Bull Environ Contam Toxicol DOI 10.1007/s00128-015-1513-9
Hydrochemistry Indicating Groundwater Contamination and the Potential Fate of Chlorohydrocarbons in Combined Polluted Groundwater: A Case Study at a Contamination Site in North China Shuang-bing Huang1,2 • Zhan-tao Han1,2,3 • Long Zhao1 • Xiang-Ke Kong1,2
Received: 27 November 2013 / Accepted: 2 March 2015 Ó Springer Science+Business Media New York 2015
Abstract Groundwater contamination characteristics and the potential fate of chlorohydrocarbons were investigated at a combined polluted groundwater site in North China. Groundwater chemistry and 2D and 18O isotope compositions indicated that high salination of groundwater was related with chemical pollution. The elevated salinity plume was consistent with the domain where typical chlorohydrocarbon contaminants occurred. The concentrations of heavy metals, oxidation–reduction potential, and pH in organic polluted areas significantly differed from those in peripheral (background) areas, indicating modified hydrochemistry possibly resulting from organic pollution. Under the presented redox conditions of groundwater, monochlorobenzene oxidation may have occurred when the trichlorohydrocarbons underwent reductive dechlorination. These findings suggested that inorganic hydrochemistry effectively indicated the occurrence of chemical contamination in groundwater and the potential fate of chlorohydrocarbons. Keywords Salination Hydrochemistry Chlorohydrocarbon Redox condition North China
& Shuang-bing Huang
[email protected] & Zhan-tao Han
[email protected] 1
Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences, CAGS, Shijiazhuang 050061, People’s Republic of China
2
Hebei Key Laboratory of Groundwater Remediation, Shijiazhuang 050061, People’s Republic of China
3
School of Civil Engineering and Geosciences, Newcastle University, Newcastle upon Tyne NE1 7RU, UK
Chlorinated solvents are a typical group of volatile organic contaminants (VOCs) prevalent in groundwater (USEPA 2009; Zogorski et al. 2006). Numerous works involving site-specific and/or areal groundwater contamination investigations (Zogorski et al. 2006; Rivett et al. 2012), have been conducted for more than 40 years in the US and European countries to mitigate the negative effects. It has been recognized that the surveys of chlorinated solvents face various challenges, such as difficulties in characterizing subsurface site parameters (Stroo et al. 2012), delineating dense non-aqueous phase liquids (DNAPL) (Dekker and Abriola 2000), and in developing accurate conceptual site models (National Research Council 2004). Concerns over typical organic contaminants (i.e. chlorohydrocarbons, CHCs for short) in groundwater were raised since the early twenty-first century in China, when National Drinking Water Quality Standards GB 5749-2006 (Ministry of Health and Standardization Administration of PR China 2006) were updated to incorporate organic components and the National Groundwater Quality Investigation and Assessment Program of China Geological Survey 2006–2010 was initiated (Zhang et al. 2013). The completed areal investigation for the North China Plain (Zhang et al. 2009) revealed a similar scenario to that in the USA and UK, with CHCs and gasoline compounds having the most frequent occurrence in groundwater (Lawrence 2006). However, tracing groundwater contamination sources with contaminants like CHCs and corresponding site characterization are currently severely lacking in China. Groundwater is a major water source in the North of China, constituting 70 %–87 % of the total water consumption among different cities (Zhang et al. 2009). However, groundwater chemistry is complex and influenced both naturally and anthropogenically (Zhang et al. 2013). To conduct site-specific groundwater investigations in this area
123
Bull Environ Contam Toxicol
would not only benefit an authentic understanding of contaminant levels on a site-specific basis, but also contribute to a clear recognition of groundwater chemical evolution under coupled contaminated condition. This study took a chemically contaminated site in North China as an example to explore CHC contamination of groundwater downstream from a surface tank once used for waste discharge storage at a former chemical plant. The data collected have shown that the chemicals used by the plant were mainly CHC compounds including monochlorobenzene (MCB), dichlorobenzenes (DCBs), choroethane, and chloroform, as well as inorganic materials with MgO, HCl, H2SO4, and NaOH. Recognition of contaminant occurrence and groundwater chemical characteristics were the main concerns at the initial stage of investigation. We employed hydrochemical methods, including salinity tracing, isotopic methods, and redox thermodynamics theory to assist in estimating the scope of distribution of possible organic contaminants, and to thermodynamically understand the conditions that may affect their degradation. The objectives of this study were as follows: (1) to determine the occurrence of chemical pollution in the groundwater downstream from the tank, and (2) to evaluate the geochemical conditions where CHCs occurred in the groundwater relative to their possible fates.
Materials and Methods The studied site is located in the southeast region of Hebei province, North China, with a semi-arid climate and intensive evaporation. The tank was initially dug by local residents to provide the clay materials for brick production. With a size of length 240 m, width 150 m and depth approximately 1.2 m, the formed tank was used for storage of discharges from the plant during 1997–1998, when the chemical oxygen demand and pH detected for the water in the tank were 800 mg/L and *2 respectively and thus the tank become a suspect pollution source. The study field was the area covered by Fig. 1a, mainly consisting of widespread farm lands and marginally distributed villages and rivers, with a total investigated area of approximately 10 km2. The Xuanxuan River in the region flows from south to north. Groundwater was mainly discharged from the center of the area toward the peripheral Shang and Xuanxuan Rivers during the survey (Fig. 1a). The measured hydraulic gradient of groundwater was within the range of 1/10,000–1/1000. The main stratum materials within the explored area included clay, clayey silts and fine sands, with an increase in sands from the south to the north (Fig. 1b). Three detailed water-bearing layers were identified (Fig. 1b). The major water-extracting layers for agricultural irrigation are at depths between 13 and 20 m which match well with those
123
of our defined primary aquifer termed as unit II in the profile of Fig. 1b. Aquifers I and III are comparatively much less continuously developed or of a less significant water-bearing (Fig. 1b) nature, and were not of concern for sampling. Existing agricultural irrigation wells were utilized to collect groundwater samples to explore the distribution of dissolved-phase contaminants in the area. Ten sampling wells were also built in the primary aquifers (II in Fig. 1b) near the north edge of the tank (NT) according to the salinity distribution and the estimated potential pollution range. Soil/sediment samples were collected to investigate the transport signatures of typical pollutants between soil and groundwater utilizing a geotome or PowerProbe 9500-VTR (AMS Inc., American Falls, ID, USA) at the bottom of the tank (BT) and in the NT, respectively. The sites for groundwater sampling and soil drilling are shown in Fig. 1a. Groundwater was withdrawn by a flow rate-adjustable peristaltic pump. Physical and chemical parameters, including dissolved oxygen (DO), specific electronic conductance (EC), oxidation–reduction potential (ORP), and pH were measured on site. Groundwater samples were collected at a flow rate of 600 ml/min under stable readings of the parameters (i.e., ORP pH, DO, and EC). During sediment/soil collection, the original sampling pipes mounted on the PowerProbe were cut open for an on-spot concentration test using a MiniRAE PGM-7300 gas detector (RAE Systems, Sunnyvale, CA, USA). Next, a total of 15 screened soil samples were sampled and transferred with a disposable syringe into a brown glass bottle added with HCl. Water samples for CHCs analysis were collected using 1 L brown glass bottles rinsed with nitric acid, whereas those for inorganic analysis were stored in high density polyethylene (HDPE) bottles. The samples for cation analysis were filtered by a 0.45 lm cellulose acetate membrane, and the pH was adjusted to \2 with ultra-pure HNO3 (Huang et al. 2012). Samples for DOC analysis were adjusted to pH 2 with HCl. Samples for anions, as well as 2D and 18O isotope analyses, were directly stored into bottles. All water and soil samples were kept in a -4°C incubator prior to analysis. The major cations and anions were determined by an inductively coupled plasma atomic emission spectrometer (ICPAES) (Thermo Electron Corporation, Madison WI, USA) and an ion chromatograph (IC) (Metrohm, Herisau, Switzerland), respectively. The trace elements (i.e., Cu, Ni, Zn, Cr) were measured by an inductively coupled plasma mass spectrometer (ICP-MS) (PerkinElmer SCIEX Instruments, ON, CA). Alkalinity was measured by the titration method. Samples were also shipped to ALS Chemical Analytic Testing Corporation (Shanghai center) for CHC analyses within 1 week. The charge balance error of the inorganic ions was within 5 % for all samples. The reported recovery rate of standard addition for CHCs ranged from 88.5 % to 125 %.
Bull Environ Contam Toxicol
Fig. 1 a Location of the study area and sites of groundwater sampling, soil drilling, and TDS contour. JC denotes the newly built monitoring well, and the other symbols are agriculture irrigation wells. Shadow blocks represent villages and all other blank areas are farm lands. b Schematic cross section showing stratum materials,
aquifer formations and groundwater sampling depth (screen location) along the profile line in a; unbroken horizontal lines denote the boundaries of generalized aquifer units; wells and drilling sites near the profile line are all projected to the profile; two boreholes (EB01&EB02) for lithology exploration are also displayed
Results and Discussion
be related to evaporation effects as groundwater fell away to the lower right of the local precipitation line on the 2D and 18O plot showing extensive evaporation (Fig. 2a). However, TDS diffused around the tank as a central source (Fig. 1a) (5.9–11.2 g/ L). The TDS in groundwater near the tank (GNT; i.e., JC2, JC3) (1.5–3.2 g/L) was much higher than in groundwater far from the tank (GFT; i.e., G01, G22) (Table 1), suggesting the impacts of the chemical pollution in the tank on GNT.
The lower levels of total dissolved solids (TDS) of groundwater samples (i.e.G01, G22, G19) were at levels between 1 and 2 g/L, which are consistent with the hydrochemical characteristics of the background water (Zhang et al. 2009). More frequently, however, higher salinities with big variations were observed in groundwater (Table 1). The elevated salinity of groundwater can
123
123
6.8
6.8
6.8
7.0
7.0
7.0 6.7
7.0
7.1
7.3
7.6
7.2
6.9
7.1
7.0
7.4
7.2
JC3
JC4
JC5
JC6
JC7 JC8
JC9
JC10
G30
C06
G19
GB42
C01
G05
G31
G34
-48.6
-88.2
-44.3
-46.7
-13.5
-58.3
-77.5
-128
-49.9
-42.7
-11.5 -34.5
21
-50.2
-58.5
4.3
-18.1
2.8
-34.2
-32.3
-2.7
-6.9
-103
-26.6
-4.3
ORP mv
12.98
6.21
6.46
4.16
6.56
3.44
5.80
4.28
12.96
11.72
13.66 17.87
11.94
18.10
17.65
19.86
22.33
19.97
2.95
4.79
5.96
4.13
3.49
3.34
3.63
EC mS/cm
0
0
0
0
0
0.76
0.14
0.62
0.67
0
0.62 0.76
1.1
0.8
0.26
0.5
0.16
0.95
0
0.24
0
0.45
0.84
2.39
0.04
DO mg/l
The symbol ‘‘–’’denotes undetermined values
7.1
JC2
7.1
G36
JC1
7.4
G37
7.0
7.9
G21
7.3
7.4
G22
G35
7.3
G01
G40
pH
Well Ids unit
6.50
3.10
3.23
2.08
3.27
1.72
2.90
2.13
6.48
5.86
6.83 8.93
5.97
9.04
8.83
9.93
11.2
9.99
1.48
2.39
2.98
2.07
1.74
1.67
1.82
TDS g/L
6.44
2.59
170
4.98
0.98
14.1
4.07
2.15
3.52
4.89
1.02 4.73
2.22
5.12
1.97
1.15
2.55
4.94
0.56
1.12
1.63
0.82
14.9
0.44
0.72
K mg/L
314
230
333
254
318
149
86.1
265
262
212
282 580
283
615
576
296
546
424
98.1
169
202
136
87.5
128
104
Ca mg/L
2410
979
637
430
780
452
1020
346
2180
1980
2630 2540
1950
2760
4250
3950
3850
1780
312
573
739
487
566
429
390
Na mg/L
Table 1 Physical-chemical measurements of groundwater from the study area
529
212
268
183
302
140
135
170
425
417
422 904
527
845
808
516
932
698
128
183
220
155
72
105
118
Mg mg/L
1.85
0.29
–
7.70
5.44
3.45
–
–
–
–
– 0.90
–
–
–
–
0.69
–
–
–
2.25
–
–
23.4
–
NO3 mg/L
0.09
0.10
–
0.26
0.17
0.07
–
–
–
–
– 0.49
–
–
–
–
0.25
–
–
–
0.01
–
–
0.07
–
NH4 mg/L
3510
1170
638
511
737
319
778
565
2440
2420
2960 4330
3080
4900
2800
2530
5190
2970
354
639
1040
516
542
510
382
SO4 mg/L
2750
1180
1520
773
1610
582
770
890
2830
2380
3310 4430
2350
4410
7630
5340
5880
3430
427
889
1130
724
423
424
554
Cl mg/L
710
678
518
630
641
783
1380
496
1020
960
1320 949
1160
900
1530
1910
1050
699
640
762
684
678
774
631
775
HCO3 mg/L
0.03
0.277
0.025
0.012
0.064
0.159
515
152
177
954
422
100
203
326
\0.05 0.62
4650
0.14
1750
1050 1700 3780
\0.05 \0.05 0.41 0.12
3460
0.38
1690
1660
\0.05 0.32
2220
2940
\0.05 0.07
266 891
\0.05 \0.05
652
273
\0.05 483
202
0.12
631
\0.05 \0.05
\0.05
Mn lg/L
Fe mg/L
243
2
2
4
2
1
4.1
14.4
11
12.2
54.8 45.6
9.1
25
12.3
59.3
42.2
25.8
4
3.6
4.9
3.7
3.1
2.9
3
Cu lg/L
7
3
4
3
3
2
3.5
10
13.2
11.8
8.5 24
9.4
24.8
21.2
6.6
12.4
9.3
5.6
3.5
7.6
4.9
4.1
4.2
4.1
Ni lg/L
4 5
\5 240
6 4
7 6
7 \5 \5
12.4
8
10.1
35
31.3
33.9 43.7
27.1
43.2
66.5
37.7
48.1
27
12.2
6.7
14.8
11.1
7.1
6.6
9.6
Cr lg/L
\5
43
47
29
64 84
21
56
22
31
92
83
14
8
28
10
9
8
15
Zn lg/L
Bull Environ Contam Toxicol
Bull Environ Contam Toxicol
Fig. 2 Typical hydrochemical characteristics of groundwater in the study area: a plot of d18O (%) versus dD (%); b piper diagram of groundwater; c relationship between SO42- and Cl-, and d relationship between Mg2? and Na?
The water types of GNT and GFT (representing waters from monitoring and irrigation wells, respectively) were obviously different. The chemical type of GFT (e.g., samples WG21 and WG01) was more similar to that of the entire regional background (Fig. 2b), which is rich in Ca2?, Mg2?, and HCO3- (Zhang et al. 2009). However, the dominant ions for GNT were Cl-, SO42-, Mg2?, and Na? (Fig. 2b). Correspondingly, the concentrations of GNT ions were much higher than for GFT ions (Table 1). Good correlations were also observed between Cl- and SO42-, as well as between Mg2? and Na? (Fig. 2c, d), indicating their accompanying occurrence. The information regarding hydrochemistry and salt pollution suggests that GNT samples, namely those in the high salinity area (Fig. 1a), may have suffered chemical pollution. Monochlorobenzene was found in almost all wells with only four exceptions (Table 2). The high concentrations were only found from the monitoring wells (i.e., JC3, JC4, JC1, and
JC7) near the tank (Fig. 1a). 1,4-Dichlorobenzene and 1,2DCB were also detected mainly from the monitoring wells near the tank (Table 2, Fig. 1a). The highest concentrations of 1,4-DCB and 1,2-DCB were also in well JC4 (Table 2), and the only detected 1,2,4-trichlorobenzene (1,2,4-TCB) was also from this well. Well JC4 was also found to contain phenol and 2, 4-dichlorophenol (2, 4-DCP) (Table 2). Soils/sediments from the BT or the NT were detected with chlorinated benzenes (CBs, i.e., MCB, 1,4-DCB, 1,2-DCB and 1,2,4-TCB), trichloromethane (TCM), and 1,2-dichloroethane (1,2-DCA). Similar with the pollution of groundwater, MCB had the highest detection rate and was found in both types of soils. 1,4-Dichlorobenzene, 1,2-DCB, and 1,2,4-TCB were detected in soils with a successive decreasing frequency. However, these CBs were found in almost all BT soils (Table 3). Trichloromethane was also found in all BT soils. Monochlorobenzene, 1,4-DCB and 1,2-DCB were detected
123
Bull Environ Contam Toxicol Table 2 Summary of concentrations of clorinated compounds detected in groundwater in comparison to U.S. Environmental Protection Agency (USEPA) Maximum Contaminant Levels (MCLs) for regulated compounds, with detected concentrations of two phenol compounds also presented (lg/L)
Well IDs detection limit
MCB 0.5 lg/L
1,4-DCB 1 lg/L
1,2-DCB 1 lg/L
1, 2, 4-TCB 0.5 lg/L
1,1,1-TCA 0.5 lg/L
Phenol 1 lg/L
2, 4-DCP 1 lg/L
MCLs
100
75
600
70
200
–
–
G01
–
1
–
–
–
–
–
G22
–
–
–
–
–
–
–
G21
–
–
–
–
–
–
–
G37
2
–
–
–
–
–
–
G36
1.9
–
–
–
–
–
–
G35
1.9
–
–
–
–
–
–
G40
2.3
–
–
–
–
–
–
JC1
22.9
6.8
2.2
–
–
–
–
JC2
0.7
1.0
–
–
–
–
JC3
765
3.7
1.4
–
–
JC4
785
138
39.3
1.7
JC5
0.8
–
–
–
JC6
1.9
–
–
–
JC7
24.9
1.6
–
JC8
1.5
3.6
JC9
0.6
3.9
JC10
1.3
1.2
G30
4.3
–
C06
5.1
G34
3.2
G19
–
–
1.3
7
0.6
–
–
–
–
–
–
–
–
–
1.4
–
–
–
–
1.5
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
2.2
–
–
–
–
–
1.4
1
–
–
–
–
–
GB42
0.9
–
–
–
–
–
–
C01
0.6
–
–
–
–
–
–
G05
1
1.2
–
–
–
–
–
G31
–
–
–
–
–
–
–
The symbol ‘‘–’’ denotes undetected values or not presented. The same meanings are maintained for the symbols in the following table. MCLs are from U.S. Environmental Protection Agency ‘‘List of Contaminants and their MCLs.’’ (US EPA 2009) MCB monochlorobenzene, 1,4-DCB 1,4-dichlorobenzene, 1,2-DCB 1,2-dichlorobenzene, 1,2,4-TCB 1,2,4trichlorobenzene, 1,1,1-TCA 1,1,1-trichloroethane, 2,4-DCP 2,4-dichlorophenol
both in soil and groundwater, and included in the list of the chemical solvents employed during the previous chemical production. Although 1,2,4-TCB was not a raw material in the production, it was also observed both in soil and groundwater. This condition suggests that 1,2,4-TCB may be the derivatives from the chlorination of MCB and/or DCB. The 1, 2-dichloroethane (1,2-DCA), trichloromethane (TCM), and 1,1,1-trichloroethane (1,1,1-TCA) were observed by chance in soil or groundwater. These could be presumed as the derivatives of chloroethane. The only detected phenol and 2, 4-DCP in groundwater may be the intermediates of CBs degradation that will be discussed later. Groundwater in the contaminated area (GNT, representing monitoring well waters) was observed with a significantly lower pH and higher Eh (Fig. 3a) and higher concentrations of heavy metals (e.g., Mn, Cu, Ni, Zn, and Cr) (Fig. 3b; Table 1). The lower pH could be linked with the carboxylic acid degradation products of chlorinated organic compounds (Appelo and Postma 2005; Sander et al. 1991) as listed from Eq. 4 through Eq. 6 in Fig. 4, whereas the higher Eh may be ascribed to the presence of the oxidative highly
123
chlorinated compounds. The higher concentrations of heavy metals could also be related to the stronger acidity at these sites. All of these possibilities suggest that modified hydrochemical conditions in high salinity areas may have resulted from organic chemical contamination. Reduction processes of oxidative species in groundwater such as O2, NO3-, MnOx, FeOOH, and SO42-, and the generation of CH4 and NH4?, generally occur in sequence (Huang et al. 2012; Stumm and Morgan 1996). Groundwater has no significant amounts of DO and low concentration of NO3-, but high concentration of dissolved manganese (Table 1). This scenario indicates that the redox status was qualified for manganese oxides to be reduced to dissolved Mn2?. High concentrations of SO42- from heavy SO42pollution are difficult to be eliminated by the reduction. Therefore, the reducing environment of groundwater remains at a level after which the sulfur reduction could happen. Iron was not detectable or detected with very low concentration (Table 1). This condition suggests that the redox status of groundwater was inadequate for the reduction of Fe(III) oxides.
Bull Environ Contam Toxicol Table 3 Organic contaminants in soil and their concentrations (mg/kg) Sample type or borehole No.
Soil sampling site or depth (m)
MCB
1,4-DCB
1,2-DCB
1,2,4-TCB
TCM
1,2-DCA
S1
Tank bottom
0.57
1.12
0.43
–
2.11
–
S2
Tank bottom
0.15
0.44
0.21
0.06
0.07
–
S3
Tank bottom
0.44
0.81
0.39
0.21
0.06
–
S4
Tank bottom
6.16
18.1
7.2
1.2
0.3
–
S5
Tank bottom
2.67
4.71
1.87
0.18
0.07
–
Z1
4.7–5.9
0.09
0.06
–
–
–
Z2
Z3
2.4–3.4
0.14
0.07
–
–
–
–
5.9–7.1
–
–
–
–
–
–
8.3–9.5
0.2
–
–
–
–
–
14–16
–
–
–
–
–
–
3.5–4.7 4.7–7.1
0.42 0.66
0.15 0.07
0.06 –
– –
– –
– –
7.2–8.3
0.64
0.07
–
–
–
–
8.3–9.5
0.45
–
–
–
–
–
23.5–24
–
–
–
–
–
0.21
S1–S5 are the mud samples from the BT, Z1–Z3 are soil/sediment samples collected from the borehole within a distance of 15 m west or NT (Fig. 1). TCM is trichloromethane
Fig. 3 Contrasting hydrochemical conditions in different groundwater samples. a ORP–pH: groundwater from the monitoring well marked in a circle has higher Eh and lower pH; b concentrations of
heavy metal ions, taking Mn and Ni as examples: groundwater from monitoring wells within rectangles has higher concentrations of Mn and Ni
Potential redox pairs in the groundwater system were drawn in Fig. 4 according to their reported values for Eh of half-reaction (Stumm and Morgan 1996). The Eh satisfying the oxidation of CBs is observed to be in the range bounded by the Eh threshold for the reduction of Mn(IV) oxides and that of SO42-, respectively. This value is the superior redox domain of the groundwater. However, Ehs for the reductive dechlorination of TCM and TCA are higher than the Eh favoring Mn(IV) reduction (Fig. 4). Monochlorobenzene should occur at a significant concentration within a certain area in the groundwater because of its high solubility, low soil sorption coefficient, and low density (EPI suite 2011; Lide 2007) suggesting a better
occurrence/mobility property compared to the other CBs. However, samples were not detected with a significant concentration except for only several GNTs. Two phenolic compounds (phenol and 2,4-DCP) that belong to hydroxylated aromatic compounds which could have been formed by oxidative dechlorination of CBs (IPCS 1991) were found in the well with highly contaminated CBs (JC4). This information suggests an oxidative degradation occurrence of MCB (or DCB). Given that no other electron acceptor is available in the groundwater suitable for the reduction of trichlorohydrocarbons (TCHs), the reductive dechlorination of TCHs could be
123
Bull Environ Contam Toxicol Fig. 4 Comparative oxidation– reduction reaction advantages of redox pairs in groundwater
triggered by the electron supply from MCB oxidation. This process is based on the energy yield (Stumm and Morgan 1996) suggested by the Eh difference between TCHs and MCBs (Fig. 4). This oxidation–reduction model makes sense because the reactions of MCB oxidation and chloralkane dechlorination could produce H? (Fig. 4). Highly chlorinated hydrocarbons can maintain the ORP of groundwater at a high level, which agrees well with the lower pH and higher Eh of groundwater within the area of high contamination with organic chemicals. Acknowledgments This research was financially supported by the National Natural Science Foundation of China (41302187), China Post-doctoral Science Foundation (2014M552105), the Institute of Hydrogeology and Environmental Geology, Chinese Academy of Geological Sciences (SK201303, SK201412), and the Chinese Geological Survey (12120113102700).
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