3.1. Grain size distribution
Six types of sediment were distributed in the Mong Cai coastal area: coarse sand, medium sand, fine sand, very fine sand, very coarse silt, and coarse silt. On the surface, fine sand predominated, followed by very fine sand and then coarse sand and medium sand (Fig. 2a). C1 was dominated by very fine sand (Fig. 2c), while very coarse silt and coarse silt were abundant in C2 (Fig. 2e).
Surface sediments comprised four types: coarse sand, medium sand, fine sand, and very fine sand. Coarse sand was found distributed in two samples from Ha Coi Bay, while gravel, sand, silt, and clay fractions were 17.20–20.48%, 73.13–79.52%, 0.0–9.07%, and 0.0–0.56%, respectively (Fig. 2a, 2b). Medium sand was found distributed in the Ka Long mouth, Mui Ngoc, and Ha Coi Bay; the gravel fraction ranged from 0 to 4.4%, the sand fraction ranged from 88.2 to100.0%, the silt fraction ranged from 0.0 to 9.9%, and the clay fraction ranged from 0.0 to 0.6% (Fig. 2a, 2b). Fine sand was common in the study area, gravel fraction ranged from 0 to 3.6%, the sand ranged 80.8 to 100.0%, the silt ranged from 0. 0 to 15.7%, and clay fraction ranged from 0.0 to 1.4% (Fig. 2a, 2b). Very fine sand was distributed to the front of the Tra Co peninsula, where the gravel fraction ranged from 0.0 to 0.55%, sand fraction ranged from 63.0 to 100%, the silt fraction ranged from 0 to 34.6%, and the clay fraction ranged 0 to 1.7% (Fig. 2a, 2b).
C1 contained fine sand, very fine sand, and coarse silt, of which very fine sand predominated (Fig. 2c, 2d). Fine sand was distributed at 62–70 cm; the sand fraction ranged from 69.8 to 79.8%, the silt fraction ranged from 19.0 to 27.5%, and the clay fraction ranged from 1.2 to 1.8.% (Fig. 2c, 2d). Very fine sand was distributed at 0–40 cm and from 70–80 cm; the sand fraction ranged from 49.0 to 87.6%, the silt fraction ranged from 11.4 to 48.7%, and the clay fraction ranged from 0. 6 to 2.4% (Fig. 2c, 2d). Very coarse silt was distributed at 40–60cm; the sand fraction ranged from 25.80 to 68.74%, the silt fraction ranged from 27.69 to 69.67%, and the clay fraction ranged from 2.81 to 5.50% (Fig. 2c, 2d).
Four types of sediment were distributed in C2: fine sand, very fine sand, coarse silt, and very coarse silt. Coarse silt and very coarse silt were abundant (Fig. 2e, 2f). Fine sand was only distributed at 8–10 cm; the sand fraction was 63.7%, the silt fraction was 34.0%, and the clay fraction was 1.6% (Fig. 2e, 2f). Very fine sand was distributed at 4–6 cm and 29–32 cm; the sand fraction ranged from 39.4 to 47.0%, the silt fraction ranged from 49.9 to 55.7%, the clay fraction ranged from 2.7 to 3.0% (Fig. 2e, 2f). Very coarse silt was distributed at 0–12cm, 25cm, and 40–52cm; the sand fraction ranged from 26.76 to 42.01%, the silt fraction ranged from 50.06 to 69.84%, and the clay fraction ranged from 2.06 to 13.41% (Fig. 2e, 2f). Coarse silt was distributed at 12–35cm; the sand fraction ranged from 9.45 to 20.20%; the silt fraction ranged from 67.45 to 81.55%, and the clay fraction ranged from 3.95 to 8.23% (Fig. 2e, 2f).
3.3. Heavy metals in sediments
In surface sediments, heavy metals were lower than the ISQG levels (Fig. 6, Table 2). Heavy metals—Fe, Cu, Pb, Zn, Cr, Ni, Mn, Li, and V—were distributed in higher concentrations in the Ka Long estuary, followed by the front of the Tra Co peninsula, with the lowest concentration in Ha Coi Bay. Fe, Cu, Pb, Zn, Cr, Ni, Mn, Li, V, Mo, and Co contents were within the following ranges: 0.34–1.26%, 2.11–8.21mg/kg, 4.48–16.02mg/kg, 12.81–30.06mg/kg, 3.63–12.18mg/kg, 1.76–6.09mg/kg, 34.67–213.76mg/kg, 5.02–16.34mg/kg, 5.16–17.57mg/kg, 0.02–0.24mg/kg, and 1.38–3.74mg/kg, respectively. Cd content ranged from 0.01 to 0.12mg/kg, with higher concentrations in Ha Coi Bay and lower concentrations to the front of the Tra Co peninsula and in the Ka Long estuary. As was uniformly distributed in the Ka Long estuary, Ha Coi Bay and to the front of the Tra Co peninsula, ranging from 2.54 to 8.90 mg/kg.
C1 was divided into three periods: prior to 1961, 1961–2008, and 2008–2019 (Fig. 7a). Heavy metal concentrations were highest before 1961 (36–80cm). Concentrations between 1961 and 2008 (15–36cm) were lower but showed a light increase over time. The 2008–2019 (0–15cm) heavy metals decreased over time. The ranges were as follows: Cu 6.96–23.24 mg/kg; Pb 11.01–28.51 mg/kg; Zn 25.75–67.48 mg/kg; Cd 0.01–0.22 mg/kg; As 5.09–21.82 mg/kg; Cr 9.77–37.40 mg/kg; Co 2.76–8.86 mg/kg; Ni 5.19–15.91 mg/kg; Fe 0.92–6.44%; Mn 71.62–202.36 mg/kg; Mo 0.12–0.79mg/kg; Li 12.30–41.62mg/kg; and V 12.97–59.34mg/kg (Table 2).
C2 encompassed three periods: prior to 1944, 1944–1996, and 1996–2019 (Fig. 7b). Before 1944, the heavy metal concentrations increased over time. Between 1944 and 1996, the concentrations decreased. In 1996–2019, heavy metals increased over time. The ranges were as follows: Cu 12.26–27.87mg/kg; Pb 19.76–38.01 mg/kg; Zn 39.10–68.87 mg/kg; Cd 0.01–0.18 mg/kg; As 12.47–29.99 mg/kg; Cr 17.71–44.19 mg/kg; Co 5.18–13.35 mg/kg; Ni 7.80–21.03 mg/kg, Fe 2.14–8.44%, Mn 175.79–426.83 mg/kg, Mo 0.47–2.52 mg/kg, Li 18.03–71.45 mg/kg, V 20.22–55.12 mg/kg
Heavy metal concentrations in the surface sediment were lower than the ISQGs. In C1, the concentration of As was higher than the ISQG (7.2 mg/kg) (Fig. 7a); Cu was higher than ISQGs in some layers at 36–80 cm (18.7 mg/kg). In C2, Cu, Pb and As were higher than ISQGs in some levels before 1944; during 1944–1996, only As was higher than the ISQG level. Meanwhile. only As and Cu in the top layer exceeded ISQG levels during 1996–2019. Neither the surface nor core sediments contained heavy metals that exceeded the PEL level (Table 2).
Table 2. Grain sizes, stable isotopes, and heavy metals on the Mong Cai coast
(% - gravel, sand, silt, clay, TOC, Fe; ‰ – ẟ15N, ẟ13C; mg/kg – Cu, Pb, Zn, Cd, As, Cr, Co, Ni, Mn, Mo, Li, V)
Position
|
Level
|
Gravel
|
Sand
|
Silt
|
Clay
|
TOC
|
ẟ13C*
|
ẟ15N*
|
Cu
|
Pb
|
Zn
|
Cd
|
As
|
Cr
|
Co
|
Ni
|
Fe
|
Mn
|
Mo
|
Li
|
V
|
Surface
(n=20)
|
Min.
|
0.00
|
63.0
|
0.00
|
0.00
|
0.12
|
-27.43
|
3.12
|
2.11
|
4.48
|
12.81
|
0.01
|
2.54
|
3.63
|
1.38
|
1.76
|
0.34
|
34.67
|
0.02
|
5.02
|
5.16
|
Max.
|
20.48
|
100.00
|
34.62
|
1.67
|
1.21
|
-17.02
|
6.05
|
8.21
|
16.02
|
30.06
|
0.12
|
8.90
|
12.18
|
3.74
|
6.09
|
1.26
|
213.76
|
0.24
|
16.34
|
17.57
|
Aver.
|
2.6
|
90.2
|
6.6
|
0.5
|
0.40
|
-24.13
|
4.96
|
4.61
|
7.83
|
18.82
|
0.04
|
4.19
|
7.49
|
2.62
|
3.33
|
0.78
|
121.00
|
0.06
|
9.13
|
8.69
|
SD
|
5.7
|
10.9
|
9.2
|
0.6
|
0.25
|
4.77
|
1.33
|
1.85
|
2.95
|
5.56
|
0.03
|
1.35
|
2.30
|
0.79
|
1.18
|
0.28
|
56.30
|
0.05
|
3.14
|
3.16
|
C1
(n=28)
|
Min.
|
-
|
25.80
|
11.40
|
0.62
|
0.09
|
-
|
-
|
6.96
|
11.01
|
25.75
|
0.01
|
5.09
|
9.77
|
2.76
|
5.19
|
0.92
|
71.62
|
0.12
|
12.30
|
12,97
|
Max.
|
-
|
87.58
|
69.67
|
5.50
|
0.71
|
-
|
-
|
23.24
|
28.51
|
67.48
|
0.22
|
21.82
|
37.40
|
8.86
|
15.91
|
6.45
|
202.36
|
0.79
|
41.62
|
59,34
|
Aver.
|
-
|
60.79
|
36.25
|
2.23
|
0.31
|
-
|
-
|
14.00
|
17.40
|
41.35
|
0.09
|
12.49
|
20.60
|
5.33
|
9.22
|
2.54
|
118.52
|
0.34
|
23.15
|
30,73
|
SD
|
-
|
16.27
|
15.29
|
1.16
|
0.18
|
-
|
-
|
4.40
|
4.94
|
12.18
|
0.05
|
4.43
|
7.04
|
1.68
|
2.95
|
1.56
|
38.65
|
0.14
|
6.85
|
11,43
|
C2
(n=21)
|
Min.
|
-
|
9.5
|
34.0
|
1.6
|
0.63
|
-
|
-
|
12.26
|
19.76
|
39.10
|
0.01
|
12.47
|
17.71
|
5.18
|
7.80
|
2.14
|
175.79
|
0.47
|
18.03
|
20,22
|
Max.
|
-
|
63.7
|
81.5
|
13.4
|
2.86
|
-
|
-
|
27.87
|
38.01
|
68.87
|
0.18
|
29.99
|
44.19
|
13.35
|
21.03
|
8.44
|
426.83
|
2.52
|
71.45
|
55,12
|
Aver.
|
-
|
28.9
|
64.1
|
5.4
|
1.81
|
-
|
-
|
17.84
|
27.92
|
56.42
|
0.09
|
20.10
|
34.29
|
10.50
|
16.18
|
5.37
|
289.00
|
1.33
|
47.84
|
43,22
|
SD
|
-
|
13.2
|
12.1
|
2.9
|
0.76
|
-
|
-
|
3.57
|
5.18
|
9.27
|
0.05
|
4.27
|
6.79
|
2.16
|
3.46
|
1.88
|
54.75
|
0.60
|
13.03
|
9,41
|
ISQG
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
30.2
|
18.7
|
124
|
0.7
|
7.2
|
52.3
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
PEL
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
112
|
108
|
271
|
4.2
|
41.6
|
160
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
Earth crust (Rudnick et al 2014)
|
-
|
-
|
-
|
-
|
-
|
-
|
-
|
17.0
|
28.0
|
67.0
|
0.09
|
4.8
|
92.0
|
17.3
|
47
|
3.92
|
774.6
|
1.1
|
24
|
97
|
N = number samples; ẟ
13C* and ẟ
15N* with n = 4; Min. – minimum; Max. – maximum; Aver. – Average; SD – standard deviation
3.4. Correlation between sediment parameters
The surface sediment parameters indicated a positive correlation between heavy metals and silt and clay fractions and a negative correlation between heavy metals and gravel and sand fractions (Fig. 8a).
A strong positive correlation (R 0.75–0.95) was observed between silt and clay, Cu, Pb, Zn, Cr, Ni, Fe, Li, and V; between clay and Cu, Zn, Cr, Ni, Fe, Li, and V; between Cu and Zn, Ni, and Li; between Pb and Cr, Ni, Li, and V; between Zn and Cr, Ni, Fe, and Mn; between Cr and Co, Ni, Fe, Li, and V; between Co and Fe; between Ni and Fe, Li, and V; and between Fe and Li. A moderate correlation (R 0.5–0.75) was observed between silt and Co and Mo; between clay and Pb and Co; between Cu and Pb, Cr, Fe, and V; between Pb and Zn, As, Fe, and TOC; between Zn and Co, Li, V, TOC; between As and Ni, V; between Cr and TOC; between Co and Ni, Li, and TOC; between Ni and TOC; between Fe and Mn, V, and TOC; between Mo and V; between Li and TOC; and between V and TOC. A weak correlation (R 0.25–0.5) was observed between silt and As and Mn; between clay and As, Mn, Mo, and TOC; between Cu and Cd, Co, Mn, Mo, and TOC; between Pb and Co, Mo, and TOC; between Zn and Mn and TOC; between Cd and Cr, Ni, V, and TOC; between As and Cr, Co, Ni, Fe, Mo, and Li; between Cr and Ni, Mn, and Mo; between Co and V; between Mn and Li; between Mo and Mn and Li; and between V and TOC.
A strong negative correlation (R − 0.75 to − 0.95) was observed between sand and silt, Mo, Li, and V; a moderate negative correlation (R − 0.5 to − 0.75) was observed between gravel and Mn; between sand and silt, Cu, Pb, Zn, Cr, Ni, Fe, and TOC; and a weak negative correlation (R − 0.25 to − 0.5) was observed between gravel and sand, Cu, Zn, and Co and between sand and Cd, As, and Co.
In C1, the sediment parameters showed positive and negative correlations: a positive correlation was observed between silt and clay and heavy metals while a negative correlation was observed between sand and heavy metals (Fig. 8b). TOC showed no significant relationship to heavy metals, sand, silt or clay.
Strong positive correlations (R 0.75–0.95) were observed between silt and Zn, As, Cr, Co, Ni, Fe, Li, and V; between clay and Cu, Pb, Zn, As, Cr, Co, Ni, Fe, Mn, Mo, Li, and V; between Cu and Pb, Zn, As, Cr, Co, Ni, Fe, Mn, Mo, Li, and V; between Pb and Zn, As, Cr, Co, Ni, Fe, Mn, Mo, Li, and V; between Zn and As, Cr, Co, Ni, Fe, Mn, Mo, Li, and V; between As and Cr, Co, Ni, Fe, Mn, Mo, Li, and V; between Cr and Co, Ni, Fe, Mn, Mo, Li, and V; between Co and Ni, Fe, Mn, Mo, Li, and V; between Ni and Fe, Mn, Mo, Li, and V; and between Fe and Mn, Mo, Li, and V; between Mn and Mo, Li, and V; between Mo and Li and V; and between Li and V. A moderate correlation (R 0.5–0.75) was observed between silt and clay, Cu, Pb, Mn, and Mo; between Zn and Cd; and between Cd and Mn, Co, Ni, and Fe (Fig. 8b). A weak correlation (R 0.25–0.5) was observed between Cd and silt, clay, Cu, Pb, As, Cr, Mo, Li, and V.
A strong negative correlation (R − 0.75 to − 0.95) was observed between sand and Cu, Pb, Zn, As, Cr, Co, Ni, Fe, Mn, Li, and V; a moderate negative correlation (R − 0.5 to − 0.75) was observed between sand and clay and Mo; and a weak negative correlation (R − 0.25 to − 0.5) was observed between sand and Cd (Fig. 8b).
In C2, a negative correlation was observed between sand and heavy metals while positive correlations were among the heavy metals and between heavy metals and silt and clay.
A strong positive correlation (R 0.75–0.95) was observed between Pb and Zn, Cr, Co, Ni, Li, and V; between Zn and Cr, Co, Ni, Fe, Mn, Li, and V; between As and Fe, Mn, and Mo; between Cr and Co, Ni, Fe, Mn, Li, and V; between Ni and Fe, Mn, Li, and V; between Fe and Mn, Mo, Li, and V; between Mn and V; and between Li and V (Fig. 8c). A moderate correlation (R 0.5–0.75) was observed between silt and Cr, Co, Ni, Li, and V; between clay and TOC; between Cu and Pb, Cr, Mn, and V; between Pb and As and TOC; between Zn and As; between As and Cr, Co, Ni, Li, and V; between Cr and Mo and TOC; between Co and Mo and TOC; between Ni and Mo and TOC; between Mn and Mo and Li; between Mo and V; and between Li and TOC (Fig. 8c). A weak correlation (R 0.25–0.5) was observed between silt and clay, Pb, Zn, Fe, Mn, Mo, and TOC; between clay and Pb, Cr, Co, Ni, Li, and V; between Cu and As, Co, Fe, and Li; between Pb and Mo; between Zn and Mo and TOC; between Fe and TOC; between Mo and Li; and between V and TOC (Fig. 8c).
A strong negative correlation (R − 0.75 to − 0.95) was observed between sand and silt and Cu. A moderate negative correlation (R − 0.5 to − 0.75) was observed between sand and Cr, Co, Ni, Li, and V. A weak negative correlation (R − 0.25 to − 0.5) was observed between sand and clay, Pb, Zn, Fe, Cd, Mn, Mo, and TOC.