*3.2. The Concentration of PCDD/Fs and DL-PCBs in Market Crabs*

The PCDD/F and DL-PCB contents in 37 crab samples from fresh markets in Shanghai were determined, and the specific results are shown in Table S8. The concentration range and average level of PCDD/Fs in these crab samples were 223.62–1934.88 and 24.27 ± 53.05 pg/g ww, respectively. The concentration range and mean values of the DL-PCBs were 228.85–2163.32 pg/g ww and 628.19 ± 375.48 pg/g ww, respectively. OCDD (375.48 pg/g ww) and 1,2,3,4,6,7,8-HpCDD (3.98 pg/g ww) were the most abun-

dant PCDD/Fs. For the DL-PCBs, 2,3 ,4,4 ,5-PeCB (321.68 pg/g ww) and 2,3,3 ,4,4 -PeCB (115.10 pg/g ww) exhibited the highest abundance (Figure 4).

**Figure 4.** Concentrations of PCDD/F (**A**) and DL-PCB (**B**) in Chinese mitten crabs from markets (*n* = 555). PCDD/F (**C**) and DL-PCB (**D**) ratios in Chinese mitten crabs from markets.

The PCDD/F and DL-PCB detection rates in the market crab samples were 100%, and in some samples, the TEQ ranges of the PCDD/Fs and DL-PCBs exceeded the relevant limits [27]. The total PCDD/F and DL-PCB TEQ levels in the market crab samples ranged from 0.33 to 10.80 pg TEQ/g, and the mean was 2.67 pg TEQ/g. The total PCDD/F TEQs ranged from 0.11 to 8.80 pg TEQ/g, and the mean was 1.56 pg TEQ/g. The total DL-PCB TEQs ranged from 0.23 to 2.35 pg TEQ/g, and the mean was 1.11 pg TEQ/g. TEQs of the total PCDD/F and DL-PCB contents in 8% of the samples were higher than the relevant limit (6.5 pg TEQ/g for the total PCDD/Fs and DL-PCBs) [27]. The TEQ of the PCDD/Fs in 14% of the samples was higher than the relevant limit (3.5 pg TEQ/g for PCDD/Fs) [27]. The TEQs of the total PCDD/Fs and DL-PCBs found in market crabs are shown in Table S6. TEFs were used to determine the TEQ distribution of individual congeners. 1,2,3,7,8-PeCDD; 2,3,4,7,8-PeCDF; and 2,3,7,8-TCDF contributed 22%, 19%, and 12%, respectively, and the TEQs of the remaining congeners accounted for less than 10%. Among the DL-PCBs, 3,3 ,4,4 ,5-PeCB (89%) are the main contributors to the TEQs of the DL-PCBs, followed by 3,3 ,4,4 ,5,5 -HxCB (8%) and 2,3 ,4,4 ,5-PeCB (0.8%). The toxicity equivalent factors of the 17 PCDD/Fs and 12 DL-PCBs were determined according to the WHO-TEF standards (Table S2) that were revised by WHO in 2005.

### *3.3. The Concentration of PCDD/Fs and DL-PCBs in Potential Sources*

To determine the major sources of PCDD/Fs and DL-PCBs in crabs, we further examined the concentrations of PCDD/Fs and DL-PCBs in each stage of crab farming. For specific results, see Table S5. The potential sources of dioxins consist of feed, including fishmeal, soybean meal, rapeseed meal, cottonseed meal, soybean phospholipid oil, dihydrogen calcium phosphate, and other chemicals to suit the crab's development requirements. In addition, other sources included water and plants from farmed crab ponds, plants on the shore of farmed crab ponds, and sediments at the bottom of farmed crab ponds.

Some substances in aquaculture water, such as impurities, food residues, and feces, eventually turn into sediments that may be more susceptible to contamination. The experimental results showed that the concentrations of PCDD/Fs and DL-PCBs in sediment were the highest on the farms, with concentrations ranging from 109.02 to 35,301.67 pg/g dw. The mean of the PCDD/F and DL-PCB concentrations in the sediment samples was 13,050.71 ± 14,855.84 pg/g dw. The concentration of the PCDD/Fs and DL-PCBs in aquaculture water ranged from 8.30 to 574.71 pg/L, with a mean concentration of 211.27 ± 217.07 pg/L. The concentrations of the PCDD/Fs and DL-PCBs in aquatic plants ranged from 3.28 to 419.12 pg/g ww, with a mean concentration of 139.83 ± 159.68 pg/g ww. The concentrations of the PCDD/Fs and DL-PCBs in shore plants ranged from 2.17 to 194.30 pg/g ww, with a mean concentration of 52.48 ± 76.80 pg/g ww. The concentration of the PCDD/Fs and DL-PCBs in feed ranged from 7.26 to 68.37 pg/g dw, with a mean concentration of 40.22 ± 23.19 pg/g dw.

The TEQs of the potential sources of PCDD/Fs and DL-PCBs are shown in Table S7. On the farms, sediments showed the highest TEQs, ranging from 0.48 to 31.51 pg TEQ/g dw. The total TEQs of the PCDD/Fs and DL-PCBs in aquaculture water, aquatic plants, and shore plants ranged from 0.23 to 1.06 pg TEQ/L, from 0.05 to 0.83 pg TEQ/g ww, and from 0.06 to 0.34 pg TEQ/g ww, respectively. The TEQs of the PCDD/Fs and DL-PCBs in feed ranged from 0.22 to 0.60 pg TEQ/g dw, in compliance with EU limit regulation EU NO 277/2012 (feed) (the maximum residue limit was 1.75 pg TEQ/g for PCDD/Fs and 5.5 pg TEQ/g for PCDD/Fs + DL-PCBs). The total TEQs of the PCDD/Fs and DL-PCBs from the three farms in Qingpu district (MH, RJ, and NXC) were greater than those found in the three farms in Chongming district (YF, HK, and ZH) (Figure 5).

**Figure 5.** The total PCDD/F and DL-PCB TEQs in the potential sources.

The congeners of PCDD/Fs and DL-PCBs in the potential sources and in the crabs were similarly distributed. OCDD and 1,2,3,4,6,7,8-HpCDD were dominant in aquaculture water, aquatic plants, and, especially, in sediments. 2,3 ,4,4 ,5-PeCB was abundant in sediment, aquaculture water, and feed. Different congeners contribute differently to the TEQ in different samples. For PCDD/Fs, 1,2,3,4,6,7,8-HpCDD had the highest TEQ in sediment, aquatic plants, and aquaculture water, with contribution rates of 39%, 32%, and 21%, respectively. In addition, in feed, 1,2,3,7,8-PeCDD contributed to 40%. For the DL-PCBs, 3,3 ,4,4 ,5-PeCB made the most dominant contributions in almost all of the samples, followed by 3,3 ,4,4 ,5,5 -HxCB, which is consistent with our conclusion that 3,3 ,4,4 ,5-PeCB (90%) and 3,3 ,4,4 ,5,5 -HxCB (9%) predominate in crab samples.
