In January
and May 2017, surface sediment samples were collected from top 5 cm of the
sediments (n = 157) according to a systematic-random sampling  design using a stainless steel Van Veen grab
sampler (Fig. 1). All samples were stored in clean aluminium foil and send to
the laboratory in a cool box, then stored at ?20 °C until further analysis. In the laboratory, samples were freeze-dried (72 h) sieved using 63 ?m mesh to eliminate of paticle size effect.
For analysis, 100 ?L of PAH surrogate internal standard mixture (200 ng g?1 of
each component, naphthalene-d8, anthracene-d10, perylened12 and chrysene-d12) was
added to about 5 g of freeze-dried sediment for quality
control of PAH analyses. Then extraction of PAHs was carried out with a Soxhlet for
10 h into 100 mL of dichloromethane. For deletion of sulfure, added active
copper to this dilution and store in refrigerator (4 c) for 24 h. Then the
extracts were reduced to approximately 4–5 ml by rotary evaporation, and then
transferred to the top of a 5%H2O deactivated silica gel column chromatography
(1 cm i.d. 9 cm) to eradicate the polar compounds. The PAH fraction was eluted
with 20 mL of dichloromethane/hexane (1:3, v/v). The eluent after solvent lessening
was further Fractionated using fully activated silica gel column chromatography
and eluted with 14 ml dichloromethane/hexane (1:3, v/v) to get the PAH fraction.
Alkanes were eluted with 4 ml of hexane former of PAHs for other research. Then
they were concentrated under a gentle stream of nitrogen blow down and
reconstituted in a volume of 100 ?L. P-terphenyl-d14 as PAH internal standard was
added immediately prior to being injected into the GC–MS (Harris et al., 2011;
Yunker and Macdonald, 2003). All of the reliable PAH standards were procured
from Sigma Aldrich. PAH analyses were  accomplished by
an Agilent Technologies 5975C quadrupole mass spectrometer coupled with an
Agilent 7890A gas chromatograph equipped with a fused silica capillary DB-5MS
column (30 m _ 0.25 mmi.d., 0.25 lmfilmthickness. Helium gas was routined as a
carrier. For PAH analysis, the oven temperature program was set up as follows:
initial temperature 70 °C for 2 min, heated to 150 °C at 30 °C/min and then to
310 °C at 4 °C/min, and held for 10 min resulting in a 60.3 min; and for
analysis of n-alkanes: it started at 70 °C, held for 2 min, increased at 30
°C/min to 150 °C, and finally, increased at 4 °C/ min to 290 °C, held for 10
min.