26
175 CHAPTER 6 DISSOLVED, PARTICULATE AND BED SEDIMENT GEOCHEMISTRY OF TRACE ELEMENTS AND REEs IN THE SWARNA ESTUARY, SOUTHWEST COAST OF INDIA
175
CHAPTER 6
DISSOLVED, PARTICULATE AND
BED SEDIMENT GEOCHEMISTRY
OF TRACE ELEMENTS AND REEs IN
THE SWARNA ESTUARY,
SOUTHWEST COAST OF INDIA
Chapter - 6
176
ABSTRACT
The studies on tropical estuarine geochemistry are limited to heavy metal analysis. This
study focuses on the abundance of trace elements and rare earth elements (REEs) along the
salinity gradient in a tropical estuary on a seasonal scale. The Swarna estuary is studied to
understand the behaviour of trace elements and REEs in the dissolved, particulate and bed
sediment phases and their distribution fractions between the three phases along the salinity
gradient. The dissolved trace elements like B, Rb, Sr, Se, Cr, Mo, As, Cs, Pb and U
increase with salinity. The dissolved trace elements like Sr, B and Rb show conservative
nature. Although the dissolved fractions of Cr, Mo, As, V, Cs, Pb, U and Nb show an
increase along the salinity gradient, they show non-conservative behavior due to their
sorption on to the SPM under the alkaline condition. The dissolved Fe undergoes
flocculation in the Swarna estuary. The dissolved Mn is found to be controlled by
dissolved organic carbon forming colloidal organic carbon – metal complex which in turn
acts as a scavenger to other metals present in the estuarine water. This is also supported by
the dominance of Mn in the SPM phase compared to the dissolved and bed sediment
phases. Dissolved REEs show identical behaviour with changing salinity for the three
sampling seasons with their availability being controlled by the prevailing pH conditions.
It is found that the estuarine suspended particulate matter (SPM) at the sea water front
(30.4 ‰) of the Swarna estuary has higher radiogenic strontium isotopic ratio (87
Sr/86
Sr)
and lower strontium concentration compared to the estuarine water which suggest the
conservative transport of particulate material from the continent towards the sea. The
87Sr/
86Sr in the bed sediment represents the dominant silicate signature of the basin. The
study determines the dominance of coastal groundwater discharge over the dissolved trace
elements and REEs during the low river discharge season in the Swarna estuary.
Keywords: Trace element, Rare earth element, Strontium isotope, Tropical estuary,
Swarna estuary, Southwest coast of India.
Chapter - 6
177
6.1. INTRODUCTION
Estuaries play a crucial role in the geochemical cycling of elements at the earth’s surface.
The dynamicity of estuaries are not limited to river runoff or tidal mixing, but also
involves various internal physical, chemical and biological processes which govern the
chemical composition of water and sediments as well as the fate of terrestrial elements
being transported to the sea. The estuarine geochemical studies help in determining
whether the riverine flux of element goes unchanged into the ocean or gets added/removed
during its passage through the estuary (Honeyman and Santschi, 1988; Santschi et al.
1997). The biogeochemical processes occurring in the estuaries promote the inorganic or
organic removal or addition of elements from or to the solution which affect their
conservative or non conservative nature. Recent geochemical and isotopic studies on the
estuarine waters suggest submarine groundwater discharge as an additional source of
elements to the coastal waters and nearby ocean (Charette and Sholkovitz, 2006; Beck et
al. 2007; Jeong et al. 2012; Kim and Kim, 2014). Thus, the study on estuarine
geochemistry needs a careful assessment of source contribution and in-depth study of each
driving factor in the complex aquatic system. This study is intended to discuss the
geochemical behaviour and the fate of trace elements in a relatively pristine estuarine
system of west coast of India.
West coast of India is one of the rapidly developing regions of India with densely
populated cities and major industries located all along the coast. The Swarna River is
studied for its estuarine geochemical characteristics and behaviour on a seasonal basis. The
main objectives of this work are (1) to determine the role of physico-chemical parameters
and dissolved organic carbon on the behaviour and fate of trace elements and rare earth
elements along the salinity gradient, (2) to assess the distribution of metals between the
dissolved, suspended particulate matter and bed sediment phases of the estuary and (3) to
determine the sources of elements being transported by the estuarine water.
6.2. RESULTS AND DISCUSSION
6.2.1. Physico-chemical characteristics
The results on the physico-chemical characteristics of the Swarna estuary are given in
Table 6.1 and Fig. 6.1. The water samples collected from the Swarna estuary during
Chapter - 6
178
January 2011 (post-monsoonal season), May 2011 (pre-monsoonal season) and October
2011 (monsoonal season) show a significant difference in physico-chemical parameters
which could be attributed to the mixing trend of fresh water and sea water throughout the
estuary. The Swarna estuary could be categorized under the well-mixed estuary as there is
a uniform mixing of fresh water and the sea water which could be observed from the
salinity gradient (0.03 – 34.7 ‰) measured for the estuarine water during the sampling
seasons and due to the shallow depth. However, the mixing of river water with sea water is
found to be negligible during the pre-monsoon season due to low river discharge.
Moreover, the downward river flow is found to be completely obstructed by the Baje dam
at Hiriyadka, Udupi during the dry sampling season. This led to the inflow of saline water
up to 25 km inland till Baje, Hiriyadka. As a result, the minimum salinity in the estuary at
the freshwater front is measured to be 17.7 ‰ for the pre-monsoon season. The dilution in
salinity of sea water in the freshwater front would have occurred due to the inflow of
groundwater to the estuarine region during the pre-monsoon season. The temperature of
estuarine water varies from 27.3 to 29.8 °C, 30.4 to 33.9 °C and 28.3 to 31.5 °C during the
post-monsoonal, pre-monsoonal and monsoonal sampling seasons respectively. Water
temperature shows an increase by 0.5 – 1 °C between 0 and 2.5 ‰ of salinity and
thereafter gradually decreases with a slight variation due to continuous mixing of sea water
during the monsoonal and post-monsoonal sampling. There is a relatively higher
variability in temperature along the salinity gradient during the pre-monsoon season
compared to other two seasons. Although the variability of water temperature is less along
the salinity gradient during each sampling season, there is a significant difference in
temperature with season, with almost a parallel linear decreasing trend along the increasing
salinity gradient. The pH of estuarine water ranges from 7.2 to 8.1, 7.5 to 8.2 and from 7.4
to 8.1 during the post-monsoonal, pre-monsoonal and monsoonal sampling seasons
respectively. The similar increasing trends in pH along the salinity gradient are observed
for post-monsoon and monsoon seasons. Although the pH of pre-monsoon season
increases along the salinity gradient (24 to 34.7 ‰), it exhibits relatively lower values
particularly between the region of 19 and 33 ‰ salinity during the pre-monsoon season.
The contrasting less alkaline nature (relatively lower pH) during the pre-monsoon season
compared to other seasons suggests higher groundwater contribution to the estuarine water
(Pempkowiak et al. 2010). The pH of estuarine water increases sharply at the sea water
front during the pre-monsoon season to represent the alkaline nature of sea water. The
slightly alkaline pH (7.8) at 17.7 ‰ salinity could be due to the photosynthetic activity of
Chapter - 6
179
the in-situ phytoplankton at the less turbulent zone (freshwater front) of the estuary during
the pre-monsoon season. The less momentum of water would have supported the higher
phytoplankton growth, and the well lit condition (as observed from higher temperature)
would have resulted in the higher photosynthetic activity at the region of 17.7 ‰ salinity.
This is also justified with the observed higher dissolved oxygen (DO) and dissolved
organic carbon (DOC) concentration for the same season (Fig. 6.1). The DO in estuarine
water varies from 6.3 to 6.9 mg L-1
, 6.7 to 7.03 mg L-1
and from 6.3 to 7.83 mg L-1
during
the post-monsoonal, pre-monsoonal and monsoonal sampling seasons respectively. The
trend in DO variability is found to be similar during the post-monsoonal and monsoonal
seasons with lower DO at mid salinities and higher DO at both fresh and saline water end
members. DO of pre-monsoon sampling season follow the pH pattern. The DOC in
estuarine water ranges from 1.77 to 2.42 mg L-1
, 1.46 to 5 mg L-1
and from 0.68 to 4.67 mg
L-1
during the post-monsoonal, pre-monsoonal and monsoonal sampling seasons
respectively. DOC concentration measured along the salinity gradient during the post-
monsoon sampling shows a slight increase in the mid salinity. A steep decrease in DOC
concentration along the salinity gradient between 0 and 3.6 ‰ salinity is observed during
the monsoonal season which could be due to the higher removal rate of DOC from the
water column during the initial mixing whereas the DOC concentration increases at mid
salinities between 13.5 and 27 ‰ of estuarine water. The decrease in DOC concentration
along the salinity gradient during all seasons suggests the removal of terrestrial DOC
brought by the rivers in the estuarine region. The increase in DOC concentration at the mid
salinities would be due to the resuspension of DOC from the previously adsorbed metal
oxy-hydroxides or SPM. The well-lit, less turbulent zone occurring at 17.7 ‰ salinity
during the pre-monsoon season supports higher photosynthetic activity of phytoplankton
which results in the sharp increase of pH, DO and DOC values. The absence in
photosynthetic activity would have resulted in lower DO under the higher temperature
condition of pre-monsoonal season. Thus, DOC in the estuarine water is found to be more
of autochthonous in nature rather than allochthonous origin during the pre-monsoonal
season. The SPM concentration in estuarine water ranges from 5.6 to 11 mg L-1
, 10.6 to
15.4 mg L-1
and from 9.8 to 19.8 mg L-1
during the post-monsoonal, pre-monsoonal and
monsoonal sampling seasons respectively. SPM increases in estuarine water along the
higher salinity gradient in all seasons. Also, there is a significant addition of SPM at
varying salinities of each sampling season which could be attributed to the resuspension of
particulates from the bottom sediments. The high river discharge during the monsoonal
Chapter - 6
181
Salinity (‰)0 10 20 30 40
T (
°C)
26
28
30
32
34
36Jan 2011
May 2011
Oct 2011
Salinity (‰)
0 10 20 30 40
pH
7.1
7.3
7.5
7.7
7.9
8.1
8.3Jan 2011
May 2011
Oct 2011
Salinity (‰)0 10 20 30 40
EC
(m
S c
m-1
)
0
10
20
30
40
50
60
Jan 2011
May 2011
Oct 2011
Salinity (‰)0 10 20 30 40
DO
(m
g L
-1)
6.2
6.4
6.6
6.8
7.0
7.2
7.4
7.6
7.8
8.0
Jan 2011
May 2011
Oct 2011
Salinity (‰)
0 10 20 30 40
SP
M (
mg
L-1
)
5
10
15
20
25Jan 2011
May 2011
Oct 2011
Salinity (‰)0 10 20 30 40
DO
C (
mg
L-1
)
0
1
2
3
4
5
6Jan 2011
May 2011
Oct 2011
Figure 6.1: Physico-chemical parameters, SPM and DOC measured along the salient
gradient for the study period in the Swarna estuary.
6.2.2. Dissolved major ions, trace elements and rare earth elements in the Swarna
estuary
The trace elements and rare earth elements measured in the dissolved phase of the Swarna
estuary are given in Table 6.2. The dominating trace elements in the Swarna estuary are as
Chapter - 6
185
Sample
No. Sm Eu Gd Dy Ho Er Tm Yb Lu
416 46.98 53.23 59.15 26.38 50.65 77.37 44.57 74.79 30.98
417 90.52 85.60 97.31 85.08 84.77 82.18 84.46 92.90 76.70
418 77.85 52.21 52.53 54.92 52.57 57.26 47.32 93.51 42.78
419 83.42 51.21 29.58 35.96 49.10 17.97 42.49 43.37 44.32
420 93.15 106.40 129.56 78.34 96.51 93.30 103.62 89.30 98.14
421 10.90 15.43 24.20 7.14 11.50 5.27 9.76 6.54 7.23
422 12.07 7.23 19.34 3.96 7.76 11.50 11.19 7.41 2.49
423 9.51 6.00 12.03 5.72 5.98 2.95 2.84 3.06 3.96
424 7.61 5.40 5.09 4.22 3.88 5.44 2.84 4.54 3.08
425 3.90 2.47 3.93 3.25 1.74 3.24 1.73 2.44 55.35
6.2.2.1. Co-existence and behaviour of dissolved chemical species
The inter-relationships between physico-chemical parameters, trace elements and rare
earth elements present in the dissolved phase (n = 25 samples) of the Swarna estuary are
represented in the form of a dendrogram (Fig. 6.2) obtained through Centroid clustering
method of Hierarchical cluster analysis and Pearson correlation technique of SPSS v.19.
The chemical species in the dendrogram are grouped into 6 cluster memberships to explain
their geochemical behaviour and fate under changing physico-chemical conditions. The
first group of elements combines with salinity showing very high correlation (r = 0.7 –
0.99 at p < 0.05 i.e., 95 % confidence level). The salinity combines with trace elements
like Cs, Cr and Cd initially which later combines with V, Pb, Nb and Sb. These elements
in turn combine with the cluster of major and minor elements including Ca, Mg, Na, B, K,
Sr, Se and Rb. At a higher stage they group with elements like Ti, Ag, Te and Er together
to form cluster membership - 1. The elements belonging to cluster membership - 1 are
largely controlled by the salinity of estuarine water (Fig. 6.3). These elements maintain the
signatures of the freshwater in the mixing zone throughout the estuary with their
concentration being dependent mainly on the salinity of water. This suggests that the
sources of these elements could be natural processes and have no additional anthropogenic
sources in the estuarine region. The concentrations of elements like B, Sr and Rb show a
linear increase with salinity representing the conservative nature in the estuarine water
whereas Se show slightly non-conservative nature. The concentrations of other elements
clustering with salinity in the dendrogram also show an increase with salinity, but are
highly affected by the pH variability at higher salinity front and higher SPM (particularly
Chapter - 6
186
during the monsoonal season) which indicates the metal sorption behaviour mainly under
the alkaline condition in the estuary.
Most of the rare earth elements like Pr, Nd, Tm, Eu, Ho Sm, Dy, Lu and Yb show a good
correlation (r = ± 0.4 – 0.9) with trace elements like As, U, Co, Mo, Ga and P which
group to form cluster membership – 2 in the dendrogram. They combine with elements of
cluster membership – 1 at a higher stage which indicates that the elements of cluster
membership – 2 are also controlled by salinity of the estuarine water and have no
additional anthropogenic sources contributing to their concentration variability in the
estuary. These elements also show a good correlation with pH indicating the pH mediated
sorption behaviour of REEs at different salinities (Fig. 6.4). It is found that the
concentrations of REEs increase along the salinity gradient with the increase in pH up to
7.7 – 8.0 units, and thereafter a sharp decrease in REEs concentration is observed under
the alkaline condition with the increase in salinity of the estuarine water. This could be due
to the dominance of sea water signature over the REEs concentration at higher salinity.
The seasonal variability of estuarine REEs concentration and its higher variability along
the salinity during the same season are further examined by determining its possible
sources in the estuarine region. The concentrations of dissolved REEs in coastal rain water
(Santhekatte), downstream river water (Puthige; surface water source) and coastal
groundwater are compared with that of the estuarine water to understand the higher
variability of the REEs present in the estuary along the salinity gradient (Fig. 6.5). It is
observed that the REE pattern in the estuary during the monsoonal season show similar
trend with rain water. The estuarine REE patterns during the non-monsoonal seasons
follow more of the REE patterns of the coastal groundwater. This suggests that the
groundwater discharge could be the dominant source for these elements in the estuarine
region particularly during the non-monsoonal seasons. This in turn would have resulted in
higher concentration of REEs along the salinity gradient in the estuarine region.
Chapter - 6
187
Figure 6.2: Dendrogram explaining the behaviour of dissolved trace elements in the
Swarna estuary.
Chapter - 6
188
Jan 2011
Salinity (‰)0 10 20 30 40
B, S
r (m
g k
g-1
)
0
2
4
6
8
Rb
, S
e (
µg
kg
-1)
0
20
40
60
80
100
120
140B
Sr
Rb
Se
Jan 2011
Salinity (‰)0 10 20 30 40
Cr,
Pb
, T
i, M
o (
µg
kg
-1)
0
2
4
6
8
10
12
14
Nb
(µ
g k
g-1
)
0
2
4
6
8
10Cr
Pb
Ti
Mo
Nb
Jan 2011
Salinity (‰)
0 10 20 30 40
V,
As
, U
(µ
g k
g-1
)
0.0
0.5
1.0
1.5
2.0
2.5V
As
U
Jan 2011
Salinity (‰)0 10 20 30 40
Cd
, S
b, A
g (
µg
kg
-1)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Cs (
µg
kg
-1)
0.0
0.2
0.4
0.6
0.8
1.0Cd
Sb
Ag
Cs
May 2011
Salinity (‰)
15 20 25 30 35 40
B, S
r (m
g k
g-1
)
0
2
4
6
8
Rb
, S
e (
µg
kg
-1)
40
60
80
100
120
140
160
May 2011
Salinity (‰)15 20 25 30 35 40
Cr,
Pb
, T
i, M
o (
µg
kg
-1)
0
2
4
6
8
10
12
14
16
Nb
(µ
g k
g-1
)
0
2
4
6
8
10
May 2011
Salinity (‰)15 20 25 30 35 40
V, A
s, U
(µ
g k
g-1
)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
May 2011
Salinity (‰)15 20 25 30 35 40
Cd
, S
b,
Ag
(µ
g k
g-1
)
0.0
0.2
0.4
0.6
0.8
Cs
(µ
g k
g-1
)
0.6
0.8
1.0
1.2
1.4
1.6
Oct 2011
Salinity (‰)0 10 20 30 40
B, S
r (m
g k
g-1
)
0
1
2
3
4
5
6
7
Rb
, S
e (
µg
kg
-1)
0
20
40
60
80
100
120
Oct 2011
Salinity (‰)0 5 10 15 20 25 30 35
Cr,
Pb
, T
i, M
o (
µg
kg
-1)
0
2
4
6
8
10
12
14
Nb
(µ
g k
g-1
)
0
2
4
6
8
Oct 2011
Salinity (‰)0 5 10 15 20 25 30 35
V,
As
, U
(µ
g k
g-1
)0.0
0.5
1.0
1.5
2.0
2.5
Oct 2011
Salinity (‰)0 5 10 15 20 25 30 35
Cd
, S
b, A
g (
µg
kg
-1)
0.0
0.1
0.2
0.3
0.4
0.5
0.6
Cs (
µg
kg
-1)
0.0
0.2
0.4
0.6
0.8
1.0
Figure 6.3: Dissolved trace element variability along the salinity gradient during the three sampling period in the Swarna estuary.
Chapter - 6
189
Figure 6.4: Relationship of rare earth elements (ng kg
-1) with pH and salinity (‰).
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Tm
Sal
inity
pH
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Yb
Sal
inity
pH
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Lu
Sal
inity
pH
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Eu
Sal
inity
pH
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Dy
Sal
inity
pH
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Ho
Sal
inity
pH
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Pr
Sal
inity
pH
0
50
100
150
200
250
0
10
2030
40
7.07.27.47.67.88.08.28.4
Er
Sal
inity
pH
0
50
100
150
200
250
300
0
10
2030
40
7.07.27.47.67.88.08.28.4
Nd
Sal
inity
pH
0
50
100
150
200
250
300
0
10
2030
40
7.07.27.47.67.88.08.28.4
Sm
Sal
inity
pH
Chapter - 6
190
Figure 6.5: Representation of source contributions to estuarine REEs for the sampling period.
La Ce Pr Nd Sm Eu Gd Dy Ho Er Tm Yb Lu
(Dis
so
lve
d R
EE
/ UC
C)
x 1
03
1e-5
1e-4
1e-3
1e-2
1e-1
1e+0
Estuarine Water
River water
Groundwater
Rainwater
January 2011
La Ce Pr Nd Sm Eu Gd Dy Ho Er Tm Yb Lu
(Dis
so
lved
RE
E/ U
CC
) x 1
03
1e-5
1e-4
1e-3
1e-2
1e-1
1e+0
May 2011
La Ce Pr Nd Sm Eu Gd Dy Ho Er Tm Yb Lu
(Dis
so
lve
d R
EE
/ UC
C)
x 1
03
1e-5
1e-4
1e-3
1e-2
1e-1
1e+0
October 2011
Chapter - 6
191
The cluster membership – 3 shows association of DO with DOC and elements like Be and
Gd (Fig. 6.2). Although Be and Gd are associated with DOC in the dendrogram, Be shows
a good correlation with Fe (r = -0.4, p < 0.05) and other redox sensitive metals (r = 0.55 –
0.73, p < 0.05 with As and Mo) whereas Gd shows a significant correlation (r = 0.42, p <
0.05) only with Mo. DOC in the estuary is largely dependent on the DO of water (Fig.
6.6). DOC concentration decreases with increasing DO during the post-monsoonal season.
The decreasing trend of DOC with increasing DO is found to be highly affected by the
photosynthetic activity of the phytoplankton during the pre-monsoon season. This results
in the overall low positive correlation (r = 0.43, p < 0.05) between DO and DOC of the
estuarine water for the sampling period as the photosynthetic activity leads to higher DOC
along with higher DO and pH conditions.
Figure 6.6: Relationship of DO with DOC in the Swarna estuary during the sampling
period.
DOC shows a good correlation with Fe, Mn, As and Mo (r = ± 0.46 – 0.7, p < 0.05) which
are known to be sensitive to redox processes. The relationship of DOC with Mn could be
observed by the combination of cluster membership – 3 with cluster membership – 4. As
the Mn and Fe are released to the dissolved phase from the oxy-hydroxide state under the
less oxidized condition, they tend to form metal complexes with DOC. The positive
correlation of Fe (r = 0.69) and Mn (r = 0.46) with DOC suggests that the DOC involved in
metal complexation are mainly organic colloids of smaller size (Buffle and Leppard, 1995)
which allow the colloidal organic carbon – metal complexes to remain in the dissolved
mode until they coagulate to form larger aggregates (Ran et al. 2000; Pourret et al. 2007)
DO (mg L-1
)6.0 6.5 7.0 7.5 8.0
DO
C (
mg
L-1
)
0
1
2
3
4
5
6Jan 2011
May 2011
Oct 2011 Higher photosynthetic activity
Chapter - 6
192
and settle at the bottom of the estuary. However, Fe shows a significant correlation with
temperature in the estuarine water with r = 0.74 at p < 0.01. This supports the close
clustering relationship of DOC with Mn and only at higher stage with Fe in the
dendrogram. Thus, the effect of DOC is relatively higher on Mn compared to Fe in the
Swarna estuary. DOC in the estuarine water is found to be of similar characteristics as that
of river DOC with formation of labile organic carbon - metal complexes, except the
dominance of DOC over the Mn concentration in the estuary unlike the dominance of river
DOC over the Fe concentration (Tripti et al. 2013). The difference in the behaviour of Fe
in the estuarine water would have occurred due to the flocculation property of dissolved Fe
in the mixing zone (Sholkovitz 1976; Boyle et al. 1977; Crerar et al. 1981).
As and Mo show a good negative correlation (r = -0.46 and -0.48, p < 0.05) with DOC
suggesting the removal of these elements by the organic carbon - metal complexes (Fig.
6.7). However, these redox sensitive elements show a poor correlation (r < 0.4, p < 0.05)
with DO in the Swarna estuary. DO, which is mainly controlled by temperature, could be
altered by the higher photosynthetic activity of the phytoplankton. This in turn results in
higher DO, higher pH and higher DOC under higher temperature condition. The control of
DO by temperature as well as biological activities would have resulted in less correlation
of DO with metal concentration in the estuarine water as these parameters in turn have
higher effect on the metal availability in the estuarine water. As and Mo are showing a
good association with salinity (cluster membership – 1) whereas Mn with pH (cluster
membership – 4) and Fe with temperature (cluster membership – 6) in the dendrogram.
The relationship of Mn and Fe with their controlling parameters is shown in Fig. 6.8. The
association of Mn with pH and DOC suggests the effect of photo-reduction of Mn oxides
by the DOC which could supply the dissolved Mn required for the photosynthetic activities
of the phytoplankton (Sunda et al. 1983). The higher Mn concentration associated with
higher pH, DO and DOC concentrations in the freshwater front (17.7 ‰ of salinity) of the
estuarine water during the pre-monsoon season support the photo-reduction of Mn oxides
in the Swarna estuary. Mn combines with La and at a higher stage clusters with Ce and
SPM which later shows association with pH, Ta and Si, together to form the cluster
membership - 4. Mn show good correlation with Ce (r = 0.57, p < 0.01) whereas Ce shows
good correlation with Ta and La (r = 0.8 – 0.9, p < 0.01). Mn being redox sensitive, forms
colloidal oxy-hydroxide which acts as a potential source for metal cation adsorption in the
natural environment (Sholkovitz and Copland 1982; Sigg 1985; Balistrieri et al. 1994).
Chapter - 6
193
The clustering of Mn with Ce in the dendrogram and also a positive correlation between
them indicates that Mn has a significant control over the Ce abundance under the redox
conditions in the Swarna estuary (Fig. 6.9).
Figure 6.7: Relationship of DOC with As and Mo.
Figure 6.8: Representation of the controls of temperature (°C), pH and DOC (mg L-1
) on
Fe (µg kg-1
) and Mn (µg kg-1
) concentrations.
DOC (mg L-1)0 1 2 3 4 5 6
As
(µ
g k
g-1
)
0.0
0.5
1.0
1.5
2.0
2.5
Mo
(µ
g k
g-1
)
0
2
4
6
8
10
12
As
Mo
0
20
40
60
80
100
120
26
28
30
3234
36
01
23
45
6
Fe
T
DOC
0
20
40
60
80
6.9
7.2
7.5
7.88.1
8.4
01
23
45
6
Mn
pH
DOC
Chapter - 6
194
Figure 6.9: Association of Mn with redox sensitive Ce in dissolved phase of the Swarna
estuary.
SPM shows good correlation (r = 0.4 – 0.7 at p < 0.05) with most metals (Fig. 6.10)
belonging to the cluster membership – 1 which are controlled by salinity. SPM is highly
correlated with pH (r = 0.63, p < 0.01) than salinity (r = 0.55, p < 0.01) which is
represented by the cluster membership – 4 suggesting the possible effect of pH mediated
geochemical processes on the SPM concentration in the estuarine water. In addition, the
colloidal organic carbon - metal complexes would be contributing to the SPM at a higher
rate under the alkaline condition. Si shows good correlation with pH (r = -0.57 at p < 0.01)
than salinity (r = -0.37 at p < 0.05) suggesting that the Si is being removed under alkaline
condition which could be controlled even by the biological processes occurring in the
estuarine water. The variability of pH from near neutral to alkaline condition (7.2 < pH <
8.2) in the estuarine water could significantly affect the metal abundance (Shiller and
Boyle 1985; Gaillardet et al. 2003) as it controls the metal sorption processes in natural
waters. pH shows a good correlation (r = 0.4 – 0.9 at p < 0.05) with elements belonging to
cluster membership – 1 which suggests that these metals are highly affected by pH driven
sorption reactions at different salinities in the estuarine water (Fig. 6.10). This is also
supported by the concentration variability of each metal in different seasons. Although the
concentrations of trace elements belonging to cluster membership – 1 show an increase
with salinity in all the seasons, there is a significant variability in the concentration of each
metal at different salinities during the monsoonal season. This could be attributed to the
pH mediated sorption processes which affect the concentration of these metals at different
Mn (µg kg-1)0 20 40 60 80
Ce
(n
g k
g-1
)
0
50
100
150
200
250
300
DO (mg L-1)
6.2 6.4 6.6 6.8 7.0 7.2 7.4 7.6 7.8 8.0
Mn vs Ce
DO vs Ce
Chapter - 6
195
salinities. The effect of sorption processes is found to be varying with season and is
relatively higher during the monsoonal season followed by pre-monsoon season and lesser
during the post-monsoon season. The higher effect of sorption processes on the trace
elements concentration in the estuary during the monsoonal season could be due to the
higher pH variability at the mixing zone. The pH of Swarna river water is found to be
slightly acidic to near neutral (Tripti et al. 2013). The mixing of higher amount of near
neutral freshwater with the alkaline sea water within the short distance would have resulted
in the significant effect on the trace element behaviour in the estuary during the monsoonal
season. The trace elements like Cr, Pb, Ti, Nb, V, U, Cd, Sb, Ag, Cs and Se are found to
be highly affected by such processes in the Swarna estuary.
The cluster membership – 5 shows association of trace metals like Zr, Al, Hf, Tl and Ni.
Al shows no significant correlation with Zr, Hf, Tl and Ni at p < 0.05 whereas it shows a
good positive correlation (r = 0.4 – 0.6, p < 0.05) with temperature, Fe and Mn which
suggests that Al could behave similar to Fe and Mn under redox conditions. Al oxy-
hydroxide acts as the potential particle surface for the adsorption of metals like Zr, Hf, Tl
and Ni in the estuarine water. However, Al show poor correlation (r = 0.35 at p < 0.05)
with DOC unlike Fe and Mn. The cluster membership – 5 represents the group of
lithophile elements which are more resistant to weathering and highly immobile in nature.
The elements of cluster membership – 5 further combine with elements of cluster
membership – 6 at a higher level. The cluster membership – 6 in the dendrogram
represents the association of temperature with Fe, Cu, Ba and Zn. The relationship of Fe
with temperature is also shown in Fig. 6.8. This suggests that the temperature driven redox
processes control the behaviour of these elements in the estuarine water. Fe and Ba which
are sensitive to redox conditions, acts as the metal scavenger to other trace metals like Cu
and Zn respectively in the Swarna estuary (Fig. 6.11). The association of metals like Fe,
Cu and Ba is also observed in the Swarna river water (Tripti et al. 2013). Thus, the
understanding of biogeochemical processes controlling the behaviour of trace elements in
the estuarine water needs a careful assessment of each driving parameter whereas the
linear relationship of metals with single controlling parameters would not be sufficient to
explain the biogeochemistry of estuarine water.
Chapter - 6
196
Figure 6.10: Relationship of pH, SPM (mg L
-1) and trace elements (µg kg
-1) in the dissolved phase of the Swarna estuary.
pH7.0 7.2 7.4 7.6 7.8 8.0 8.2 8.4
SP
M (
mg
L-1
)
4
6
8
10
12
14
16
18
20
22Jan 2011
May 2011
Oct 2011
0
2
4
6
8
10
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Cr
SPM
pH
0
2
4
6
8
10
12
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Pb
SPM
pH
0
1
2
3
4
5
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Ti
SPM
pH
0
2
4
6
8
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Nb
SPM
pH
0.0
0.5
1.0
1.5
2.0
2.5
3.0
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
V
SPM
pH
0.0
0.5
1.0
1.5
2.0
2.5
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
U
SPM
pH
0.0
0.2
0.4
0.6
0.8
1.0
1.2
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Sb
SPM
pH
0.0
0.2
0.4
0.6
0.8
1.0
1.2
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Cs
SPM
pH
0.00
0.05
0.10
0.15
0.20
0.25
0.30
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Cd
SPM
pH
0.000.020.040.060.080.100.12
0.14
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Ag
SPM
pH
020406080
100120140160
48
1216
2024
7.07.
27.47.
67.88.
08.28.
4
Se
SPM
pH
Chapter - 6
197
Figure 6.11: Relationship of Fe with Cu (1a – Data range and 1b - Zoom-in view at low Fe
values) and Ba with Zn (2a – Data range and 2b - Zoom-in view at low Ba values).
6.2.2.2. Dissolved strontium and its isotope ratio
The Sr concentration and its isotopic ratio measured in the dissolved phase of Swarna
estuary are given in Table 6.3 and Fig. 6.12. It is observed that the dissolved Sr behaves
conservatively along the salinity gradient in the Swarna estuary (Fig. 6.12.a). The
dissolved Sr concentration increases linearly with salinity during the three sampling
periods. The concentrations of Sr are almost similar for the same salinity during the three
sampling seasons. A slight decrease in concentration at the freshwater front of the estuary
is observed during the monsoonal sampling. The decrease in dissolved Sr concentration
could be due to the dilution effect resulting from the higher river water discharge during
the monsoonal season. It is found that the radiogenic Sr isotope ratio gradually decreases
as the water moves from fresh water end member to sea water end member (Fig. 6.12.b).
For the monsoonal season, the decreasing rate is higher at the initial mixing of fresh water
with the saline water between 0.5 and 3 ‰ of salinity; thereafter the isotopic ratio
decreases slightly with a gentle linear slope (r2 = 0.84) towards the sea water end member.
Fe (µg kg-1)
0 50 100 150 200 250 300
Cu
(µ
g k
g-1
)
0.3
0.7
1.1
1.5
1.9
2.3Jan 2011
May 2011
Oct 2011
(1a)
Fe (µg kg-1)
0 10 20 30 40 50 60 70
Cu
(µ
g k
g-1
)
0.3
0.7
1.1
1.5
1.9
2.3
(1b)
Ba (µg kg-1
)
0 20 40 60 80 100 120 140
Zn
(µ
g k
g-1
)
0
2
4
6
8
10
12
14
16(2a)
Ba (µg kg-1
)
0 5 10 15 20 25
Zn
(µ
g k
g-1
)
0
2
4
6
8
10
12
14
16
(2b)
Chapter - 6
199
Salinity (‰)0 10 20 30 40
Sr
(µm
ol k
g-1
)
0
20
40
60
80
100Jan 2011
May 2011
Oct 2011
Salinity (‰)0 5 10 15 20 25 30 35
87
Sr/
86
Sr
0.7090
0.7092
0.7094
0.7096
0.7106
0.7108
0.7110
May 2011
Oct 2011(a) (b)
Figure 6.12: Strontium concentration (a) and its isotopic ratio (b) in the dissolved phase
along the salinity gradient for the sampling period.
1/Sr (kg µmol-1
)
0.0 0.1 5.0 6.0
87S
r/8
6S
r
0.7090
0.7092
0.7094
0.7096
0.7106
0.7108
0.7110May 2011
Oct 2011
Figure 6.13: Isotope systematics of dissolved strontium in the Swarna estuary during the
study period.
6.2.3. Sediment geochemistry of the Swarna estuary
The major elements, trace elements and rare earth elements measured in suspended
particulate matter (SPM) and bed sediments of the Swarna estuary are given in Table 6.4.
The concentrations measured for these elements in the estuarine bed sediment and SPM
supports the explanation on dissolved trace elements in the Swarna estuary for the study
period.
Chapter - 6
202
the bed sediments, unlike the dissolved phase of the Swarna estuary for the study period.
Strontium in the bed sediment samples behaves distinctly in different seasons. The Sr
concentration decreases with increasing salinity during the pre-monsoonal season whereas
it tends to increase along the salinity gradient during the monsoonal season. This increase
in Sr concentration could be due to the addition of Sr which first occurs at a lesser rate
between 0.4 and 13.5 ‰ of salinity and then a significant addition between 17 and 30 ‰ of
salinity during the monsoonal sampling (Fig. 6.14). The addition of Sr to the bed
sediments has not affected the conservative Sr in the dissolved phase. This suggests that
the additional Sr to the bed sediments would have been contributed by the SPM or the less
weathered sediments are being transported to greater distance during the monsoonal
season. The latter effect could be less as the bed sediments exhibit lesser radiogenic Sr
isotope ratio with increasing salinity and thus, indicates the SPM contribution. This could
be the reason for the variability of SPM concentration measured at these salinities during
the monsoonal season. The SPM of monsoonal season measured for the sea water front (at
30 ‰ of salinity) exhibit relatively lower Sr concentration (Fig. 6.14.a) and relatively
higher radiogenic Sr isotope ratio (Fig. 6.14.b and Fig. 6.15) compared to the sea water
(87
Sr/86
Sr = 0.7091) suggesting that the estuarine SPM in the sea water front is still of
terrestrial origin, and that the terrigenous SPM is being transported to the ocean. A gradual
decrease in Sr concentration of the bed sediment with increasing salinity is observed
between 17 – 34 ‰ during the non-monsoonal season which follow the similar pattern of
monsoonal sampling between 13 – 17 ‰ of salinity. This could be due to the removal of
Sr from the estuarine bottom sediments during the high tides of pre-monsoon season when
there is less fresh water input to the sea; thus leading to higher SPM concentration at the
saline water end member. Hence, the Sr concentration and its isotopic ratio in SPM of the
Swarna estuary suggests that the increase in the SPM concentration along the higher
salinity gradient in different seasons could be due to the resuspension of particulates from
the bottom sediments, and the marine particulate input is negligible. The 87
Sr/86
Sr
measured in bed sediments and SPM of the Swarna estuary explains that the terrestrial
materials brought by the river are being transported through the estuarine region towards
the ocean and the marine influx is less.
Chapter - 6
204
1/Sr (kg mmol-1)
0.0 0.5 1.0 1.5 2.0 2.5
87
Sr/
86
Sr
0.70
0.72
0.74
0.76
0.78May 2011 - BS
Oct 2011 - BS
Oct 2011 - SPM
Figure 6.15: Isotope systematics of strontium in bed sediments (BS) and SPM of the
Swarna estuary during the study period.
6.2.3.2. Elemental distribution between dissolved and particulate phases
The partitioning of trace elements between the dissolved and particulate phases results in
the redistribution of trace elements in the aquatic system. The factors controlling the
partitioning of trace elements between surface water and sediments in the Swarna estuary
mainly include pH, redox condition, organic content, temperature and biological activities.
The distribution coefficient for the chemical species is calculated as follows (Valenta et al.
1986): Kd = CSPM/CDP, where CSPM is the concentration (µg kg-1
) of trace elements in the
suspended particulate matter and CDP is the concentration (µg kg-1
) of trace elements in the
dissolved phase. As the trace elements are measured only in one SPM sample of 30 ‰
salinity (sea water front) collected during the monsoonal season, the Kd is measured for
this sample. The sample of sea water front in the estuary shows different Kd values for
different elements which could be classified into seven groups (Table 6.6). The Kd values
suggest that the SPM is dominated by colloidal Fe oxy-hydroxides followed by Al and Mn
oxy-hydroxides or the colloidal metal-organic carbon complexes in the sea water end
member of the Swarna estuary. The ratio of chemical species between bed sediment and
SPM in the sea water end member shows that the trace elements like Zn, Te and Mn are
relatively enriched in SPM than bed sediments whereas Sn, Cu, Tl, As, Pb, Cs, B and Mo
in SPM have almost the bed sediment composition. The depletion in SPM relative to bed
Chapter - 6
205
sediments are found to be 50 % for elements like P, Ni, Sb, Fe, Mg, Ga, Al, Cd, Rb, Co,
W, Tm, V, Er, Dy, Ho, Yb, Eu, Gd and Be and 70 % for Lu, Ti, Sm, Nd, Pr, Cr, Na, La,
Zr, Th, K and U. The bed sediments are found to be three times enriched with Nb, Sr, Ce,
Hf, Ba, Ag, Ta and Ca relative to its suspended particulate phase. The dominance of
colloidal metal oxy-hydroxides and metal-organic carbon complexes support the relatively
higher SPM concentration measured at the sea water front than the fresh water front during
its transport in the Swarna estuary for the study period.
Table 6.6: Distribution coefficient (Kd) for trace elements and REEs at the sea water front
in the Swarna estuary.
Group Ratio Elements
1 0.1 < Kd < 10 Se, Mg, B
2 10 < Kd < 102 Sr, K, Ca, Mo
3 102 < Kd < 10
3 Rb, Csd, Tl, Te, W, U, Sb
4 103 < Kd < 10
4
Pb, Tm, Cs, P, Ta, Lu, Nb, Ag, Be, Ho, Eu, As, Yb, Er,
Cr, Ba, Hf
5 104 < Kd < 10
5 Gd, Ga, Pr, Sm, Zn, Dy, Co, Nd, Cu, V, Ni, La
6 105 < Kd < 10
6 Ce, Zr, Mn, Ti
7 106 < Kd > 10
7 Al, Fe
6.3. CONCLUSIONS
The study shows that most of the elements in the estuarine water maintain similar
association with other metals as that of river water in the Swarna basin. This include: i) Pb
and V which are discharge driven elements in the river water are controlled by the mixing
of freshwater with sea water (salinity) in the estuary, ii) dominance of smaller size
colloidal fractions of DOC both in river and estuary, iii) formation of colloidal organic
carbon - metal complexes, particularly with Mn and iv) control of oxy-hydroxides of Fe
and Ba over the heavy metals like Cu and Zn under redox conditions in the river and
estuarine waters. The difference in the inter-metal association in the estuarine water and
river water would have occurred mainly due to the pH variability. The near neutral to
alkaline conditions and the relatively higher variability of pH with space in the estuary
compared to that of the river water would have resulted in the adsorption/ desorption of
Chapter - 6
206
trace elements in the estuarine system. The coastal groundwater discharge forms the main
source of trace elements and REEs in the estuarine water, which is at a higher rate
particularly during the pre-monsoon season.
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