Status, sources, and human health risk assessment of DDT pesticide residues in river sediments in a highly developed agricultural region in the upper Yangtze River in China.

The concentrations of DDT and its metabolites in 19 sediment samples from a highly developed agricultural region in the upper reaches of the Yangtze River were measured. Non-carcinogenic hazard quotient for different age groups was evaluated using reference doses provided by the USEPA, and the excess lifetime cancer risk due to eating fish was assessed based on the local eating habits. The results showed that this region had a high level of residual DDT (12.84 ± 8.97 ng/g), which mainly came from the historically used technical DDT in agriculture. The non-carcinogenic risk was just acceptable in the region, but 11 of the 19 sites showed an unacceptable carcinogenic risk. Although DDT has been banned for decades, there were still notable health risks, especially for children. Special attention should be given to the potential health risks in historically developed agricultural regions.

Keywords: DDT; Human health risk; Sediment; Non-carcinogenic risk; Carcinogenic risk; Biota-sediment accumulation factor

Highlights • The concentration of DDT in Yangtze River sediments is relatively high. •DDT was derived from technical DDT historically used. •Only site 2 (Luzhou) has a new DDT input. •The non-carcinogenic risk was higher in children than in adults and young adults. •Eleven of the 19 sites presented an unacceptable risk of cancer by eating fish.

1,1,1-Trichloro-2,2 bis (p-chlorophenyl) ethane (DDT; including o,p′-DDT, p,p′-DDT, o,p′-DDD, p,p′-DDD, o,p′-DDE, and p,p′-DDE) is a man-made pesticide that does not occur naturally in the environment (Gwiazda et al. [21]). The production cost of DDT is very low, and the effect is long-lasting and efficient (Grung et al. [18]; Özcan [40]). Therefore, DDT is widely used in the control of malaria vectors and agricultural pests (WHO [59]). The successful promotion of DDT is attributed to its persistent nature in the environment, making it an attractive choice despite its harmful effects (Beard and Collaboration [5]). The half-life of DDT in surface sediments is about 14 years, while the half-life of DDT in deep sediments may exceed more than 30 years (Zhang et al. [66]). Therefore, although China banned the use of DDT after 1983 (Zhang et al. [66]) and signed the Stockholm Convention in 2001, DDT could still be detected in soil and sediment (Fiedler et al. [15]). After DDT was banned as a pesticide, dicofol, which contains large amounts of DDT impurities, was widely used as an alternative (Zhang et al. [67]). At the same time, the production and application of DDT continued for the control of mosquitoes and disease vectors (Tao et al. [52]). This also exacerbates the residue of DDT in water, sediments, and soils, and studies have shown that DDT is widely found in rivers, lakes, reservoirs, ponds, ocean bodies of water, and sediments (Tao et al. [53]; Zhou et al. [69]).

DDT has strong hydrophobicity and high affinity for organic carbon, which makes it deposit in riverbed sediments, then bioaccumulates, and enriches through the food chain (Pontolillo and Eganhouse [41]). Researches have shown that eating fish and other aquatic products containing DDT increases cancer and non-cancer risk (Klasing et al. [29]; Wilson et al. [60]). The hazards of DDT include neurotoxicity, endocrine disorders, slow development of the reproductive system, and other non-cancer diseases (Faroon and Harris, [14]) and cancers such as breast cancer (Falck Jr et al. [13]) and pancreatic cancer (Cocco et al. [9]).

In view of the high risk and high toxicity of DDT, the international academic community has paid enough attention to them and carried out a lot of research on them (Bao et al. [4]; He et al. [22]). United States Environmental Protection Agency (USEPA) has evaluated the recommended daily intake and carcinogenic potency of DDT, and established a health risk assessment model to calculate the cancer risk and non-cancer risk of human exposure to organic pollutants (USEPA [56]). Office of Environmental Health Hazard Assessment (OEHHA) proposed a calculation method of DDT intake through eating fish (OEHHA [37]), which was also widely used to calculate dietary intake of other organic matter (Babut et al. [3]). However, due to the extremely high uncertainty in calculating the concentration of DDT in fish through field fishing, and the need for a large amount of fishing and subsequent measurement and calculation, it is even more practicable to simulate bioaccumulation to make predictions (USEPA [55]). Biota-sediment accumulation factor (BSAF) proved to be an effective method for simulating organochlorine bioaccumulation in sediments, which can estimate the concentration of contaminating organic matter in fish (Sparling [48]).

The Cheng-Yu Economic Zone in southwest China has rainy and hot seasons, sufficient sunshine, suitable temperature, superior natural conditions, developed agriculture, and a permanent population of nearly 100 million. The cultivated land area of the Cheng-Yu Economic Zone is about 57,866.6 km2 now, and the area used to be much large before the urbanization transformation in recent decades. As a result, a large amount of pesticide was used in this region; the annual pesticide application rates in Sichuan and Chongqing reached 1.24 and 2.47 kg/ha, respectively (Zhang et al. [65]).

There are already a number of reports on the historical residue and risk assessment of DDT in the Yangtze River Delta (Hu et al. [26], [27]), but there is little information on the environmental burden and human health risks of DDT in the Cheng-Yu Economic Zone in the upper Yangtze River (Li et al. [34]). This study measured the concentrations of DDT and its metabolites in sediments along the Yangtze River and its tributaries in the Cheng-Yu Economic Zone, and evaluated and elaborated on the sources, recent investments, and carcinogenic and non-carcinogenic risks of DDT sediments.

Grab samplers were used to collect river sediment samples from 19 sites in the Cheng-Yu Economic Zone (see Fig. 1 for the location of the sampling points). Of the 19 sampling sites, 1, 2, 4, 5, 8, 9, 10, 12, 14, 17, and 19 are in Sichuan Province, and the other sampling sites are in Chongqing. The collected samples were placed in a sealed box, transported to the laboratory in an ice pack, stored at −20 °C, and pretreated within 7 days.

Graph: Fig. 1Map of study area and excess lifetime cancer risk values of each sampling site

After freeze-drying, the sediment samples were ground through a 178-μm sieve and stored at 4 °C in zip lock bags. Approximately 20 g of each sample was weighed and mixed with surrogate standards of a mixture of 20 ng of 2,4,5,6-tetrachloro-m-xylene (TCmX) and decachlorobiphenyl (PCB209) before being extracted using a Soxhlet apparatus with 200 mL of n-hexane and acetone mixture (volume ratio 1:1) for 48 h. The resulting extract was concentrated to 5~10 mL and replaced with n-hexane. Separation and purification were performed using an alumina silica composite chromatography column (12 cm × 6 cm). The concentrate was passed through a glass chromatography column and washed with a mixture of 10 mL of n-hexane and dichloromethane (volume ratio of 1:1) to extract the organochlorine pesticide components. The eluent solution was then concentrated to 0.5 mL under nitrogen and transferred to a 2-mL vial. And then, 20 ng of pentachloronitrobenzene (PCNB) was added as an internal standard.

In this study, the detection instrument was an Agilent HP6890 gas chromatograph with a 63Ni electron capture detector (GC-ECD). The chromatographic column was HP-5MS, 30 m × 0.25 mm × 0.25-μm capillary column, and high-purity helium was used as the carrier gas. The injection port temperature was 315 °C, and the column heating program was initial temperature 60 °C standing for 2 min, 6 °C·min−1 heated to 200 °C, 1 °C·min−1 heated to 210 °C, and 10 °C·min−1 heated to 290 °C. The split-free injection was performed for 10 min, the column flow rate was 1.0 mL·min−1, and the injection volume was 1.0 μL. DDT standard sample US-1128 was purchased from Accustandard Company in the USA, containing 6 target compounds p,p′-DDE, o,p′-DDE, p,p′-DDD, o,p′-DDD, p,p′-DDT, and o,p′-DDT.

Surrogates were added to each of the 19 samples to monitor program performance and matrix effects. Spiked samples and solvent blanks were analyzed to check 19 samples for interference and cross-contamination. No peaks of DDTs were detected in the solvent blanks and no significant peaks were identified in the procedural blank samples. The recoveries of the surrogates for DDTs (TCmX and PCB209) were 76~106%; concentrations of the samples were corrected based on those results. Method detection limits (MDLs) were defined as the average of all blanks plus three times of the standard deviation, estimated from signal-to-noise (S/N) of DDTs > 3; the MDLs of DDTs in the sediments ranged from 0.01 to 0.20 ng/g dry weight (Qian et al. [42]).

According to the 2020 China Fishery Statistical Yearbook, fish consumption in Sichuan Province and Chongqing City accounts for about 96.2% of all aquatic product consumption. Therefore, eating fish is the main way for humans to ingest DDT from sediments. The per capita daily fish intake in Sichuan Province and Chongqing Municipality was 15 g/day and 67 g/day, respectively (Wang et al. [58]). The fish intake of children aged 1–2 and adolescents aged 2–16 is about 26.3% and 54% of adults, respectively (Guo et al. [20]). Carp, crucian carp, grass carp, bighead carp, mullet, yellow catfish, loach, and eel are among the most frequently consumed fish species in China. These eight species were selected for estimating the human dose of DDT ingested from fish, based on consumption ratio data from the "2020 China Fishery Statistical Yearbook" (Xiujuan Yu [62]).

BSAF was the ratio of the concentration of chemical substances in the organism to the concentration of chemical substances in the sediment (Burkhard et al. [7]). The value of BSAF was calculated by Eq. (1) (Ankley et al. [2]).

1 $$BSAF=\frac{\left(C_{\mathrm{f}},/,F_{\mathrm{lipid}}\right)}{\left(C_{\mathrm{sed}},/,F_{\mathrm{toc}}\right)}$$

Graph

where Cf is the DDT concentration in the fish (mg/kg of wet wt). Flipid is the lipid fraction of the fish (kg lipid/kg wet wt). Csed is the chemical concentration in surficial sediment (mg/kg dry wt). Ftoc is the fraction of total organic carbon (TOC) in the Yangtze River sediment (kg organic carbon/kg dry wt).

BSAF could be used to assess the relationships between organochlorine pesticide residues in sediments and pesticide concentrations in fish, and to provide fish consumption recommendations (Sparling, [48]). By calculating the value of BSAF, the bioavailability of organic compounds in sediments could be estimated effectively and reliably (Coffin et al. [10]). In this study, the DDT concentrations of 8 species of fish in 19 sampling sites were estimated by the calculation formula of BSAF, as shown in Table 1.

Table 1 Flipid, Ftoc, BSAF, and DDT concentrations in fish (Cfish) at each sampling point of 8 species of fish

There were three routes of human exposure to DDT, namely, maternal transfer during pregnancy, breast milk intake 1 year after birth, and dietary intake after 2 years of age, respectively.

• Maternal transfer and exposure 2 years and older during pregnancy

DDT ingested by eating fish in humans over 2 years of age was calculated using Eq. (2) (OEHHA [37]).

2 $${\mathit{ADD}}_{\mathrm{f}\mathrm{ish}}\left(\mathrm{mg},/,\mathrm{kg},-,\mathrm{day}\right)=\frac{C_{\mathrm{f}}\times I_{\mathrm{f}\mathrm{ish}}\times GI\times L\times EF\times ED}{BW\times AT\times {10}^6}$$

Graph

where ADDfish is the dose of contaminant via ingestion of fish (mg/kg-day). Cf is the concentration of DDT in fish (μg/kg). GI is the gastrointestinal absorption fraction, unitless (default = 1). L is a site-specific factor assessing how much fish for human consumption came from the affected water body; usually L is set to 1. EF is the exposure frequency (365 days/year). AT is the averaging time (life time in years × 365 days/year); 10−6 is the conversion factor (μg/mg) (kg/g) (Hickox and Denton, [23]). ED is the exposure duration (adult: 54 years, adolescents: 14 years, children: 1 year) (Coffin et al. [10]). Ifish is the fish intake rate (g/day). BW is the body weight (kg). The DDT dose for maternal transfer during pregnancy is roughly the same as the maternal DDT intakes (Hickox and Denton, [23]). The parameters and references required in this study are shown in Table 2. Each sample point used the parameters for its province.

Table 2 Parameters and references required for this study

• Exposure during 1 year of age

Infants mainly feed on breast milk during their first year of life, so DDT intake was different from 2-year-olds and above. The concentration of DDT in human breast milk was the same as that in human body fat (Cao et al. [8]). Therefore, the DDT infant daily intake for breastfed infants aged 1–2 years was calculated by Eq. (3) (Coffin et al. [10]).

3 $$IDI\left(\mathrm{mg},/,\mathrm{kg},-,\mathrm{day},-,\mathrm{BW}\right)=\left(0.7,ln,\left({\mathit{ADD}}_{\mathrm{fish}},\times ,B,W\right),+,3\right)\times BMI\times PMF$$

Graph

where ADDfish is the average daily dose of DDT per kilogram of maternal body weight mg/kg-day-BW (calculated above). BW is the average weight of the province to which the survey area belongs. PMF is the percent milk fat in breast milk. BMI is an infant's daily breast milk intake; the average BMI for an infant is 0.101 kg-milk/kg BW/day (Hickox and Denton [23]). PMI is the percentage of fat in breast milk; the PMI is about 4% (Mariën and Laflamme [35]).

The non-carcinogenic risk hazard quotient (HQ) of DDT to humans was assessed by Eq. (4) (USEPA [56]), where the reference daily intake dose was 5 × 10−4 mg/kg-day-BW (USEPA, [56]). When the value of HQ was less than 1, we can assume that there was no non-cancer risk of DDT ingested by eating fish.

4 $$HQ=\frac{AD{D}_{\mathrm{fish}}}{Referencedose}$$

Graph

The carcinogenic risk of DDT was different for different life stages, so the carcinogenic risk was relatively difficult to assess (OEHHA, [38]). To address this, Office of Environmental Health Hazard Assessment (OEHHA) recommended adding a weighting factor to the cancer risk assessment for each age group to describe the effect of DDT on exposure at various stages of life, termed the Age Sensitivity Factor (ASF) (OEHHA, [38]). ASF values are 10 from late pregnancy to under 2 years of age, 3 for adolescents (2–16 years), and 1 for adults. The excess lifetime cancer risk (ELCR) is then calculated by Eq. (5) (Coffin et al. [10]).

5 $$ELCR=\frac{\begin{array}{c}({\mathit{ADD}}_{\mathrm{fish}\left(17,-,70\right)}\times ASF(10)\times 0.3\mathrm{year}\mathrm{s}+IDI\times ASF(10)\times 1\mathrm{year}+{\mathit{ADD}}_{\mathrm{fish}(2)}\times ASF(10)\times 1\mathrm{year}\\ +{\mathit{ADD}}_{\mathrm{fish}\left(3,-,16\right)}\times ASF(3)\times 14\mathrm{years}+{\mathit{ADD}}_{\mathrm{fish}\left(17,-,70\right)}\times ASF(1)\times 54\mathrm{year}\mathrm{s})\times CSF\end{array}}{70\mathrm{year}\mathrm{s}}$$

Graph

where CSF is the cancer slope factor for DDT that is 0.34 (mg/kg-day-BW)−1 (USEPA [56]).

The concentrations of DDT and its metabolites in 19 sites are shown in Fig. 2. The results showed that the average composition of DDT and its metabolites in sediment samples were p,p′-DDE (29.94%) > p,p′-DDT (28.4%) > p,p′-DDD (20.23%) > o,p′-DDE (8.83%) > p,p′-DDT (6.66%) > o,p′-DDT (5.94%). The concentrations of the p,p′ isomers were remarkably higher than that of the o,p′ isomers. The concentrations of DDTs in 19 regions were all lower than the risk screening value of 0.1 ng/g in GB15618-2018, issued by the Ministry of Ecology and Environment of the People's Republic of China. The concentrations of DDT in site 6 and site 15 were much higher than those in the other 17 regions, while the concentrations of DDT in site 9 and site 11 were lower. This may be because the agriculture in site 6 and site 15 was highly developed, and the large agricultural land area led to the application of a massive amount of organochlorine pesticides, which increased the residual amount of DDT.

Graph: Fig. 2Concentrations of DDT and its metabolites and values of DDE + DDD/DDTs and o,p′-DDT/p,p′-DDT at sampling sites in Cheng-Yu Economic Zone

We selected 17 rivers and reservoirs (including 9 domestic locations and 8 foreign locations) and compared the concentrations of six DDTs and their metabolites (Table 3). The concentration of DDT in sediments of the Cheng-Yu region was higher than that in the vast majority of the rivers, except for Yalta town in Ukraine. Especially, DDE was at a notably higher level than other aquatic environments. It suggested that a probable heavy use of organochlorine pesticides in the study region in the last decades has led to severe residue of DDT, which likely result in potential ecological and human health risks.

Table 3 Comparison of DDT concentrations in sediments in the Cheng-Yu Economic Zone with other sites

ND: not detected or measured, /: information not calculated in respective publication or not applicable

The detection of high levels of DDT in sediments may be attributed to historical applications of technical DDT or current use of dicofol (Qiu and Zhu, [44]; Qiu et al. [45]). Technical DDT typically consists of p,p′-DDT (77.1%), o,p′-DDT (14.9%), p,p′-DDD (0.3%), o,p′-DDD (0.1%), p,p′-DDE (4.0%), o,p′-DDE (0.1%), and unidentified compounds (3.5%) (WHO, [59]). During the synthesis of dicofol from technical DDT, the conversion ratio of o,p′-DDT to o,p′-dicofol is much lower than that of p,p′-DDT to p,p′-dicofol, resulting in a higher o,p′-DDT/p,p′-DDT ratio in dicofol (approximately 7) (Qiu et al. [45]). Meanwhile, the o,p′-DDT/p,p′-DDT ratio in technical DDT is typically around 0.2–0.3 (Venkatesan et al. [57]). Therefore, the o,p′-DDT/p,p′-DDT ratio is often used to determine the origin of the DDT pollution at the sampling point, whether it is from dicofol or technical DDT. As shown in Fig. 2, the o,p′-DDT/p,p′-DDT ratios in the Cheng-Yu Economic Zone ranged from 0.1 to 0.4, indicating that the DDT residues in this area mainly originated from technical DDT. This finding differs from the results of the Tianjin and Yibin studies presented in Table 4, suggesting that the causes of DDT residues vary regionally, and thus, pollution prevention and control measures should be tailored to local characteristics.

Table 4 Cheng-Yu Economic Zone (DDE + DDD)/DDTs and o,p-DDT/p,p′-DDT compared to other regions

The values in () represent averages

In aquatic environments, DDT will decompose into DDD and DDE over time in the absence of new DDT inputs, causing the ratio of DDD + DDE/DDTs to gradually increase (Doong et al. [11]). Therefore, the DDD + DDE/DDT ratio can be used to determine whether there has been recent DDT input at the sampling point (Lee et al. [32]). A ratio greater than 0.5 indicates that the local DDT has undergone long-term biotransformation, while a lower ratio suggests recent DDT input (Hitch and Day [24]). As shown in Fig. 2, the ratios of DDE + DDD/DDTs in the Cheng-Yu Economic Zone were all greater than 0.5, except for site 2, suggesting strict enforcement of the DDT ban in this region. However, site 2 may have other activities besides agriculture that contribute to DDT contamination in the river. These findings are similar to those obtained in other regions (Table 4), where few areas still have DDT inputs, while most regions have not received any DDT inputs recently.

When the daily intake of DDT exceeds the reference dose, it can cause chronic toxicity and adversely affect human systems such as the reproductive system (USEPA [56]). In this study, we calculated the daily dose of DDT ingested by people in Cheng-Yu Economic Zone by eating fish, and compared it with the reference dose to obtain HQ. As shown in Fig. 3, The HQ values of all 19 sampling sites were found to be less than 1, indicating that non-carcinogenic risks were acceptable in the region. The HQ values of sites 6 Lize and 15 Fuling were significantly higher than other sites, which was due to the higher concentration of DDT residues in the local river sediments. As can be seen, the risk value was slightly higher in children than in adults and young adults due to the lower weight of children and higher daily intake of DDT per kilogram of body weight. Among the eight types of fish in this study, crucian carp and yellow catfish have relatively high contributions to DDT intake. Yellow catfish is one of the fish species that were found to have a higher concentration of DDT residue due to its higher BSAF value. And crucian carp is due to local taste preferences that people eat more crucian fish than other fish. Moreover, the HQ values of the Cheng-Yu Economic Zone are found to be significantly higher than those of other regions, as shown in Table 5. Therefore, although there is no non-carcinogenic risk caused by DDT in the Cheng-Yu area, it is necessary to pay attention to the situation of DDT in the river sediments of the Lize and Fuling areas. Furthermore, it is crucial to monitor the amount of DDT ingested by children in the Cheng-Yu Economic Zone through fish and the effects of crucian carp and yellow catfish on human intake of DDT.

Graph: Fig. 3 HQ values for adults, adolescents, children (a) and the contribution of 8 species of fish to intake (b) in the Cheng-Yu Economic Zone

Table 5 Comparison of carcinogenic and non-carcinogenic risks in the Cheng-Yu Economic Zone with other regions

Animal studies had shown that DDT, DDE, and DDD were all human carcinogens; excessive exposure to DDT increases the risk of cancers such as pancreatic cancer (USEPA [56]). ELCR evaluated the level of exposure to a certain compound that can cause cancer in a person's lifetime. When the ELCR was less than 1 × 10−6, the cancer risk was negligible, and when the ELCR was between 1 × 10−4 and 1 × 10−6, the cancer risk was acceptable, and when the ELCR was greater than 1 × 10−4, the impact on human cancer was great and the cancer risk is unacceptable (Boström et al. [6]).

The ELCR values of the 19 sampling sites in this study were represented by the size of the circle in Fig. 1. A yellow circle indicated that the risk is acceptable, and red indicates that the risk exceeds an acceptable value. The ELCR values indicate that the cancer risks at sites 4, 5, 8, 9, 10, 12, 14, and 17 were acceptable, but at other sites, the risks exceeded the acceptable range. The ELCR values at the other sites were 1 to 2 orders of magnitude higher than those in Table 5, which was obviously unacceptable. This may be due to the combination of regional fish consumption habits in the Cheng-Yu region and a historical legacy of high DDT usage. In this case, we believe that long-term monitoring of DDT in river sediments in areas with acceptable risk should be carried out to prevent increased risk due to new sources of DDT. For areas with unacceptable risks, we believe that the concentration of DDT in common fish should be further tested, DDT in rivers and sediments should be degraded, and some fish with excessive DDT content should be restricted from fishing and eating, and the use of DDT pesticides and dicofol should be strictly limited.

Since determining DDT concentrations in fish by wild fishing requires a large number of samples and was highly variable and contingent (Coffin et al. [10]), this study uses BSAF that has been shown to reliably estimate organic matter accumulation in fish and further estimate the health risks of DDT (Sparling [48]). The estimation of the average daily fish consumption for the 19 sampling sites was based on the provincial fish consumption data, which may not reflect the actual consumption at each sampling point, introducing potential uncertainties in the assessment of the health risks associated with DDT exposure.

The results of this study show that the concentration of DDTs in the sediments of the upper reaches of the Yangtze River in the Cheng-Yu Economic Zone was at a relatively high level, among which the content of DDE is the highest. Compositional analysis of DDT showed that DDT in the study area derived from technical DDT historically used in agriculture, and except for the recent DDT investment in site 2, there were no new sources in other regions. According to the daily reference dose of DDT of USEPA, there was no significant non-carcinogenic risk in this region through dietary fish, slightly higher than other regions but at a relatively safe level, and it was also concluded that the risk value was slightly higher in children than in adults and adolescents. However, during the excess lifetime cancer risk evaluation, it was found that 11 out of 19 sites had carcinogenic risks exceeding acceptable levels through eating fish, which should be paid enough attention, and the necessity for continuous monitoring and research on DDT was also emphasized.

Yutong Zhu: data analysis, software, writing — original draft, writing — review and editing. Yongzhen Chai: writing — review and editing. Fei Guo: supervision, writing — review and editing, funding acquisition, project administration. Chengbin Xu: manuscript writing instructions.

This work was supported by Innovation Community Project of Chinese Research Academy of Environmental Sciences (grant number 2021-JY-02).

The datasets used or analyzed during the current study are available from the corresponding author on reasonable request.

Neither the full text nor part of the paper was submitted or published elsewhere. Papers will not be submitted elsewhere until the editorial procedures of the journal are completed.

Human subjects are not covered herein.

The author agrees to publication in the Environmental Science and Pollution Research and also to publication of the article in English by Springer in Springer's corresponding English-language journal, and agreed to publish their data in journal articles.

The authors declare no competing interests.

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