Environmental quality assessment of sandy beaches in Ha Long - Cat Ba area (Viet Nam)

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VIỆN HÀN LÂM KHOA HỌC VÀ CÔNG NGHỆ VIỆT NAM
VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY
HÀ NỘI
2022
1(T.22)
ISSN 1859-3097
VIETNAM JOURNAL OF MARINE SCIENCE AND TECHNOLOGY

©2022 Vietnam Academy of Science and Technology (VAST) 65
Vietnam Journal of Marine Science and Technology; Vol. 22, No. 1; 2022: 65–76
DOI: https://doi.org/10.15625/1859-3097/16061
https://www.vjs.ac.vn/index.php/jmst
Environmental quality assessment of sandy beaches in Ha Long - Cat Ba
area (Viet Nam)
Cao Thi Thu Trang*, Tran Dinh Lan, Do Thi Thu Huong, Tran Manh Ha,
Nguyen Thi Thu Ha, Pham Thi Kha, Nguyen Van Bach, Le Van Nam, Dinh Hai Ngoc
Institute of Marine Environment and Resources, VAST, Vietnam
*E-mail: trangct@imer.vast.vn
Received: 12 May 2021; Accepted: 15 August 2021
ABSTRACT
The environmental quality of 6 famous sand beaches of the Ha Long - Cat Ba area was surveyed in July
2018 (the rainy season) and March 2019 (the dry season). The results showed that the pore water layer
below the sand surface contained some pollutants exceeding the critical value of QCVN 10:2015/BTNMT
for coastal water and ASEAN-criteria, including TSS, ammonium and nitrate. In the rainy season, the pore
water quality was lower than that in the dry season, indicating that the tourism influenced the sandy beach
water quality. Among the studied beaches, Ha Long and Bai Dai (Van Don) showed pore water pollution;
the rest beaches were low to moderate pollution. Pore water in sandy beaches was charecterized by low
dissolved oxygen saturation and high mineralization compared to sea water, thus confirming the role of
sandy beaches in providing mineral nutrients to the sea. Analysis of the sediment grain size at the sandy
beaches showed that the beach quality was medium to good. Forty-seven benthic fauna species were found
on sandy beaches of the Ha Long - Cat Ba area, of which the dominant species were mollusks, followed by
crustaceans, polychaetes, and echinoderms. The biodiversity of sandy beaches in study area was at an
average level.
Keywords: Sandy beaches, pore water quality, sediment, benthic fauna.
Citation: Cao Thi Thu Trang, Tran Dinh Lan, Do Thi Thu Huong, Tran Manh Ha, Nguyen Thi Thu Ha, Pham Thi Kha,
Nguyen Van Bach, Le Van Nam, and Dinh Hai Ngoc, 2022. Environmental quality assessment of sandy beaches in Ha
Long - Cat Ba area (Viet Nam). Vietnam Journal of Marine Science and Technology, 22(1), 65–76.

Cao Thi Thu Trang et al.
66
INTRODUCTION
Sandy beach is one of the critical marine
resources types due to its benefits to human
life: it is home to many species of economic
value, serving the development of tourism for
swimming and resorts. Sandy beach is also a
soft belt to protect the coast from the impact of
waves and sea processes [1–3]. Beach
ecosystems provide numerous other goods and
services [4]. Many beaches are berths for
anchoring fishing boats to avoid storms for
fishers. This type of resource is also sensitive
and vulnerable to natural and human activities,
especially the impacts of climate change and
sea-level rise. The value of sandy beaches has
not been paid much attention to by the human
being and the scientific community. Sand beach
is a reasonably popular resource all over
Vietnam, but, research on this resource is still
limited; many beaches are not managed and are
used arbitrarily, thus leading to the risk of
landscape deformation and degradation of the
beach [5].
Pore water in sandy beaches is the osmotic
water in the sediment of sea sand. The sediment-
sand area of beaches is considered a filter
containing or concentrating organic particles [6].
Ehrenhauss and Huettel (2004) [7] had shown
that organic matter was degraded rapidly in sub-
tidal permeable coastal sands, indicating that
these sediments were very active sites of nutrient
recycling. Pore water in sandy beaches is the
osmotic water in sediment of sea sand. Pore
water was released to the sea during ebb tide
with mineralized organic matter [8]. Gibbes et
al., (2008) [9] demonstrated that tidally driven
pore water exchange within an offshore
sandbank might be capable of delivering bio-
available iron across the sediment-water
interface. Sandy beaches also act as
biogeochemical reactors, in which organic
matter supplied by seawater that enters the pore
space during a flood is trapped long enough to
be mineralized. Recycled nutrients are
efficiently brought back to seawater during ebb
through tidally driven flow; the role of sandy
beaches in nutrient biogeochemistry should not
be overlooked in sandy tidal coasts [8].
Sandy beaches are vulnerable to erosion
and sea-level rise or inland influences. A study
on biogeochemical processes and dynamics in
the sandy beaches can help preserve ecosystem
services and assess beach ecosystems’
vulnerablity [10]. Studying the characteristics
of environment quality at beaches can evaluate
the impact of inland sources on beaches.
The Ha Long - Cat Ba coastal area is
located in the northeast of Vietnam, under the
administrative management of Quang Ninh
province and Hai Phong city. The coastline of
the Hai Phong city - Quang Ninh province is
about 375 km, with lots of beautiful beaches.
Along with tourism - services, infrastructure,
sand beaches have attracted many tourists to
visit, swim and entertain. The number of
tourists coming to Quang Ninh and Hai Phong
city constantly increases over time, from nearly
3 million visitors in 2001 to 20 million visitors
in 2018. For this reason, we selected several
famous sand beaches in the Ha Long - Cat Ba
area to be the study object and initial
assessment of the environment quality and
ecosystem of some sandy beaches in the area.
The study area in the region that affected
by the tropical monsoon climate with two main
seasons: the winter lasts from November to the
following March, characterized by cold and dry
weather; the summer lasts from April to
October with hot and humid weather [11]. The
average annual rainfall in Ha Long - Cat Ba
coastal areas is very high, reaching more than
2,000 mm [12]. The tide of the study area is a
typical diurnal tide [13] with the highest
amplitude range up to 4.6 m. There are two
spring tide cycles in a month with an average
water level of 3.9 m and two neap tide cycles
with an intermediate water level of 1.9 m.
The sandy beaches are distributed around
small islands and coral reefs in the Ha Long -
Cat Ba area. The beach is often exposed to the
air at low tide. The components of beaches are
sand, shells of bivalve (snails, oysters, clam),
and dead coral. Although many sandy beaches
are high, their area is small with quite a steep
slope. According to Huong and Ve (2013) [5],
there are 53 sandy beaches around Cat Ba
Island, most of which are minor. There are 104
sandy beaches with very small area in Ha Long
bay area, mainly distributed around small
islands in the bay. This ecosystem is poor in

Environmental quality assessment of sandy beaches
67
biological composition, but it has the most
critical tourism value thanks to its sandy beach
structure and materials.
In the context of this paper, some initial
research results on the environment in sandy
beaches in Ha Long - Cat Ba area (Viet Nam)
have been assessed, serving as a basis for
further studies on environment and dynamics of
sand beaches.
METHODS
Studied area
Some sandy beaches in Ha Long - Cat Ba
coastal area had been surveyed include Ha
Long, Hong Van, Bai Dai, Quan Lan, Cat Co
and Monkey islands (Figure 1). These sandy
beaches are very famous and attract a high
number of tourists every year.
Figure 1. Sampling location for environment quality and biodiversity at the sandy beaches
of Ha Long - Cat Ba area
Sampling of pore water samples and
sediment samples
Pore water samples on the sand beaches
were collected, following Jenifer E. D et al.
(2011) [14]. Three sites were chosen for
sampling at each sand beach, including a low
tidal area (near the water edge), a mid-tidal area
(in the middle of the sand beach), and a high
tidal area (top upper of the sand beach).
Besides, two water samples were taken in the
sub-tidal zone at Ha Long and Cat Co beaches
in July 2018, named Ha Long 3 and Cat Co 4
(see Table 1 for coordinate of sampling points).
Measurement of water quality parameters
such as pH, temperature, dissolved oxygen
(DO), turbidity, and salinity was done in situ
using portable meters. The sampling point is a
quantitative square size of 25 cm by 25 cm with
a depth of up to 60 cm to form holes dug by a
stainless-steel shovel. Water from the sediment

Cao Thi Thu Trang et al.
68
pores slowly filling the holes was collected in
plastic bottles to analyze for COD, TSS, N-NO2,
N-NO3-, N-NH4+, P-PO43-, N-T, P-T, chlorophyll
a, oil and grease, and coliform. Pore water
samples were fixed with reagents according to
TCVN 5993:1995 (ISO 5667-3:1985) - Water
quality - Sampling - Instructions for preservation
and handling of samples [15], then put in cold
boxes at a temperature of 4oC until analysis. The
value of oxygen saturation (OS) was calculated
from salinity, temperature and DO, using an on-
line program of USGS namely DOTABLES
(https://water.usgs.gov/water-resources/softwar-
e/DOTABLES).
Table 1. Coordinates of sampling points at sandy beaches
No.
Sampling points
Coordinates
No.
Sampling points
Coordinates
1
Ha Long 1
20o56’48.7”N-107o02’52”E
11
Bai Dai 3
21o6’31’’N-107o29’24’’E
2
Ha Long 2
20o56’48.7”N-107o02’51.8”E
12
Quan Lan 1
20o54’25’’N-107o31’36’’E
3
Ha Long 1-2
20o56’51.7”N-107o03’02.3”E
13
Quan Lan 2
20o54’24’’N-107o31’37’’E
4
Ha Long 3
20o56’39”N-107o03’80”E
14
Quan Lan 3
20o54’23’’N-107o31’38’’E
5
Hong Van 1.1
21o0’23.56”N-107o45’54.32”E
15
Cat Co - 1
20o43’04.8”-107o03’10.4”E
6
Hong Van 1.2
21o0’23.84”N-107o45’54.76”E
16
Cat Co - 2
20o43’03”N-107o03’11.3”E
7
Hong Van 2.1
20o59’57.31’’N-107o46’17.36’’E
17
Cat Co - 2
20o43’3’’N-107o3’12’’E
8
Hong Van 2.2
20o59’57.50’’N-107o46’18.06’’E
18
Cat Co - 4
20o43’00.4”N-107o03’13.4”E
9
Bai Dai 1
21o6’36’’N-107o29’21’’E
19
Monkey island 2
20o43’55’’N-107o4’47’’E
10
Bai Dai 2
21o6’34’’N-107o29’22’’E
20
Monkey island 3
20o43’55’’N-107o4’46’’E
Besides, sediment sample were collected
and put into plastic bags and kept in cold
boxes for analysis of grain size and oil and
grease content.
Pore water samples and sediment samples
were collected in July 2018 (representing for
rainy season) and March 2019 (representing for
dry season). Samples were taken at the ebb tide
of the spring tide period. There were 2–4 pore
water samples and sediment samples collected
at each sand beach.
Sampling of benthic fauna
Benthic fauna samples were collected
following the methods of Eleftheriou and
McIntyre, 2005 [16] and “Procedures for
investigation, a survey of marine resources and
environment” of Institute of Marine
Environment and Resources (IMER), 2014 [17].
The benthic fauna was sampled by transects and
quantitative squares, whose sampling site was
similar to pore water and sediment samples, as
shown in Figure 1. Each transect is divided into
three zones, high-, mid-, and low-tide and
samples were collected in the zones by surface
space, using a square of 0.5 × 0.5 m. Samples
were kept in plastic boxes in ethanol 70% until
analysis in the laboratory [17].
Analysis in the laboratory
Analysis of pore water quality parameters
complies with standard analysis documents of
Viet Nam and the United States [18]. Methods
were used as follow: Chemical oxygen
demand (COD) - TCVN 6186:1996; Total
suspended solids (TSS) - TCVN 6625:2000;
Oil and grease in water - TCVN 5070:1995;
Phosphate (P-PO43-) and total Phosphorus -
TCVN 6201:2008; nitrite (N-NO2-) - TCVN
6187:1996, ammonium (N-NH3 + NH4+) and
total Nitrogen - SMEWW4500-NH3 F;
Chlorophyll a - TCVN 6662:2000 (ISO
10260:1992).
The grain size of sediment was determined
by pipetting method [19]. Oil and grease in
sediment are determined following TCVN
5070:1995 after extraction with n-hexane.
The zoobenthos samples in the laboratory
were analyzed using a stereomicroscope
according to the authors Abbott R. T. (1967)
[20]; Springsteen F. J and Leobreca C. B.
(1986) [21]; Jorgen and Richard (2003) [22].
Data Processing
Up to now, no criteria have been given to
assess the pore water quality. For beaches in
the Ha Long - Cat Ba area, the National

Environmental quality assessment of sandy beaches
69
Technical Regulation on Marine water quality
QCVN 10-MT:2015/BTNMT to protect
aquaculture and aquatic life and for
entertainment was used to assess the quality of
pore water [23]. In addition, use criteria of
ASEAN for nitrite, nitrate [24], and Hong
Kong [25] for chlorophyll a that are not
specified in the National Technical Regulation.
Besides, using the risk quotient RQ - the ratio
of the number of pollutants in water divided by
regulated critical value - for assessing water
quality. When RQ < 0.1: The water is minimal
risk, 0.1≤ RQ < 1: The water is a medium risk
and RQ ≥ 1: The water is a high risk [26, 27].
As for DO, the RQ value of DO was calculated
by dividing the regulated limitation value by
the measured value.
For sediment grain size, we used the
classification of Lisitizin (1986) [28] to classify
sediments. The good quality beaches are those
with medium sand structure (the mean particle
diameter (Md) is in the range of 0.25–0.5 mm),
fine sand (Md from 0.1–0.25 mm), and coarse
sand (Md from 0.5–1 mm). The low-quality
beach is the one structured from very fine sand
(Md from 0.05–0.1 mmm) and gravel sand (Md
> 1 mm).
The Shannon-Weiner (H′) diversity index
was used to assess the biodiversity of benthic
fauna on beaches. The Shannon-Weiner (H′)
diversity index was calculated as follows [29]:
1ln


S
ii
i
H p p
in which: S: total number of species in the
community (richness); pi: proportion of S made
up of the ith species.
RESULTS AND DISCUSSION
Pore water quality in the Ha Long - Cat Ba
area
The analytical results on pore water
quality through two surveys in July 2018 and
March 2019 were summarized and presented
in Table 2.
July 2018 (the rainy season)
In July 2018, the water quality parameters
measured in the field changed sharply between
beaches in which the pore water temperature
was in the range of 21.3–30.1oC, the pH value
was from 7.38–8.39, the salinity ranged wildly
from 12–31‰, and the turbidity was in the
range of 1.3 to 117.7 NTU. TSS concentration
in pore water was from 25.6–448.4 mgL-1, the
TTS concentration was over the critical value
regulated in QCVN 10:2015/BTNM (50 mgL-1)
for coastal water from 3.8–8.9 times at Ha
Long beach and 2.8 times at Hong Van beach
(high-tide area).
The dissolved oxygen concentrations in pore
water at beaches was quite low, ranging from
1.77 mgL-1 (Ha Long, high-tide area) to
6.82 mgL-1 (Hong Van 2, low-tide area),
corresponding to saturation from 24.1–100%. It
is noted that the dissolved oxygen concentration
in pore water at Ha Long beach was very low;
the average value was 3.19 mgL-1, indicating
that organic waste from inland sources has a
significant influence on the quality of pore water
at this beach. Chemical oxygen demand ranged
from 2.42–7.26 mgL-1, the higher value recorded
in Ha Long beach, reconfirming the pollution of
organic matter at this beach.
The nitrite and nitrate concentrationin pore
water ranged from 2.80–56.33 gL-1 and
28.18–204.26 gL-1, respectively. Compared to
ASEAN criteria for coastal water, the pore
water in most beaches was polluted by nitrate
from 1.1–3.4 times. Ammonium concentration
was quite high, ranging from 30.34–617.78
gL-1, compared to the critical value in QCVN
10:2015/BTNMT for entertainment purposes
(500 μgL-1), only one pore water sample in Ha
Long beach in July 2018 exceeded the
limitation about 1.2 times. However, if
compared to the critical value of aquatic life
protection (100 gL-1), 54.5% of the pore water
samples was over the critical value.
The N-T and P-T concentrations ranged
from 1.14–2.25 mgL-1 and 0.06–0.21 mgL-1,
respectively. Chlorophyll a concentration was
low, ranging from 1.14–3.34 gL-1, lower than
the critical value of Hong Kong. Oil and grease
content ranged from 0.23–0.55 mgL-1 of which
there was 1 sample at Ha Long beach - a high-
tide area over critical value.
The coliform density ranged from less than
10–150 CFU/100 mL, lower than the critical
value in QCVN 10:2015/BTNMT (1,000
CFU/100 mL).

Cao Thi Thu Trang et al.
70
Table 2. Surveyed results of pore water quality in studied beaches
Parameters
Unit
July 2018 (rainy season)
March 2019 (dry season)
Critical
value
Cat Co
(n = 2)
Cat Co -
sub-tidal
(n = 1)
Ha Long
(n = 3)
Ha Long -
sub-tidal
(n = 1)
Hong Van (n
= 4)
Cat Co
(n = 3)
Monkey
island
(n = 2)
Bai Dai
(n = 3)
Quan Lan
(n = 3)
Hong Van
(n = 4)
Temperature
oC
28.15±1.48
29.2
28.23±0.59
30.1
23.78±2.69
21.10±0.10
21.40±0.10
19.97±0.21
21.20±0.44
18.78±0.05
NR
pH
7.68±0.42
8.32
8.04±0.18
8.39
8.21±0.12
7.72±0.06
7.79±0.07
7.90±0.25
7.95±0.20
8.09±0.07
6.5–8.5
Salinity
‰
27.2±3.1
30
18.0±7.2
25
27.5±4.1
31.0±0.6
31.5±0.7
27.3±1.5
29.3±2.52
30.3±1.5
NR
Turbidity
NTU
6.0±5.9
1.3
65.9±45.1
2.5
7.1±9.0
2.4±0.3
3.9±2.0
154.2±164.4
9.70±12.65
29.4±40.0
NR
TSS
mgL-1
66.0±29.1
25.6
286.8±140.9
48.4
63.15±
32.6±2.7
19.20±1.41
496.33±479.4
46.20±46.46
145.8±161.3
50
DO
mgL-1
5.74±0.37
5.74
3.19±2.25
6.54
5.82±0.88
5.10±0.96
5.61±1.00
4.48±0.66
5.33±1.33
4.84±0.91
≥ 5
COD
mgL-1
3.13±0.22
3.9
6.95±0.27
4.45
2.67±0.24
2.86±0.10
2.05±0.08
4.14±0.10
2.12±0.73
2.23±0.12
NR
N-NO2-
μgL-1
33.13±32.82
5.84
9.13±9.14
7.96
9.84±1.60
24.8±23.1
5.92±1.77
14.84±9.50
3.67±1.07
9.37±7.42
55*
N-NO3-
μgL-1
128.8±51. 5
63.63
110.0±81.6
78.46
55.30±37.75
106.9±32.6
84.6±11.0
95.20±22.38
82.87±11.64
148.1±37.3
60*
N-NH4+
μgL-1
42.23±9.71
50.18
365.8±227.7
52.78
171.2±94.97
80.1±28.4
48.6±11.4
127.4±106.3
54.73±3.18
41.93±17.30
100
P-PO43-
μgL-1
38.35±20.41
20.63
36.38±10.42
33.29
26.31±6.81
20.68±3.33
16.92±3.14
44.21±10.20
20.12±18.27
31.17±3.83
200
N-T
mgL-1
1.30±0.23
1.34
2.02±0.15
1.35
1.85±0.29
1.13±0.11
0.99±0.07
1.34±0.37
1.25±0.07
1.13±0.10
NR
P-T
mgL-1
0.08±0.02
0.07
0.18±0.02
0.10
0.15±0.05
0.06±0.01
0.06±0.01
0.10±0.02
0.09±0.04
0.09±0.03
NR
Chlorophyll a
μgL-1
1.68±0.22
1.78
3.21±0.13
3.34
1.61±0.39
2.41±0.16
1.64±0.12
5.40±0.61
2.39±1.20
3.69±1.72
10**
Oil and grease
mgL-1
0.32±0.0
0.27
0.40±0.13
0.36
0.30±0.08
0.21±0.03
0.21±0.04
0.33±0.02
0.20±0.03
0.27±0.04
0.5
Coliform
CFU/100
mL
120±42
90
60±17
40
25±17
20±35
85±64
273±412
483±144
170±112
1,000
Note: Critical value is regulated in QCVN 10:2015/BTNMT for coastal water in protection of aquatic life; (*) ASEAN criteria; (**) Hong Kong criteria; NR-not regulated; n - number
of sample.

Environmental quality assessment of sandy beaches
71
Assessing the quality of pore water at
beaches shows that: in the rainy season, pore
water was contaminated by TSS and
ammonium; the water was deficient in
dissolved oxygen and oil pollution risk at Ha
Long beach. In general, an average RQ index
of pore water in Hong Van beach was 0.78 -
the water was a median risk; the RQ index of
pore water in Ha Long beach was greater than
1 - the water was a high risk; RQ index of
pore water in Cat Co beach was 0.57 - the
water was safe.
March 2019 (the dry season)
In March 2019, the pore water temperature
was in the range of 18.7–21.5oC, the pH value
was lower than that in the rainy season and
narrow changed from 7.67–8.19, the salinity
ranged from 26–32‰, and the turbidity
changed sharply from 2.0–341.0 NTU in
which, the high turbidity was recorded at Bai
Dai and Hong Van beaches. TSS concentration
in pore water was from 15.0–1,045.0 mgL-1; the
beaches that polluted by TSS were Bai Dai
(overcritical value near 10 times) and Hong
Van (overcritical value near 3 times).
The concentration of dissolved oxygen in
pore water at beaches was low, ranging from
3.53 mgL-1 (Hong Van, high-tide area) to
6.78 mgL-1 (Quan Lan, low-tide area),
corresponding to saturation from 44.8–
91.2%. However, these values were within
the critical limitation.
Chemical oxygen demand ranged from
1.52–4.21 mgL-1, lower than in the rainy
season.
The nitrite concentration in pore water
ranged from 1.86–51.39 gL-1, lower than
ASEAN criteria for coastal water. The nitrate
concentration in pore water ranged from
72.19–187.86 gL-1, higher than ASEAN
critical from 1.2–3.1 times. Ammonium
concentration was different between beaches,
ranging from 32.44–249.68 gL-1, lower than
the critical value in QCVN 10:2015/BTNMT
for entertainment purposes (500 μgL-1).
However, compared to critical value for
protecting aquatic life (100 gL-1), two
beaches (Cat Co and Quan Lan) were
polluted by ammonium in pore water in the
high-tide area.
The N-T and P-T concentrations ranged
from 0.44–1.75 mgL-1 and from 0.05–
0.14 mgL-1. Chlorophyll a concentration
ranged from 1.48–5.99 gL-1, lower than the
critical value of Hong Kong. Oil and grease
content ranged from 0.18–0.36 mgL-1, lower
than the necessary value. The coliform
density ranged from 0–750 CFU/100 mL,
lower than the critical value.
In the dry season, the level of pollution
decreases. The pore water still showed TSS
pollution and was at risk of dissolved oxygen
deficiency at all beaches. Except for Bai Dai
(Van Don) had an average RQ of 1.59 - the
water was a high risk, the left beaches was a
median risk.
Figure 2 illustrates the risk quotient for
pore water quality in the beaches of the Ha
Long - Cat Ba area in 2018–2019, showing that
the pore water at Bai Dai (Van Don) and Ha
Long were at high risk, other beaches were a
median risk.
Figure 2. Risk quotient value of pore water
quality in studied beaches in 2018–2019
Studies on other beaches worldwide show
that the oxygen saturation (OS) in pore water is
always lower than that in seawater [8, 30]. The
survey results of the OS in pore water on
beaches of Ha Long - Cat Ba area show an
increase of OS in pore water from the high tide
area to the low tide area (Figure 3). In the rainy
season, the oxygen saturation in pore water at
Cat Co and Hong Van beaches was quite high,
from over 65–100%. But in Ha Long beach, the
pore water of the beach was seriously lacking
dissolved oxygen, with an oxygen saturation
level of 24.1% in the high tide area to 28.5% in

Cao Thi Thu Trang et al.
72
the low tide area. However, it still
demonstrated the increase of oxygen saturation
from the intertidal area to under tidal area.
Figure 3. Oxygen saturation in pore water at
the studied beaches in 2018–2019
The dissolved oxygen concentration in
the seawater sample of Ha Long beach in
July 2018 was 6.54 mgL-1 (corresponding to
99.39% saturation), so the difference in
oxygen concentration compared to pore water
was 3.35 mgL-1, which means there were
104 μM of organic carbon mineralized.
Due to the organic mineralization, the
concentration of mineral nutrients of nitrogen
and phosphorus in water also increases in
pore water samples compared to seawater
samples. Pierre A. et al., (2008) [8] studied
the concentrations of phosphorus and nitrate
in pore water and seawater at Truc Vert
Beach and found that the concentration of
these substances in pore water exceeded their
value in seawater in most cases. At the Ha
Long beach area, in the sampling in July
2018, the total concentration of dissolved
nitrogen in pore water ranged from 374.0–
682.0 gL-1, which was much higher than
seawater -139.2 gL-1 (the Ha Long 3
sample). The phosphate concentration in the
pore water in Ha Long beach ranged from
28.17–48.10 gL-1, an average of 36.38 gL-
1, higher than the phosphate concentration in
seawater -33,29 gL-1 (the Ha Long 3
sample). Similar, at Cat Co beach in July
2018, the total dissolved nitrogen
concentration in pore water was higher than
that in seawater (257.84 gL-1 compared to
119.65 gL-1); and concentration of
phosphate in pore water was higher than that
in seawater (52.78 gL-1 compared to
20.63 gL-1). The difference between mineral
nutrients in pore water and seawater indicates
the sandy beaches’ role in providing recycle
nutrients to the sea [7]. This mechanism was
confirmed by Gibbes et al., (2008) [9] in
which sandy beaches act as biochemical
reactors; organic matter provided by seawater
enters pore space during the high tide period
is trapped long enough to be mineralized.
Sedimentary quality of sandy beaches
Grain size
Sediment grain size is an essential
parameter for assessing sediment quality and
beach quality. The analytical results of grain
size of sandy beaches in the area in 2018–2019
showed that sediments were structured mainly
from medium and fine sands with Md in the
range of 0.17–0.47 mm, which means that the
quality of the beach was from medium to good.
Most of the sandy beaches in the Ha Long - Cat
Ba area are small (less than 1 ha) distributed
along with the limestone islands.
Oil content
The analysis of oil content in sediments at
the sandy beach shows that the value of oil was
in the range of 5.00–68.69 mg/kg dry weight
(Fig. 4), lower than the critical limit of Vietnam
regulated in QCVN 43:2012/BTNMT - 100
mg/kg dry weight for total hydrocarbon [31]. In
the dry season, the oil content in sediments at
sandy beaches tended to increase compared to
that in the rainy season.
Figure 4. Oil content in sediment of sandy
beach ecosystem of Ha Long - Cat Ba area

Environmental quality assessment of sandy beaches
73
Benthic fauna
Through this study, 47 species of benthic
fauna were investigated on the sandy beaches
of the Ha Long - Cat Ba area, in which the
dominant species were mollusks, followed by
crustaceans, polychaetes, and echinoderms. The
number of species of benthic fauna on the sand
beaches was quite poor compared to the benthic
fauna in Ha Long bay 571 species [32] and Cat
Ba of 210 species [33].
The diversity of benthic groups at the
studied sandy beaches varied slightly, the area
with the most diverse number of species was
Cat Co, and Quan Lan with 16–17 species, Bai
Dai (Van Don) with 11 species, the remaining
beaches had less than 10 species. The average
density of benthic groups in the studied beaches
reached 30 individuals/m2. The dominant
density was mollusks group with an average of
39 individuals/m2, followed by the polychaetes
group - 29 individuals/m2, crustacean - 27
individuals/m2. Typical species on sandy
beaches are Donax semigranosus, Tellina
autralia (mollusks group), Mictyris longicarpus
(crustaceans group), Laonice sp. (polychaetes
group) - Figure 5.
ind.m-2
Figure 5. Diversity and density of benthic
groups in studied sandy beaches
The research results show that the diversity
of benthic species composition was
significantly different according to the
distribution of intertidal zones. The area with
high variety was the low-tidal area with 33
species; the lowest diversity was a high-tidal
area with 15 species. The dominant species in
sub-tidal areas were of polychaetes group such
as Namalycastis indica, Nephtys dibranchis,
Glycinde sp., Terebellides stroemi,
Polyophthalmus pictus. The dominant species
in the mid-tidal area were crustacean group
include Scopimera longidactyla, Mictyris
longicarpus and Hemigrapsus penicilatus.
Meanwhile, the mollusks species were more
prevalent in the high-tidal area, including
Donax semigranosus, Meretrix meretrix.
Assessment of the Shannon-Weiner (H′)
diversity index in the study areas shows that the
H′ index in the sandy beaches in Bai Dai, Quan
Lan, and Cat Co areas was higher than 2. In
general, the biodiversity of sandy beaches in
the study area was in average level (Figure 6).
H′ (loge)
Figure 6. Biodiversity index of studied
sandy beaches
CONCLUSION
Research results on zoo benthos’
environmental quality and biodiversity at six
sandy beaches in the Ha Long - Cat Ba area
show dissolved oxygen deficiency in pore water,
especially in the rainy season. Pore water in
some beaches was turbidity, and high TSS
concentrations, such as Ha Long, Bai Dai and
Hong Van beaches, probably related to the
beaches’ reclamation. Pore water was high
concentration of ammonium, affecting to sandy
beach ecosystems. In the rainy season, the pore
water quality was lower than that in the dry
season, indicating that the source of tourism
waste influenced the sandy beach water quality.
Among the studied beaches, Ha Long and Bai
Dai (Van Don) showed pore water pollution; the
rest beaches were low to moderate pollution.
The grain size of sediment at the sandy
beaches shows that the quality of beach was
from medium to good. The survey results have

Cao Thi Thu Trang et al.
74
recorded 47 species of zoobenthos in the Ha
Long - Cat Ba area, of which the dominant
species are molluscs, followed by polychaetes
and echinoderm.
Studies have shown that pore water on
beaches was characterized by high mineral
nutrient concentration and lower oxygen
saturation than seawater. They act as a filter to
provide minerals for the sea. They are
distributed from high tide to low tide and the
sea, mineralization level decreases, and oxygen
saturation increases, indicating that sandy
beaches are the environment for
biogeochemical reactions of the sea. Therefore,
the protection of sandy beaches from pollution
and urbanization should be considered and paid
more attention to in the coming time.
Acknowledgements: The project “Study of a
scientific basis to develop indices for sustainable
use assessment of tourist sea beaches in
Northern Vietnam” coded KHCBBI.01/18–20
was funded by the Vietnam Academy of Science
and Technology. We are appreciated for
allowing publish the project data.
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VIETNAM JOURNAL OF MARINE SCIENCE AND TECHNOLOGY
Vol. 22, No. 1 - March 2022
CONTENTS
Research results of marine biodiversity and biological resources in Vietnam: current
status, threats, proposed solutions for sustainable use
Do Cong thung
1
A study on the spawning season of 3 Acropora species in Nha Trang bay, Southern
waters of Vietnam
Vo Si Tuan, Ho Son Lam, Dang Tran Tu Tram, Phan Kim Hoang, Doan Van Than,
Mai Xuan Dat
21
Response to salinity of the submerged aquatic vegetation species Najas indica (Willd.)
Cham.
Dang Thi Le Xuan, Phan Thi Thuy Hang, Ton That Phap, Hoang Cong Tin, Luong
Quang Doc
29
Assessment of changes in the structure and distribution of mangroves caused by
aquaculture activities at the Bach Dang estuary, Vietnam
Hung Manh Vu, Quang Van Pham, Linh Manh Nguyen, Thao Van Dau
37
Evaluation of cytotoxic activity of marine fungi isolated from sponges in Nha Trang
bay
Phan Thi Hoai Trinh, Ekaterina A. Yurchenko, Anton N. Yurchenko, Ngo Thi Duy
Ngoc, Vo Thi Dieu Trang, Cao Thi Thuy Hang, Tran Thi Thanh Van, Pham Duc Thinh,
Huynh Hoang Nhu Khanh, Le Dinh Hung, Nguyen Ho Cong Dung
51
Study on the year-round fluctuation of total lipid content and lipid classes composition
in soft coral Sinularia flexibilis from Nha Trang, Khanh Hoa coastal
Dao Thi Kim Dung, Pham Minh Quan, Pham Quoc Long, Nguyen Thi Nga, Dinh Thi
Tu, Sikorskaya T. V., Ermolenko E. V., Dang Thi Phuong Ly, Nguyen Anh Hung
57
Environmental quality assessment of sandy beaches in Ha Long - Cat Ba area
(Vietnam)
Cao Thi Thu Trang, Tran Dinh Lan, Do Thi Thu Huong, Tran Manh Ha, Nguyen Thi
Thu Ha, Pham Thi Kha, Nguyen Van Bach, Le Van Nam, Dinh Hai Ngoc
65
Water quality at Cai river mouth and Tac river mouth, Nha Trang bay (2015–2019)
Pham Hong Ngoc, Le Hung Phu, Do Anh Van, Nguyen Hong Thu, Le Trong Dung,
Dao Viet Ha
77
Deep based learning: ebin sorting system development
Dang Vu Kim Ky, Nguyen Huynh Thong, Au Thuy An, Pham Tan Hung
83
Effects of some plant protection chemicals OCPs on the growth of two crustancean
species D. magna and M. macrocopa
Tran Thi Thu Huong, Nguyen Xuan Tong, Le Hung Anh, Le Van Hau
91