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Fs in a reservoir system in northern Taiwan
Chemosphere 83 (2011) 745–752
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Deposition fluxes of PCDD/Fs in a reservoir system in northern Taiwan Kai Hsien Chi a,⇑, Shih-Chieh Hsu b, Chuan-Yao Lin b, Shuh Ji Kao b, Tzu Yi Lee c a
Institute of Environmental and Occupational Health Sciences, National Yang Ming University, Taipei 112, Taiwan Research Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan c Environmental Analysis Laboratory, Taiwan Environmental Protection Administration, Chungli 320, Taiwan b
a r t i c l e
i n f o
Article history: Received 20 August 2010 Received in revised form 30 December 2010 Accepted 27 February 2011 Available online 23 March 2011 Keywords: Dioxin Water column Deposition Atmosphere Long-range transport
a b s t r a c t In this study, polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) concentrations and depositions in ambient air, water column and sediment were measured at a coupled reservoir-watershed system in northern Taiwan. The atmospheric PCDD/F concentration measured in the vicinity of the reservoir ranged from 4.9 to 39 fg I-TEQ m3 and the Asian dust storm in February accounted for the peak value, which corresponded to a total suspended particle concentration of 128 lg m3. The atmospheric PCDD/F deposition ranged from 1.4 to 19 pg I-TEQ m2 d1, with higher deposition occurring during winter and spring (long-range transport events). During summer, when atmospheric deposition is lower, consecutive tropical cyclones (typhoons) bring heavy rainfall that enhances soil erosion and creates turbidity-driven intermediate flow. This results in significantly higher PCDD/F deposition in water column of the reservoir at 70 m water depth (179 pg I-TEQ m2 d1) than at 20 m (21 pg I-TEQ m2 d1) during typhoon event. The accumulation rate of PCDD/Fs (9.1 ng I-TEQ m2 y1) in the reservoir sediments (depth: 0–2 cm) was consistent with PCDD/F deposition obtained from water column (6.1 and 8.3 ng ITEQ m2 y1); however, it is significantly higher when compared to the atmospheric deposition (2.0 ng I-TEQ m2 y1). Based on the mass balance between the measurements of atmospheric deposition and sinking particles in water column, around 54–74% of PCDD/F inputs into the reservoir were contributed by the catchment erosion during normal period. However, the PCDD/F input contributed by the enhanced catchment erosion significantly increased to 90% during intensive typhoon events. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ Fs) are classified as persistent organic pollutants (POPs). They are unwanted by-products of various combustion processes that can be emitted directly into the atmosphere, if not properly treated (Correa et al., 2006). Previous study (US EPA, 1989) also indicated that PCDD/Fs are the cancer hazard to humans; that exposure to PCDD/Fs can cause severe reproductive and developmental problems and PCDD/Fs can damage the immune system and interfere with regulatory hormones. In general, sediment serves as the ultimate sink for POPs, including PCDD/Fs and trace metal. Therefore, atmospheric transport and deposition serve as the main pathway transporting PCDD/Fs from emission sources to various environmental compartments (Lohmann et al., 1999; Ogura et al., 2001). Previous studies (Meijer et al., 2006; Bogdal et al., 2010) also indicated that high altitude lakes, which receive their contaminant inputs uniquely from the atmosphere through long-range atmospheric transport, provide ideal controlled environments for the study of the interactions between atmospheric depositional ⇑ Corresponding author. E-mail address: [email protected] (K.H. Chi). 0045-6535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2011.02.069
and water column biogeochemical processes. Taiwan is an island in the subtropics, located off the southeast coast of Mainland China. In the winter and spring, the country and its surrounding areas are often influenced by northeasterly winter monsoon winds originating from Central Asia. The winter monsoon not only brings cold air but transports air pollutants and dust over long distances to Taiwan (Hsu et al., 2008) and even to the Northwestern Pacific area. Due to the high persistence of PCDD/F compounds, concentrations of those contaminants in sediments decrease very slowly. Previous study (Oram et al., 2008) indicated that extensive monitoring of POPs in estuarine water and sediment, freshwater, air, and wastewater effluents and sludge were integrated with a mass budget model to provide a synthetic view of these emerging contaminants. In this study, the Feitsui Reservoir is the major source of domestic water for Taipei citizen, the potential risks of PCDD/F contaminated in this watershed is very important for public health. Hence, the deposition Fluxes of PCDD/Fs measured in this reservoir system was useful to evaluate the effectiveness of legislative actions on the inputs of contaminants. In order to quantify the relative PCDD/F deposition fluxes and identify probable sources, a comprehensive and systematic investigation was conducted to measure atmospheric deposition, settling, and sedimentary deposition in the Feitsui reservoir.
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2. Experimental 2.1. Sampling sites The Feitsui Reservoir is the source of domestic water supply for the Taipei Metropolitan Area, which has a population of over five million. The reservoir with a storage capacity of 406 million m3 is approximately 170 m above sea level. The Feitsui reservoir has a surface area of 10.3 km2, a mean depth of 40 m and a maximum depth of 114 m near the dam. The catchment area is 30 times greater than the reservoir surface area. Reservoir construction began in mid-1979 and was completed for water storage in mid1987. Based on the meteorological data collected at the control center of the investigated reservoir, the prevailing wind for all four seasons is from east and the average wind speed is less than 2 m s1. During the northeastern monsoon episode, the prevailing wind changes to a northeast wind and the average wind speed increases to greater than 4 m s1. The mean annual precipitation rate in this area is approximately 2500 mm. In the northwest of the reservoir, there are two municipal wastes incinerators about 15 km from the reservoir. Those two incinerators could be the possible emission sources of PCDD/Fs in the vicinity of the Feitsui Reservoir. 2.2. Sample collection and analysis To measure atmospheric PCDD/F deposition and determine the ambient air concentration of PCDD/Fs in northern Taiwan, a sampling site was established at the control center of the
investigated reservoir (Fig. 1). Ambient air (n = 11) and atmospheric deposition (n = 12) PCDD/F samples were obtained monthly in 2008. Ambient air samples for both vapor and solid phases of PCDD/Fs were collected using sampling trains (Sibata HV-700F) for semi-volatile compounds. Fiber glass filters were utilized for collection of all particle-bound compounds, while polyurethane foam (PUF) plugs were utilized for retention of PCDD/F compounds in the vapor phase. The total volume of the air sampled was greater than 1000 m3 for a typical sampling duration of 7 d (gas flow rate: 100 L min1). The ambient air samples were taken during the second or third week of the month at the control center of the reservoir. At the same location, the monthly (30 d) measurement of atmospheric PCDD/F deposition was using mirror-polished stainless steel cylindrical vessels (D: 500 mm, H: 600 mm). Prior to sampling, approximately 10 L of deionized water were added to the vessel to cover the surface of the bottom. In order to measure the PCDD/F deposition flux in the water column, sinking particles in the reservoir were monthly collected at water depths of 20 and 70 m (n = 12 2 = 24), respectively, near the dam site (Fig. 1). A sediment trap array consisting of four cylindrical plastic core tubes 10 cm in diameter and 70 cm in length was used. Additionally, the PCDD/F concentrations in the surface water (n = 6) and sediments (n = 12) were measured at upstream, the middle and lower reaches of the reservoir (Fig. 1). The surface water (water depth: 0.5 m) was collected using a portable sampling system with a fiber glass filter, PUF and gear pump. Sediment collection was performed directly by cautiously inserting plastic tubes into the bottom sediment by hand. In June 2008, a 32 cm
Taiwan
Taipei county
Up stream Control center Middle reaches Lower reaches
Dam
Feitsui Reservoir Ambient air Sampling point Surface water and sediment sampling points Fig. 1. Sampling locations and scope of the catchment area of the Feitsui Reservoir situated in northern Taiwan.
K.H. Chi et al. / Chemosphere 83 (2011) 745–752
sediment core was collected at a location downstream near the reservoir. The site was at the former site of a tea farm along riverbanks that were flooded after the reservoir’s completion. The core was delivered to the laboratory within 2 h and was sectioned immediately to slices with thickness of 1–2 cm. In this study, sediment samples were freeze-dried and then ground to 100–200 mesh-sized powder using an agate mortar and pestle. The water content of the sediment was calculated based on the variation of the mass weight of sediment before and after freeze-dried process. In addition, the total organic carbon (TOC) was analyzed with a HORIBA model EMIA CS500 equipped with a resistance furnace and an ND-IR detector. About 100 mg of sediment was fumed with HCl to remove carbonates. The de-carbonated sample was then combusted at 1350 °C in the analyzer. For PCDD/Fs analysis, the ambient (air and deposition) and environment (water trap and sediment) samples were then spiked with known amounts of internal quantification standards according to USEPA method 23 and 1613, respectively. The detail information regarding the extraction and clean-up procedure of PCDD/F samples was provided elsewhere (Chi et al., 2008, 2009). Finally, the PCDD/F samples were analyzed with high-resolution gas chromatography (HRGC)/high-resolution mass spectrometry (HRMS) (Thermo DFS) equipped with a fused silica capillary column DB-5 MS (60 m 0.25 mm 0.25 lm, J&W). For metal analysis, 0.2 g aliquots of the sediments collected by sediment trap in water were digested in an acid mixture of concentrated HNO3/HF (4 mL/2 mL) using an ultra-high-throughput microwave digestion system (MARSXpress, CEM Corporation, Matthews, NC, USA). Details of microwave digestion are provided elsewhere (Hsu et al., 2008). Digested solutions were analyzed for five major metals (Al, Fe, Na, Mg and Ca) using inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 2100DV, Perkin– Elmer™ Instruments, USA). 2.3. Quality control and data analysis A laboratory blank and matrix spike sample (40–400 pg PCDD/ Fs) were used in the analytical procedure for every eight samples for quality control. Method detection limits (0.04–1.3 pg g1) were determined from the blanks and quantified as three times the standard deviation of the mean concentration in the blanks. In this study, the concentrations of all laboratory blank samples are less than 1.0 pg (PCDD/Fs). The mean recoveries of standards for all 13 C12-2,3,7,8-chlorosubstituted PCDD/Fs range from 52% to 109%. The analyzed results are all within the acceptable 40–130% range, set by the US EPA in Method 23 and Method 1613. For data analysis, International Toxic Equivalent Factors (I-TEFs) are adopted to compare the potential toxicity of each PCDD/F congener in a mixture to the well-studied and understood toxicity of TCDD, the most toxic member of the group (US EPA, 1989). The I-TEF of each congener present in a mixture is multiplied by the respective mass concentration, and the products are summed to yield the 2,3,7,8TCDD International Toxic Equivalence (I-TEQ) of the mixture. 3. Results and discussion 3.1. Atmospheric PCDD/F concentration and deposition flux over the reservoir As shown in Fig. 2a, the atmospheric PCDD/F concentration and deposition flux measured within the vicinity of the reservoir ranged from 4.9 to 39 fg I-TEQ m3 and 1.4 to 19 pg I-TEQ m2 d1, respectively. The annual atmospheric PCDD/F input flux in the vicinity of the reservoir was 2.0 ng I-TEQ m2 y1. The results of our previous study (Chi et al., 2008) indicated that the atmospheric
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PCDD/F concentrations measured in the urban area of Taiwan ranged from 20 to 110 fg I-TEQ m3. In some Asian countries (such as Korea and Japan), the atmospheric PCDD/F concentrations in the urban area have been shown to range from 28 to 120 fg I-TEQ m3 (Makiya, 1999; Lee et al., 2007). The atmospheric PCDD/F concentrations measured in the vicinity of the reservoir were considerably lower than those measured in other countries. Compared to other relevant studies measured in Taiwan, the atmospheric PCDD/F depositions (1.4–19 pg I-TEQ m2 d1) measured in the vicinity of Feitsui Reservoir in northern Taiwan were significantly lower than that measured in an industrial area (1826 pg I-TEQ m2 d1) in northern Taiwan (Chi et al., 2009) and urban area (3.1–19 pg ITEQ m2 d1) in southern Taiwan (Shih et al., 2006). The low atmospheric PCDD/F deposition can be attributed to the lack of emissions and combustion sources within almost 10 km of the Feitsui Reservoir. However, significant increases in PCDD/F compounds and total suspended particles in ambient air were measured during an Asian dust storm episode (February 2008), and the highest deposition flux of atmospheric PCDD/Fs was observed during the northeast monsoon episode (December 2008). Recently, studies have shown concentrations of 0.97–51.2 pg I-TEQ m3 and 0.22– 3.45 pg I-TEQ m3 PCDD/Fs in the vicinity of electric waste processing facilities in the coastal provinces of Southeast China (Li et al., 2007a,b). Additionally, the results of our previous study indicated that atmospheric PCDD/F concentrations increased 2.5 and 3.2 times in northern Taiwan, during the Asian dust storm episode (Chi et al., 2008). In the winter and spring, Taiwan and the surrounding area is often under the influence of northeasterly monsoon winds originating in central Asia. After passage of the front, the strong northeasterly winds can occasionally carry dust and/or air pollutants that have undergone long-range transport (LRT) to Taiwan (Lin et al., 2004, 2007). To classify the air mass following each frontal passage in northern Taiwan according to the amounts of LRT dust and air pollutants, we have developed an objective method (Lin et al., 2004). The meteorological parameters, i.e., surface temperature, wind direction, wind speed, rainfall, and atmospheric concentrations of PM10 (fine particulate matter with a diameter smaller than 10 lm), NOX, CO, and SO2 observe at coastal stations and a mountain station in northern Taiwan are used to identify the time and intensity of the frontal passage. In this work, we develop an objective method to classify the air mass following each frontal passage in northern Taiwan according to the amounts of long-range transported dust and air pollutants. The meteorological parameters observed near the coasts such as temperature and winds can be used to identify the time and intensity of the frontal passage. The air mass behind a front usually contains Asian continental air, while winds before the frontal passage tend to be weak and thus local emissions can dominate distributions of air pollutants. For contrast purposes, analysis starts 1 d before the frontal passage and lasts for 4 d to cover the entire period of a typical frontal passage. The results indicated that the significant dust storm events and LRT of pollutants occurred during the northeasterly monsoon period (January to May and October to December) in 2008. The results shown in Fig. 2b clearly demonstrate that the most frequent occurrence of the events in 2008 was in February and December (16 and 14 d, respectively). Fig. 2b also shows the daily accumulation rainfall monitored by Central Weather Bureau station which located around our sampling site. During the summer season in 2008, there were 4 typhoons passing through Taiwan. The heavy daily precipitation rate was observed during the typhoon periods. The relationship between the daily rainfall amount and the significant LRT events observed in 2008 is irrelevant. However, the variations of atmospheric PCDD/F measurements are consistent with the identifier significant LRT events (Fig. 2a). Therefore, the significant amounts of PCDD/Fs observed in the present study may have been
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150
Ambient air
Atmospheric deposition
TSP 125
40
100 30 75 20 50 10
25
0 ar -0 8 A pr -0 8 M ay -0 8 Ju n08 Ju l-0 8 A ug -0 8 Se p08 O ct -0 8 N ov -0 8 D ec -0 8
M
Fe
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0
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100 Significant LRT dust & air pollutants events
20
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Daily rainfall (mm)
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Total suspended particle (TSP) concentration (µg/m3)
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n08
PCDD/F deposition flux (pg I-TEQ/m 2/day)
PCDD/F concentration (fg-I-TEQ/m 3)
50
400
5 0 Ja
08 n-
8 8 08 -0 y-08 n-08 l-08 g-08 p-08 t-08 v-08 c-08 -0 bc ar Apr a Ju Au O Se Ju Fe De No M M
500
Fig. 2. (a) Total suspended particle (TSP), atmospheric PCDD/F concentration and deposition flux measured within the vicinity of the investigated reservoir, (b) the monthly variation of the significant dust storm cases and long-range transport with pollutants events in 2008.
transported from China across the Taiwan Strait, reaching northern Taiwan during LRT episodes. 3.2. Comparison of PCDD/F deposition in the atmosphere and in the water column of the reservoir The sampling results indicated that the PCDD/F levels in the surface water (water depth: 0.5 m) measured at upstream, middle and lower reaches of the reservoir ranged from 14.9 to 15.2 fg ITEQ L1. The results indicate that the PCDD/F concentrations in the surface water were uniform and sufficiently mixed at different locations in the reservoir. However, the PCDD/F contents in the sediments changed significantly at different locations. The highest PCDD/F contents (2.0 ± 0.1 ng I-TEQ kg1 dry weight, d.w., n = 4) in the sediment were measured at lower reaches at the reservoir. The average PCDD/F content (0.53 ± 0.2 ng ITEQ kg1 d.w., n = 4) measured upstream was significantly lower than that measured at the middle reaches (1.1 ± 0.3 ng ITEQ kg1 d.w., n = 4). To further investigate the transportation of PCDD/Fs in the reservoir, PCDD/F depositions in the water column at two water depths (20 and 70 m) were monthly measured at lower reaches from January to December 2008. Fig. 3a shows that PCDD/F deposition fluxes measured in the atmosphere (1.4– 19 pg I-TEQ m2 d1) were lower than that measured in the water
column (10–30 pg I-TEQ m2 d1) at 20 m water depth during the same period. Interestingly, the temporal trend of PCDD/F deposition flux in water at 20 m is quite similar to the atmospheric PCDD/F deposition flux before the typhoon season (July, August and September). Fig. 3b indicated that PCDD/F deposition flux in the water at 70 m ranged from 9.1 to 179 pg I-TEQ m2 d1. These values were significantly higher than the PCDD/F deposition flux measured at 20 m water depth, especially during the specific typhoon events. Annual PCDD/F input fluxes in the water of the investigated reservoir were 6.1 and 8.3 ng I-TEQ m2 y1 at water depths of 20 and 70 m, respectively. The higher PCDD/F deposition flux in the water as compared with atmospheric deposition may be attributed to the catchment erosion at lower reaches of the reservoir. In September 2008, PCDD/F deposition flux measured at 70 m was 8.5 times higher than that measured at 20 m during the intensive typhoon event (total rainfall 1200 mm month1). To further identify the characteristics of sinking sediments collected in the water, crust-derived elements in water column collected at 20 m and 70 m were also analyzed. Table S3 in Supplementary Material indicated that the deposition fluxes of crust-derived elements (Al, Fe, Na, Mg and Ca) measured at 20 m water depth were quite similar to those measured at 70 m water depth during the normal periods. However, the deposition fluxes of those elements measured at
PCDD/F deposition flux (pg-I-TEQ/m 2/day)
K.H. Chi et al. / Chemosphere 83 (2011) 745–752
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40
(a)
Deposition in water column (20 m water depth) Atmospheric deposition
30
20
10
0
Jan-08 Feb-08 Mar-08 Apr-08 May-08 Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 Nov-08 Dec-08
PCDD/F deposition flux in water body (pg-I-TEQ/m2/day)
200
(b) 70 m water depth 150
20 m water depth
100
50
0
Jan-08 Feb-08 Mar-08 Apr-08 May-08 Jun-08 Jul-08 Aug-08 Sep-08 Oct-08 Nov-08 Dec-08
Fig. 3. (a) Comparison of PCDD/F deposition flux in atmosphere and water column (b) PCDD/F deposition flux in water column at different depth at lower reaches of the investigated reservoir.
70 m water depth during the intensive typhoon period were over 20 times higher than those measured at 20 m. These results suggest that the sharp increases in deposition fluxes of PCDD/F and mineral-derived elements at 70 m water depth may have resulted from a deep turbid layer that formed upstream owing to landslide and/or mud flows during the typhoon period. Fig. 4 shows the contributions of PCDD/F input sources to the reservoir during certain periods based on the PCDD/F deposition flux measured in the atmosphere and water column (20 m depths). The contribution of atmospheric PCDD/F deposition in the investigated reservoir can be obtained from the atmospheric deposition flux divided by the deposition flux collected in water column at 20 m water depth. Based on the mass calculations, the results indicate that 26–46% of the PCDD/F input flux in the reservoir was contributed by atmospheric deposition during normal periods. However, the contribution of atmospheric PCDD/F deposition increased to 57% and 66% during the Asian dust storm and the northeasterly monsoon episodes, respectively. In contrast, the contribution of atmospheric PCDD/F deposition decreased to 10% during the typhoon event, presumably owing to the enhanced catchment erosion around the reservoir. Fig. 5 demonstrates the distribution of sum-PCDDs and sum-PCDFs (based on seventeen 2,3,7,8-chlorosubstituted congeners) in ambient air, atmospheric deposition, surface water, deposition in the water column and
sediment samples, respectively. The results revealed that the highest distribution of PCDFs (80%) was observed in the vapor-phase ambient air sample. Increased distributions of PCDDs were also observed in the samples of aerosols, atmospheric deposition, trapped particles, and sediments. The origins of the increased trend of PCDD distributions are twofold. First, PCDDs are primarily distributed in the solid phase in ambient air (Lohmann et al., 1999; Chang et al., 2004). Therefore, deposited particles collected in the atmosphere and water enhance the PCDD distribution. The second factor refers to the effect of catchment erosion around the reservoir. PCDD/F sources in the reservoir included atmospheric deposition and soil erosion of the reservoir’s catchment. Previous studies (Jou et al., 2007; Kiguchi et al., 2007) have demonstrated that there is a higher distribution of PCDDs in soils. The results of our measurements suggest that around 54–74% of the PCDD/F input flux was contributed by catchment erosion during normal periods. Therefore, catchment erosion of the reservoir would increase the distribution of PCDDs in sediments. 3.3. PCDD/F input flux derived from a sediment core Table 1 lists the total organic carbon (TOC) and water contents at various depths of a sediment core collected at lower reaches of the reservoir. The TOC contents increase with depth and are thus
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Distribution of PCDD/F input source (%)
100
(a)
Catchment erosion
75
50
25
0
Distribution of PCDD/F input source (%)
Atmospheric deposition
Spring season
Summer season
Fall season
Winter season
100
Atmospheric deposition
(b)
Catchment erosion
75
50
25
0 Typhoon event
Asian dust stormepisdoe
Northeast monsoon episode
Fig. 4. Contribution of PCDD/F input sources of the investigated reservoir during (a) normal periods and (b) specific events.
Distribution of PCDD/F congener (%)
100
PCDD
PCDF
80
60
40
20
0 Ambient air Ambient air Atmospheric Surface Deposition in (vapor phase, (solid phase, deposition water (n=6) water body n=11) n=11) (n=12) (n=24)
Sediment (n=12)
Fig. 5. Distribution of sum-PCDDs and sum-PCDFs in different environmental matrices.
higher at the bottom of the core. Relevant studies regarding the sedimentation rate in the Feitsui Reservoir (Lo, 1994; Chen et al., 2006) have indicated that the 20 cm modern sediments correspond to a time span of 15 years (from June 1987 to August 2002), which yielded a mean sedimentation rate of 1.3 cm y1. In the present
study, the core bottom (depth: 29–30 cm) contained root debris exhibiting a yellowish color, distinguishing it from the grayish sediments of the upper core. The bottom layer is apparently the old soil, marking the year in which the reservoir was filled (A.D. 1987). Therefore, the mean sedimentation rate was determined
K.H. Chi et al. / Chemosphere 83 (2011) 745–752 Table 1 PCDD/F concentrations, total organic carbon and water contents in sediment core at different depths in the investigated reservoir. Depth (cm)
Estimated TOC content yeara (%)
Water PCDD/F PCDD/F input content (%) concentration flux (ng I(ng ITEQ m2 y1) TEQ kg1 d.w.)
0–2 2–4 4–6 6–8 8–10 10–12 12–14 14–16 16–18 18–20 20–22 22–24 24–26 26–28 28–29 29–30 30–32
2008 2007 2005 2004 2003 2001 2000 1999 1997 1996 1995 1993 1992 1991 1989 1987 –
71 60 54 50 53 50 55 57 56 54 42 37 39 44 41 45 34
1.2 1.2 1.0 0.9 1.0 0.9 1.2 1.4 1.2 1.1 1.2 1.3 1.6 1.5 1.6 2.2 2.0
2.0 1.7 2.9 2.8 1.8 1.7 2.7 3.0 3.1 2.6 2.1 1.8 2.9 1.8 1.3 1.6 0.80
9.1 8.1 15 16 9.9 10 16 16 15 13 14 13 20 12 8.8 9.5 –
a
The estimated year of each depth of sediment core is calculated by the mean sedimentation rate 1.4 ± 0.1 cm y1.
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sinking particles in water column, around 90% of the PCDD/Fs input into the reservoir were contributed by the enhanced catchment erosion during intensive typhoon period. These findings imply that pollutants deposited in the watershed during winter and spring must be transported to the reservoir by typhoon runoff and related physical erosion and that both of these phenomena play a major role in PCDD/F accumulation in the downstream reservoir. Acknowledgements The authors acknowledge the financial supports provided by National Science Council (NSC 98-2111-M-001-015-MY3) and Taiwan EPA (EPA-97-E3S4-02-04) of the Republic of China. Assistance provided by Prof. M.B. Chang, Mr. S.H. Chang and Mr. P.C. Hung of National Central University in analyzing the samples is also acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemosphere.2011.02.069. References
to be 1.4 ± 0.1 cm y1. The representative age of the sediment core at various depths was estimated from the sedimentation rate (Table 1). Table 1 also lists the PCDD/F concentrations at various depths of a sediment core collected at lower reaches of the reservoir. The results revealed that the PCDD/F concentrations ranged from 0.8 to 3.1 ng I-TEQ kg1. However, the variations of PCDD/F concentrations measured in different depths of the sediment core are within the narrow range. Based on the PCDD/F concentrations at different depths and the mean sedimentation rate, the annual input fluxes of PCDD/F to the reservoir can be calculated as follows:
PCDD=F input fluxðng I TEQ m2 y1 Þ ¼ PCDD=F concentrationðng I TEQ kg
1
d:w:Þ of each sediment core
Dry weight of each part in sediment core ðkg d:w:Þ Height of each part in sediment core ðcmÞ Sedimentation rateð1:4 0:1 cm y1 Þ Area of sediment coreð6:36 103 m2 Þ As shown in Table 1, the PCDD/F input flux into sediment of the reservoir ranged from 8.8 to 20 ng I-TEQ m2 y1. The PCDD/F input flux (9.1 ng I-TEQ m2 y1) obtained from the surface sediment (depth: 0–2 cm) was significantly higher than the atmospheric PCDD/F deposition (2.0 ng I-TEQ m2 y1); however, it was similar to the settling PCDD/F flux in the water column (6.1 and 8.3 ng ITEQ m2 y1 at depths of 20 m and 70 m, respectively). The catchment area of the reservoir is around 330 km2, which is 30 times greater than that of the reservoir area (10.2 km2). Therefore, the PCDD/F input flux measured in the reservoir is the sum of the PCDD/F input flux in the reservoir and that in its catchment area. 4. Conclusions According to the results obtained from the measurements of the investigated reservoir, the PCDD/F input flux (9.1 ng ITEQ m2 y1) obtained from the surface sediment was significantly higher than the atmospheric PCDD/F deposition (2.0 ng ITEQ m2 y1); however, it was similar to the settling PCDD/F flux in the water columns (8.3 ng I-TEQ m2 y1 at 70 m water depth). Based on the mass balance between atmospheric deposition and
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Deposition fluxes of PCDD/Fs in a reservoir system in northern Taiwan Kai Hsien Chi a,⇑, Shih-Chieh Hsu b, Chuan-Yao Lin b, Shuh Ji Kao b, Tzu Yi Lee c a
Institute of Environmental and Occupational Health Sciences, National Yang Ming University, Taipei 112, Taiwan Research Center for Environmental Changes, Academia Sinica, Taipei 115, Taiwan c Environmental Analysis Laboratory, Taiwan Environmental Protection Administration, Chungli 320, Taiwan b
a r t i c l e
i n f o
Article history: Received 20 August 2010 Received in revised form 30 December 2010 Accepted 27 February 2011 Available online 23 March 2011 Keywords: Dioxin Water column Deposition Atmosphere Long-range transport
a b s t r a c t In this study, polychlorinated dibenzo-p-dioxin and dibenzofuran (PCDD/F) concentrations and depositions in ambient air, water column and sediment were measured at a coupled reservoir-watershed system in northern Taiwan. The atmospheric PCDD/F concentration measured in the vicinity of the reservoir ranged from 4.9 to 39 fg I-TEQ m3 and the Asian dust storm in February accounted for the peak value, which corresponded to a total suspended particle concentration of 128 lg m3. The atmospheric PCDD/F deposition ranged from 1.4 to 19 pg I-TEQ m2 d1, with higher deposition occurring during winter and spring (long-range transport events). During summer, when atmospheric deposition is lower, consecutive tropical cyclones (typhoons) bring heavy rainfall that enhances soil erosion and creates turbidity-driven intermediate flow. This results in significantly higher PCDD/F deposition in water column of the reservoir at 70 m water depth (179 pg I-TEQ m2 d1) than at 20 m (21 pg I-TEQ m2 d1) during typhoon event. The accumulation rate of PCDD/Fs (9.1 ng I-TEQ m2 y1) in the reservoir sediments (depth: 0–2 cm) was consistent with PCDD/F deposition obtained from water column (6.1 and 8.3 ng ITEQ m2 y1); however, it is significantly higher when compared to the atmospheric deposition (2.0 ng I-TEQ m2 y1). Based on the mass balance between the measurements of atmospheric deposition and sinking particles in water column, around 54–74% of PCDD/F inputs into the reservoir were contributed by the catchment erosion during normal period. However, the PCDD/F input contributed by the enhanced catchment erosion significantly increased to 90% during intensive typhoon events. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/ Fs) are classified as persistent organic pollutants (POPs). They are unwanted by-products of various combustion processes that can be emitted directly into the atmosphere, if not properly treated (Correa et al., 2006). Previous study (US EPA, 1989) also indicated that PCDD/Fs are the cancer hazard to humans; that exposure to PCDD/Fs can cause severe reproductive and developmental problems and PCDD/Fs can damage the immune system and interfere with regulatory hormones. In general, sediment serves as the ultimate sink for POPs, including PCDD/Fs and trace metal. Therefore, atmospheric transport and deposition serve as the main pathway transporting PCDD/Fs from emission sources to various environmental compartments (Lohmann et al., 1999; Ogura et al., 2001). Previous studies (Meijer et al., 2006; Bogdal et al., 2010) also indicated that high altitude lakes, which receive their contaminant inputs uniquely from the atmosphere through long-range atmospheric transport, provide ideal controlled environments for the study of the interactions between atmospheric depositional ⇑ Corresponding author. E-mail address: [email protected] (K.H. Chi). 0045-6535/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2011.02.069
and water column biogeochemical processes. Taiwan is an island in the subtropics, located off the southeast coast of Mainland China. In the winter and spring, the country and its surrounding areas are often influenced by northeasterly winter monsoon winds originating from Central Asia. The winter monsoon not only brings cold air but transports air pollutants and dust over long distances to Taiwan (Hsu et al., 2008) and even to the Northwestern Pacific area. Due to the high persistence of PCDD/F compounds, concentrations of those contaminants in sediments decrease very slowly. Previous study (Oram et al., 2008) indicated that extensive monitoring of POPs in estuarine water and sediment, freshwater, air, and wastewater effluents and sludge were integrated with a mass budget model to provide a synthetic view of these emerging contaminants. In this study, the Feitsui Reservoir is the major source of domestic water for Taipei citizen, the potential risks of PCDD/F contaminated in this watershed is very important for public health. Hence, the deposition Fluxes of PCDD/Fs measured in this reservoir system was useful to evaluate the effectiveness of legislative actions on the inputs of contaminants. In order to quantify the relative PCDD/F deposition fluxes and identify probable sources, a comprehensive and systematic investigation was conducted to measure atmospheric deposition, settling, and sedimentary deposition in the Feitsui reservoir.
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2. Experimental 2.1. Sampling sites The Feitsui Reservoir is the source of domestic water supply for the Taipei Metropolitan Area, which has a population of over five million. The reservoir with a storage capacity of 406 million m3 is approximately 170 m above sea level. The Feitsui reservoir has a surface area of 10.3 km2, a mean depth of 40 m and a maximum depth of 114 m near the dam. The catchment area is 30 times greater than the reservoir surface area. Reservoir construction began in mid-1979 and was completed for water storage in mid1987. Based on the meteorological data collected at the control center of the investigated reservoir, the prevailing wind for all four seasons is from east and the average wind speed is less than 2 m s1. During the northeastern monsoon episode, the prevailing wind changes to a northeast wind and the average wind speed increases to greater than 4 m s1. The mean annual precipitation rate in this area is approximately 2500 mm. In the northwest of the reservoir, there are two municipal wastes incinerators about 15 km from the reservoir. Those two incinerators could be the possible emission sources of PCDD/Fs in the vicinity of the Feitsui Reservoir. 2.2. Sample collection and analysis To measure atmospheric PCDD/F deposition and determine the ambient air concentration of PCDD/Fs in northern Taiwan, a sampling site was established at the control center of the
investigated reservoir (Fig. 1). Ambient air (n = 11) and atmospheric deposition (n = 12) PCDD/F samples were obtained monthly in 2008. Ambient air samples for both vapor and solid phases of PCDD/Fs were collected using sampling trains (Sibata HV-700F) for semi-volatile compounds. Fiber glass filters were utilized for collection of all particle-bound compounds, while polyurethane foam (PUF) plugs were utilized for retention of PCDD/F compounds in the vapor phase. The total volume of the air sampled was greater than 1000 m3 for a typical sampling duration of 7 d (gas flow rate: 100 L min1). The ambient air samples were taken during the second or third week of the month at the control center of the reservoir. At the same location, the monthly (30 d) measurement of atmospheric PCDD/F deposition was using mirror-polished stainless steel cylindrical vessels (D: 500 mm, H: 600 mm). Prior to sampling, approximately 10 L of deionized water were added to the vessel to cover the surface of the bottom. In order to measure the PCDD/F deposition flux in the water column, sinking particles in the reservoir were monthly collected at water depths of 20 and 70 m (n = 12 2 = 24), respectively, near the dam site (Fig. 1). A sediment trap array consisting of four cylindrical plastic core tubes 10 cm in diameter and 70 cm in length was used. Additionally, the PCDD/F concentrations in the surface water (n = 6) and sediments (n = 12) were measured at upstream, the middle and lower reaches of the reservoir (Fig. 1). The surface water (water depth: 0.5 m) was collected using a portable sampling system with a fiber glass filter, PUF and gear pump. Sediment collection was performed directly by cautiously inserting plastic tubes into the bottom sediment by hand. In June 2008, a 32 cm
Taiwan
Taipei county
Up stream Control center Middle reaches Lower reaches
Dam
Feitsui Reservoir Ambient air Sampling point Surface water and sediment sampling points Fig. 1. Sampling locations and scope of the catchment area of the Feitsui Reservoir situated in northern Taiwan.
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sediment core was collected at a location downstream near the reservoir. The site was at the former site of a tea farm along riverbanks that were flooded after the reservoir’s completion. The core was delivered to the laboratory within 2 h and was sectioned immediately to slices with thickness of 1–2 cm. In this study, sediment samples were freeze-dried and then ground to 100–200 mesh-sized powder using an agate mortar and pestle. The water content of the sediment was calculated based on the variation of the mass weight of sediment before and after freeze-dried process. In addition, the total organic carbon (TOC) was analyzed with a HORIBA model EMIA CS500 equipped with a resistance furnace and an ND-IR detector. About 100 mg of sediment was fumed with HCl to remove carbonates. The de-carbonated sample was then combusted at 1350 °C in the analyzer. For PCDD/Fs analysis, the ambient (air and deposition) and environment (water trap and sediment) samples were then spiked with known amounts of internal quantification standards according to USEPA method 23 and 1613, respectively. The detail information regarding the extraction and clean-up procedure of PCDD/F samples was provided elsewhere (Chi et al., 2008, 2009). Finally, the PCDD/F samples were analyzed with high-resolution gas chromatography (HRGC)/high-resolution mass spectrometry (HRMS) (Thermo DFS) equipped with a fused silica capillary column DB-5 MS (60 m 0.25 mm 0.25 lm, J&W). For metal analysis, 0.2 g aliquots of the sediments collected by sediment trap in water were digested in an acid mixture of concentrated HNO3/HF (4 mL/2 mL) using an ultra-high-throughput microwave digestion system (MARSXpress, CEM Corporation, Matthews, NC, USA). Details of microwave digestion are provided elsewhere (Hsu et al., 2008). Digested solutions were analyzed for five major metals (Al, Fe, Na, Mg and Ca) using inductively coupled plasma optical emission spectrometry (ICP-OES) (Optima 2100DV, Perkin– Elmer™ Instruments, USA). 2.3. Quality control and data analysis A laboratory blank and matrix spike sample (40–400 pg PCDD/ Fs) were used in the analytical procedure for every eight samples for quality control. Method detection limits (0.04–1.3 pg g1) were determined from the blanks and quantified as three times the standard deviation of the mean concentration in the blanks. In this study, the concentrations of all laboratory blank samples are less than 1.0 pg (PCDD/Fs). The mean recoveries of standards for all 13 C12-2,3,7,8-chlorosubstituted PCDD/Fs range from 52% to 109%. The analyzed results are all within the acceptable 40–130% range, set by the US EPA in Method 23 and Method 1613. For data analysis, International Toxic Equivalent Factors (I-TEFs) are adopted to compare the potential toxicity of each PCDD/F congener in a mixture to the well-studied and understood toxicity of TCDD, the most toxic member of the group (US EPA, 1989). The I-TEF of each congener present in a mixture is multiplied by the respective mass concentration, and the products are summed to yield the 2,3,7,8TCDD International Toxic Equivalence (I-TEQ) of the mixture. 3. Results and discussion 3.1. Atmospheric PCDD/F concentration and deposition flux over the reservoir As shown in Fig. 2a, the atmospheric PCDD/F concentration and deposition flux measured within the vicinity of the reservoir ranged from 4.9 to 39 fg I-TEQ m3 and 1.4 to 19 pg I-TEQ m2 d1, respectively. The annual atmospheric PCDD/F input flux in the vicinity of the reservoir was 2.0 ng I-TEQ m2 y1. The results of our previous study (Chi et al., 2008) indicated that the atmospheric
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PCDD/F concentrations measured in the urban area of Taiwan ranged from 20 to 110 fg I-TEQ m3. In some Asian countries (such as Korea and Japan), the atmospheric PCDD/F concentrations in the urban area have been shown to range from 28 to 120 fg I-TEQ m3 (Makiya, 1999; Lee et al., 2007). The atmospheric PCDD/F concentrations measured in the vicinity of the reservoir were considerably lower than those measured in other countries. Compared to other relevant studies measured in Taiwan, the atmospheric PCDD/F depositions (1.4–19 pg I-TEQ m2 d1) measured in the vicinity of Feitsui Reservoir in northern Taiwan were significantly lower than that measured in an industrial area (1826 pg I-TEQ m2 d1) in northern Taiwan (Chi et al., 2009) and urban area (3.1–19 pg ITEQ m2 d1) in southern Taiwan (Shih et al., 2006). The low atmospheric PCDD/F deposition can be attributed to the lack of emissions and combustion sources within almost 10 km of the Feitsui Reservoir. However, significant increases in PCDD/F compounds and total suspended particles in ambient air were measured during an Asian dust storm episode (February 2008), and the highest deposition flux of atmospheric PCDD/Fs was observed during the northeast monsoon episode (December 2008). Recently, studies have shown concentrations of 0.97–51.2 pg I-TEQ m3 and 0.22– 3.45 pg I-TEQ m3 PCDD/Fs in the vicinity of electric waste processing facilities in the coastal provinces of Southeast China (Li et al., 2007a,b). Additionally, the results of our previous study indicated that atmospheric PCDD/F concentrations increased 2.5 and 3.2 times in northern Taiwan, during the Asian dust storm episode (Chi et al., 2008). In the winter and spring, Taiwan and the surrounding area is often under the influence of northeasterly monsoon winds originating in central Asia. After passage of the front, the strong northeasterly winds can occasionally carry dust and/or air pollutants that have undergone long-range transport (LRT) to Taiwan (Lin et al., 2004, 2007). To classify the air mass following each frontal passage in northern Taiwan according to the amounts of LRT dust and air pollutants, we have developed an objective method (Lin et al., 2004). The meteorological parameters, i.e., surface temperature, wind direction, wind speed, rainfall, and atmospheric concentrations of PM10 (fine particulate matter with a diameter smaller than 10 lm), NOX, CO, and SO2 observe at coastal stations and a mountain station in northern Taiwan are used to identify the time and intensity of the frontal passage. In this work, we develop an objective method to classify the air mass following each frontal passage in northern Taiwan according to the amounts of long-range transported dust and air pollutants. The meteorological parameters observed near the coasts such as temperature and winds can be used to identify the time and intensity of the frontal passage. The air mass behind a front usually contains Asian continental air, while winds before the frontal passage tend to be weak and thus local emissions can dominate distributions of air pollutants. For contrast purposes, analysis starts 1 d before the frontal passage and lasts for 4 d to cover the entire period of a typical frontal passage. The results indicated that the significant dust storm events and LRT of pollutants occurred during the northeasterly monsoon period (January to May and October to December) in 2008. The results shown in Fig. 2b clearly demonstrate that the most frequent occurrence of the events in 2008 was in February and December (16 and 14 d, respectively). Fig. 2b also shows the daily accumulation rainfall monitored by Central Weather Bureau station which located around our sampling site. During the summer season in 2008, there were 4 typhoons passing through Taiwan. The heavy daily precipitation rate was observed during the typhoon periods. The relationship between the daily rainfall amount and the significant LRT events observed in 2008 is irrelevant. However, the variations of atmospheric PCDD/F measurements are consistent with the identifier significant LRT events (Fig. 2a). Therefore, the significant amounts of PCDD/Fs observed in the present study may have been
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Fig. 2. (a) Total suspended particle (TSP), atmospheric PCDD/F concentration and deposition flux measured within the vicinity of the investigated reservoir, (b) the monthly variation of the significant dust storm cases and long-range transport with pollutants events in 2008.
transported from China across the Taiwan Strait, reaching northern Taiwan during LRT episodes. 3.2. Comparison of PCDD/F deposition in the atmosphere and in the water column of the reservoir The sampling results indicated that the PCDD/F levels in the surface water (water depth: 0.5 m) measured at upstream, middle and lower reaches of the reservoir ranged from 14.9 to 15.2 fg ITEQ L1. The results indicate that the PCDD/F concentrations in the surface water were uniform and sufficiently mixed at different locations in the reservoir. However, the PCDD/F contents in the sediments changed significantly at different locations. The highest PCDD/F contents (2.0 ± 0.1 ng I-TEQ kg1 dry weight, d.w., n = 4) in the sediment were measured at lower reaches at the reservoir. The average PCDD/F content (0.53 ± 0.2 ng ITEQ kg1 d.w., n = 4) measured upstream was significantly lower than that measured at the middle reaches (1.1 ± 0.3 ng ITEQ kg1 d.w., n = 4). To further investigate the transportation of PCDD/Fs in the reservoir, PCDD/F depositions in the water column at two water depths (20 and 70 m) were monthly measured at lower reaches from January to December 2008. Fig. 3a shows that PCDD/F deposition fluxes measured in the atmosphere (1.4– 19 pg I-TEQ m2 d1) were lower than that measured in the water
column (10–30 pg I-TEQ m2 d1) at 20 m water depth during the same period. Interestingly, the temporal trend of PCDD/F deposition flux in water at 20 m is quite similar to the atmospheric PCDD/F deposition flux before the typhoon season (July, August and September). Fig. 3b indicated that PCDD/F deposition flux in the water at 70 m ranged from 9.1 to 179 pg I-TEQ m2 d1. These values were significantly higher than the PCDD/F deposition flux measured at 20 m water depth, especially during the specific typhoon events. Annual PCDD/F input fluxes in the water of the investigated reservoir were 6.1 and 8.3 ng I-TEQ m2 y1 at water depths of 20 and 70 m, respectively. The higher PCDD/F deposition flux in the water as compared with atmospheric deposition may be attributed to the catchment erosion at lower reaches of the reservoir. In September 2008, PCDD/F deposition flux measured at 70 m was 8.5 times higher than that measured at 20 m during the intensive typhoon event (total rainfall 1200 mm month1). To further identify the characteristics of sinking sediments collected in the water, crust-derived elements in water column collected at 20 m and 70 m were also analyzed. Table S3 in Supplementary Material indicated that the deposition fluxes of crust-derived elements (Al, Fe, Na, Mg and Ca) measured at 20 m water depth were quite similar to those measured at 70 m water depth during the normal periods. However, the deposition fluxes of those elements measured at
PCDD/F deposition flux (pg-I-TEQ/m 2/day)
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Fig. 3. (a) Comparison of PCDD/F deposition flux in atmosphere and water column (b) PCDD/F deposition flux in water column at different depth at lower reaches of the investigated reservoir.
70 m water depth during the intensive typhoon period were over 20 times higher than those measured at 20 m. These results suggest that the sharp increases in deposition fluxes of PCDD/F and mineral-derived elements at 70 m water depth may have resulted from a deep turbid layer that formed upstream owing to landslide and/or mud flows during the typhoon period. Fig. 4 shows the contributions of PCDD/F input sources to the reservoir during certain periods based on the PCDD/F deposition flux measured in the atmosphere and water column (20 m depths). The contribution of atmospheric PCDD/F deposition in the investigated reservoir can be obtained from the atmospheric deposition flux divided by the deposition flux collected in water column at 20 m water depth. Based on the mass calculations, the results indicate that 26–46% of the PCDD/F input flux in the reservoir was contributed by atmospheric deposition during normal periods. However, the contribution of atmospheric PCDD/F deposition increased to 57% and 66% during the Asian dust storm and the northeasterly monsoon episodes, respectively. In contrast, the contribution of atmospheric PCDD/F deposition decreased to 10% during the typhoon event, presumably owing to the enhanced catchment erosion around the reservoir. Fig. 5 demonstrates the distribution of sum-PCDDs and sum-PCDFs (based on seventeen 2,3,7,8-chlorosubstituted congeners) in ambient air, atmospheric deposition, surface water, deposition in the water column and
sediment samples, respectively. The results revealed that the highest distribution of PCDFs (80%) was observed in the vapor-phase ambient air sample. Increased distributions of PCDDs were also observed in the samples of aerosols, atmospheric deposition, trapped particles, and sediments. The origins of the increased trend of PCDD distributions are twofold. First, PCDDs are primarily distributed in the solid phase in ambient air (Lohmann et al., 1999; Chang et al., 2004). Therefore, deposited particles collected in the atmosphere and water enhance the PCDD distribution. The second factor refers to the effect of catchment erosion around the reservoir. PCDD/F sources in the reservoir included atmospheric deposition and soil erosion of the reservoir’s catchment. Previous studies (Jou et al., 2007; Kiguchi et al., 2007) have demonstrated that there is a higher distribution of PCDDs in soils. The results of our measurements suggest that around 54–74% of the PCDD/F input flux was contributed by catchment erosion during normal periods. Therefore, catchment erosion of the reservoir would increase the distribution of PCDDs in sediments. 3.3. PCDD/F input flux derived from a sediment core Table 1 lists the total organic carbon (TOC) and water contents at various depths of a sediment core collected at lower reaches of the reservoir. The TOC contents increase with depth and are thus
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Distribution of PCDD/F input source (%)
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Fig. 5. Distribution of sum-PCDDs and sum-PCDFs in different environmental matrices.
higher at the bottom of the core. Relevant studies regarding the sedimentation rate in the Feitsui Reservoir (Lo, 1994; Chen et al., 2006) have indicated that the 20 cm modern sediments correspond to a time span of 15 years (from June 1987 to August 2002), which yielded a mean sedimentation rate of 1.3 cm y1. In the present
study, the core bottom (depth: 29–30 cm) contained root debris exhibiting a yellowish color, distinguishing it from the grayish sediments of the upper core. The bottom layer is apparently the old soil, marking the year in which the reservoir was filled (A.D. 1987). Therefore, the mean sedimentation rate was determined
K.H. Chi et al. / Chemosphere 83 (2011) 745–752 Table 1 PCDD/F concentrations, total organic carbon and water contents in sediment core at different depths in the investigated reservoir. Depth (cm)
Estimated TOC content yeara (%)
Water PCDD/F PCDD/F input content (%) concentration flux (ng I(ng ITEQ m2 y1) TEQ kg1 d.w.)
0–2 2–4 4–6 6–8 8–10 10–12 12–14 14–16 16–18 18–20 20–22 22–24 24–26 26–28 28–29 29–30 30–32
2008 2007 2005 2004 2003 2001 2000 1999 1997 1996 1995 1993 1992 1991 1989 1987 –
71 60 54 50 53 50 55 57 56 54 42 37 39 44 41 45 34
1.2 1.2 1.0 0.9 1.0 0.9 1.2 1.4 1.2 1.1 1.2 1.3 1.6 1.5 1.6 2.2 2.0
2.0 1.7 2.9 2.8 1.8 1.7 2.7 3.0 3.1 2.6 2.1 1.8 2.9 1.8 1.3 1.6 0.80
9.1 8.1 15 16 9.9 10 16 16 15 13 14 13 20 12 8.8 9.5 –
a
The estimated year of each depth of sediment core is calculated by the mean sedimentation rate 1.4 ± 0.1 cm y1.
751
sinking particles in water column, around 90% of the PCDD/Fs input into the reservoir were contributed by the enhanced catchment erosion during intensive typhoon period. These findings imply that pollutants deposited in the watershed during winter and spring must be transported to the reservoir by typhoon runoff and related physical erosion and that both of these phenomena play a major role in PCDD/F accumulation in the downstream reservoir. Acknowledgements The authors acknowledge the financial supports provided by National Science Council (NSC 98-2111-M-001-015-MY3) and Taiwan EPA (EPA-97-E3S4-02-04) of the Republic of China. Assistance provided by Prof. M.B. Chang, Mr. S.H. Chang and Mr. P.C. Hung of National Central University in analyzing the samples is also acknowledged. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.chemosphere.2011.02.069. References
to be 1.4 ± 0.1 cm y1. The representative age of the sediment core at various depths was estimated from the sedimentation rate (Table 1). Table 1 also lists the PCDD/F concentrations at various depths of a sediment core collected at lower reaches of the reservoir. The results revealed that the PCDD/F concentrations ranged from 0.8 to 3.1 ng I-TEQ kg1. However, the variations of PCDD/F concentrations measured in different depths of the sediment core are within the narrow range. Based on the PCDD/F concentrations at different depths and the mean sedimentation rate, the annual input fluxes of PCDD/F to the reservoir can be calculated as follows:
PCDD=F input fluxðng I TEQ m2 y1 Þ ¼ PCDD=F concentrationðng I TEQ kg
1
d:w:Þ of each sediment core
Dry weight of each part in sediment core ðkg d:w:Þ Height of each part in sediment core ðcmÞ Sedimentation rateð1:4 0:1 cm y1 Þ Area of sediment coreð6:36 103 m2 Þ As shown in Table 1, the PCDD/F input flux into sediment of the reservoir ranged from 8.8 to 20 ng I-TEQ m2 y1. The PCDD/F input flux (9.1 ng I-TEQ m2 y1) obtained from the surface sediment (depth: 0–2 cm) was significantly higher than the atmospheric PCDD/F deposition (2.0 ng I-TEQ m2 y1); however, it was similar to the settling PCDD/F flux in the water column (6.1 and 8.3 ng ITEQ m2 y1 at depths of 20 m and 70 m, respectively). The catchment area of the reservoir is around 330 km2, which is 30 times greater than that of the reservoir area (10.2 km2). Therefore, the PCDD/F input flux measured in the reservoir is the sum of the PCDD/F input flux in the reservoir and that in its catchment area. 4. Conclusions According to the results obtained from the measurements of the investigated reservoir, the PCDD/F input flux (9.1 ng ITEQ m2 y1) obtained from the surface sediment was significantly higher than the atmospheric PCDD/F deposition (2.0 ng ITEQ m2 y1); however, it was similar to the settling PCDD/F flux in the water columns (8.3 ng I-TEQ m2 y1 at 70 m water depth). Based on the mass balance between atmospheric deposition and
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