a considerable time after fertilizer application, whereas the higher concentrations  of SO2 4 and Cl in the lower part of the oxic groundwater may represent recharge that occurred during peak fertilizer application in February to early March. The decrease of NO 3 concentrations with depth in the lower part of the oxic groundwater suggests that NO 3 was removed by biogeochemical reactions such as denitrification. This process can occur coupled with pyrite oxidation, which can increase SO2 4 concentrations further in groundwater. This hypothesis will be discussed later in more detail using isotope data. At site B2 located in a sandy vegetable field near the Keum River, the marked change from the upper oxic groundwater to the lower sub-oxic groundwater occurred at depths of 11.0–14.1 mbgl (Fig. 4). However, the profile of NO 3 concentrations in the oxic groundwater was different compared to those at the MLSs B1

and B3 (Figs 3 and 5). Nitrate concentrations at a depth of 3 mbgl (B2-1) and 5.5 mbgl (B2-2) were comparatively low (<10 mg/L NO3 AN). The low concentrations of NO 3 between 3 and 5.5 mbgl may indicate rapid recharge of low- NO 3 water in the months after intense fertilizer application enhanced by the high permeability of the sandy soil at this site. However, NO 3 concentrations in the oxic groundwater at site B2 increased up to 35 mg/L NO3 AN at a depth of 7.5 mbgl (B2-3) and then decreased to values below 20 mg/L NO3 AN at a depth of 11 mbgl (B2-5). At site B3 located within the area dominated by paddy fields, the marked change from oxic groundwater to the sub-oxic groundwater occurred at depths of 7.8–9.8 mbgl (Fig. 5). In the oxic groundwater, NO 3 concentration also decreased markedly with depth while SO2 and Cl concentrations increased. In addition, 4  NO3 at the bottom of the oxic groundwater zone had the lowest

1256

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

concentrations among all water samples obtained from the MLSs, which is possibly the result of a higher rate of denitrification in the silt-rich paddy fields. 3.1.2. Deeper sub-oxic groundwater Sub-oxic groundwater occurred under shallow oxic groundwater in all MLSs below an impermeable layer (i.e., silty sand with a mud cap containing organic debris) in 8–12 mbgl and was char2 acterized by lower Eh and DO values and lower NO 3 and SO4 concentrations than oxic groundwater, while pH values, alkalinity, and dissolved Fe concentrations were higher (Figs 3–5). According to Choi et al. (2010), deeper sub-oxic groundwater largely constitues a groundwater mass which had mainly recharged through nearby rice paddies with low permeability soils. It is, therefore, feasible that the recharging water experiences significant denitrification, Fe reduction and bacterial (dissimilatory) SO4 reduction during the slow recharge through flooded rice paddies. This suggests that 2 sub-oxic groundwater having low NO concentrations 3 and SO4 has a different biogeochemical history due to its different recharge area in rice paddies. In geochemically reducing environments, dissolved Fe concentrations can increase in groundwater as a result of Fe reduction. In the study area, the concentrations of dissolved Fe are higher in groundwater at B1 and B3 penetrating a silt-rich soil than at B2 under a sand soil (Figs. 3–5). Alkalinity of groundwater also showed a similar trend. It is also noteworthy that at B3 the change of dissolved Fe is similar to that of Cl, a conservative element that is insensitive to redox reaction. It is hypothesized that sub-oxic groundwater in the silty alluvium has a longer residence time after the infiltration because of the lower permeability of this flow unit. Previous research has also shown that groundwater can evolve distinctly along different paths because of the presence of low permeability sequences that prevent the movement of water across low permeability geological units (Hendry and Wassenaar, 2000; Hendry et al., 2000; Bartlett et al., 2010).

3.2. Sources of nitrate and its biogeochemical fate in the aquifer The d15N and d18O values of NO 3 are summarized in Table 1. For the shallow oxic groundwater, d15N and d18O values of NO 3 range from 6.4‰ to 15.8‰ and from 2.0‰ to 8.5‰, respectively. Plots of the dual isotopic composition fall within or near the general ranges  of NO 3 formed via nitrification of soil organic-N and NO3 from manure or sewage (Fig. 6). As shown in Fig. 6, most synthetic N fertilizers have d15N values in the range of 3.9–4‰ (Wassenaar, 1995; Iqbal et al., 1997; Cey et al., 1999; Bateman and Kelly, 2007). In contrast, NO 3 derived from manure or sewage usually has d15N values ranging from 10‰ to 20‰ and from 7‰ to 15‰, respectively (Heaton, 1986; Mariotti et al., 1988; Wassenaar, 1995; Fogg et al., 1998; Bateman and Kelly, 2007) and is typically characterized by d18O values below 15‰ as a result of nitrification (Wassenaar, 1995; Mayer et al., 2002). Soil organic-N has d15N values generally ranging from 3‰ to 8‰ with agricultural soils often falling near the high end of this range (Mayer et al., 2002). Choi et al. (2007) reported the d15N values for manure (e.g. from pigs, cattle, etc.; see also Fogg et al. (1998) for general chemical composition of animal manure), synthetic N fertilizers (urea and ammonium sulfate), and compound fertilizers (NPK 21-17-17) used in rural areas of Korea. The d15N values of total N reported by Choi et al. (2007) ranged from 6.8‰ to 9.2‰ for manure, from 3.9‰ to 0.5‰ for synthetic N fertilizers, and from 1.9‰ to 0.5‰ for compound fertilizers, respectively. In the study area, sewage does not represent a major NO 3 source since waste water effluents from nearby small villages are treated in a sewage treatment facility. In addition, nitrification of soil organic-N would be unable to generate the very high NO 3 concentrations observed in parts of the aquifer. Therefore, the elevated d15N values and comparatively low d18O values of NO 3 in oxic groundwater appear to reflect the dominance of manure-derived NO 3 . In this area, animal manure is applied on agricultural fields in addition to fertilizers. Considering the high use of urea- and ammonia-containing fertilizers in the

Fig. 6. Plot of N versus O isotope ratios of NO3 in the shallow oxic groundwater, showing the general d15N and d18O ranges for diverse sources of NO3 (after Kendall, 1998; see also text), The enlarged diagram (top right) shows a trend of increasing d15N and d18O values as well as the depth (in meters below ground level) and sampling date for each sample. The number in the rectangles denotes the NO3–N concentrations (mg/L).

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

study area, it is also feasible that the major sources of groundwater  NO 3 are a mixture of manure-derived NO3 and urea- and ammonia-containing fertilizers. Careful examination shows that the data in oxic groundwater can be divided into two groups, as shown in the inset in Fig. 6. Group 1 samples have lower d15Nnitrate and d18Onitrate values (6.4– 8.1‰ and 2.0–5.7‰, respectively) than group 2 samples (11.1– 15.8‰ and 6.0–8.5‰, respectively) (Table 1). A t-test revealed that the difference in N and O isotope ratios between the two groups was statistically significant (p = 0.006 for N isotope ratios and p = 0.029 for O isotope ratios) at the 95% confidence interval. For each MLS, group 1 samples were obtained from shallower depths within the oxic groundwater zone than group 2 samples. Thus, with increasing depth in the oxic zone, increasing d15N and d18O 15 18 values for NO 3 (>10‰ for d N and >6‰ for d O; Table 1 and Fig. 6) were found in groundwater at each MLS. Fig. 6 reveals a linear trend (slope = 1.5, r2 = 0.8) between d15N and d18O values suggesting that denitrification is responsible for the increasing d15N and d18O values for group 2 samples from deeper parts of the oxic zone. The slope (1.5) is similar to those reported in previous denitrification studies (i.e., 1.4 in Cey et al., 1999; 1.5 in Mengis et al., 1999; and 2.1 in Böttcher et al., 1990; and Aravena and Robertson, 1998). For the sub-oxic groundwater, NO 3 concentrations were below the detection limit. However, elevated pH values and alkalinity and Fe concentrations (Figs. 3–5) suggest that significant denitrification and Fe reduction had occurred during infiltration into the sub-oxic groundwater mainly through nearby paddy fields with } ber and Conrad, silty soils containing abundant organic matter (Klu 1998; Xing et al., 2002; Min et al., 2003; Kim et al., 2009; Choi et al., 2010). 3.3. Sources of sulfate and its biogeochemical fate in the aquifer The isotopic composition of SO2 4 in groundwater is mainly controlled by its sources and subsequent biogeochemical processes such as bacterial SO4 reduction (cf. Krouse and Mayer, 2000). Groundwater SO2 4 is derived from diverse sources, such as atmospheric deposition (e.g. acid rain), oxidation of reduced inorganic S compounds (e.g. pyrite), dissolution of sulfate minerals (e.g., gypsum and anhydrite) and agricultural fertilizers, and mineralization of organic soil S. Alluvial groundwater collected from the MLSs in this study had high SO2 4 concentrations (up to 55–130 mg/L; Figs. 3–5). Atmospheric deposition had SO2 4 concentrations of ca. 1.7 mg/L (unpublished data) and hence cannot solely explain the elevated SO2 4 concentrations in groundwater even if evaporation and transpiration are taken into account. Sulfate minerals are unlikely to occur in the alluvial aquifer of the study area because the study area is located within a granitic terrain without sulfate minerals and under a monsoonal climate without a lengthy dryn period forming evaporitic minerals. Therefore, the two potential sources for elevated SO2 concentrations in the aquifer are syn4 thetic S-containing fertilizers and pyrite oxidation. Various SO4containing fertilizers such as K2 SO4 and MgSO4 are applied on agricultural fields in the study area. Pyrite oxidation is a potential SO2 4 source consistent with the hydrochemical data, since SO2 4 concentrations in all MLSs within the shallow oxic zone tend to increase in concert with decreasing pH and DO values with increasing depth (Figs 3–5). However, the groundwater samples showed only small decreases of DO concentrations (by less than 4 mg/L; Figs 3–5). When 4 mg/L DO is consumed for pyrite oxidation, only 6.4 mg/L SO2 is produced 4 according to a balanced equation for this process (Chae et al., 2001). Therefore, pyrite oxidation solely by DO cannot explain the substantial increases of SO2 4 concentrations of 30–60 mg/L, with the largest increase at site B1 (Figs. 3–5). Hence, NO 3 may be considered as an alternate electron acceptor for pyrite oxida-

1257

tion, as shown in the following equation (Appelo and Postma, 1994; Moncaster et al., 2000; Pauwels et al., 2000; Schwientek et al., 2008): þ 10FeS2 þ 30NO3 þ 20H2 O ! 10FeðOHÞ3 þ 20SO2 4 þ 15N2 þ 10H

The above reaction shows that pyrite oxidation occurs in tandem with NO 3 removal via denitrification. To test this hypothesis, S isotope ratios of SO2 4 for a total of 19 groundwater samples were determined to evaluate the sources of SO2 4 and its biogeochemical fate in the aquifer (Table 1, Fig. 7). At site B1, the B1-2 sample (6 mbgl) from the shallow oxic zone was characterized by low d34Ssulfate values (14.4‰ in July 2003 and 13.4‰ in May 2004; Table 1). The negative d34S values seem to indicate that the SO2 4 is derived from oxidation of pyrite (Krouse and Mayer, 2000). Pyrite is common in fine-grained, organic-rich sediments (Hartog et al., 2002) and organic-rich silty soil occurs in the vicinity of site B1 (Chae et al., 2004). However, the B1-4 sample (about 8 mbgl) was characterized by a higher d34Ssulfate value of 34 1.4‰ while SO2 4 concentration also increased. This d Ssulfate value falls within the general range reported for SO4-containing fertilizers (Table 2; Fig. 7). Previous studies investigating S-bearing fertilizers (Mizota and Sasaki, 1996; Robinson and Bottrell, 1997; Moncaster et al., 2000; Vitòria et al., 2004; Hosono et al., 2007) reported d34Ssulfate values between 6.5‰ and 21.6‰ (with a median value around 5.7‰; Vitòria et al., 2004) (Fig. 7). The d34Ssulfate values of SO2 4 -containing fertilizers used in this study area had a range from 2.0‰ to 10.5‰ (Table 2). Thus, the d34Ssulfate value of 1.4‰ is more consistent with a fertilizer source since increased SO2 4 concentrations caused by pyrite oxidation coupled with denitrification should have resulted in more negative d34Ssulfate values (Krouse and Mayer, 2000). Similarly, the oxic groundwater samples from the other MLSs had d34Ssulfate values ranging between 0.5‰ and 2.4‰, consistent with those for fertilizers (Table 2), even though SO2 4 concentrations increased in concert with decreasing NO3 concentrations with depth due to denitrification (Table 1). 2 At site B2, both NO 3 and SO4 concentrations of B2-3 (7.5 mbgl) were higher than those of B2-2 (5.5 mbgl). With increasing depth, 2 NO 3 concentrations decreased but SO4 concentrations further increased at B2-5 (11 mbgl) (Fig. 4). However, d34Ssulfate values varied by only <1‰ between the depth intervals. At site B3, SO2 4 concentrations at B3-2 (6.3 mbgl) were higher than those at B3-1 (4.3 mbgl) concomitantly with lower NO 3 concentrations, while d34Ssulfate values changed by only <2.2‰. These observations further suggest that sulfates predominantly originate from synthetic

Fig. 7. General ranges of d34S values for different sulfur sources (Krouse and Mayer, 2000; see also text). Sulfur isotope values of SO4 in groundwater from the study area are also shown.

1258

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

Table 2 The d34S values of sulfur-bearing fertilizers being used in the study area. Fertilizer

Company

d34S (‰)

MgSO4 MgSO4 K2MgSO4 Granular SO4 Manufactured MgSO4 Manufactured granular MgSO4

Namhae Keondo Jobi KG Chemical Keondo Daehan

9.2 (0.6) 0.4 (0.3) 9.5 (0.6) 2.0 (0.1) 5.2 (0.5) 10.5 (0.2)

The values are the averages (n = 2) with standard deviation in parentheses.

Fig. 8. The relationship between SO4 concentrations and d34Ssulfate values for alluvial groundwater in the study area.

fertilizers, and not from pyrite oxidation. Therefore, it is concluded that the elevated SO2 concentrations in the oxic groundwater 4 with depth are predominantly caused by the high input of synthetic fertilizers. In contrast, SO2 4 concentrations in the deeper sub-oxic groundwater were found to be low (0.1–23.6 mg/L). High d34Ssulfate values (2.5–65.4‰) and low SO2 4 concentrations (Table 1; Figs. 3–5) suggest that this is due to bacterial (dissimilatory) SO4 reduction under reducing conditions (e.g., Strebel et al., 1990; Spence et al., 2001). In particular, samples from the deepest MLSs at B1 and B3 showed high d34Ssulfate values (43.2–65.4‰; Table 1), which are comparable to those reported for bacterial SO4 reduction in other aquifers (e.g., Schüring et al., 2000; Dogramaci et al., 2001). The highly elevated d34Ssulfate values in the sub-oxic zone in concert with markedly decreased SO2 4 concentrations (Fig. 8) clearly indicate that bacterial SO4 reduction is occurring, since such elevated d34Ssulfate values cannot be derived from marine or evaporitic SO4 sources. Therefore, S isotope data indicate that SO2 in the oxic 4 zone is predominantly derived from synthetic fertilizers while bacterial SO4 reduction is an important process affecting the fate of SO2 4 in the sub-oxic zone. 4. Conclusion and implications An integrated hydrochemical and environmental isotopic (d15Nnitrate, d18Onitrate, and d34Ssulfate) study was performed to eluci2 date the sources and biogeochemical behavior of NO 3 and SO4 in shallow (<25 mbgl) alluvial groundwater underneath a riverside agricultural area in Korea. Special emphasis was placed on evaluating the changes of chemistry and groundwater quality during and after the inflow of NO 3 contaminated agricultural water into the

groundwater system. For this study, depth-specific groundwater samples were collected from multi-level samplers that were installed in three boreholes. The alluvial groundwater in the study area is divided into two distinct compartments, based on the redox states and hydrochemical changes: (1) shallow (above 8–10 mbgl) NO 3 contaminated oxic groundwater and (2) deeper, NO 3 poor sub-oxic groundwater. 2 In particular, the concentrations of NO 3 and SO4 significantly decrease in sub-oxic groundwater: from about 35–0.02 mg/L for NO3 AN, and from about 130–0.14 mg/L for SO2 4 . According to the changes of N and O isotope ratios of NO , the shallow oxic 3 groundwater can be further sub-divided into two groups (group 1 and group 2). Nitrates in group 1 water originated mainly from manure and some urea- or ammonia-containing fertilizers during infiltration, whereas NO 3 in group 2 water was affected by denitrification. Sulfur isotope ratios of SO2 4 in the oxic groundwater suggest that SO2 was predominantly derived from synthetic 4 fertilizers. Increasing concentrations of SO2 4 with depth in the oxic groundwater are caused by high fertilizer inputs during previous fertilizer application campaigns, not by pyrite oxidation coupled with denitrification. In contrast, SO2 4 in the sub-oxic groundwater is removed by bacterial SO4 reduction, as indicated by very high d34Ssulfate values (up to 65.4‰) in concert with low SO2 4 concentrations in deeper levels of the sub-oxic zone. This study demonstrates that careful examination of the vertical changes in hydrochemical and environmental isotopic data using MLSs can be very useful for better understanding the sources and 2 biogeochemical processes that affect NO in alluvial 3 and SO4 groundwater systems underneath agricultural fields. In particular, isotope data were shown to be highly effective in discriminating between different possible explanations for the occurrence of high  SO2 4 and low NO3 waters. Such information is required for a better understanding of sources and processes governing water quality in shallow aquifers affected by agricultural land use. This knowledge will be highly valuable for sustainable management of the quality of alluvial groundwater (and nearby streams) and its associated ecosystems in Korea and elsewhere. Acknowledgements This work was supported by the Environmental Geosphere Research Laboratory (EGRL) of Korea University (KU), which was funded from Korea Research Foundation (KRF). Financial support from the 2010 Radioactive Waste Management research program of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge and Economy (No. 201017102002D) and from the Natural Sciences and Engineering Research Council of Canada (NSERC) are also gratefully acknowledged. Many graduate students of KU helped during well installation and field surveys. Constructive comments provided by Prof. Simon Bottrell and seven anonymous reviewers helped to clarify and improve this manuscript. References Appelo, C.A.J., Postma, D., 1994. Geochemistry, Groundwater and Pollution. Balkema, Rotterdam. 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. Balci, N., Shanks III, W.C., Mayer, B., Mandernack, K.W., 2007. Oxygen and sulfur isotope systematics of sulfate produced by bacterial and abiotic oxidation of pyrite. Geochim. Cosmochim. Acta 71, 3796–3811. Bartlett, R., Bottrell, S.H., Sinclair, K., Thornton, S., Fielding, I.D., Hatfield, D., 2010. Lithological controls on biological activity and groundwater chemistry in quaternary sediments. Hydrol. Process. 24, 726–735. Bateman, A.S., Kelly, S.D., 2007. Fertilizer nitrogen isotope signatures. Isotopes Environ. Health Stud. 43, 237–247.

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260 Böhlke, J.K., 2002. Groundwater recharge and agricultural contamination. Hydrogeol. J. 10, 153–179. Böttcher, J., Strebel, O., Voerkelius, S., Schmidt, H.L., 1990. Using isotope fractionation of nitrate-nitrogen and nitrate-oxygen for evaluation of microbial denitrification in a sandy aquifer. J. Hydrol. 114, 413–424. Brenot, A., Carignan, J., France-Lanord, C., Benoît, M., 2007. Geological and land use control on d34S and d18O of river dissolved sulfate: the Moselle river basin, France. Chem. Geol. 244, 25–41. 2 Campbell, J.L., Mitchell, M.J., Mayer, B., 2006. Isotopic assessment of NO 3 and SO4 mobility during winter in two adjacent watersheds in the Adirondack Mountains, New York. J. Geophys. Res. 111, G04007. Cey, E.E., Rudolph, D.L., Aravena, R., Parkin, G., 1999. Role of riparian zone in controlling the distribution and fate of agricultural nitrogen near a small stream in southern Ontario. J. Contam. Hydrol. 37, 45–67. Chae, G.T., Kim, K., Yun, S.T., Kim, K.H., Kim, S.O., Choi, B.Y., Kim, H.S., Rhee, C.W., 2004. Hydrogeochemistry of alluvial groundwaters in an agricultural area: an implication for groundwater contamination susceptibility. Chemosphere 55, 369–378. Chae, G.T., Yun, S.T., Kim, S.R., Hahn, C., 2001. Hydrogeochemistry of seepage water collected within the Youngcheon diversion tunnel, Korea: source and evolution of SO4-rich groundwater in sedimentary terrain. Hydrol. Process. 15, 1565– 1583. Chae, G.T., Yun, S.T., Mayer, B., Choi, B.Y., Kwon, J.S., Yu, S.Y., 2009. Hydrochemical and stable isotopic assessment of nitrate contamination in an alluvial aquifer underneath a riverside agricultural field. Agric. Water Manage. 96, 1819–1827. Chapelle, F.H., 2001. Ground-water Microbiology and Geochemistry. Wiley, New York. Choi, B.Y., Yun, S.T., Mayer, B., Chae, G.T., Kim, K.H., Kim, K., Koh, Y.K., 2010. Identification of groundwater recharge pathways and processes in a heterogeneous alluvial aquifer: results from multi-level monitoring of hydrochemistry and environmental isotopes in a riverside agricultural area in Korea. Hydrol. Process. 24, 317–330. Choi, W.J., Han, G.H., Lee, S.M., Lee, G.T., Yoon, K.S., Choi, S.M., Ro, H.M., 2007. Impact of land-use types on nitrate concentration and d15N in unconfined groundwater in rural areas of Korea. Agric. Ecosyst. Environ. 120, 259–268. Cooper, C.M., 1993. Biological effects of agriculturally derived surface water pollutants on aquatic systems – a review. J. Environ. Qual. 22, 402– 408. Deutsch, B., Liskow, I., Kahle, P., Voss, M., 2005. Variations in the d15N and d18O values of nitrate in drainage water of two fertilized fields in Mecklenburg– Vorpommern (Germany). Aquat. Sci. 67, 156–165. Dogramaci, S.S., Herczeg, A.L., Schiff, S.L., Bone, Y., 2001. Controls on d34S and d18O of dissolved sulfate in aquifers of the Murray Basin, Australia and their use as indicators of flow processes. Appl. Geochem. 16, 475–488. Doussan, C., Poitevin, G., Ledoux, E., Detay, M., 1997. River bank filtration: modeling of the changes in water chemistry with emphasis on nitrogen species. J. Contam. Hydrol. 25, 129–156. Engesgaard, P., Kipp, K.L., 1992. A geochemical transport model for redox-controlled movement of mineral fronts in groundwater flow systems: a case of nitrate removal by oxidation of pyrite. Water Resour. Res. 28, 2829–2843. Fogg, G.E., Rolston, D.E., Decker, D.L., Louie, D.T., Grismer, M.E., 1998. Spatial variation in nitrogen isotope values beneath nitrate contamination sources. Ground Water 36, 418–426. Giesemann, A., Jäger, H.J., Norman, A.L., Krouse, H.R., Brand, W.A., 1994. Online sulphur isotope analysis using an elemental analyzer coupled to a mass spectrometer. Anal. Chem. 66, 2816–2819. Gillham, R.W., 1991. Nitrate contamination of groundwater in southern Ontario and the evidence for denitrification. In: Bogadri, I., Kuzelka, R.D. (Eds.), Nitrate Contamination. Springer-Verlag, Berlin, pp. 181–198. Hartog, N., Griffioen, J., van der Weijden, C., 2002. Distribution and reactivity of O2reducing components from a layered aquifer. Environ. Sci. Technol. 36, 2338– 2344. Heaton, T.H.E., 1986. Isotopic studies of nitrogen pollution in the hydrosphere and atmosphere: a review. Chem. Geol. 5, 87–102. Hendry, M.J., Wassenaar, L.I., 2000. Controls on the distribution of major ions in pore waters of a thick surficial aquitard. Water Resour. Res. 36, 503–513. Hendry, M.J., Wassenaar, L.I., Kotzer, T., 2000. Chloride and chlorine isotopes (36Cl and d37Cl) as tracers of solute migration in a thick, clay-rich aquitard system. Water Resour. Res. 36, 285–296. Hosono, T., Nakano, T., Igeta, A., Tayasu, I., Tanaka, T., Yachi, S., 2007. Impact of fertilizer on a small watershed of Lake Biwa: use of sulfur and strontium isotopes in environmental diagnosis. Sci. Total Environ. 384, 342–354. Iqbal, M.Z., Krothe, N.C., Spalding, R.F., 1997. Nitrogen isotope indicators of seasonal source variability to groundwater. Environ. Geol. 32, 210–218. Jacobs, T.C., Gilliam, J.W., 1985. Riparian losses of nitrate from agricultural drainage waters. J. Environ. Qual. 14, 472–478. Jones, A.L., Smart, P.L., 2005. Spatial and temporal changes in the structure of groundwater nitrate concentration time series (1935–1999) as demonstrated by autoregressive modelling. J. Hydrol. 310, 201–215. Kaown, D., Koh, D.C., Mayer, B., Lee, K.K., 2009. Identification of nitrate and sulfate sources in groundwater using dual stable isotope approaches for an agricultural area with different land use (Chuncheon, mid-eastern Korea). Agric. Ecosyst. Environ. 132, 223–231. Kendall, C., 1998. Tracing nitrogen sources and cycling in catchments. In: Kendall, C., McDonnell, J.J. (Eds.), Isotope Tracers in Catchment Hydrology. Elsevier, Amsterdam, pp. 521–576.

1259

Kim, K.H., Yun, S.T., Choi, B.Y., Chae, G.T., Joo, Y., Kim, K., Kim, H.S., 2009. Hydrochemical and multivariate statistical interpretations of spatial controls of nitrate concentrations in a shallow alluvial aquifer around oxbow lakes (Osong area, central Korea). J. Contam. Hydrol. 107, 114–127. } Kluber, H.D., Conrad, R., 1998. Effects of nitrate, nitrite, NO, and N2O on methanogenesis and other redox processes in anoxic rice soil. FEMS Microbiol. Ecol. 25, 301–318. KMA (Korea Meteorological Administration), 2002. Annual Climate Report. KMA (in Korean). Krouse, H.R., Mayer, B., 2000. Sulphur and oxygen isotopes in sulphate. In: Cook, P.G., Herczeg, A.L. (Eds.), Environmental Tracers in Subsurface Hydrology. Kluwer Academic, pp. 195–231. Mantovi, P., Fumagalli, L., Beretta, G.P., Guermandi, M., 2006. Nitrate leaching through the unsaturated zone following pig slurry applications. J. Hydrol. 316, 195–212. Mariotti, A., Landreau, A., Simon, B., 1988. 15N isotope biogeochemistry and natural denitrification process in groundwater: application to the chalk aquifer of northern France. Geochim. Cosmochim. Acta 52, 1869–1878. Massmann, G., Tichomirowa, M., Merz, C., Pekdeger, A., 2003. Sulfide oxidation and sulfate reduction in a shallow groundwater system (Oderbruch Aquifer, Germany). J. Hydrol. 278, 231–243. Mayer, B., Bollwerk, S.M., Mansfeldt, T., Hütter, B., Veizer, J., 2001. The oxygen isotope composition of nitrate generated by nitrification in acid forest floors. Geochim. Cosmochim. Acta 65, 2743–2756. Mayer, B., Boyer, E.W., Goodale, C., Jaworski, N.A., Breemen, N.V., Howarth, R.W., Seitzinger, S., Billen, G., Lajtha, K., Nadelhoffer, K., Dam, D.V., Hetling, L.J., Nosal, M., Paustian, K., 2002. Sources of nitrate in rivers draining sixteen watersheds in the northeastern US: isotopic constraints. Biogeochemistry 57 (58), 171–197. Mengis, M., Schiff, S.L., Harris, M., English, M.C., Aravena, R., Elgood, R.J., MacLean, A., 1999. Multiple geochemical and isotopic approaches for assessing ground water NO 3 elimination in a riparian zone. Ground Water 37, 448–457. Meredith, J., Metcalf, J., Robbins, G.A., 2007. Comparison of water quality profiles from shallow monitoring wells and adjacent multilevel sampler. Groundwater Monitor. Remed. 27, 84–91. Min, J.H., Yun, S.T., Kim, K., Kim, H.S., Kim, D.J., 2003. Geologic controls on the chemical behavior of nitrate in riverside alluvial aquifers, Korea. Hydrol. Process. 17, 1197–1211. Mizota, C., Sasaki, A., 1996. Sulfur isotope composition of soils and fertilizers: differences between Northern and Southern hemispheres. Geoderma 71, 77–93. Moncaster, S.J., Bottrell, S.H., Tellam, J.H., Lloyd, J.W., Konhauser, K.O., 2000. Migration and attenuation of agrochemical pollutants: insights from isotopic analysis of groundwater sulphate. J. Contam. Hydrol. 43, 147–163. Otero, N., Vitòria, L., Soler, A., Canals, A., 2005. Fertiliser characterization: major, trace and rare earth elements. Appl. Geochem. 20, 1473–1488. Park, Y.H., Doh, S.J., Yun, S.T., 2007. Geoelectrical resistivity sounding of riverside alluvial aquifer in an agricultural area at Buyeo, Geum River watershed, Korea: an application to groundwater contamination study. Environ. Geol. 53, 849– 859. Pauwels, H., Foucher, J.-C., Kloppmann, W., 2000. Denitrification and mixing in a schist aquifer: influence on water chemistry and isotopes. Chem. Geol. 168, 307–324. Pauwels, H., Lachassagne, P., Bordenave, P., Foucher, J.-C., Martelat, A., 2001. Temporal variability of nitrate concentration in a schist aquifer and transfer to surface waters. Appl. Geochem. 16, 583–596. Postma, D., Boesen, C., Kristiansen, H., Larsen, F., 1991. Nitrate reduction in an unconfined sandy aquifer: water chemistry, reduction processes, and geochemical modeling. Water Resour. Res. 27, 2027–2045. Powlson, D.S., Addiscott, T.M., Benjamin, N., Cassman, K.G., de Kok, T.M., van Grinsven, H., L’hirondel, J.-L., Avery, A.A., van Kessel, C., 2008. When does nitrate become a risk for humans? J. Environ. Qual. 37, 291–295. Rajagopal, R., Tobin, G., 1989. Expert opinion and ground-water quality protection: the case of nitrate in drinking water. Ground Water 27, 835–847. Robinson, B.W., Bottrell, S.H., 1997. Discrimination of sulfur sources in pristine and polluted New Zealand river catchments using stable isotopes. Appl. Geochem. 12, 305–319. Schüring, J., Schlieker, M., Hencke, J., 2000. Redox fronts in aquifer systems and parameters controlling their dimensions. In: Schüring, J., Schulz, H.D., Fischer, W.R., Böttcher, J., Duijnisveld, W.H.M. (Eds.), Redox – Fundamentals, Processes and Applications. Springer, Berlin, pp. 135–151. Schwientek, M., Einsiedl, F., Stichler, W., Stögbauer, A., Strauss, H., Maloszewski, P., 2008. Evidence for denitrification regulated by pyrite oxidation in a heterogeneous porous groundwater system. Chem. Geol. 255, 60– 67. Shanley, J.B., Mayer, B., Mitchell, M.J., Michel, R.L., Bailey, S.W., Kendall, C., 2005. Tracing sources of stream water sulfate during snowmelt suing S and O isotope ratios of sulfate and 35S activity. Biogeochemistry 76, 161–185. Silva, S.R., Kendall, C., Wilkison, D.H., Ziegler, A.C., Chang, C.C.Y., Avanzino, R.J., 2000. A new method for collection of nitrate from fresh water and the analysis of nitrogen and oxygen isotope ratios. J. Hydrol. 228, 22–36. Spence, M.J., Bottrell, S.H., Thornton, S.F., Lerner, D.N., 2001. Isotopic modeling of the significance of bacterial sulphate reduction for phenol attenuation in a contaminated aquifer. J. Contam. Hydrol. 53, 285–304. Spoelstra, J., Schiff, S.L., Hazlett, P.W., Jeffries, D.S., Semkin, R.G., 2007. The isotopic composition of nitrate produced from nitrification in a hardwood forest floor. Geochim. Cosmochim. Acta 71, 3757–3771.

1260

B.-Y. Choi et al. / Applied Geochemistry 26 (2011) 1249–1260

Strebel, O., Böttcher, J., Firtz, P., 1990. Use of isotope fractionation of sulfate-sulfur and sulfate-oxygen to assess bacterial desulfurication in a sandy aquifer. J. Hydrol. 121, 155–172. Taylor, R.G., Cronin, A.A., Lerner, D.N., Tellam, J.H., Bottrell, S.H., Rueedi, J., Barrett, M.H., 2006. Hydrochemical evidence of the depth of penetration of anthropogenic recharge in sandstone aquifers underlying two mature cities in the UK. Appl. Geochem. 21, 1570–1592. Trettin, R., Knöller, K., Loosli, H.H., Kowski, P., 2002. Evaluation of the sulfate dynamics in groundwater by means of environmental isotopes. Isotopes Environ. Health Stud. 38, 103–119. US Environmental Protection Agency, 1995. Drinking Water Regulations and Health Advisories. Office of Water, Washington, DC.

Vitòria, L., Otero, N., Soler, A., Canals, À., 2004. Fertilizer characterization: isotopic data (N, S, O, C, and Sr). Environ. Sci. Technol. 38, 3254–3262. Wassenaar, L.I., 1995. Evaluation of the origin and fate of nitrate in the Abbotsford aquifer using the isotopes of 15N and 18O in NO 3 . Appl. Geochem. 10, 391–405. Xing, G.X., Cao, Y.C., Shi, S.L., Sun, G.Q., Du, L.J., Zhu, J.G., 2002. Denitrification in underground saturated soil in a rice paddy region. Soil Biol. Biochem. 34, 1593– 1598. Yanagisawa, F., Sakai, H., 1983. Thermal decomposition of barium sulfate-vanadium pentaoxide-silica glass mixtures for preparation of sulfur dioxide in sulfur isotope ratio measurements. Anal. Chem. 55, 985–987.