Chemosphere 77 (2009) 459–464
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Modelling historical budget of a-hexachlorocyclohexane in Taihu Lake, China Chongguo Tian a, Yi-Fan Li a,b,*, Hongliang Jia c, Hongli Wu c, Jianmin Ma b a
International Joint Research Center for Persistent Toxic Substances (IJRC-PTS), State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China b Science and Technology Branch, Environment Canada, 4905 Dufferin Street, Downsview, Ontario, Canada M3H 5T4 c IJRC-PTS, Dalian Maritime University, Dalian, China
a r t i c l e
i n f o
Article history: Received 6 April 2009 Received in revised form 26 July 2009 Accepted 28 July 2009 Available online 22 August 2009 Keywords: a-Hexachlorocyclohexane (a-HCH) Taihu Lake China Dynamic modeling Mass balance
a b s t r a c t Historical a-hexachlorocyclohexane (a-HCH) budget in Taihu Lake (TL), China has been simulated by a Gridded Basin-based Pesticide Mass Balance Model (GB-PMBM). Using annual usage of a-HCH from 1952 to 1984 as input, the model outputs included annual concentrations in air, water and sediment in TL, and annual cumulative burden of a-HCH in the lake water and sediment from 1952 to 2008. Model results showed that the modeled a-HCH in the air, water and sediment matched their corresponding measured data well, and the current levels of a-HCH in the air, water and sediment in TL in 2008 are 11.7 (3.4–22.7) pg m 3, 0.8 (0.3–1.5) ng L 1 and 0.18 (0.04–0.46) ng g 1 dw (dry weight), respectively. The a-HCH burden in TL water started to accumulate after 1952, reached the highest value of 11 000 kg in 1972, decreased very quickly since the beginning of 1980s, reduced to 200 kg in 1984 and 3 kg in 2008. It was found that TL water played a role of ‘‘distributor” in process of transport of aHCH. Before 1980, TL water took a large amount of a-HCH from atmosphere through a huge air–water interface and carried a major portion of it out of the lake through water current. After 1980, TL water took a-HCH from lake sediments and river water entering the lake, and released almost all of it to air. The lake water itself cannot hold a large portion of the chemical due to its shallow depth and short residence time. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Technical hexachlorocyclohexane (HCH) was a widely used pesticide all over the world (Li, 1999a,b). It contains a-isomer (60– 70%) as its main components (Kutz et al., 1991). In China alone, the total usage of technical HCH from 1952 to 1984 was approximately 4.5 million tons, approximately half of total global use (Li et al., 1998b, 2001). South and southeast of China were the regions with heavy usage of this chemical (Li et al., 2001; Cao et al., 2007). Taihu Lake Basin (TLB) is situated in one of these regions. Located in the Yangzi River Delta, the basin covers the City of Shanghai, parts of Jiangsu, Zhejiang, and Anhui provinces. As one of China’s most developed areas, TLB, with only 0.4% of total country area, has 3% of the grain production (Qiu et al., 2004; Peng et al., 2005). TLB was also one of the regions with signiﬁcant use of organochlorine pesticides (OCPs) before 1984, with approximately 7% for technical HCH and 10% for DDT of total national usage (Li et al., 1998a, 2001). It has been found that, after use restriction
* Corresponding author. Address: International Joint Research Center for Persistent Toxic Substances (IJRC-PTS), State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, China. Tel./fax: +86 451 8628 2099. E-mail address: [email protected]
(Y.-F. Li). 0045-6535/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2009.07.061
of technical HCH in 1980 and ofﬁcial ban in 1983, the residue levels of -HCH (or total HCHs) in soil (in ng g 1 dw (dry weight)) in TLB have declined dramatically, from 870 in 1979 (Cai et al., 1983), to 310 in 1982–83 (Ma et al., 1986), and to present level of 29 (Wang et al., 2007). Taihu Lake (TL) is the third largest fresh water lake in China and the main drinking water source of the megalopolis in TLB, including Shanghai, Wuxi, and Suzhou, all have millions of population (Qiu et al., 2008). Technical HCH applied in TLB entered TL through air transport and water current, causing a serious problem for TL water quality (Qiu et al., 2004). There have been many studies to investigate the occurrence, pathways, and fate of HCHs in TL. These studies have focused on a certain matrix or matrixes for a certain year or years. According to our knowledge, however, there has not been any systematic study to describe the historical budget of the OCPs in TL. A mass balance model, Gridded Basin-based Pesticide Mass Balance Model (GB-PMBM), was developed to investigate the longterm fate of a-HCH in TLB (Tian et al., in press). Modeled results indicated a good agreement between the simulated and measured concentrations in soils, air, water and sediment in TLB in the 1980s and 2000s (Tian et al., in press). The objective of this study is to use the results given by GB-PMBM to construct a mass balance for aHCH in TL from 1952 to 2008 to understand how a-HCH has loaded
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into the lake, what their fate has been, and whether there are knowledge gaps critical to prediction of future trends of the chemical in TL.
and the physico-chemical property parameters of a-HCH are showed in Table S3, SI. 3. Results and discussion
2. Materials and methods 2.1. Model GB-PMBM employed in this investigation has been developed to describe transport and transfer of pesticides in TLB (Tian et al., in press). Detailed description of the model can be found in the Supplementary Information (SI) of this paper, and only a brief introduction is given here. The model domain (MD) covers the area from 29° to 33° North and from 118° to 123° East (see Fig. S1, SI), where a grid system on a horizontal resolution of 1/6° latitude by 1/4° longitude with a total of 480 (20 24) grid cells was established within the MD. The TLB lies in the central section of the MD, and contains 103 grid cells, among which TL has 14. The whole domain is divided into 5 portions: TL, west of TL (TLW), east of TL (TLE), west of TLB (TLBW), and east of TLB (TLBE) (see Fig. S1, SI). GB-PMBM considers four matrixes, air (air, particulates), soil (air, water and solids), water (water, suspended solids), and sediment (water and solids), and includes 2 components, transfer and transport modules. The transfer module describes the changes of a-HCH concentrations and inter-compartmental transfer of the pesticide within each grid cell using a level IV fugacity method (Mackay, 2001). The transport module depicts mass exchange of a-HCH between different grid cells due to advection in air layers and water current. While only TLB was considered for water currents, the entire MD was included in air advection in the transport model. The area of MD is large enough to take into account the input of a-HCH via atmospheric transport from outside of TLB. 2.2. Historical a-HCH usage and other input data Gridded pesticide usage inventories are a critical input to the GB-PMBM. Gridded annual usage of a-HCH from 1952 to 1984 in the MD, shown in Fig. S5, SI, was extracted from the gridded technical HCH usage inventories in China developed by Li et al. (2001). By assuming 67.5% composition of a-HCH in technical HCH (Li et al., 1998b), approximately 350 and 160 kiloton of a-HCH entered environment in the MD and TLB, respectively. The temporal trend of total a-HCH usage in the MD is displayed in Fig. S6, SI. The highest usage of a-HCH happened in 1974. Technical HCH was applied only on cropland (including paddy ﬁeld and dry cropland) in whole MD by different application modes, such as spray, seed treatment, and soil corporation (Table S4, SI). After application, the chemical entered soil and air, and was then brought by short- and long-range atmospheric transport and runoff to other areas and compartments where it had not been applied directly. All these processes were simulated by the model described in the previous section. Gridded daily air temperature and precipitation data before 2006 was obtained by interpolating the monthly data from 14 observation stations in southeast of China within or close to the grid domain (see Fig. S7A, SI). Gridded daily horizontal wind components before 2006 were collected from the Unites States National Center for Environmental Prediction (NCEP) reanalysis (Kalnay et al., 1996) which are then interpolated into the grid cells (see Fig. S7B, SI). The gridded daily air temperature, precipitation and wind components in 2007 and 2008 were assumed to be equal to the daily arithmetical mean during 2000 and 2006. Other environmental parameters used in the model are listed in Table S2, SI,
The model outputs include gridded annual concentrations in air, water and sediment in TL, annual -HCH loading to, removal from TL, and annual burden of -HCH in TL water and sediment from 1952 to 2008. We also take into account air–water and water–sediment exchanges, diffusion between adjacent soil layers and the downwards leaching from each soil layer for -HCH. 3.1. a-HCH concentrations Gridded a-HCH concentrations in air, water and sediment of TL (14 grid cells in total) from 1952 to 2008 were calculated by the model, and the results for 1972 and 2004 are shown in Fig. S12, SI. It is worthwhile to note that the locations with the highest aHCH concentrations in air and water switched from the eastern side (the area with the highest technical HCH usage around TL, see Fig. S5, SI) in 1972 to northern side of TL, the Meiliang Bay in particular, in 2004 (see Fig. S9, SI). This is in agreement with the result from previous study (Qiu et al., 2008). The reason for this is the difference of air temperature shown in Fig. S7A and East Asian summer monsoon, which has brought a-HCH from the warmer eastern side to the colder northern side and deposited there for decades. Fig. 1 illustrates the annual maximum, minimum, and mean a-HCH concentrations among the 14 cells over TL given by the model from 1952 to 2008. Measured data collected from previous studies are also displayed in Fig. 1, which shows a good agreement with the modeled data. Detailed comparisons are depicted in Fig. S9–S11, SI. It is notable in Fig. 1 that the minimum air concentrations showed larger ﬂuctuations compared to the maximum and mean air concentrations during the application period of technical HCH. The maximum concentrations in air, water, and sediment were measured in the east side of the TL (Grid cell 8, Fig. S12, SI) where there was the heaviest technical HCH usage around TL (Fig. S5, SI), while the minimum concentrations were located in the center of TL (Grid 6, Fig. S12, SI). The center of TL is relatively farther away from the source region, and the concentrations of a-HCH in this area were associated with the usage of a-HCH and also the wind speed and direction, leading larger ﬂuctuations than the maximum concentrations. It shows that the temporal trends of a-HCH concentrations in air and water follows actually the use trend of technical HCH in TLB (Fig. S6, SI) before 1983, and have decreased monotonously after, while the temporal trend of a-HCH concentrations in sediment is relatively smoother, increased since 1952, reaching the peak in 1976, and decreased thereafter, which was also reported by other study (Lin, 2007). The model results indicated that, since 1983 the level of -HCH in all 3 matrixes in TL decreased dramatically. The estimated concentrations of -HCH in air, water and sediment in TL were 77 000 pg m 3, 540 ng L 1, and 17 ng g 1 dw in 1980, and decreased to 11.7 pg m 3, 0.8 ng L 1 and 0.18 ng g 1 dw in 2008, respectively. 3.2. Dynamics of a-HCH in TL 3.2.1. Air–water and water–sediment exchanges Net ﬂux of a-HCH from air to water is illustrated in Fig. 2. Air– water exchange includes dry deposition, wet deposition, rain dissolution and diffusion (absorption and volatilization). Maximum deposition ﬂux was 16 tons in 1972, corresponding to the maximum concentration in air (Fig. 1) and the maximum usage in
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Fig. 1. Modeled annual maximum, minimum, and mean a-HCH concentrations in air, water, and sediment in TL during 1952 and 2008 and also the measured data (sources of measured data: Cai et al., 1983; Jiang et al., 1989; Yuan et al., 2003; Qiao et al., 2004; Huang et al., 2006; Qiu et al., 2008; Zhao et al., 2008).
Fig. 2. Net ﬂuxes of a-HCH from air to water (+) and from sediment to water (+) in TL during 1952 and 2008 (the negative values mean the opposite net ﬂuxes direction). The inset ﬁgure is the enlarged portion from 1993 to 2008.
the same year (Fig. S6, SI). The highest volatilization was 2.7 tons in 1980. The reversal of the net ﬂux direction of a-HCH from deposition to volatilization in 1979–1980 was resulted from the sharp decrease in the use of technical HCH (Fig. S6, SI) which, in turn, led to the considerable decline in the atmospheric level of a-HCH (Fig. 1). Concentrations of a-HCH in water have declined at a relative slower rate since 1980. Net ﬂux of a-HCH from sediment to water is also illustrated in Fig. 2. Water–sediment ex-
change includes sedimentation of solid in water, resuspending of sediment and diffusion (absorption and resolution) between water and sediment. The year of change in the direction of aHCH net ﬂux was the same as that in the air–water interface, from water to sediment before 1980, and a reversal in 1979– 1980, suggesting that sediment has become a source of a-HCH in the lake water since 1980, and this ﬂux reversal was directly due to reversal of air–water exchange.
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Fig. 3. Annual change of the a-HCH inﬂow (+), outﬂow ( ) and net ﬂow due to water current in TL from 1952 to 2008 (inﬂow: river current, outﬂow: lake current). The inset ﬁgure is the enlarged portion from 1993 to 2008.
3.2.2. Inﬂow and outﬂow due to lake water current Fig. 3 displays the annual inﬂow, outﬂow and the net ﬂow of aHCH due to lake water current. The annual variation of the ﬂuxes correlates well with the annual usage of a-HCH, showing the highest inﬂow and the lowest outﬂow and net ﬂow in 1972, when the maximum amount of a-HCH was applied. It is interesting to note that the direction of net ﬂux due to water current reversed in 1983, before which water current removed a-HCH from TL water, and has brought more a-HCH to lake water after (see inset ﬁgure of Fig. 3). 3.3. Budget of a-HCH in TL GB-PMBM yields annual a-HCH loading to and removal from TL, which enables us to calculate annual a-HCH budget and burden in the lake. In this analysis, the net ﬂow of a-HCH due to water current is considered as one process. Fig. 4 summarizes the percentages of a-HCH removal from (left panel) and loadings to (right panel) lake waters during three periods. The years between 1952 and 1979 is regarded as the pollution
1985-2008 1980-1984 1952-1979
Air Water -100
Loading(+) & Removal(-) (%) Fig. 4. Percentage of a-HCH loading to (>0) and removal from (<0) the TL for 1952– 1979 (polluting period), 1980–1984 (transition period), and 1985–2008 (Cleaning period).
period, when the loading through the air–water interface was a sole contributor to annual loadings of a-HCH (100%, 127.6 tons) and net ﬂux through water currents was the primary removal process (37.2%, 45.8 tons), followed by degradation due to microbes and hydrolysis (32.2%, 39.7 tons), and loadings to sediment (30.6%, 37.8 tons). In this period the total loading of a-HCH was 127.6 tons and the total removal was 123.2 tons, yielding a net loading of only 4.3 tons to lake water. The period from 1980 to 1984 is designated as the transition period, when the direction of ﬂuxes between the air–water and the water–sediment reversed and the effect of the net ﬂux of a-HCH through lake currents has changed from the removal to the loading. The a-HCH loading to water from sediment became the sole contributor to annual loadings of the compound (100%, 3.9 tons), and the removal through air–water interface was the major contributor to annual removal (64.3%, 5.1 tons), followed by degradation due to microbes and hydrolysis (23.5%, 1.9 tons), and net ﬂux through lake currents (12.2%, 1.0 tons). In this period the total loading of a-HCH was 3.9 tons and the total removal was 8.0 tons which yields a net removal of 4.1 tons from lake water. The duration after 1985 is deﬁned as the cleaning period, during which the net ﬂux of a-HCH through lake currents entering to the lake water brought a total of 0.4 tons (8.6%), following the major loading to water from sediment (91.4%, 4.3 tons). The removal of a-HCH through air–water interface has still been the major portion (96.2%, 4.7 tons), followed by degradation (3.8%, 0.2 tons). In this period the total loading of a-HCH was 4.7 tons and the total removal was 4.9 tons, yielding a net removal of 0.2 tons from lake water. Our results showed that both exchange from sediment to water and water current were two pathways for aHCH to enter TL water during the 2000s, the former of which was pointed out by Qiu et al. (2008) and the latter by Jia et al. (2007). In all these three periods, the exchange of a-HCH through air– water surface played a prominent role. It was the only contributor of a-HCH to the lake water in pollution period, and the major process to remove a-HCH during the transition and cleaning periods. Sediment was a sink of a-HCH from the lake water in the pollution period, and became the sole source in the transition period, and the major source in the cleaning period. The water current was a
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removal process of a-HCH in the pollution and transition periods, but the contributor of a-HCH to the lake in the cleaning period. Degradation due to microbes and hydrolysis has always been the removal process in all three periods. Fig. 5 depicts the temporal trends of a-HCH loading to, removal from, net ﬂux and total burden in TL water and sediment from 1952 to 2008, and also the annual usage in TLB from 1952 to 1984. While these trends for sediments are smoother, those for lake water show a similar pattern as that of the annual usage of a-HCH, suggesting that there is quick response of a-HCH budget in water body to its usage, which was reported by other study (Tao et al., 2006). As shown in Fig. 5, the burden of a-HCH in TL water and sediment are similar, increased almost monotonically since 1952, reached the highest value of 8 tons and 22 tons, respectively, in 1974, and decreased monotonically after. The loading of HCH to the sediment from water reached the maximum of 5.0 tons in 1972, and the removal of -HCH from sediment reached the maximum of 2.3 tons in 1980. Although exchange between the water and the sediment is a sole pathway of the pesticide loading to the sediment, the burden in the sediment has overtaken quickly than that in the water since 1957. 3.4. The role of TL water As shown in Fig. 5, the amounts of annual loading to and removal from the lake water were almost in equal amount from 1952 to 2008, leading a very small amount of net increase or decrease of a-HCH in TL water, which indicates a very special feature of TL water in transporting of a-HCH. TL is a very special water body characterized by a large surface area with 2300 km2 and a shallow layer with an averaging depth 1.9 m (Qin et al., 2004). Water in TL changes fast with a short residence time of approximately 309 days (Qin et al., 2004). Hence, the lake can exchange a large amount of chemicals with air through the broad water surface, but cannot hold this large amount of chemicals due to its shallow depth and short residence
time. Unlike the Arctic Ocean, which acts as a large sink to store aHCH entering the Ocean through long-range transport before 1990, and a large source of a-HCH for releasing this compound through air–water exchange and sea current after 1990 (Li et al., 2004). TL has acted as a ‘‘distributor”, rather than a sink or a source, in the process of transporting a-HCH. Before 1980 it took a large amount of a-HCH from atmospheric and water current, carried almost all of them (>94%) out of the lake through the water current, and input a certain amount to the sediment. Consequently, the lake left only a very small portion of the substance (<6%) to its burden. After 1980, TL water took a-HCH from sediments and river waters that injected to the lake, and released a major portion of it (>80%) to air through air–water exchange. The contribution to the release of the compound from its own burden is very small (<6% of the total removal). We further compared the model results for 1972 and 2004 shown in Fig. 6. In 1972, the TL took 16.4 tons (58%) of -HCH from air through air–water exchange, 11.7 tons (42%) from the upstream current entering the lake, and released 17 tons (61%) of the compound through down-stream water current, 5 tons (18%) to sediment, 3.8 tons (13%) due to degradation. As a result, there was only 2.3 tons (8%) left in the lake water. The burden of -HCH in the lake water was 8.5 tons at the beginning of 1972, and 10.7 tons at the end of the year. Clearly, the TL behaves more likely to be a ‘‘distributor”, rather than a sink of a-HCH. Signiﬁcant change occurred in 2004. In this year, the TL took 33.8 kg (79%) of a-HCH from sediment through sediment–water exchange, and 9.1 kg (21%) from the up-stream current that injects to the lake. In the same year, 36 kg (83%) of the compound was released to the air through water outgasing, 6.2 kg (14%) was carried out of the lake through down-stream water current, 1.3 kg (3.0%) was lost due to degradation. The contribution to the release of the compound from the burden in the lake water was only 0.6 kg (1.4%). Therefore, the burden of a-HCH in the lake water was 6.5 kg in the beginning of 2004 and 5.9 kg in the end of the year. In this sense, the TL behaves more likely to be a ‘‘distributor”, rather than a
Fig. 5. Temporal trends of a-HCH loading to, removal from, net ﬂux and total burden of a-HCH in TL water and sediment from 1952 to 2008, and also the annual usage in TLB from 1952 to 1984. The inset ﬁgures are the enlarged portion from 1990 to 2008.
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Loading (+) & removal (-) (kg) in 2004 -40
40 1972 2004
Degradation Water-sedi. Outflow Inflow Air-water -20
Loading (+) & removal (-) (t) in 1972 Fig. 6. Loading to, removal from, net ﬂux in TL water, sediment and total burden of a-HCH in the water and sediment from 1952 to 2008.
source. Given that there was only 3.0 kg of a-HCH left in the lake water but 150 kg in sediment by the end of 2008, it is expected that the TL, performing as a distributor, would input a-HCH continuously from its a-HCH burden in the sediments to water. Acknowledgments This work was supported by State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology (Project #: 2008DX01). Valuable comments from two anonymous reviewers are highly appreciated.
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