Using Isomeric and Metabolic Ratios of DDT to Identify the Sources

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Using Isomeric and Metabolic Ratios of DDT to Identify the Sources and Fate of DDT in Chinese Agricultural Topsoil Chong Zhang, Li Liu, Yan Ma, and Fasheng Li Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b05877 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018

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Environmental Science & Technology

Using Isomeric and Metabolic Ratios of DDT to Identify the Sources and Fate of DDT in Chinese Agricultural Topsoil Chong Zhang, Li Liu*, Yan Ma, and Fasheng Li State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese Research Academy of

Environmental Sciences, Beijing 100012, China

*Corresponding Author Phone: (+86-10)84915216; E-mail: [email protected]

TOC Art

ACS Paragon Plus Environment


1

ABSTRACT

2

The metabolic ratio of (p,p'-DDE + p,p'-DDD)/p,p'-DDT or p,p'-DDE/p,p'-DDT has been used

3

previously to estimate the approximate half-life of p,p'-DDT, with a relatively unclear concept of

4

“old” and “new” sources of p,p'-DDT and without paying attention to the influence by dicofol-type

5

DDT contributed from the more recent usage of dicofol. Based on the isomeric ratio of o,p'-

6

DDT/p,p'-DDT to distinguish the sources of DDT, this study used the corrected metabolic ratio of

7

(p,p'-DDE + p,p'-DDD)/p,p'-DDT to estimate a more accurate half-life of p,p'-DDT using a model-

8

based approach. This indicates the average half-life of p,p'-DDT in Chinese topsoils was 14.2 ± 0.9

9

years with dicofol-type DDT input considered. In deeper soil, the half-life was > 30 years and the

10

metabolic pathway of p,p'-DDT was significantly different with topsoil's. Further analysis on the

11

fraction of DDT from technical DDT suggested that a region that had been sprayed with technical

12

DDT was likely to have been sprayed with dicofol as well, but the monitoring residues of DDT in

13

topsoil mainly derive from historical use of technical DDT.

14

INTRODUCTION

15

China is a large agricultural country and a primary manufacturer, consumer and supplier of

16

pesticides. The country began to produce dichlorodiphenyltrichloroethane (DDT) in the 1950s.

17

Subsequently DDT's use as a pesticide was banned in May 1983 by the Chinese government1, 2. By

18

1983, 4.6 × 105 tons of DDT had been produced in China, which was 20% of the global total3. DDTs

19

have relatively stable physicochemical properties and can persist a long time in the environment4, 5

20

and present risks6, 7. Over the past 30 years, extensive studies have reported high concentrations and

21

long persistence of DDT residues in the environment8-14. On May 23, 2001, one hundred and twenty-

22

seven countries and regions, including China, jointly signed the Stockholm Convention, which


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strictly prohibited or restricted the use of twelve persistent organic pollutants (POPs), and thereby

24

DDT became one of the first controlled POPs by the time that the Convention entered into force on

25

May 17, 2004.

26

Residues of DDTs in the environment mainly included p,p'-DDT, its isomer (o,p'-DDT) and

27

metabolites (p,p'-DDE, p,p'-DDD). In the ecosystem, p,p'-DDT firstly and principally degrades to

28

p,p'-DDE and/or p,p'-DDD by various environmental factors4, 15. Researchers have usually used

29

metabolic ratio of (p,p'-DDE + p,p'-DDD)/p,p'-DDT or p,p'-DDE/p,p'-DDT to estimate the

30

residence time of p,p'-DDT in the environment3, 13, 16-18. A ratio > 1 or < 1, respectively suggest that

31

the residues of p,p'-DDT came from “old” or “new” sources. However, the boundary between “old”

32

and “new” is relatively unclear. In addition, the fate of DDT has different tendencies in different

33

environmental media. Cortes et al.19 measured gas-phase concentrations of chlorinated pesticides at

34

the shores of the Great Lakes, and hypothesized that atmospheric concentrations of DDTs decreased

35

by first-order dynamics, which indicated that the half-life of p,p'-DDT was 2.5–2.7 years in the

36

atmosphere. Dimond and Owen20 carried out a long-term study of sprayed forest soil in Maine, and

37

reported that the half-life of DDT was 20–30 years in the soil.

38

Another problem with the ratio approach described above is that dicofol-type DDT, as a

39

relatively new input source of DDT into the environment, has been ignored by some researchers

40

when evaluating these ratios. Although China banned the agricultural use of DDT in the 1980s, the

41

country still produced DDT for disease control and as a raw material for synthesis of dicofol21,

42

which contains p,p'-DDT as impurity. Qiu et al.8, 21 considered that high concentration residues of

43

DDT came from the agricultural use of dicofol with impurity of DDT; Jaward et al.22 found higher

44

levels of atmospheric o,p'-DDT in sampling sites in the southeast of China, suggesting that the



45

atmospheric concentration of DDTs was mainly derived from dicofol. However, Jin et al.23 found

46

old sources of technical DDTs predominated in the study area for the sampling period. Hence, both

47

technical DDT and dicofol-type DDT are the main sources of the monitoring residues of DDT.

48

The agricultural use of DDT had a long history from 1951 to 1983 in China, but the production

49

of technical dicofol was stopped on May 17, 201424. Regarding the influence of dicofol formulation

50

in soil, it is important to reexamine the sources and fate of DDT, and to thereby help provide one

51

reference to perform future work aimed at DDT in agricultural soil during the soil survey committed

52

to in the "13th Five-Year Plan" of China25.

53

This study fully considered the influence of dicofol-type DDT and built a newly developed

54

methodology to identify the sources and fate of DDT: the isomeric ratio of o,p'-DDT/p,p'-DDT was

55

used to distinguish the sources of DDT based on literature data on the distribution of DDT residues

56

in soil from 1983 to 2014 in China; Based on the corrected metabolic ratio of (p,p'-DDE + p,p'-

57

DDD)/p,p'-DDT, the first-order kinetic model was developed to calculate the degradation period of

58

DDT in soil. Combining our previous studies of pesticide-producing sites26, a detailed comparison

59

between topsoil and deep soil was made to evaluate the persistence of DDT in order to reexamine

60

the distribution characteristics of DDTs and the half-life of p,p'-DDT in soil in China.

61

MATERIALS AND METHODS

62

Derivation of Equation for the Fraction of DDT from Technical DDT

63

This study combined literature surveys, field surveys and personal communication to establish

64

a database of DDTs in soil from 1983 to 2014; only the DDT-related literature after 2000 had detailed

65

concentrations of DDTs in soil in China. The details of the database and sampling locations are

66

described in Table S1 and Figure S1 of the Supporting Information (SI). In this study, it is


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67

hypothesized that only two sources of DDT occurred in soil: historical residues of technical DDT

68

and new input of dicofol formulation. To obtain the DDT fraction based on actual measurements in

69

soil, the molar value of Ns was calculated by eq. (1): Ns = Nt + Nd

(1)

70

where Ns is the sum of p,p'-DDT and o,p'-DDT in soil with contributions from Nt (technical DDT)

71

and Nd (dicofol formulation). If x is the fraction of DDT from technical DDT, then the above

72

equation expresses the mass balance by the following: Nt = x × Ns = Nt,o,p'-DDT + Nt,p,p'-DDT

(2)

Nd = (1− x) × Ns = Nd,o,p'-DDT + Nd,p,p'-DDT

(3)

73

In the actual soil samples, the molar ratio of o,p'-DDT/p,p'-DDT (Rs,o,p'/p,p') is influenced by both

74

technical DDT and dicofol formulation: Rs,o,p'/p,p' =

Nt,o,p'-DDT + Nd,o,p'-DDT Nt,p,p'-DDT + Nd,p,p'-DDT

(4)

75

For technical DDT and dicofol formulation respectively, Rt,o,p'/p,p' and Rd,o,p'/p,p' are defined as

76

the molar ratio of o,p'-DDT/p,p'-DDT from technical DDT and dicofol formulation by the following:

77

Rt,o,p'/p,p' = Nt,o,p'-DDT/Nt,p,p'-DDT

(5)

Rd,o,p'/p,p' = Nd,o,p'-DDT/Nd,p,p'-DDT

(6)

Combining Eq. (4) with Eqs. (2), (3), (5), (6): Rt,o,p'/p,p' Rd,o,p'/p,p' x+ (1 – x) 1 + Rt,o,p'/p,p' 1 + Rd,o,p'/p,p' Rs,o,p'/p,p' = 1 1 x+ (1 – x) 1 + Rt,o,p'/p,p' 1 + Rd,o,p'/p,p'

78

Rearranging Eq. (7) gives: x=

79

(7)

(1 + Rt,o,p'/p,p')(Rd,o,p'/p,p' – Rs,o,p'/p,p') (1 + Rs,o,p'/p,p')(Rd,o,p'/p,p' – Rt,o,p'/p,p')

(8)

where both Rt,o,p'/p,p' and Rd,o,p'/p,p' can be measured from technical DDT and dicofol



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80

formulation. On the basis of the content of DDTs in dicofol formulation, Qiu et al.21 systematically

81

studied the content and composition of DDTs impurities from dicofol formulation in the Chinese

82

market, which suggested that Rd,o,p'/p,p' was 7.0 ± 2.2 from the formulated dicofol samples

83

collected. Meanwhile, technical DDT was composed of 65–80% p,p'-DDT and 15–21% o,p'-DDT

84

from Metcalf’s study27. In this study, it is assumed that Rt,o,p'/p,p' is 0.25 and Rd,o,p'/p,p' is 728.

85

Hence, we can accurately quantify the contribution of technical DDT in soil, combining Eq. (8) with

86

the environmental measurement of Rs,o,p'/p,p' from our collected data (Detailed information is

87

described in SI Table S2). Also, when Rs,o,p'/p,p' is < 0.25, it is assumed that there is no DDT input

88

from dicofol formulation and the mean of x is equal to 1. Conversely, the mean of x is equal to zero

89

if Rs,o,p'/p,p' is > 7.0. Bring two parameters Rt,o,p'/p,p' and Rd,o,p'/p,p' into Eq. (8) results in the

90

following equation: x=

91 92 93

1.25(7 – Rs,o,p'/p,p') 6.75(1 + Rs,o,p'/p,p')

(9)

Kinetic Model for the Degradation Period of DDT in Soil For actual measurements in soil, the molar ratio of (p,p'-DDE + p,p'-DDD)/p,p'-DDT (Rp,p'/ p,p') can indicate the degradation period. Rp,p'/p,p' =

Np,p'-DDE + Np,p'-DDD Np,p'-DDT

(10)

94

In soil, p,p'-DDT principally degrades to p,p'-DDE and/or p,p'-DDD by various environmental

95

factors4, 10, 29: 𝑘𝑘

p,p'-DDT → p,p'-DDE and/or p,p'-DDD

(11)

96

given that p,p'-DDE and p,p'-DDD have similar physicochemical properties and the degradation

97

rate k is slow enough to regard metabolites of p,p'-DDT as a whole. Furthermore, the first-order

98

kinetic model is widely used to evaluate the degradation of p,p'-DDT4, 11, 19.


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CDDT(t) = CDDT(0) exp(−kt) 99 100

(12)

where k is the first-order rate constant, CDDT(t) and CDDT(0) are the molar concentrations in sampling year t and in the starting year respectively. As to the residues of technical DDT in soil: CDDT(t) = Nt,p,p'-DDT

(13)

CDDT(0) − CDDT(t) = Nt,p,p'-DDE + Nt,p,p'-DDD

(14)

101

Eq.14 describe that p,p'-DDT transformed to p,p'-DDE and/or p,p'-DDD equimolarly with time

102

from the starting year to sampling year t. Nt,p,p'-DDE and Nt,p,p'-DDD are the monitoring

103

concentration of p,p'-DDE and p,p'-DDD respectively in soil, the value of p,p'-DDE and p,p'-DDD

104

is given in Supporting Information Table S2.

105

Dividing Eq. (14) by Eq. (13),

106

CDDT (0)/CDDT (t) - 1 =

Nt,p,p'-DDE + Nt,p,p'-DDD Nt,p,p'-DDT

(15)

Combining Eq. (15) with Eq. (8), (12):

Rt,p,p'/p,p' = exp(kt) − 1

(16)

107

Although Rt,p,p'/p,p' cannot be directly calculated, an approach can be used to estimate

108

approximately Rt,p,p'/p,p' by Rs,p,p'/p,p'. Rs,p,p'/p,p' = (Ns,p,p'-DDE + Ns,p,p'-DDD)/ Ns,p,p'-DDT

(17)

Nt,p,p'-DDE + Nt,p,p'-DDD = Ns,p,p'-DDE + Ns,p,p'-DDD + C

(18)

109

where C is a correction concentration for the sums (p,p'-DDE and p,p'-DDD) between residues of

110

technical DDT and actual measurement in soil, which is influenced by various environmental factors

111

such as volatilization, runoff, adsorption, long-range transport and so on.

112

Combining Eq. (16) with Eqs. (10) and (18): exp(kt) − 1 =

Ns,p,p'-DDE + Ns,p,p'-DDD C + Nt,p,p'-DDT Nt,p,p'-DDT


(19)


113

114

115

116 117

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Combining Eq. (19) with Eqs. (2) and (8): exp(kt) − 1 =

1.25 Ns,p,p'-DDE + Ns,p,p'-DDD C ( + ) x Ns Ns

(20)

Rearranging Eq. (20)

1.25/x·Rs,p,p'/p,p' = exp(kt) − 1 − 1.25C/(x·Ns)

(21)

y = exp(kt) − b

(22)

Simplifying Eq. (21):

where the correction ratio y = 1.25/x·Rs,p,p'/p,p' and correction factor b = 1 + 1.25C/(x·Ns) .

Given that DDTs have similar relative molecular weights ranging from 318 to 354.530, this study

118

sets the mass ratio approximately equal to the molar ratio in the corresponding calculations.

119

Commercial technical DDT was used from 1951 to 1983 in China1. Hence, this study assumed that

120

the starting year was 1967 (the median of 1951 and 1983, detailed explanation is given in Text S1).

121

As to the first-order kinetic model, the half-life is only related to first-order rate constant,

122 123

Data Analysis

𝑡𝑡1/2 = In2/𝑘𝑘

(23)

Statistical analyses were performed with IBM SPSS Statistics 19.0 software. The kinetic fitting

124

was performed with OriginPro 9.1 software.

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RESULTS AND DISCUSSION

126

Spatial Distribution and Temporal Variation of DDTs

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China started to produce commercial technical grade DDT in 19511. Technical DDTs was a

128

mixture of four forms, mainly including p,p'-DDT (65–85%), o,p'-DDT (15–21%), p,p'-DDE (~5%)

129

and p,p'-DDD (< 5%)27. When it was sprayed on farmland, forests and disease-control regions, some

130

DDTs entered environmental media such as air, soil and water.


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Figure 1. Spatial distribution of DDTs in topsoil from 2001 to 2013 in China. 131

Figure 1 presents an overview of the spatial distribution of DDTs in topsoil in China. Due to

132

the lack of annual reports on residues of DDTs in soil in China, literature surveys, field surveys and

133

personal communication were combined to estimate the overall residues of DDTs since 1983. As

134

shown in Figure 1, residues of DDTs are mainly distributed in Hebei, Shandong, Jiangsu, Zhejiang,

135

Fujian, and Guangdong provinces and major metropolitan cities of Beijing, Tianjin, and Shanghai.

136

These nine regions belong to the eastern/southern economic zone in China, which indicates that

137

historical usage and current research on DDTs are closely related to national economic development.

138

As shown in Figure 2 (Detailed information is described in Table S1), the sum (N = 60) of different

139

forms of DDT ranges from 0.54 ng/g to 364 ng/g with arithmetic mean 56.2 ± 69.6 ng/g, geometric

140

mean 26.0 ± 4.2 ng/g and median mean 31.5 ng/g. The monitoring levels of ΣDDTs across sampling

141

sites between 2001 and 2012 remained at 10-100 ng/g. It is not obvious that the levels of ΣDDTs

142

declined over time from 2001 to 2013. The possible reasons are that the degradation of DDTs could



143

take decades in topsoil20. If only using the results of ΣDDTs from 2001 to 2013, it is difficult to

144

illustrate the degradation of DDT in topsoil. Given that the concentration of DDTs was

145

approximately 800 ng/g in cotton field soils during the 1970s and 419 ng/g in arable layer soils in

146

198031, the level of ΣDDTs has declined remarkably from the 1970s to now.

Figure 2. Temporal variation of ∑DDTs (the sum of o,p'-DDT, p,p'-DDT, p,p'-DDE and p,p'DDD) in topsoil from 2001 to 2013 in China. The bottom left diagram is the frequency distribution for the levels of ∑DDTs. 147

Sources of DDT in China

148

Though the agricultural use of DDT was banned in China in 1983, the DDT production capacity

149

still remained for disease control and as a raw material for other pesticide synthesis. Dicofol, a

150

nonsystemic acaricide with 2,2,2-trichloro-1,1-bis (4-chlorophenyl) ethanol as its acaricidal active

151

ingredient, is significantly related to DDT because DDT was used as raw material for dicofol

152

synthesis. Due to incomplete chlorination during the production process of dicofol, commercial


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dicofol usually has some DDT impurities contributing to DDT residue21. After extensive spraying

154

of dicofol to cotton, vegetables and fruit trees for controlling mites beginning in 1976 in China24,

155

some researchers found that there was a large area of new DDT input partially originating from

156

dicofol formulation in China11, 21, 32-34. Based on the database of DDTs in topsoil as shown in Figure

157

3, most of the Rs,o,p'/p,p' are > 0.25, which indicates that dicofol formulation has a contribution to

158

the monitoring DDT levels in most contaminated soil samples. Four of the samples even have values

159

than 7.0 (Detailed explanation is given in Text S2). Meanwhile, most of the Rs,p,p'/p,p' are > 1,

160

indicated that more than half of p,p'-DDT has transformed to p,p'-DDE and/or p,p'-DDD. It is

161

inferred that the half-life of DDT could be < 30 years. In addition, the concentrations of ΣDDTs had

162

no significant correlation with Rs,p,p'/p,p' or Rs,o,p'/p,p'. The possible reason is that the variation

163

of spatial distribution had a significant impact on the concentrations of ΣDDTs in topsoil. Hence,

164

this study tends to determine the ratio of Rs,p,p’/p,p’ and Rs,o,p'/p,p' to minimize the impact of

165

regional differences on the environmental fate of DDT in soil.

Figure 3. Different ratios of DDTs in 60 groups of samples from 2001 to 2013.



166

For topsoil, the fraction of DDT from technical DDT (x) was calculated based on Eq. (9). The

167

mean values of x range from 0.11 to 1.0 with arithmetic mean 0.74 ± 0.30 and median mean 0.84

168

(Detailed data are listed in Table S2, and the regularity of the variation in x is shown in Figure S2).

169

Hence, the monitoring residues of DDT in topsoil are composed of 84% technical DDT and 16%

170

dicofol-type DDT. However, the detection frequency of dicofol-type DDT reaches up to 75%. This

171

suggests that China has used dicofol widely, but the monitoring residues of DDT in topsoil are

172

mainly made up of historical technical DDT.

Figure 4. The correlation between Nt and Nd in 60 groups of samples from 2001 to 2013. (For 41 groups of samples derived from Nt & Nd, the curve has 95% prediction confidence ellipse, and the dotted line is a linear fitting with R2 = 0.509 for Nt vs. Nd. The inset is a partially enlarged view only for much lower ranges.) 173

As the sources of DDT come from both technical DDT and dicofol-type DDT, it is necessary

174

to consider the influence on the fate of DDT from different sources. The correlation between Nt and

175

Nd as shown in Figure 4 (Detailed information is described in Table S2), the Pearson correlation


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coefficient is 0.438 (P < 0.01) for all 60 groups of samples, which is significant at the 0.01 level (2-

177

tailed). Disregarding the samples having Nt = 0 or Nd = 0, the Pearson correlation coefficient

178

between Nt and Nd rises to 0.714 (P < 0.01) for the remaining 41 groups of samples. This indicates

179

that a region that had been sprayed with technical DDT before 1983 was likely to have been sprayed

180

with dicofol as well.

181

Degradation Period of DDT in Topsoil

182

Based on the previous analysis, the concentration of DDT in soil from dicofol formulation

183

should be separated out. Since soil from urban and suburban regions may be influenced by some

184

human factors, and contaminated soil near chemical industrial districts may be overestimated by

185

high levels of DDTs, this study only used data from agricultural soil samples to explore the

186

degradation period of DDT. In topsoil, p,p'-DDT principally degraded to p,p'-DDE and/or p,p'-

187

DDD4, 15, and the degradation rate of metabolites was too slow4 to be considered in contrast to the

188

degradation rate of p,p'-DDT. When p,p'-DDE and/or p,p'-DDD are substantially derived from the

189

degradation of p,p'-DDT in the environment, Rt,p,p'/p,p' should increase over the degradation time.

190

As shown in Figure 5 with correction ratio y =1.25/x·Rs,p,p'/p,p' as Y-axis and sampling year

191

as X-axis, the correction ratio y increases from 2001 to 2009 in agricultural topsoil (The details are

192

described in Table S3). Given that the data came from different regions, this study used the annual

193

arithmetic mean to perform first-order kinetic fitting with degradation time, t = sampling year –

194

starting year. The result of curve fitting (R2 = 0.801, P < 0.01) gives a first-order kinetic rate constant

195

k = 4.9 × 10-2 ± 3.2 × 10-3 year-1 and correction factor b = 3.4 ± 0.8. Based on Eq. (23), the half-life

196

of p,p'-DDT is 14.2 ± 0.9 years in agricultural topsoil, which is consistent with the prior inferences

197

from this study, and is longer than the value of 9 years derived from Zhang’s study11 in agricultural



198

topsoil of Zhejiang Province, China.

Figure 5. Temporal variation of correction ratio of Rs,p,p'/p,p' in agricultural topsoil from 2001 to 2009. (The dotted line is a first-order kinetic fitting curve for annual arithmetic mean vs. degradation time, and the shadowed region shows values within a prediction band at a confidence level of 95%.) 199

However, the above result might be influenced by some environmental factors such as

200

volatilization, runoff, long-range transport and so on. Kurt-Karakus et al.35 found that the half-life

201

of volatilization loss was ~200 years, which was estimated for ΣDDTs in the upper 5 cm of the soil

202

column. It is inferred that the volatilization process will persist a long time and have only minor

203

influence. Due to the hydrophobicity of DDTs, runoff also has a limited influence on the

204

hydrodynamic migration of DDTs, and loss of DDTs in runoff primarily takes place via migration

205

of DDT-adsorbed particulate matter4. Furthermore, the kinetic study excluded the area significantly

206

affected by long-range transport (Detailed explanation is given in Text S3). In addition, dicofol has


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207

been found to mainly degrade into p,p'-dichlorobenzophenone (DBP) via alkaline hydrolysis or

208

photocatalytic reaction36-39. Although dicofol has been widely used in China and is structurally

209

analogous to DDTs, DDTs including p,p'-DDT, p,p'-DDE and p,p'-DDD are not the main

210

degradation products of dicofol. Hence, it is indicated that the degradation of dicofol in soil has little

211

influence on the degradation of technical DDT. In short, the half-life of 14.2 ± 0.9 years in

212

agricultural topsoil can be considered valid and trustworthy.

213

Degradation Period of DDT in Deep Soil

214

For deep soil, the half-life of p,p'-DDT may display significant differences from that in topsoil26.

215

Literature surveys found that most researchers collected ~2 m soil cores to explore the fate of DDT

216

in deep soil or its vertical migration40-43. However, our prior study26 indicated that the shallow soils

217

at depths of 0–2 m were mostly backfill soils, which cannot accurately represent the concentration

218

of DDTs in situ. Rs,p,p'/p,p' in deep soil at depths of 5–21.5 m ranged from 0.1–1.1 with arithmetic

219

mean 0.34 ± 0.23, including ~82% of Rs,p,p'/p,p' less than 0.5 (The details are described in Table

220

S4). Because these six sampling sites were located at a DDT production factory, which stopped the

221

production of DDT in 1983 and had no new DDT input since then, these samples had almost no

222

influence from usage of dicofol and human activities, and the deep soil maintained relatively steady

223

physicochemical properties against environmental effects such as volatilization and long-range

224

transport. Hence, Rs,p,p'/p,p' of the deep original soil could indicate the degradation period of DDT,

225

which numerically had the same value as Rt,p,p'/p,p'. Wang et al.44 also found that the ratio of (p,p'-

226

DDE + p,p'-DDD)/p,p'-DDT was < 0.5 in deep soil at depths of 1.8–4.0 m at two other chemical

227

plants with DDT production from 1958–1965 and 1965–1981. Based on the above analysis, it

228

suggests that the half-life of p,p'-DDT is > 30 years in deep soil as DDT has been forbidden for such



229

a long time in China.

230

Further comparing topsoil and deep soil as shown in Figure 6, the fate of DDT has significant

231

differences in the levels of ΣDDTs and the ratio of metabolites (RDDE/DDD) between deep soil and

232

topsoil. As the soil samples in deep soil were collected from a DDT production factory in this study,

233

the levels of ΣDDTs in deep soil were much higher than those in topsoil. When the soil samples

234

were collected from different sources, the comparison of the levels seemed to be meaningless,

235

without any information on environmental factors. However, analytical method based on the ratio

236

of RDDE/DDD may reveal the difference in the environment fate of p,p'-DDT between deep soil and

237

topsoil. The major metabolites of p,p'-DDT are formed under both aerobic conditions (p,p'-DDE)

238

and anaerobic conditions (p,p'-DDD)4, 10, 29. The ratio of RDDE/DDD was 91% > 1 and 70% > 2 in

239

topsoil, while it was only 42% > 1 and 23% > 2 in deep soil. This indicates that p,p'-DDT is mainly

240

transformed into p,p'-DDE aerobically in topsoil, instead of both p,p'-DDE and p,p'-DDD

241

anaerobically in deep soil. The topsoil has more frequent and active interaction with the

242

surroundings in comparison to deep soil. The component in topsoil tends to exchange with the

243

atmosphere32, 45, 46, and continuous soil-air exchange benefits aerobic transformation of DDT by

244

microorganisms4. Human activities and agricultural practices can also influence the degradation of

245

DDT in the topsoil47. For example, the degradation of DDT in fallow land was slower than that in

246

paddy fields and wood lands2.

247

This study clearly shows the persistence of DDT in soils. A more accurate half-life of p,p'-DDT

248

in topsoil is estimated by a newly developed approach, and the application can be extended to other

249

areas of DDT contaminated soils. It is a challenging task to remediate the soil contaminated from

250

the usage of technical DDT and dicofol formulation. It is therefore imperative to consider whether


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251


persistent residues remain potentially bioavailable and present a risk or not.

Figure 6. The comparison with the fate of p,p'-DDT between topsoil and deep soil. (Note: RDDE/DDD > 100, p,p'-DDD undetected; RDDE/DDD < 0.01, p,p'-DDE undetected) 252 253

ASSOCIATED CONTENT

254

Supporting Information

255

Additional material as described in the text: Detailed explanation for the starting year in kinetic

256

model, Rs,o,p’/p,p’ higher than 7.0 and the influence of long-range transport; Detailed information

257

about the concentrations of DDTs(p,p'-DDT, p,p'-DDE, p,p'-DDD, o,p'-DDT), isomeric and

258

metabolic ratios of DDT; The location of sampling points in topsoil in China; Detailed information

259

about the fraction of DDT from technical DDT.



260

AUTHOR INFORMATION

261

Corresponding Author

262

*Phone: (+86-10)84915216. E-mail: [email protected].

263

Notes

264

The authors declare no competing financial interest.

265 266

ACKNOWLEDGMENTS

267

This study was financially supported by the National Key Research and Development Program of

268

China (2017YFA0207002), the National Natural Science Foundation of China (NSFC, Project No.

269

41373130) and the National Key Research and Development Plan (2016YFD0800905). We thank

270

Huang Tao and Baolin Liu for providing their own detailed data of DDTs.

271 272

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Using Isomeric and Metabolic Ratios of DDT to Identify the Sources

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