Identification of Multiple Sources of Groundwater Contamination by Dual Isotopes by Dugin Kaown1 , Orfan Shouakar-Stash2 , Jaeha Yang1 , Yunjung Hyun3 , and Kang-Kun Lee4

Abstract Chlorinated solvents are one of the most commonly detected groundwater contaminants in industrial areas. Identification of polluters and allocation of contaminant sources are important concerns in the evaluation of complex subsurface contamination with multiple sources. In recent years, compound-specific isotope analyses (CSIA) have been employed to discriminate among different contaminant sources and to better understand the fate of contaminants in field-site studies. In this study, the usefulness of dual isotopes (carbon and chlorine) was shown in assessments of groundwater contamination at an industrial complex in Wonju, Korea, where groundwater contamination with chlorinated solvents such as trichloroethene (TCE) and carbon tetrachloride (CT) was observed. In November 2009, the detected TCE concentrations at the study site ranged between nondetected and 10,066 μg/L, and the CT concentrations ranged between nondetected and 985 μg/L. In the upgradient area, TCE and CT metabolites were detected, whereas only TCE metabolites were detected in the downgradient area. The study revealed the presence of separate small but concentrated TCE pockets in the downgradient area, suggesting the possibility of multiple contaminant sources that created multiple comingling plumes. Furthermore, the variation of the isotopic (δ 13 C and δ 37 Cl) TCE values between the upgradient and downgradient areas lends support to the idea of multiple contamination sources even in the presence of detectable biodegradation. This case study found it useful to apply a spatial distribution of contaminants coupled with their dual isotopic values for evaluation of the contaminated sites and identification of the presence of multiple sources in the study area.

Introduction Chlorinated solvents are one of the most commonly observed groundwater contaminants in industrial areas worldwide (Lenczewski et al. 2006; Gerhard et al. 2007; Lojkasek-Lima et al. 2012). Assessment and characterization of contaminated sites are major tasks that must be addressed by consultants and government agencies. These tasks include identifying the contaminant sources as well as determining the polluters. According to the Korean Groundwater Law, the party responsible for the contamination is liable for remediating the contaminated site; therefore, in the contaminated sites, it is critical to identify the various contamination sources as well as the parties responsible for discharging these contaminants to accurately assign appropriate remediation responsibilities. 1 School

of Earth and Environmental Sciences (BK21 SEES), Seoul National University, Seoul 151-747, Korea. 2 Department of Earth and Environmental Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. 3 Environmental Policy Research Group, Korea Environment Institute, Seoul 122-706, Korea. 4 Corresponding author: School of Earth and Environmental Sciences, Seoul National University, Seoul 151-747, Korea; +82 2 880 8161; fax: +82 2 873 3647; [email protected] Received September 2012, accepted September 2013. © 2013, National Ground Water Association. doi: 10.1111/gwat.12130

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In recent years, a number of studies have used compound-specific isotope analysis (CSIA) to identify contaminant sources by quantifying in situ biodegradation patterns of chlorinated solvent plumes (Hunkeler et al. 1999; Song et al. 2002; Kirtland et al. 2003; Vieth et al. 2003; Chartrand et al. 2005; Chapman et al. 2007; Imfeld et al. 2008). In general, CSIA has been applied as a component in a multiple-lines-of-evidence approach that includes evaluation of historical, hydrological, geological, and statistical analyses to recreate contamination scenarios in industrialized areas (Blessing et al. 2009). As such, CSIA can provide useful information on the delineation of contamination plumes as well as the identification of contamination sources in areas where multiple sources of contamination are present (Slater 2003). The application of CSIA in identification of different sources was driven by the fact that organic compounds including chlorinated solvents produced by different manufacturers tend to be isotopically different (Van Warmerdam et al. 1995; Jendrzejewski et al. 2001; Slater 2003). Different chemical, biological, and physical processes (e.g., reductive dechlorination of chlorinated solvents) could affect the organic contaminants in the subsurface, and these substances are usually associated with unique and predictable isotopic fractionations (Hunkeler et al. 1999; Bloom et al. 2000; Sherwood Lollar et al. 2001; Slater et al. 2001; Song et al. 2002; Hunkeler et al. 2004). Therefore, the use

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of CSIA at any contaminated site can provide valuable information on the source and fate of contaminants. More recently, researchers have considered application of the dual isotope approach because single isotope analysis can lead to uncertain results for certain complicated cases in which the contaminants undergo various environmental processes such as biodegradation (Hunkeler et al. 2008). In such cases, additional information can be gained from dual isotope measurements to accurately identify different sources and separate the effects of biodegradation (Hunkeler et al. 2001, 2011; LojkasekLima et al. 2012). Several studies have demonstrated the usefulness of the dual isotope approach in assessing contaminated sites. For example, CSIA for carbon and hydrogen isotopes was used to delineate different sources of contamination at sites where biodegradation was observed (Hunkeler et al. 2001), and CSIA for carbon and chlorine isotopes was used to evaluate the fate of large-scale chlorinated ethene plumes originating from a tetrachloroethene (PCE) source in a sandy aquifer in Denmark (Hunkeler et al. 2011). Additionally, CSIA for carbon and chlorine was used to identify the sources of TCE in an extensive fractured bedrock aquifer beneath the city of Guelph, Ontario (Lojkasek-Lima et al. 2012). Groundwater contamination due to several chlorinated solvents such as trichloroethene (TCE) and carbon tetrachloride (CT) was observed in an industrial area of Wonju, Korea. During the groundwater quality monitoring survey completed in November 2009, the measured TCE concentration levels at the site ranged between nondetected and 10,066 μg/L, and the CT concentrations ranged between nondetected and 985 μg/L. Wonju City and EMC (2003) and Yu et al. (2006) suggested a single source for the TCE contamination, whereas Yang et al. (2003) and Baek and Lee (2011) suggested two contaminant sources. Yang and Lee (2012) analyzed the influences of rainfall recharge events on the concentrations of the contaminants in groundwater and found multiple residual sources in the study area. Although the probable existence of multiple dense nonaqueous phase liquid (DNAPL) sources that created multiple comingling plumes was proposed, the degrees of confirmation for the existence of each source point were different, with strongly or weakly supporting evidences. This study was intended to show how CSIA (δ 13 C and δ 37 Cl) coupled with other hydrogeochemical parameters could provide strong evidence for locating the source points. Thus, the objectives of this study are (1) the evaluation of the effectiveness of using a dual CSIA approach combined with historical, hydrological, and geochemical analyses to identify the potential contaminant sources, and (2) the assessment of the fate and transport of contaminants based on the source identification.

Study Area The study area is located approximately 75 km east of Seoul, Korea in an industrial area bounded by low hills along the western boundary and by a stream in the northeastern boundary (Figure 1). The area of the site 876

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is about 0.65 km2 and contains approximately 40 public, commercial, and residential buildings. The Road Administrative Office (RAO) of Gangwon Province (see Figure 1), located in the western part of the study area, has historically used chlorinated solvents during asphalt testing and had dumped the solvents without appropriate treatment from 1982 through the 1990s (Gangwon Province 2005; Baek and Lee 2011). The RAO used CT to perform their tests from 1982 to the 1990s and replaced it thereafter with TCE after the Korean Ministry of Environment restricted the use of CT due to its potential health risks. The estimated annual consumption of TCE by the RAO is close to 500 L. Throughout the RAO operations, the produced TCE waste was disposed by dumping the waste on the surface next to the institute until 1997 (Yu et al. 2006); however, the amount of CT discharged at this site has not been estimated. In July 1995, TCE was detected in two pumping wells owned by a food processing company located in the area (see Figure 1), and the observed concentrations exceeded the MCL levels of Korea (30 μg/L) (Baek and Lee 2011). In 1999, TCE was detected in all the pumping wells used by the food company, and hence, all wells were shut down and the company began to use municipal water supplies for their water needs. These contamination findings led to the installation of a new set of monitoring wells for a comprehensive investigation of the entire industrial area that was intended to delineate the contamination plume and identify the causes and the sources of the TCE contamination problem. The aquifer in this area consists of weathered and fractured Jurassic biotite granite overlain by soil and alluvial deposits that are 10-15 m thick (Yu et al. 2006; Baek and Lee 2011). The alluvial deposits consist primarily of two types of sediments: silty sand and coarse sand. The hydraulic conductivity of the upgradient and downgradient regions were measured by conducting slug tests, and the measured conductivity values ranged between 2.0 × 10-4 cm/s and 2.4 × 10-3 cm/s. During the summer, the depth to the water table was measured as approximately 9-10 m in the western area and 2-3 m in the southeastern area. The measured water table levels during the winter ranged from 10 to 11 m in the western area and close to 2 to 3 m in the southeastern area. The water table data show significant seasonal variations, mostly in the western part of the study area, and the values varied in a vertical range of approximately 2 m throughout the year. Seasonal variations can be attributed to the fact that greater than 66% of the annual precipitation (approximately 1359 mm/yr) is concentrated during the wet season (June to August), and only a small amount of rainfall (about 10%) occurs during the dry season (November to January). The regional groundwater flow direction at this site is from the west to the east. The subsurface geologies of the western and eastern regions of the study area are considerably different. Steep slopes occur in the western part of the study area, and gentle slopes occur in the eastern part of the study area (Figure 1). Most of the wells in the western part are NGWA.org

(a)

Elevation (m)

(b)

A'

A 140

K DPW-2 GW-1

120

140 M W-2 2 M W-2 1

GW-3

M W-1 1

GW-1 2

M W-5 GW-1 1

We a t h e r e d zo n e

100

120 100

biot ot e granit e

0

200

400 Distance (m)

600

800

Figure 1. Location map of the Wonju site and layout of the groundwater sampling wells (a). Blue lines indicate the contour lines of the measured groundwater table in meters for November 2009. Numbers on the contour lines indicate altitude in meters a.s.l. Hydrogeological cross sections of A-A (b). The locations of the Road Administrative Office (RAO) and the food company are indicated by red stars.

located in a weathered granite aquifer with depths of greater than 20 m. These wells are directly affected by rainfall because most of the wells are surrounded by unpaved forest area. The wells in the eastern part of the study area are located in an alluvial deposit with shallower depths of less than 20 m and are only affected indirectly by rainfall processes because most of the area is covered by asphalt, concrete, and buildings.

Groundwater samples were collected for chemical and isotopic analyses from 50 wells used for monitoring industrial water supplies during the period from November 2009 to August 2010. The sampling locations are shown in Figure 1. The groundwater samples were collected at the monitoring wells using a submersible pump and a peristaltic pump. The samples were filtered through a 0.45 μm membrane and collected in 60-mL bottles for chemical analysis. Samples for cation analysis were preserved using

ultrapure HNO3 . The dissolved oxygen (DO), pH, electrical conductivity (EC), and temperature were measured in situ using standard field probes. Alkalinity was determined in the field by the Gran titration method. Samples for isotope analyses (δ 13 C and δ 37 Cl) were collected in triplicate in 40-mL amber vials fitted with septa and were preserved by the addition of NaOH. Cations were analyzed by ICP-OES (VISTA-MPX, Varian, Palo Alto, California), and anions were analyzed by ion chromatography (761 Compact, Metrohm, Herisau, Switzerland). Concentrations of chlorinated solvents were determined using a Varian Saturn 2100T GC/MS equipped with an electron capture detector at Sangji University, Korea. Both the δ 13 C and δ 37 Cl values of the contaminant compounds were determined at the Environmental Isotope Laboratory (EIL) at the University of Waterloo, Canada. Compound-specific carbon isotopic ratios of TCE were determined using a gas chromatography (GC) (Agilent 6890, Santa Clara, California) fitted to an isotope ratio mass spectrometer (IRMS) with a combustion interface

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Methods

877

(Isoprime, Micromass, Manchester, United Kingdom). The analytical steps are described in Hunkeler and Aravena (2000). The system was equipped with a purgeand-trap concentrator connected to the GC via a cryogenic trap. The minimum concentrations for TCE for carbon isotope analysis were approximately 30 μg/L. Compoundspecific chlorine stable isotopes of TCE were determined using GC-IRMS (Isoprime) via direct injection following the methodology described in Shouakar-Stash et al. (2006). Samples were extracted using a solid phase micro extraction (SPME) fiber automated with a CTC Combipal sampler (CTC, Zwingen, Switzerland). The minimum concentrations for TCE for chlorine isotope analysis were approximately 15 μg/L. Isotope compositions are reported in per mil (‰) deviation from the isotopic standard reference materials using conventional δ notation δ = [(R sample /R standard ) − 1] × 1000, where R is the 13 C/12 C or 37 Cl/ 35 Cl ratio. The δ 13 C values of TCE are reported relative to Vienna Pee Dee Belemnite (δ 13 CVPDB ) (Coplen 1996) with an analytical precision of

±0.3‰. The δ 37 Cl values of TCE are reported relative to the Standard Mean Ocean Chloride (δ 37 ClSMOC ) (Long et al. 1993) with an analytical precision of ±0.1‰.

Results Spatial Distribution of Contaminants The groundwater quality in Wonju has been monitored since 2004. High concentrations of TCE (ranging from 10,066 to 10,444 μg/L) were observed in the KDPW-2 and SKW-1 wells located next to the RAO of Gangwon Province in 2009. The RAO is located in the western part of the study area, which is characterized by higher elevations and steeper slopes. The TCE concentrations were observed to decrease along the flow path toward the eastern part of the study area (Figure 2). The TCE plume was also observed to move toward the north due to artificial pumping (60 m3 /d) effects from PW-1, where a laundry facility is located. Concentrations of TCE between 60 and 70 μg/L were observed in the GW-10 and

Figure 2. Spatial distributions of TCE (a), cis-DCE (b), carbon tetrachloride (c), and chloroform (d) in groundwater samples obtained in November 2009.

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PW-7 wells located in the downgradient area outside the main plume (Figure 2). The spatial distribution of cisdichloroethene (cis-DCE), a byproduct of TCE-reductive dechlorination, is shown in Figure 2. The concentrations of cis-DCE ranged between nondetected and 54 μg/L in November 2009. The cis-DCE was detected only in the upgradient and downgradient areas and was not detected in the middle part of the study area. High concentrations (33–54 μg/L) of cis-DCE were also observed in the GW-19 well, whose location falls outside the main plume boundary in the downgradient area. However, VC (a byproduct of DCE-reductive dechlorination) was observed only in the MW-3 and GW-19 wells in the downgradient area. The spatial distributions of CT and chloroform (CF) observed at this site are shown in Figure 2. The concentrations of CT and CF ranged between nondetected and 1048 μg/L and were observed only in the upgradient area (western part of the study area) within 400 m of the TCE plume, where the highest concentrations of TCE were

observed. The separate occurrences of both TCE and cisDCE away from the main plume in the downgradient area indicate the possibility of multiple contamination sources. Although the results obtained from this study strongly suggest the presence of multiple sources of contamination in the study area, additional follow-up fieldwork will be required to identify the exact number of contamination sources within this study area. Redox Conditions The concentrations of DO were determined four times during the year from November 2009 to August 2010, and the DO values ranged from 0.2 to 7.23 mg/L (Tables S1 to S5). The redox conditions indicate that the groundwater system is characterized by aerobic conditions beneath most of the study area, except for the downgradient area (Figure 3). This is most likely due to the fact that most of the upgradient portions of the flow system are shallow and are directly recharged by rainfall infiltrating forested

Figure 3. Seasonal variations of DO, NO3 − , Fe2+ , and SO4 2− concentrations in groundwater samples obtained in November 2009 and March, May, and August 2010.

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and grassed areas, while, most of the downgradient area is covered by asphalt and cement; thus it is not directly affected by rainfall events. Partially anaerobic conditions were found in the downgradient area, which was mostly used as a paddy field before the industrial complex was built (Baek and Lee 2011). The concentrations of NO3 − ranged between nondetected and 1.8 mg/L in the downgradient area and between 5.8 and 16.4 mg/L in the upgradient area; thus, NO3 − reducing conditions were observed in the downgradient area (Figure 3). The Fe-reducing condition was observed only in the downgradient area where the cis-DCE was detected. The Fe2+ concentration ranged between 0.5 and 1.76 mg/L in the downgradient area and less than 0.2 mg/L in the upgradient area. The concentrations of SO4 2− ranged between 1.8 and 79.2 mg/L, and thus, the SO4 2− reducing condition was not observed. The upgradient parts of the flow system are aerobic, with high DO and concentrations of TCE at 99% of the molar fraction of total volatile organic compounds (VOCs). Further downgradient areas, anaerobic conditions exist with low DO and low NO3 − concentrations and elevated Fe2+ concentrations and concentrations of TCE at 22% to 75% of the molar fraction of total VOCs (Figure 4a). Compound-Specific Carbon Stable Isotope Analysis Compound-specific carbon isotopic ratios of TCE and cis-DCE were used to identify the source(s) of various contaminants in the study area. The δ 13 C values of TCE in the source area with known sources (KDPW-2 and SKW-1) in the upgradient area were used to investigate the presence of other TCE sources in the area. The measured δ 13 C values of TCE are presented in Table 1. Most of the measured δ 13 C values for TCE ranged from −26.9‰ to −22.3‰ in November 2009, −28.2‰ to −22.3‰ in March 2010, and −27.8‰ to −23.8‰ in August 2010. The δ 13 C values of TCE obtained from three sampling periods showed a certain level of variability. In general, the δ 13 C values of TCE seemed to be depleted in the upgradient area, whereas they were enriched in the downgradient area, most likely due to reductive dechlorination of TCE in the downgradient areas. Most of the δ 13 C values of TCE in the source area of the upgradient part of the study area ranged from −26.8‰ to −25.5‰ (Figure 4c). The depleted δ 13 C values of TCE in most wells located in the upgradient area suggest very little degradation of TCE in this part of the study area. The isotope values of TCE in certain wells of the upgradient area were slightly enriched in aerobic conditions, indicating the possibility of aerobic cometabolic TCE degradation processes in these localities. Isotopic fractionation of TCE under aerobic conditions may lead to only slightly enriched δ 13 C values (Chu et al. 2004), which agrees with the isotopic observations in this area. In the downgradient parts of the flow system, TCE was found to be enriched in δ 13 C when sampled from wells with low DO concentrations, and furthermore, cisDCE and VC were found to be present in these wells. The most enriched δ 13 C values of TCE were found in 880

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the downgradient area. For example, MW-5 consistently showed enriched values (−22.3‰ to −23.8‰) over the investigation period (Table 1). Similarly, GW-10 showed enriched values during two of the three sampling events. The increase of TCE concentration in well GW-10 in August 2010 (Figure 4b) compared with the previous TCE concentration levels was associated with an isotopic depletion where the δ 13 C values of TCE were more than 2‰ depleted compared with previous measurements, which is most likely a reflection of mixing with freshly dissolved material and an indication of the possibility of a DNAPL source near well GW-10. Generally, the isotopic enrichments of TCE together with the decrease of TCE concentrations and the presence of by-products such as cis-DCE representing 75% and 22% of the remaining total VOC in wells MW-5 and GW-10 are good indicators of biodegradation (Figure 4a). Depleted δ 13 C values of cisDCE compared with the initial TCE values are expected to be observed during TCE degradation and the production of cis-DCE in an incomplete sequential degradation (Hunkeler et al. 1999; Bloom et al. 2000; Sherwood Lollar et al. 2001). In general, the downgradient wells show larger variation in both cis-DCE concentrations and δ 13 C values. The δ 13 C value of cis-DCE in well MW-5 (a downgradient well) is enriched (approximately −22‰) and is nearly quite similar to the δ 13 C value of TCE, which is a strong indication of biodegradation processes in that area. Similar characteristics were also observed in other downgradient wells, such as GW-12 (Figure 4d). Generally, although the carbon isotopic values of both TCE and cis-DCE were found to be enriched in most downgradient wells compared with those found in the upgradient wells, the enrichments are relatively limited, which indicates that the biodegradation processes in the area are moderate. An anomaly was observed in one of the downgradient wells (GW-19) in which both cis-DCE and VC (TCE degradation products) were observed. High cis-DCE concentrations (75% of the remaining total VOC) (Figure 4a) and low DO concentration (less than 1 mg/L) are good indicators of TCE degradation. Despite the TCE and cisDCE degradation in GW-19, the δ 13 C value of TCE was slightly more depleted than those found in the known source area (Figure 4c). The TCE concentration in GW19 fluctuated during the year and was higher than those in any wells in the downgradient area (Figure 4b); nonetheless, the δ 13 C values of TCE were consistently more depleted than the values assigned to the known source. The δ 13 C value of TCE in well GW-19 was more depleted in August 2010 than that in November 2009, which might indicate the presence of a secondary DNAPL source of TCE in the aquifer or production of TCE from PCE degradation. Compound-Specific Chlorine Stable Isotope Analysis The δ 37 Cl values of TCE were determined to investigate the presence of multiple contamination sources as well as to gain a better understanding of the fate of TCE in the subsurface. The combination of compound-specific NGWA.org

Figure 4. Molar fractions (a) and seasonal variations of TCE (b), δ 13 C value of TCE (c) and cis-DCE (d), δ 37 Cl of TCE (e) and cis-DCE (f).

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Table 1 Concentrations and Isotopic (δ 13 C and δ 37 Cl) Values of TCE from Sampled Groundwater in the Study Area November 2009 13

Samples

TCE (μg/L)

δ C (‰)

KDPW-2 SKW-1 GW-1 GW-3 GW-8 GW-9 GW-10 GW-12 GW-19 MW-5 MW-9 MW-11 MW-12 MW-21 MW-22 PW-5 PW-7

10,066 N.M. 243 518 195 109 62 159 105 27 95 340 462 265 548 393 73

−26.7 N.M. −25.1 −25.6 −26.2 −24.6 −22.3 −25.8 −26.9 −23.3 −24.6 −25.6 −25.9 −26.2 −25.5 −26.0 −26.1

March 2010 δ

37

Cl (‰)

−0.3 N.M. 0.8 N.M. N.M. N.M. −0.3 −0.3 0.3 2.4 0.2 N.M. N.M. N.M. −0.5 N.M. 20.6

13

August 2010

TCE (μg/L)

δ C (‰)

TCE (μg/L)

δ 13 C (‰)

7269 11,304 203 356 213 119 80 168 166 20 70 230 378 179 405 161 73

−25.7 −25.8 −25.6 −25.7 −26.2 −26.1 −22.8 −24.9 −28.2 −22.3 −25.2 −25.7 −24.9 −25.9 −25.8 −26.3 −26.1

6842 10,444 108 361 199 140 140 227 76 20 80 40 578 N.M. 438 N.M. 56

−24.2 −25.5 −25.3 −25.6 N.M. −24.4 −24.9 −25.4 −27.8 −23.8 −25.5 −24.9 −25.5 N.M. −25.1 N.M. −24.9

δ

37

Cl (‰)

−0.1 −0.5 −0.9 −0.6 N.M. −1.1 −0.8. −1.2 0.8 10.7 −0.8 −0.7 −0.9 N.M. −1.6 N.M. 24.6

N.M., not measured.

chlorine and carbon isotope analyses increases the resolution of the data for use of CSIA as a fingerprinting tool even when reductive dechlorination processes are present in the plume. Recent studies have recommended the use of dual isotope measurements to differentiate between contaminant sources and to distinguish between variations that are due to biodegradation from variations that are due to the presence of multiple sources (Hunkeler et al. 2008; Blessing et al. 2009; Hunkeler and Aravena 2010; Lojkasek-Lima et al. 2012). The δ 37 Cl values of TCE ranged from −0.5‰ to +20.6‰ in November 2009 and −1.6‰ to +24.6‰ in August 2010 (Table 1). The δ 37 Cl values of TCE obtained from two sampling campaigns showed variations of nearly 3‰ except for MW-5 and PW-7, which showed larger δ 37 Cl variations (Figure 4e). The measured δ 37 Cl values of TCE ranged from −0.5‰ to −0.3‰ in the main source areas (KDPW-2 and SKW-1) where the TCE concentrations were also high (10,000 μg/L). However, the isotopically enriched δ 37 Cl values (+2.4 to +24.6‰) of TCE were mainly concentrated in downgradient wells, especially in MW-5 and PW-7. The enriched δ 37 Cl value of TCE in MW-5 is consistent with the enriched δ 13 C value of TCE in that well. The dual enrichment is a good indicator of the presence of biodegradation in that well, which is in line with previously presented evidence. In general, all analyzed wells showed enriched δ 37 Cl values of cis-DCE compared with δ 37 Cl values of TCE from the same wells (Figure 4f). These observations are similar to those reported by Lojkasek-Lima et al. (2012) in their field study, and they attributed these enrichments to the degradation of TCE. The δ 37 Cl value (2.9‰) of cis-DCE in MW-5 is slightly more enriched than the δ 37 Cl value (2.4‰) of TCE of the same well, which is an indication that the TCE is undergoing degradation 882

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and the degradation process is slightly extended beyond cis-DCE. This observation is supported by the redox conditions (high concentrations of ferrous iron and low concentrations of nitrate and sulfate concentrations) as well as the VOC molar distribution. The GW-12 well shows enriched δ 37 Cl value of cis-DCE (2.9‰) associated with depleted δ 37 Cl of TCE (−0.3‰). The enriched δ 37 Cl value of cis-DCE values are a strong indication of degradation in the area even if the δ 37 Cl value of TCE in GW-12 does not show signs of enrichment. The depleted δ 37 Cl of TCE can be attributed to TCE input from a localized DNAPL source or from the degradation of PCE that masks any isotopic shifts due to degradation. The VOC molar distribution presented in Figure 4a shows the presence of PCE in GW-12. In MW-5, the large variation (approximately 8‰) of the δ 37 Cl values of TCE between two rounds of sampling events (November 2009 and August 2010) cannot be easily explained by biodegradation. The carbon isotopic variation is much smaller than the chlorine isotope variation, which is not common. Further investigation will be necessary to better understand this observation and determine whether this variation is related to a process or is simply due to a different source of contaminant in the study area. Identification of Contaminant Sources Table 1 and Figure 5 show the large variation of δ 13 C and δ 37 Cl values of TCE in the contaminated site (greater than 12‰ for chlorine isotopes and close to 6‰ for carbon isotopes). These large variations as well as the clustering of certain isotopic data (Figure 5) are indicative of an active site where a complex scenario of transformation processes with multiple sources and multiple processes take place. NGWA.org

The primary “known” contamination source is located at the RAO property in Gangwon Province in the western part of the study area. The δ 13 C and δ 37 Cl values of TCE at the main source can be represented by the isotopic values of several wells, i.e., KDPW-2 and SKW-1. Although, the isotopic values in these wells are generally tagged with a tight range for both chlorine and carbon isotopes (Figure 5), slight enrichments of both carbon and chlorine isotopic values of KDPW-2 can be explained by isotopic shifts caused by degradation processes in the source area. All known degradation processes tend to enrich both carbon and chlorine isotopes but at different rates, which results in different slopes of enrichments if chlorine and carbon isotopes are plotted against each other (e.g., Figure 5). The presence of isotopic values that are significantly different from those of the known source area is a strong indication of degradation processes and/or the presence of multiple sources. Furthermore, the spatial distributions of TCE, cis-DCE, CT, and CF in Figure 2 show discontinuity in the contamination plume, which might suggest the presence of multiple sources of contamination in the area. The cis-DCE and VC were detected only in the upgradient and the downgradient areas, whereas cis-DCE and VC were not detected in the middle of the study area, which could be attributed to spatial variations in the biodegradation rates in various parts of the study area. The spatial distribution of CT and CF also indicate the possible existence of multiple contaminant sources other than the main TCE source because CT and CF were observed only in the upgradient area (Baek and Lee 2011). Well GW-19 is located in the downgradient part of the study area. In both sampling events (November 2009 and August 2010), the TCE δ 13 C and δ 37 Cl values at GW-19 were more depleted in carbon and more enriched

in chlorine isotopes compared with those of the known source area. These isotopic values of GW-19 cannot be linked to the isotopic values found in the known source area by any known process. The measured geochemical parameters in GW-19 showed anaerobic conditions with low concentrations of DO. The TCE in GW-19 represented 75% of the remaining total of VOC, and cis-DCE was present with concentrations of 33-51 μg/L. The typical behavior of δ 13 C and δ 37 Cl values during any degradation process is enrichment of both isotopes; however, GW-19 does not fall in line with any possible degradation trend that can be originated from the known source area (Figure 5). The isotopic values illustrated in Figure 5 points toward a different source of TCE in GW-19 than that of the known source area. Similarly, the isotopic values in MW-22 do not fit on any possible degradation trend that can be originated for the known source area. The isotopic values of MW-22 are more enriched in the δ 13 C values and more depleted in the δ 37 Cl values than can be attributed to the known source area (Figure 5). Well MW-22 showed aerobic conditions with high concentrations of DO (4.0-5.1 mg/L) and no presence of cis-DCE during all sampling periods. This observation further supports the idea of an additional source that may have produced the contamination evidence in MW-22 (TCE concentrations of 438–548 μg/L), which is located at a dry cleaning agency. The concentration of TCE sampled in GW-10 increased from 80 to 140 μg/L between the two sampling events in March and August 2010 (Figure 4b), and the δ 13 C values of TCE were depleted from −22.3‰ to −24.9‰. The measured geochemical parameters in GW-10 showed anaerobic conditions with low concentrations of DO and measurable cis-DCE concentrations during the sampling periods. In November 2009, GW-10 showed enriched δ 13 C of TCE and depleted δ 37 Cl of TCE compared with those isotopic values of the main known source even though other parameters of the water sample from this well support the presence of biodegradation (Figure 5). The redox and VOC data from GW-10 located near MW-5 clearly indicated reductive dechlorination activity. However, the δ 13 C and the δ 37 Cl values of TCE in GW-10 are not in line with the process occurring in well MW-5. This observation suggests that the TCE in GW-10 underwent a different degradation pathway that is localized in the GW-10 area, or alternatively, that belongs to a different source of contamination. In August 2010, the depleted δ 13 C and δ 37 Cl values of TCE observed in GW10 could be due to mixing with freshly recharged water in the upgradient area that carried over contaminants with depleted δ 13 C and δ 37 Cl of TCE (i.e., similar to the main contamination source). Well PW-7 represents another possibility of a separate contamination source; this well showed a small separate plume with concentrations of 56-140 μg/L of TCE, as shown in Figure 2. Furthermore, the δ 13 C and δ 37 Cl values of TCE plotted in Figure 5 show that PW-7 falls in a separate domain from all other wells. The δ 37 Cl values of TCE in PW-7 are the most enriched values in the entire

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Figure 5. Values of δ 13 C vs. δ 37 Cl in TCE for groundwater sampled in November 2009 and August 2010.

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site; however, the δ 13 C values of TCE in PW-7 were similar to those in the known TCE source (Figure 5). The geochemical parameters of PW-7 revealed anaerobic conditions with low concentrations of DO. The δ 13 C and δ 37 Cl values of TCE in PW-7 (Figure 5) point toward the presence of an end member. There are no known processes that can enrich chlorine isotopes to that extent without shifting the carbon stable isotope ratios. The δ 13 C and δ 37 Cl values in PW-7 are either an indicator of a separate source or could be caused by a process that has yet to be determined. Further investigation is needed to better understand the outcome results found in PW-7. It is worth noting that PW-7 is located near the paper manufacturer in the downgradient area. The enriched δ 37 Cl values of TCE and cis-DCE in MW-5 indicated the presence of biodegradation. However, the δ 37 Cl value of TCE in MW-5 was drastically enriched by 8‰ between the two sampling periods (Table 1). This observation cannot be easily explained by a simple biodegradation process. This isotopic variation might indicate a different contaminant source and possibly different degradation pathways from that of the primary contamination source. In general, Figure 5 shows that the sampling points are not aligned with a single trend as expected in the presence of a single source with a single process scenario.

the δ 13 C and δ 37 Cl values of TCE in the local source zones. Despite the complicated states of contamination, dual isotope analysis data can be usefully applied to evaluate the fate and transport of the contaminants as well as to differentiate among the five contaminant sources. These results support the usefulness of CSIA as a method for differentiating multiple plume sources. The identified contaminant sources and the behavior of trapped DNAPL obtained from this study can be used to assign remediation responsibilities to the responsible parties.

Acknowledgments This work was supported by the National Research Foundation of Korea Grant funded by the Korean Government (NRF-2009-351-2-C00168), “The GAIA Project (173-092-009)” by the Korea Ministry of Environment, and the Brain Korea 21 Project (through the School of Earth and Environmental Sciences, Seoul National University). Suggestions provided by Professor Prabhakar Clement at Auburn University, Alabama, are kindly acknowledged. The authors also thank the three anonymous reviewers for valuable comments that improved the quality of this article.

Supporting Information Conclusion In the study area, the upgradient areas occur mostly under aerobic conditions with high concentrations of DO, whereas the downgradient areas occur generally under anaerobic conditions with low concentrations of DO, NO3 − , and elevated concentrations of Fe2+ . This observation suggests that reductive dechlorination of TCE can be present in the downgradient areas. The use of the dual isotopic approach was useful for determining of the presence of trapped and dissolved contaminant source(s) in the study area. Furthermore, isotope data were also employed to evaluate the fate of contaminants in the subsurface. The enriched δ 13 C and δ 37 Cl values of TCE compared with those of the main known source indicated that anaerobic degradation takes place in the downgradient area. This observation was supported by the geochemical data and the presence of degradation by-products such as cis-DCE and VC. It is quite difficult to allocate the contaminant sources in the study area because the area is located in an industrial complex, and several small contaminant sources produce small pockets of plumes that are mixed with dissolved TCE from the trapped residual DNAPLs that are affected by freshly recharged water. Although it is rather difficult to differentiate among the contaminant sources in a complicated industrial area, several small sources and one main source were identified in the study area using δ 13 C and δ 37 Cl analyses combined with hydrogeochemical data. Freshly recharged water with depleted δ 13 C and δ 37 Cl values of TCE from the upgradient recharge zone of the main sources affected the plumes and changed 884

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Additional Supporting Information may be found in the online version of this article: Table S1. Specifications of the wells. Table S2. Measured hydrogeological samples collected in August 2009. Table S3. Measured hydrogeological samples collected in November 2009. Table S4. Measured hydrogeological samples collected in March 2010. Table S5. Measured hydrogeological samples collected in August 2010.

parameters from parameters from parameters from parameters from

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