Assessment of nitrogen nutrient sources in aquatic environment of Tuyen Lam sub-catchment based on it’s stable isotopes ratio (δ¹⁵N-NO₃) combined with geochemical parameters
Nuclear Science and Technology, Vol.10, No. 2 (2020), pp. 32-43
Assessment of nitrogen nutrient sources in aquatic environment
of Tuyen Lam sub-catchment based on it’s stable isotopes ratio
(δ15N-NO3) combined with geochemical parameters
Nguyen Thi Huong Lan, Nguyen Minh Dao, Phan Quang Trung, Vo Thi Mong Tham,
Le Xuan Thang, Phan Son Hai, Nguyen Van Phuc
Dalat Nuclear Research Institute
*Corresponding author: Email: [email protected] | Tel:+84975078864
(Received 25 May 2020, accepted 26 June 2020)
Abstract: Overloading and erosion of pollutants, especially nitrogen (nitrate and ammonium)
nutrient, in catchment of multiple land use put pressure on the quality of surface water. Excess of
nitrogen nutrient in the aquatic environment causes eutrophication leading to adverse effects to the
habitat and ultimately it could affect the human health through the food chain. The aim of this study is
to assess the sources of nitrogen nutrient that could affect the quality of surface water at a sub-
catchment of Tuyen Lam reservoir in Lam Dong province by the use of water’s physico-chemical
parameters combined with stable isotope ratio of nitrogen in nitrate (δ15N-NO3) dissolved in the water.
It was revealed that nitrogen nutrient in water from springs in the study area came from at least three
sources: decomposition of soil organic matters, inorganic fertilizers and water waste from residential
areas. In points of annual crop farming and close to residential the contribution of inorganic fertilizers
to the total concentration of nitrate was estimated, based on the δ15N-NO3 values, to be as high as
more than 60% and the contribution of water waste was around 35-40%. This is the first time isotopic
signature of nitrogen-15 in nitrate (δ15N-NO3) was applied as a tool to assess and identify sources of
the nutrient in aquatic environment. Results of this study, although it was initial but it would be useful
for the local environmental managers in developing suitable strategies to control over nitrogen-
nutrient releases in order to keep the aesthetic appeal of Tuyen Lam Reservoir.
Keywords: Water quality, Nitrate, Stable isotope ratio.
also causes surface water to be polluted in
I. INTRODUCTION
downstream areas. Among other pollutants in
Over the years, agriculture including
livestock and crop production, deforestation,
mining activities cause pollution to rivers
basins. This situation has become more serious
in recent years due to the intensive farming,
urbanization and industrialization that have led
to major changes and increased pressure on
soil and water resources. This is because the
above mentioned activities cause erosion
which increases sedimentation rate in
reservoirs and water distribution canals,
reduces water quality and degrades the habitat
of aquatic ecosystems. Not only, but erosion
the aquatic environment, nitrogen nutrient in
the nitrate form (N-NO3) and organic matters
possess special concerns because of their direct
impacts to human health.
The sources of nitrogen nutrient
pollutant in surface water are mainly from
atmospheric deposition, inorganic fertilizer
used in agriculture, industrial wastewater,
domestic wastewater from septic systems,
animal waste, and soil organic matter
degradation. Assessment and identification of
nitrogen nutrient sources in surface water
©2020 Vietnam Atomic Energy Society and Vietnam Atomic Energy Institute
NGUYEN THI HUONG LAN et al.
would help to strengthen strategies for the
The aim of this study is an attempt to
assess and identify nitrate sources in the
water body of a sub-catchment of Tuyen Lam
reservoir in Lam Dong province (Vietnam)
by using isotopic composition of nitrogen in
nitrate combined with physic-chemical
parameters of the aquatic environment.
environment management towards improving
water quality.
Sources nitrogen nutrient are related to
each other in the nitrogen biochemical cycle
and can be differentiated by isotopic
composition of nitrogen [1, 2, 3]. N-NO3 is
the dominant form of N in oxidized
environment, has good solubility and easily
penetrates into soil [4]. Excess nitrate enters
the water bodies mainly through diffuse/non-
point sources, e.g. run off and soil erosion,
rather than point sources. Identifying the
source of nitrate causing water pollution in a
multiple land-use area has been a concern
over years. Analyzing the natural abundance
of stable isotopes can help identify both the
origin of nitrate and their distribution and
modification mechanism [3].
II. MATERIALS AND METHODS
A. Study area
The study area was Tuyen Lam
Reservoir located in the Tuyen Lam – Dinh
An – Quang Hiep hydraulic system of Da Lat
city, Lam Dong province. The storage
capacity of the reservoir is more than 31
million m3 of water discharged from a basin
of 33 km2 area. Water in the reservoir is
currently used for irrigation to agricultural
lands, supply domestic water, for electricity
generation, and tourism services.
The isotopic composition of nitrogen
(δ15N) in most terrestrial matter ranges
from - 10‰ to + 25‰ vs. air. Variation in
δ15N-values allows to differentiate several
nitrate sources. However, there are several
sources that are indistinguishable on this
basis due to the overlap in the value of
δ15N [5,6,7]. Moreover, the identification
of nitrate pollution sources often faces
other difficulties such as: heterogeneity in
landscape, the spatial correlation of the
study area may overlap the possible
impacts in the basin, the influence of
seasonality on the sources of nitrate and on
the response of the aquatic system, the
presence of multiple nitrate sources in
landscape [2,5,8,9].
In Da Lat city, there are two main types
of landscape, mountainous and plain-
mountainous ones. Rainfall in the area
concentrates seasonally with high intensity so
soil erosion occurs strongly, especially from
sloping land, bare land and bare hills
throughout the city. The landscape of Tuyen
Lam Reservoir’s basin has a significant
impact on the water quality of the reservoir
due to the multiple land-use and residences
around. Figure 1 and Figure 2 depict sketches
of the study area.
Table I presents meteorological data
collected in Da Lat city representing for the
study area.
33
ASSESSMENT OF NITROGEN NUTRIENT SOURCES IN AQUATIC ENVIRONMENT …
Fig. 1. A map showing the study area
Fig. 2. The map showing land use in Tuyen Lam catchment
34
NGUYEN THI HUONG LAN et al.
Table I. Meteorological data in Da Lat collected for the 2019 year
12
1
2
3
4
5
6
7
8
9
10
11
Month
Temperature,
17
16 17
19
20
21
20
20
19
19
19
18
oC
Monthly
rainfall, mm
0
0
7
5
0
0
100 189 538 560 560 284 655 165 168
Number of
rainy days, day
11
16
20
13
13
20
24
22
8
farming areas and residential areas (Figure 3).
Water samples were taken 2 times, in late dry
season (May 2019) and at the end of rainy
season (October 2019). Table II presents
sampling points and their tributaries.
B. Sampling design
The study area was selected based on the
map of hydro system of Da Lat city. Water
samples were taken along the main tributaries
that flow through typical land-use areas: forest,
Fig. 3. Study area (left) and sampling points (right)
Table II. Sampling points and land-use around the points
Tributary
Forest
Sample
N1
No.
1
Forest and agriculture
Agriculture and domestic waste water
Mix N1, N2, N3
N2
2
N3
3
N4
4
Agriculture
N5
5
Mix N1, N2, N3, N4, N5
Agriculture and domestic waste water
MixN1, N2, N3, N4, N5, N6, N7
N6
6
N7
7
N8
8
35
ASSESSMENT OF NITROGEN NUTRIENT SOURCES IN AQUATIC ENVIRONMENT …
in nitrate (15N-NO3). In that laboratory
C. Samples treatment and analyses
nitrate was separated from water samples by
Physico-chemical parameters of water in
anion-exchange method [12] then it was
the sampling points were measured directly in-
reduced to ammonium in acidic solution (pH
situ. The water’s temperature, dissolved
1) by Dewada’s alloy. The ammonium
oxygen (DO) were measured by SensoDirect
formed was then converted into ammonia by
Oxi200 device (Aqualytic
–
Germany)
raising the pH to 10 in bottles tightly capped
to avoid the loss of ammonia released. The
ammonia released was entrapped by KHSO4
(1M solution) that was impregnated into
What man filter chips. Amount of nitrogen
needed for its isotopic analysis was
calculated in order to have at least 50 g
ammonia be entrapped into KHSO4 to satisfy
the sensitivity of the analytical procedure.
The filter chips were encapsulated both sides
with Teflon tape and allowed to float on the
surface of the solution in the bottles. The
content in the bottles was occasionally
shaken. The ammonia was completely
entrapped into the filter chips during several
equipped with appropriate probes. Electric
conductivity (EC), total dissolved solid (TDS)
were measured by AL20Con device (Aqualytic
– Germany); and turbidity was measured by
Turbidity meter AL250T-IR (Aqualytic –
Germany). Procedures of the measurements
were
complied
with
the
Technical
Requirements for Environmental Monitoring
set by the Ministry of Natural Resources and
Environmen of Vietnam in the Circular No
24/2017/BTNMT.
Water samples were filtered through
0.45 mm mesh polycarbonate membranes
and stored in bottles made of high density
polyethylene (HDPE) resin and tightly
capped then transported to the laboratory for
days, usually it took
5
days. Upon
completion of the absorption, the filter chips
were taken out from the bottles and the
Teflon tape was removed. The filter chips
contained KNH4SO4 were pyrolyzed at
1050oC on the Ni-cobaltous catalyst to
nitrogen gas (N2) in the Elemetal Analyzer
(Eurovector, Italy) of an Isotope Ratio Mass-
Spectrometer (IR MS, IsoPrime, Micro-mass,
GV, UK supplier). The N2 gas was carried by
a continuous flow of helium carrier gas
through chromatographic column to purify
from contaminants before entering the
ionization chamber of the IR MS to ionize.
The continuous He-carrier gas carried the
chemical
analysis.
Concentrations
of
+
-
ammonium (NH4 ), nitrate (NO3 ), phosphate
3-
(PO4 ) in water were quantified by ion
chromatography (IC) technique in the
laboratory of Da Lat Nuclear Research
Institute following the Vietnam National
Technical Standard TCVN:6494-2-2000
(ISO10304-2 : 1995)[11]. A quality control
program was applied for the ionic content
determination
by
analyzing
standard
solutions supplied by the IC supplier. The
standard deviation of results derived by the
laboratory was within ±5% from the certified
value for respective constituents.
+
ions of masses 28 (14N2 ) and 29 (14N15N+)
An aliquot (300 ml) of the filtered
water samples were shipped to Isotope
Hydrology Lab of the Institute for Nuclear
Science and Technology in Hanoi for
measuring isotopic composition of nitrogen
into the mass-separator of the IR MS where
they were separated from each other. The
+
ions 14N2 and 14N15N+ were collected and
counted by respective Faraday cups installed
on the exits from the separator.
36
NGUYEN THI HUONG LAN et al.
To calculate the δ15N the Mass Lynx
III. RESULT AND DISCUSSION
A. Results
software supplied by the IR MS supplier was
used. The δ 15N notation was defined as:
1. Physical parameters
15
R
sample
δ15N (
1)x1000
, ‰
(1)
Table III presented physical parameters
of water samples that were measured in situ.
15
R
Std
15
15
Where Rsample and RStd are respectively
the ratios of [29N+/28N+] in samples and that in
standard. The standard used in this measurement
is nitrogen in the ambient air. The measurement
was conducted thrice. Precision of δ15N-NO3
determination was ±0,3‰.
2. Chemical and biological parameters
Table IV presented concentration of
chemical and biological parameters measured
in-situfor water samples collected in May and
October 2019.
Table III. Physical parameters of water samples
Temperature,
oC
DO,
mg/l
EC,
μS/cm
TDS,
mg/l
Turbidity,
NTU
No. Sample
May, 2019
1
2
3
4
5
6
7
8
N1
N2
N3
N4
N5
N6
N7
N8
21.7
29.2
26.2
27.8
26.7
25.8
24.6
25.8
4.51
3.90
4.40
3.85
3.90
3.97
3.57
3.52
24.7
53.7
138.9
86.6
124.5
85.2
378.0
33.8
18
36
94
59
84
59
267
161
7.03
30.3
28.6
30.1
31.2
22.1
7.17
19.7
October, 2019
1
2
3
4
5
6
7
8
N1
N2
N3
N4
N5
N6
N7
N8
22.1
27.2
25.9
26.3
26.6
26.0
25.5
25.7
4.34
5.26
4.99
4.41
3.88
4.04
3.96
3.56
19.8
46.1
117.6
79.1
136.0
82.5
249.0
166.6
15
35
89
60
103
63
6.29
18.2
6.41
12.0
35.2
14.3
9.57
12.5
189
126
In table III: DO, EC, TDS denote for dissolved oxygen, electrical conductivity and
total dissolved solid, respectively.
Table IV. Chemical and biological parameters of study water samples
3-
PO4
+
-
P,
mg/L
NH4 ,
NO3 ,
Coli form,
N,
Sample
,
No
mg/L
mg/L MPN/100mL mg/L
mg/L
May, 2019
-
-
-
-
-
N1
N2
N3
N4
N5
0.03
0.05
0.12
0.08
1.00
0.34
0.26
0.24
0.26
0.24
0.28
1.50
2.43
0.45
1.02
-
-
-
-
-
-
-
-
-
-
1
2
3
4
5
37
ASSESSMENT OF NITROGEN NUTRIENT SOURCES IN AQUATIC ENVIRONMENT …
3-
PO4
+
-
P,
mg/L
NH4 ,
NO3 ,
Coli form,
N,
Sample
,
No
mg/L
mg/L MPN/100mL mg/L
mg/L
-
-
-
N6
N7
N8
0.10
6.30
3.50
0.41
0.39
0.54
2.12
1.24
0.83
-
-
-
-
-
-
6
7
8
October, 2019
0.22
0.10
0.10
0.33
0.17
0.10
0.11
0.12
N1
N2
N3
N4
N5
N6
N7
N8
1
2
3
4
5
6
7
8
0.03
0.17
0.35
0.17
2.70
0.03
0.25
0.11
0.09
0.16
0.08
0.14
0.20
0.14
0.73
0.39
0.21
2.49
5.55
6.66
4.80
6.41
8.48
6.45
460
2400
2400
4600
7500
2100
1100
11000
1.52
1.61
1.60
1.91
1.53
1.52
1.50
1.51
QCVN08-
MT:2015/BTNMT
(Column B1)
-
0.9
0.3
10
7500
-
Value of δ15N-NO3 in water samples
were determined for October 2019
sampling campaign and shown in Table V.
3. Isotopic composition of nitrogen-15
in nitrate (δ15N-NO3) dissolved in in
water samples
Table V. The value of δ15N-NO3 in water samples collected in October 2019
SD
Sample
N1
δ15N vs. air N2, ‰
No.
1
± 0,14
± 0,16
± 0,35
± 0,21
± 0,17
± 0,11
± 0,26
± 0,23
-1.44
-1.83
8.27
3.08
1.15
6.09
6.99
4.02
N2
2
N3
3
N4
4
N5
5
N6
6
N7
7
N8
8
all other sites. Turbidity in water from site N1
in May and October was lowest, it was around
7 NTU, however, at the same time in sites N2
and N5 where agricultural activities are
conducted which probably made local soil to
be disturbed frequently and soil easily eroded
with run-off water. This erosion led the
turbidity in water to increase up to more than 4
times higher than that observed for site N1.
The turbidity in water from sites N2 and N5
was respectively 28 and 312 NTU in May and
B. Discussion
Results in Table IV showed that water in
site N1 came from sub-surface of the
surrounding forest as water’s temperature in
that site was lower than that in all other sites, it
was 21.7 and 22.1oC in the dry and rainy
seasons,
respectively.
It
seems
that
contribution of run-off water to stream in site
N1 was insignificant compared to those of
groundwater, so this made erosion (soil loss)
around site N1 to be lowest compared to that in
38
NGUYEN THI HUONG LAN et al.
18 and 35 NTU in October (Table I).
N6, N7, N8 (May) and N7 (October); coli
forms at sites N5 and N8 (October).
Obviously, soil treatment (tillage for crops
planting) increased rate of erosion made
turbidity in water to be high.
Water in sites N5 and N7 was the run-off
from agricultural areas where annual crops
such as strawberries, vegetables (cauliflower,
cabbage, etc...) were planted but in site N8
water was a composite of water from all the
sites studied (Fig. 3 and Table II). It was
thought that farmers around sites N5, and N7
fertilized their annual crops with N-P-K and
nitrogen fertilizer was urea. In soil and during
the run-off urea would hydrolize according to
the equation:
As seen from Table IV that water
flowing along all the springs was well mixed
so that oxygen content in water was almost at
the equilibrium at temperatures from 20 to
30oC, ranging from 3.5 to 5.56 mg/L (Table
IV), meaning that water was fresh.
From Table IV it was clear a fact that
water waste from human activities contributed
a significant portion of dissolved substances in
water. The content of total dissolved solid
(TDS) in water from sites N3 and N7 where it
received water waste from residential areas in
May were 94 and 267 mg/L respectively, i.e. 5
and 15 times higher than TDS content in water
from site N1 (forest area) or 2.6 and 7.4 times
higher than TDS content in water from site
N2,a composite of forest and perennial crop
farming areas. Associated with TDS is electric
conductivity (EC) in sites N3 and N5 in May
was 138.9 and 378 S/cm, respectively, i.e.
roughly 5 and 15 times higher than that for
water in site N1 (Table IV).The similar
situation was also observed in October (Table
IV). In the study area, a part of waste water
was not completely collected for centralized
treatment therefore it could much affect to the
quality of surface water.
(NH2)2CO + H2O → 2NH3 + CO2
(1)
CO2 formed in water (reaction 1)
would continue hydrolyze to carbonic acid
-
following dissociation to bicarbonate (HCO3 )
and protonium ion (H+) as follows:
-
CO2 + H2O → H2CO3→ H+ + HCO3
(2)
Ammonia formed in reaction (1) could
partly escape from the aquatic environment in
to the atmosphere due to evaporation and
partly associate with ion protonium to stabilize
in the form of ammonium (process 3):
+
NH3 + H+→ NH4
(3)
To address the fertilizer source of
ammonium in water of the study region a graph
+
of concentration of ammonium [NH4 ] vs.
3-
[PO4 ] was built and depicted in Figure 4.
As seen from Fig. 4 there was no any
trend of relationship between ammonium and
phosphate concentrations in water samples
collected. However, this could be understood
from the point of view that the hydro
geochemical behaviour of ammonium and
phosphate in aquatic environment was quite
different from each other. In aquatic
environment, ammonium could be subjected to
Results in Table V, for both 2 times of
sampling in May and October 2019,revealed
that the quality of surface water in the study
region mostly met the requirements of National
Technical Regulation set by the Ministry of
Natural Resources and Environment of
Vietnam for surface water (QCVN 08-MT:
+
2015/ BTNMT), excepted forNH4 at sites N5,
N7, N8 in May and N5 (October); PO43- at sites
39
ASSESSMENT OF NITROGEN NUTRIENT SOURCES IN AQUATIC ENVIRONMENT …
evaporation, nitrification and denitrification
but phosphate would subject to precipitation in
the form of hydroxyapatite (Ca10(PO4)6(OH)2)
with calcium ion dissolved in water. Therefore,
both the constituents could be lost from water
by different mechanisms and as a consequence
they did not closely correlated each other as it
was in the case. Unfortunately, in this work
concentration of Ca2+ in study water samples
was not available to assess whether
hydroxyapatite is precipitating or not, however
it believed that it is precipitating because the
dissociation product of this substance is very
low, it was 10-115[13].
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
0.5
1
3-
[PO4 ], mg/L
+
3-
Fig. 4. A scatter plot of relationship between [NH4 ] vs. [PO4 ] in surface water from Tuyen Lam sub-
catchment in October 2019 to show the fertilizer source of ammonium and phosphate released into the
aquatic environment of the region
The nitrification of ammonium in
almost saturated at the temperature range of
25-30oC (Table III). The final product of the
nitrification process (reaction series 4) is
nitrate, therefore it expected that ammonium
concentration in water would correlate well
with that of nitrate. Figure 5 showed a
scatter plot of the relationship between
aquatic environment proceeds as follows [14]:
+
NH4 → NH2OH → [HNO] → NO2NHOH →
-
-
NO2 → NO3
(4)
Nitrification much facilitates by the
presence of oxygen as in this case when
oxygen content in water was found to be
+
-
[NH4 ] and [NO3 ].
10.0
N7
N4, N6,
8.0
N8
6.0
N3
N5
4.0
N2
2.0
N1
0.0
0.0
1.0
2.0
3.0
+
[NH4 ], mg/L
-
+
Fig. 5. A scatter plot of [NO3 ] vs. [NH4 ] showing nitrate was a product of the nitrification of ammonium in
aquatic environment of Tuyen Lam sub-catchment
40
NGUYEN THI HUONG LAN et al.
From Fig. 5 once can see a general trend
decomposition of soil organic matter, from
-
of positive and close correlation between [NO3
inorganic fertilizer and from water waste of
human activities.
+
] and [NH4 ] if site N5 excluded. The
+
-
significant deviation of [NH4 ] from the [NO3 ]
Assume that the 15N in inorganic
fertilizer be 1‰, and that in water waste be in
average of 20‰[5, 16] one can estimate the
contribution of nitrate from inorganic fertilizer
and water waste in site N3, N5 and N7 based
on the two end-member mixing model (Eq. 5)
as follows:
+
vs. [NH4 ] relationship (Fig. 5) was not clear
and needed to be studied further.
Another reason to explain the elevated
(in comparison with the QCVN 08-MT:
2015/BTNMT)
[15]
concentration
of
ammonium and phosphate in water from sites
N5, N6, N7 and N8 (Table V) was due to the
contribution of wastewater from residential
areas. The waste contains high organic matters
that would decompose with the assistance of
biological activity to ammonium and
phosphate as it was indicated by population
density of coli-forms in those sites during
October, the end of rainy season (Table IV).
15N-NO3, sample = x.15N-NO3, fer+ (1-x). 15N-
NO3, ww
(5)
Where 15N-NO3,sample
,
15N-NO3,fer
,
15N-NO3,ww are isotopic signatures of nitrogen
15 in nitrate found in study water sample, in
inorganic fertilizer and in water waste,
respectively; x is the contribution of nitrate
from inorganic fertilizer, and (1-x) is the
contribution of nitrate from water waster.
Results in Table V indicated that nitrate
source in the area of forest and perennial crops
(N1 and N2) was originated from the
decomposition of soil organic matter, so that
isotopic signature of N-15 in nitrate (15N-NO3)
was in the range of -1.83 to -1.43‰vs. Air.
Nitrate in water from site N5 where annual crop
farming is the only agricultural activity has
15N-NO3 of 1.15‰vs. Air, which is typical
value of 15N in inorganic nitrogen fertilizers as
it was synthesized from air-nitrogen. Nitrate in
water from annual crops farming and residential
areas, site N3 or site N7, was enriched, up to
8.27 and 6.99‰vs. Air, respectively. The reason
for the enrichment in 15N-NO3 in water from
sites N3 and N7 could be due to the contribution
of nitrate from water waste. It was reportedly
that 15N-NO3 from human water waste ranges
from 10 to 30‰ vs. Air [5].
The contribution of nitrate from
inorganic fertilizer in water of site N3, N5 and
N7 was estimated (Eq. 5) to be as high as
61.74%; almost 100% and 68.47%,
respectively. Respectively, the contribution of
water waste in site N3, N5 and N7 was as high
as 38.26, 0, and 31.53%.
It was surprisingly a fact that in Tuyen
Lam sub-catchment (Da Lat city) there is very
intensive vegetable farming that it expected a lot
of both inorganic fertilizer and manure be applied
led to high concentration of nitrogen and
phosphorus nutrient in the aquatic environment.
However, as seen from Table IV the
concentration of nitrate in all the sampling sites
was in within the national standard (QCVN 08-
MT: 2015/BTNMT) meaning that surface water
in that region is clean by the nutrients substances.
Commonly, in the study region nitrogen
nutrient (nitrate) in surface water were
originated from three sources, namely from
The low concentration of nitrate in
surface water in the study region could be
41
ASSESSMENT OF NITROGEN NUTRIENT SOURCES IN AQUATIC ENVIRONMENT …
explained by the denitrification process [14]. In
water, denitifying bacteria capable of
converting nitrate into nitrogen gas that easily
escapes from the aquatic environment as
shown in the reaction series bellows:
IV. CONCLUSIONS
The quality of water of sub-catchment of
Tuyen Lam reservoir was studied through
chemical, physical and biological parameters
as well as stable isotope ratio of nitrogen-15
(δ15N) in nitrate dissolved in water. Generally,
it was revealed that in the study area, the
content of nutrients contamination is still not
yet at the warning level according to the
-
-
NO3 → NO2 - - ->[X]- - -> N2O → N2 (6)
The denitrificationmade concentration
of nitrate in water to decrease, and as the
rule the isotopic signature of nitrogen-15 in
the remaining reactant to enrich. It was well
known that in any chemical or biological
process composition of heavy isotopes in
products will be depleted, reversely
composition of heavy isotopes in initial
National
Standard
(QCVN
08-MT-
2015/BTNMT) of the Ministry of Natural
Resources and Environment. However, if
proper strategies on better use of inorganic
fertilizers and public water waste management
not be applied right now it would cause serious
consequences to quality of the aquatic
environment and ultimately to the human
health of the locality.
reactants
will
be
enriched.
The
denitrification in this study was evident
from the correlation of 15N in nitrate and
concentrations of nitrate in water as shown
in Figure 6.
This is the first time isotopic technique
was applied to identify the source of nitrate in
the aquatic environment. However, it appeared
that the δ15N-NO3 value could be useful tool in
assessing the sources of nitrogen nutrient
released into the environment. It is advisable
that isotopic technique should be continuous
applied in environmental studies, particularly
such a kind of study addressed to the quality of
water resources in order to precisely determine
the sources of nitrogen as well as phosphate
nutrients in the aquatic environment of Da Lat
city. In this case not only 15N-NO3, but also
18N-NO3, 18O-PO4 dissolved in water. The
outcomes of these studies would assist the
local environmental managers in developing
suitable strategies for protecting water
resources of the city from nutrient pollution.
10
N3
8
15N-NO3 = 1.16.[NO3 ] - 2.67
-
N7
N6
6
4
2
0
-2
-4
R² = 0.652
N8
N4
N5
N1
N2
0
5
10
-
[NO3 ], mg/L
Fig. 6. Correlation of 15N-NO3 with nitrate
concentration remained in water
due to denitrification
A model of the denitrification process
-
expressing through the 5N-NO3vs. [NO3 ]
ACKNOWLEDGEMENT
correlation was as follows (Fig. 6)
The financial support from Vietnam
Atomic Energy Institute (Vinatom) under grant
CS/19/01-01 is acknowledged.
-
15N-NO3=1.16.[NO3 ]-2.67 (‰)
R2 = 0.652
42
NGUYEN THI HUONG LAN et al.
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