Environ Sci Pollut Res DOI 10.1007/s11356-013-2400-8
RESEARCH ARTICLE
Evaluating the exchange of DDTs between sediment and water in a major lake in North China Guo-Hua Dai & Xin-Hui Liu & Gang Liang & Wen-Wen Gong
Received: 30 August 2013 / Accepted: 25 November 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract A large-scale sampling program was conducted to simultaneously collect surface water, overlying water, pore water, and sediment samples at monthly intervals between March and December 2010 from Baiyangdian Lake, North China to assess the distribution of DDTs and determine the net direction of sediment–water exchange. Total DDT concentrations ranged 2.36–22.4 ng/L, 0.72–21.9 ng/L, 2.25–33.7 ng/L, and 4.42–7.29 ng/g in surface water, overlying water, pore water, and sediments, respectively, which were at the intermediate levels compared to those of other area around the world. Seasonal variations of DDTs were featured by higher concentration in summer. This was likely associated with (a) the increase of land runoff in the summer and (b) application of dicofol and DDT-containing antifouling paints for ships in summer. Sediment–water fugacity ratios of the DDT isomers were used to predict the direction of the sediment–water exchange of these isomers. The sediment–surface water, sediment–overlying water, and sediment–pore water fugacity ratios of DDT isomers averaged 0.34, 0.44, and 0.1, which are significantly lower than the equilibrium status (1.0), suggesting that the net flux direction were from the water to sediment and the sediment acted as a sink for the DDTs. The difference of DDT concentrations between sediment and water samples was found to be an important factor affecting the diffusion of DDT from the water to sediment.
Responsible editor: Roland Kallenborn G.p,p′-DDT (1.87±0.67 ng/g)>o,p′DDT (0.71±0.46 ng/g)>p,p′-DDD (0.20±0.12 ng/g)>o,p′DDE (0.06±0.04 ng/g)>o ,p ′-DDD (0.04±0.03 ng/g). The isomers of p,p′-DDE, p,p′-DDT, and o,p′-DDT made up of 52, 31, and 12 % of the total DDTs in sediment, respectively. Overall, the six DDT isomers were detected in almost all samples, indicating that the DDTs were ubiquitous in the Baiyangdian Lake. The concentrations of p ,p ′-DDE were higher than the other isomers in each month, followed by o,p′-DDT and p,p′-DDT; the average abundance of the other three remaining isomers was generally less than 12 % in water samples and less than 5 % in sediment samples. The predominance of p ,p ′-DDE suggests that the p ,p ′-DDT in the Baiyangdian Lake was mostly degraded. In addition, relatively high levels of o,p′-DDT indicate that new input of o,p′DDT into the Baiyangdian Lake might occur recently. Compared with the previous studies of Baiyangdian Lake, in the present study, the mean concentration of DDTs in the surface water of the Baiyangdian Lake was much lower than that found in 1995 (range 0–900 ng/L, mean 250 ng/L) (Dou and Zhao 1998). Compared with other water bodies, the concentration range of DDTs in the surface water from the Baiyangdian Lake was similar to those of Jiulong River Estuary and Lake Chaohu and slightly greater than those of Daliao River Estuary, Ebro River in Spain, and Lake Baikal in Russia but was lower than those reported in the Minjiang River Estuary, Beijing Guanting Reservoir, and Gomti River
in India (Table 2). No research data on DDT residual levels in overlying water has been presented in the previous studies, so it could not be compared with the other areas. For pore water, in comparison, the DDT level was lower than those observed in Minjiang River Estuary, Beijing Guanting Reservoir, while it was similar to those in Daliao River Estuary and Jiulong River Estuary in China. In sediments, the concentration of DDTs in Baiyangdian Lake was comparable to those of Minjiang River Estuary, Beijing Guanting Reservoir, Daliao River Estuary, Lake Chaohu, and Ebro River in Spain, and greater than those of Jiulong River Estuary and Lake Baikal in Russia, but much lower than those of Gomti River in India and Hanoi area in Vietnam (Table 2). Compared with the previous studies in Baiyangdian Lake, the DDT levels in sediment in the present study were comparable to the levels in 1995 (0.69– 2.26 ng/g) (Dou and Zhao 1998) and the levels in 2007 (2.2– 3.1 ng/g) (Hu et al. 2010). Spatial and temporal variation of DDTs Temporally, the concentrations of DDTs varied considerably in the surface water (Fig. 2). The greatest concentration of DDTs was detected in July, and relatively higher concentrations were also found in August, June, and September. Three possible explanations can be provided. First, the higher concentration levels of DDTs found in the summer might be partially attributed to the increase of land runoff during the wet season (June–September). A similar result was also obtained by Yuan et al. (2013), indicating that the increasing land runoff during the summer might bring the chemical residues from soil to the water environment. Second, the high contamination level in the summer might be linked to the emission of DDTs by ships since there are intensive boat activities for tourism and fishery in these months during sampling period. DDT, as the auxiliary material of an antifouling paint is still in use. He et al. (2012) reported that in China, from 1950s to 2005, approximately 1.1 million tons of DDT was used to produce antifouling paints. Third, the higher levels of DDTs in summer might be related to the dicofol applications for crops
Environ Sci Pollut Res Table 2 Concentrations of DDTs isomers in surface water (ng/L), pore water (ng/L) and sediments (ng/g) from lakes and rivers around the world
nd not detected
Sampling area
Surface water
Pore water
Sediment
References
Baiyangdian Lake Minjiang River Estuary Beijing Guanting Reservoir Jiulong River Estuary Daliao River Estuary
2.36–22.35 40.61–233.5 nd–528.8 June>August>September>May>October>December>March>April>November in the overlying water, and the order was July>August>June> September>May>October>April>December>November> March in the pore water (Fig. 2). No obvious temporal variations of DDT concentrations were observed in the sediment. A slightly lower concentration occurred in sediment during the periods of March and December compared to other sampling times (Fig. 2).
in the summer season. Yang et al. (2008) reported that dicofol with high impurity of DDT compounds is still widely used in agricultural practice such as cotton cultivation and becomes an important source of DDT pollution in China. Statistical data showed that from 1988 to 2002, the average annual DDT production was about 6,000 tons in China, of which nearly 80 % was for dicofol production (Qiu et al. 2005). On the contrary, lowest concentrations of DDTs in surface water were found in April and November, followed by October, December, and March. It is worth noting that during the months of April and November 2010, a great deal of fresh water was allocated to the Baiyangdian Lake by the local government to improve the hydrological and ecological conditions of the 35 site 2 site 5
site 3 site 6
Surface water
site 1 site 4
30
25
Concentration (ng/L)
Concentration (ng/L)
30
35 site 1 site 4
20 15 10 5
site 2 site 5
site 3 site 6
Overlying water
25 20 15 10 5
0 Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
0
Dec
Mar
70
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
20
site 1 site 4
60
site 2 site 5
site 3 site 6
site 1 site 4
18
Pore water
site 2 site 5
site 3 site 6
Sediments
50
Concentration (ng/g)
Concentration (ng/L)
16
40 30 20
14 12 10 8 6 4
10 2
0
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
0
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Fig. 2 Distribution of DDT concentrations in surface water, overlying water, pore water, and sediments from Baiyangdian Lake
Nov
Dec
Environ Sci Pollut Res
Overall, the concentrations of DDTs in the surface water, overlying water, pore water, and sediments varied with the sampling locations (Fig. 2). The greatest concentrations of DDTs were found at site 2 in the surface water, at sites 2 and 5 in the overlying water, and at site 2 in the pore water and sediment. Relatively higher levels were also found at sites 5 and 6 in the surface water, at sites 3 and 6 in the overlying water, and at sites 3, 5, and 6 in the pore water and sediments. Two possible explanations can be provided. First, sites 2 and 3 are located on the entrance of the Fuhe River. The higher concentrations at these two sites might be caused by high rates of influx of contaminants into the lake through the Fuhe River. This result can be confirmed by the fact that the concentration of DDTs in the sediments from the Fuhe River (range 1.3– 51.3 ng/g) was much higher than that in the Baiyangdian Lake (2.2–3.1 ng/g) (Hu et al. 2010). The Fuhe River, which runs through the heavy industrial city of Baoding, carries about 250,000 tons of untreated industrial and domestic wastewater per day produced by Baoding city into the Baiyangdian Lake (Hu et al. 2010). Second, sites 5 and 6 are all close to villages and fish or duck aquaculture areas, and the high levels at these sites may be the result of agricultural runoff entering the water environment. In addition, the higher levels at sites 5 and 6 (fish aquaculture area) may also be related to the fish feed. DDTs have been reported in commercial feed or trash fish (Meng et al. 2007; Guo et al. 2009), which will contaminate the surrounding water body of aquaculture area. On the other hand, in the relatively pristine site 4, levels of DDTs were much lower than the other sites. DOC contents in water and total organic carbon (TOC) in sediments were also investigated in this study, with the concentrations ranging from 6.78 to16.23 mg/L in surface water, 7.12 to 18.56 mg/L in overlying water, 7.28 to 22.08 mg/L in pore water, and 0.23 to 3.09 % in sediments, respectively, during the sampling periods. The distribution of HOCs such as PCBs has been shown to be related by DOC contents in water and by TOC contents in sediments (Iwata et al. 1995). However, in this study, distribution of DDTs did not show any correlation with DOC contents (r =0.415, p =0.272) in water phase and TOC contents (r =0.423, p =0.112) in sediment. This relationship was also observed by Mai et al. (2002), suggesting that many factors contributed to the DDTs' distribution in water, and the sediment contamination in the study area may be dictated more predominantly by anthropogenic inputs than by natural processes. The correlation of DDT concentrations with temperature (r =0.536, p =0.09) and dissolved oxygen (r =0.322, p = 0.302) was also investigated in this study, and no statistically significant relationship was observed. Possible sources of DDTs The ratios of the parent compound to its metabolite can provide useful information on the pollution source. The
portion of DDT metabolic products is indicative of new inputs or historical usage. A ratio of DDT/(DDE+DDD) higher than 1 is generally pointed to the fresh input of DDTs (Hitch and Day 1992). Otherwise, aged DDTs are suggested. There are two known sources of DDTs characterized by the o,p′-DDT/ p,p′-DDT ratio of 0.2 in technical DDT mixture and ~7.0 in dicofol products in China, respectively (Qiu et al. 2005). Therefore, the ratio of o,p′-DDT/p,p′-DDT is usually used to distinguish technical DDT from dicofol-type DDT (Qiu et al. 2005; Yang et al. 2008; Liu et al. 2009), and an elevated o,p′-DDT/p ,p′-DDT is an indicative of the dicofol-type DDT. In the present study, the ratios of DDT/(DDE+DDD) were 1.13±0.80, 0.66±0.48, 0.65±0.33, and 0.56±0.19 in surface water, overlying water, pore water, and sediments, respectively, during the sampling periods. As shown in Table 3, more than 70 % of the DDT/(DDE+DDD) ratios were less than 1, suggesting that DDT in Baiyangdian Lake mostly stemmed from the historical input. On the contrary, for surface water and overlying water, higher DDT/(DDE+DDD) ratios (>1.0) were found from April to September, indicating that fresh input of DDT occurred in the Baiyangdian Lake in these months. Moreover, elevated o,p′-DDT/p,p′-DDT ratios, with the mean value of 0.18±0.08 to 2.41±1.60 in different environment medium, were also observed. Recently, similar results were also reported in water from Taihu Lake (Qiu et al. 2008) and Chaohu Lake (He et al. 2012) in China, with o,p′DDT/p,p ′-DDT ratios of 1.6–5.6 and 0.40–2.18, respectively. Since o,p ′-DDT metabolizes more readily than p,p ′-DDT in the environment (Martijn et al. 1993; Qiu et al. 2005), the degradation of technical DDT is unlikely to cause increase of o,p′-DDT/p ,p′-DDT ratio in the environment that is higher than that of technical DDT. Therefore, the relatively higher values of o ,p′-DDT/p,p′-DDT in Baiyangdian Lake clearly demonstrates that the new input of DDT was from dicofoltype DDT. In summary, mixed sources of DDTs, i.e., both historical input of technical DDT and new input of dicofoltype DDT, were obvious in Baiyangdian Lake. According to the formula proposed by Liu et al. (2009), the contributions of technical DDT and dicofol-type DDT were 85 and 15 %, respectively, in the Baiyangdian Lake. Fugacity ratios calculation Fugacity is a measure of the tendency of a chemical to escape from its medium. A convenient way to estimate the net flux direction is to compare fugacity values of a chemical among the different compartments (Mackay 2001; Rowe et al. 2007). The fugacity (f ) of a chemical in any phase is defined as the concentration (C ) of the chemical in the phase divided by the fugacity capacity (Z ) of the phase (Mackay 2001): f ¼ C=Z
ð1Þ
Environ Sci Pollut Res Table 3 Ratios used in source identification of DDT with its metabolites in Baiyangdian Lake from March to December
Ratios DDT/(DDE+DDD)
o,p′/p,p′-DDT W surface water, OW overlying water, PW pore water, SED sediments
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
W OW PW SED W
0.77 0.24 0.48 0.37 1.82
1.04 0.31 0.24 0.80 1.14
1.55 0.53 0.44 0.25 3.00
1.84 1.05 0.85 0.83 2.19
2.56 1.35 1.13 0.54 4.86
1.75 1.31 1.05 0.50 5.03
1.14 0.18 0.48 0.47 3.18
0.28 0.40 0.82 0.52 1.67
0.08 1.02 0.20 0.77 0.42
0.28 0.17 0.86 0.57 0.78
OW PW SED
0.85 0.52 0.07
0.15 0.37 0.12
1.80 0.24 0.27
1.48 0.64 0.23
1.77 0.35 0.09
1.83 0.21 0.17
0.16 0.64 0.24
2.44 0.71 0.20
0.09 0.37 0.14
0.75 0.94 0.28
Z values are obtained from the following relationship (Mackay 2001): Z w ðwaterÞ ¼ 1=H
ð2Þ
Z s ðsedimentÞ ¼ 0:41 ðOCÞ K ow ρs =H
ð3Þ
so that the fugacities of a chemical in water and sediment are: f w ¼ CwH
ð4Þ
f s ¼ C s H=0:41 ðOCÞK ow ρs
ð5Þ
where f w and f s are the fugacities in water and sediment (Pa), C w and C s are the concentrations of DDT isomer in water (ng/L) and sediment (ng/g), H is the Henry's law constant at the temperature of the water (Pa m3/mol), OC is the organic carbon content (fraction ranging from 0 to 1), K ow is the octanol–water partition coefficient, and ρs is the sediment density (kg/L). The measured average density of sediment solids (2.0±0.3 kg/L) was used for the calculations in this study. The K ow values for o,p′-DDT, p,p′-DDT, p,p′-DDE, and p,p′-DDD were taken from UNEP Chemicals (2002). The sediment–water fugacity ratio (f s/f w) is: f s f w ¼ C s C w ð0:41 ðOCÞK ow ρs Þ:
ð6Þ
Here, f s/f w =1.0 implies that the DDTs in water and sediment are at equilibrium, the net flux is zero, although DDTs are exchanged between water and sediment at the same rate. Values of f s/f w >1.0 and
RESEARCH ARTICLE
Evaluating the exchange of DDTs between sediment and water in a major lake in North China Guo-Hua Dai & Xin-Hui Liu & Gang Liang & Wen-Wen Gong
Received: 30 August 2013 / Accepted: 25 November 2013 # Springer-Verlag Berlin Heidelberg 2013
Abstract A large-scale sampling program was conducted to simultaneously collect surface water, overlying water, pore water, and sediment samples at monthly intervals between March and December 2010 from Baiyangdian Lake, North China to assess the distribution of DDTs and determine the net direction of sediment–water exchange. Total DDT concentrations ranged 2.36–22.4 ng/L, 0.72–21.9 ng/L, 2.25–33.7 ng/L, and 4.42–7.29 ng/g in surface water, overlying water, pore water, and sediments, respectively, which were at the intermediate levels compared to those of other area around the world. Seasonal variations of DDTs were featured by higher concentration in summer. This was likely associated with (a) the increase of land runoff in the summer and (b) application of dicofol and DDT-containing antifouling paints for ships in summer. Sediment–water fugacity ratios of the DDT isomers were used to predict the direction of the sediment–water exchange of these isomers. The sediment–surface water, sediment–overlying water, and sediment–pore water fugacity ratios of DDT isomers averaged 0.34, 0.44, and 0.1, which are significantly lower than the equilibrium status (1.0), suggesting that the net flux direction were from the water to sediment and the sediment acted as a sink for the DDTs. The difference of DDT concentrations between sediment and water samples was found to be an important factor affecting the diffusion of DDT from the water to sediment.
Responsible editor: Roland Kallenborn G.p,p′-DDT (1.87±0.67 ng/g)>o,p′DDT (0.71±0.46 ng/g)>p,p′-DDD (0.20±0.12 ng/g)>o,p′DDE (0.06±0.04 ng/g)>o ,p ′-DDD (0.04±0.03 ng/g). The isomers of p,p′-DDE, p,p′-DDT, and o,p′-DDT made up of 52, 31, and 12 % of the total DDTs in sediment, respectively. Overall, the six DDT isomers were detected in almost all samples, indicating that the DDTs were ubiquitous in the Baiyangdian Lake. The concentrations of p ,p ′-DDE were higher than the other isomers in each month, followed by o,p′-DDT and p,p′-DDT; the average abundance of the other three remaining isomers was generally less than 12 % in water samples and less than 5 % in sediment samples. The predominance of p ,p ′-DDE suggests that the p ,p ′-DDT in the Baiyangdian Lake was mostly degraded. In addition, relatively high levels of o,p′-DDT indicate that new input of o,p′DDT into the Baiyangdian Lake might occur recently. Compared with the previous studies of Baiyangdian Lake, in the present study, the mean concentration of DDTs in the surface water of the Baiyangdian Lake was much lower than that found in 1995 (range 0–900 ng/L, mean 250 ng/L) (Dou and Zhao 1998). Compared with other water bodies, the concentration range of DDTs in the surface water from the Baiyangdian Lake was similar to those of Jiulong River Estuary and Lake Chaohu and slightly greater than those of Daliao River Estuary, Ebro River in Spain, and Lake Baikal in Russia but was lower than those reported in the Minjiang River Estuary, Beijing Guanting Reservoir, and Gomti River
in India (Table 2). No research data on DDT residual levels in overlying water has been presented in the previous studies, so it could not be compared with the other areas. For pore water, in comparison, the DDT level was lower than those observed in Minjiang River Estuary, Beijing Guanting Reservoir, while it was similar to those in Daliao River Estuary and Jiulong River Estuary in China. In sediments, the concentration of DDTs in Baiyangdian Lake was comparable to those of Minjiang River Estuary, Beijing Guanting Reservoir, Daliao River Estuary, Lake Chaohu, and Ebro River in Spain, and greater than those of Jiulong River Estuary and Lake Baikal in Russia, but much lower than those of Gomti River in India and Hanoi area in Vietnam (Table 2). Compared with the previous studies in Baiyangdian Lake, the DDT levels in sediment in the present study were comparable to the levels in 1995 (0.69– 2.26 ng/g) (Dou and Zhao 1998) and the levels in 2007 (2.2– 3.1 ng/g) (Hu et al. 2010). Spatial and temporal variation of DDTs Temporally, the concentrations of DDTs varied considerably in the surface water (Fig. 2). The greatest concentration of DDTs was detected in July, and relatively higher concentrations were also found in August, June, and September. Three possible explanations can be provided. First, the higher concentration levels of DDTs found in the summer might be partially attributed to the increase of land runoff during the wet season (June–September). A similar result was also obtained by Yuan et al. (2013), indicating that the increasing land runoff during the summer might bring the chemical residues from soil to the water environment. Second, the high contamination level in the summer might be linked to the emission of DDTs by ships since there are intensive boat activities for tourism and fishery in these months during sampling period. DDT, as the auxiliary material of an antifouling paint is still in use. He et al. (2012) reported that in China, from 1950s to 2005, approximately 1.1 million tons of DDT was used to produce antifouling paints. Third, the higher levels of DDTs in summer might be related to the dicofol applications for crops
Environ Sci Pollut Res Table 2 Concentrations of DDTs isomers in surface water (ng/L), pore water (ng/L) and sediments (ng/g) from lakes and rivers around the world
nd not detected
Sampling area
Surface water
Pore water
Sediment
References
Baiyangdian Lake Minjiang River Estuary Beijing Guanting Reservoir Jiulong River Estuary Daliao River Estuary
2.36–22.35 40.61–233.5 nd–528.8 June>August>September>May>October>December>March>April>November in the overlying water, and the order was July>August>June> September>May>October>April>December>November> March in the pore water (Fig. 2). No obvious temporal variations of DDT concentrations were observed in the sediment. A slightly lower concentration occurred in sediment during the periods of March and December compared to other sampling times (Fig. 2).
in the summer season. Yang et al. (2008) reported that dicofol with high impurity of DDT compounds is still widely used in agricultural practice such as cotton cultivation and becomes an important source of DDT pollution in China. Statistical data showed that from 1988 to 2002, the average annual DDT production was about 6,000 tons in China, of which nearly 80 % was for dicofol production (Qiu et al. 2005). On the contrary, lowest concentrations of DDTs in surface water were found in April and November, followed by October, December, and March. It is worth noting that during the months of April and November 2010, a great deal of fresh water was allocated to the Baiyangdian Lake by the local government to improve the hydrological and ecological conditions of the 35 site 2 site 5
site 3 site 6
Surface water
site 1 site 4
30
25
Concentration (ng/L)
Concentration (ng/L)
30
35 site 1 site 4
20 15 10 5
site 2 site 5
site 3 site 6
Overlying water
25 20 15 10 5
0 Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
0
Dec
Mar
70
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
20
site 1 site 4
60
site 2 site 5
site 3 site 6
site 1 site 4
18
Pore water
site 2 site 5
site 3 site 6
Sediments
50
Concentration (ng/g)
Concentration (ng/L)
16
40 30 20
14 12 10 8 6 4
10 2
0
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
0
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Fig. 2 Distribution of DDT concentrations in surface water, overlying water, pore water, and sediments from Baiyangdian Lake
Nov
Dec
Environ Sci Pollut Res
Overall, the concentrations of DDTs in the surface water, overlying water, pore water, and sediments varied with the sampling locations (Fig. 2). The greatest concentrations of DDTs were found at site 2 in the surface water, at sites 2 and 5 in the overlying water, and at site 2 in the pore water and sediment. Relatively higher levels were also found at sites 5 and 6 in the surface water, at sites 3 and 6 in the overlying water, and at sites 3, 5, and 6 in the pore water and sediments. Two possible explanations can be provided. First, sites 2 and 3 are located on the entrance of the Fuhe River. The higher concentrations at these two sites might be caused by high rates of influx of contaminants into the lake through the Fuhe River. This result can be confirmed by the fact that the concentration of DDTs in the sediments from the Fuhe River (range 1.3– 51.3 ng/g) was much higher than that in the Baiyangdian Lake (2.2–3.1 ng/g) (Hu et al. 2010). The Fuhe River, which runs through the heavy industrial city of Baoding, carries about 250,000 tons of untreated industrial and domestic wastewater per day produced by Baoding city into the Baiyangdian Lake (Hu et al. 2010). Second, sites 5 and 6 are all close to villages and fish or duck aquaculture areas, and the high levels at these sites may be the result of agricultural runoff entering the water environment. In addition, the higher levels at sites 5 and 6 (fish aquaculture area) may also be related to the fish feed. DDTs have been reported in commercial feed or trash fish (Meng et al. 2007; Guo et al. 2009), which will contaminate the surrounding water body of aquaculture area. On the other hand, in the relatively pristine site 4, levels of DDTs were much lower than the other sites. DOC contents in water and total organic carbon (TOC) in sediments were also investigated in this study, with the concentrations ranging from 6.78 to16.23 mg/L in surface water, 7.12 to 18.56 mg/L in overlying water, 7.28 to 22.08 mg/L in pore water, and 0.23 to 3.09 % in sediments, respectively, during the sampling periods. The distribution of HOCs such as PCBs has been shown to be related by DOC contents in water and by TOC contents in sediments (Iwata et al. 1995). However, in this study, distribution of DDTs did not show any correlation with DOC contents (r =0.415, p =0.272) in water phase and TOC contents (r =0.423, p =0.112) in sediment. This relationship was also observed by Mai et al. (2002), suggesting that many factors contributed to the DDTs' distribution in water, and the sediment contamination in the study area may be dictated more predominantly by anthropogenic inputs than by natural processes. The correlation of DDT concentrations with temperature (r =0.536, p =0.09) and dissolved oxygen (r =0.322, p = 0.302) was also investigated in this study, and no statistically significant relationship was observed. Possible sources of DDTs The ratios of the parent compound to its metabolite can provide useful information on the pollution source. The
portion of DDT metabolic products is indicative of new inputs or historical usage. A ratio of DDT/(DDE+DDD) higher than 1 is generally pointed to the fresh input of DDTs (Hitch and Day 1992). Otherwise, aged DDTs are suggested. There are two known sources of DDTs characterized by the o,p′-DDT/ p,p′-DDT ratio of 0.2 in technical DDT mixture and ~7.0 in dicofol products in China, respectively (Qiu et al. 2005). Therefore, the ratio of o,p′-DDT/p,p′-DDT is usually used to distinguish technical DDT from dicofol-type DDT (Qiu et al. 2005; Yang et al. 2008; Liu et al. 2009), and an elevated o,p′-DDT/p ,p′-DDT is an indicative of the dicofol-type DDT. In the present study, the ratios of DDT/(DDE+DDD) were 1.13±0.80, 0.66±0.48, 0.65±0.33, and 0.56±0.19 in surface water, overlying water, pore water, and sediments, respectively, during the sampling periods. As shown in Table 3, more than 70 % of the DDT/(DDE+DDD) ratios were less than 1, suggesting that DDT in Baiyangdian Lake mostly stemmed from the historical input. On the contrary, for surface water and overlying water, higher DDT/(DDE+DDD) ratios (>1.0) were found from April to September, indicating that fresh input of DDT occurred in the Baiyangdian Lake in these months. Moreover, elevated o,p′-DDT/p,p′-DDT ratios, with the mean value of 0.18±0.08 to 2.41±1.60 in different environment medium, were also observed. Recently, similar results were also reported in water from Taihu Lake (Qiu et al. 2008) and Chaohu Lake (He et al. 2012) in China, with o,p′DDT/p,p ′-DDT ratios of 1.6–5.6 and 0.40–2.18, respectively. Since o,p ′-DDT metabolizes more readily than p,p ′-DDT in the environment (Martijn et al. 1993; Qiu et al. 2005), the degradation of technical DDT is unlikely to cause increase of o,p′-DDT/p ,p′-DDT ratio in the environment that is higher than that of technical DDT. Therefore, the relatively higher values of o ,p′-DDT/p,p′-DDT in Baiyangdian Lake clearly demonstrates that the new input of DDT was from dicofoltype DDT. In summary, mixed sources of DDTs, i.e., both historical input of technical DDT and new input of dicofoltype DDT, were obvious in Baiyangdian Lake. According to the formula proposed by Liu et al. (2009), the contributions of technical DDT and dicofol-type DDT were 85 and 15 %, respectively, in the Baiyangdian Lake. Fugacity ratios calculation Fugacity is a measure of the tendency of a chemical to escape from its medium. A convenient way to estimate the net flux direction is to compare fugacity values of a chemical among the different compartments (Mackay 2001; Rowe et al. 2007). The fugacity (f ) of a chemical in any phase is defined as the concentration (C ) of the chemical in the phase divided by the fugacity capacity (Z ) of the phase (Mackay 2001): f ¼ C=Z
ð1Þ
Environ Sci Pollut Res Table 3 Ratios used in source identification of DDT with its metabolites in Baiyangdian Lake from March to December
Ratios DDT/(DDE+DDD)
o,p′/p,p′-DDT W surface water, OW overlying water, PW pore water, SED sediments
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
W OW PW SED W
0.77 0.24 0.48 0.37 1.82
1.04 0.31 0.24 0.80 1.14
1.55 0.53 0.44 0.25 3.00
1.84 1.05 0.85 0.83 2.19
2.56 1.35 1.13 0.54 4.86
1.75 1.31 1.05 0.50 5.03
1.14 0.18 0.48 0.47 3.18
0.28 0.40 0.82 0.52 1.67
0.08 1.02 0.20 0.77 0.42
0.28 0.17 0.86 0.57 0.78
OW PW SED
0.85 0.52 0.07
0.15 0.37 0.12
1.80 0.24 0.27
1.48 0.64 0.23
1.77 0.35 0.09
1.83 0.21 0.17
0.16 0.64 0.24
2.44 0.71 0.20
0.09 0.37 0.14
0.75 0.94 0.28
Z values are obtained from the following relationship (Mackay 2001): Z w ðwaterÞ ¼ 1=H
ð2Þ
Z s ðsedimentÞ ¼ 0:41 ðOCÞ K ow ρs =H
ð3Þ
so that the fugacities of a chemical in water and sediment are: f w ¼ CwH
ð4Þ
f s ¼ C s H=0:41 ðOCÞK ow ρs
ð5Þ
where f w and f s are the fugacities in water and sediment (Pa), C w and C s are the concentrations of DDT isomer in water (ng/L) and sediment (ng/g), H is the Henry's law constant at the temperature of the water (Pa m3/mol), OC is the organic carbon content (fraction ranging from 0 to 1), K ow is the octanol–water partition coefficient, and ρs is the sediment density (kg/L). The measured average density of sediment solids (2.0±0.3 kg/L) was used for the calculations in this study. The K ow values for o,p′-DDT, p,p′-DDT, p,p′-DDE, and p,p′-DDD were taken from UNEP Chemicals (2002). The sediment–water fugacity ratio (f s/f w) is: f s f w ¼ C s C w ð0:41 ðOCÞK ow ρs Þ:
ð6Þ
Here, f s/f w =1.0 implies that the DDTs in water and sediment are at equilibrium, the net flux is zero, although DDTs are exchanged between water and sediment at the same rate. Values of f s/f w >1.0 and
