Indian Journal of Marine Sciences
Vol. 30, June 2001, pp. 75-80
Sediment characteristics of Poonthura estuary (southwest coast of India)
in relation to pollution
K S Anila Kumary
School of Applied Life Sciences, Mahatma Gandhi University Regional Centre,
Pathanamthitta 689 645, Kerala, India
and
P K Abdul Azis & P Natarajan
Department of Aquatic Biology & Fisheries, University of Kerala, Beach P.O., Thiruvananthapuram 695 007, Kerala, India
Received 17 August 2000, revised 27 January 2001
Texture, redox potential and organic carbon of sediments in Poonthura estuary, Thiruvananthapuram, presently exposed
to sewage pollution were evaluated over a period of one year. Textural study revealed the presence of highest percentage of
finer particles at the sheltered upstream stations. Oxidation-reduction potential varied between −211 and +156. Strong
negative potentials recorded from the most polluted stations while freshwater and oceanic stations always recorded positive
values. Organic carbon widely varied between 2.4 to 83.3 mg/g, and showed enrichment always at the polluted zone.
Organic carbon content was more than the prescribed limit for unpolluted estuaries at some stations and reflects the level of
pollution in the estuary. Significant variation in organic carbon and redox potential during the monsoon season shows that
better dilution can diminish the high organic load to a great extent.
Sediments are indicators of the quality of overlying
water and its study is a useful tool in the assessment
of the status of environmental pollution. The level of
organic carbon in sediments is reported to be a
reliable index of nutrient regeneration and the
productivity of a water body. Redox potential is a
readily obtained description of organic matter within
the sediment and of its oxidizing or reducing power.
Oxidation-reduction activities at the sediment-water
interface bear marked influence upon the estuarine
chemistry and upon the type of organisms present in
the sediment. Although the components and
composition of sediments have received much
attention, most of the work in India relates to the
texture and organic contents of marine sediments1-5
and not much information is available on the estuarine
material6-8.
Poonthura estuary and the lower reaches of
Karamana river (8° 25’-8° 30’ N latitude and 76° 55’77° 00’ E longitude) on the south-west coast of India
(Fig. 1) near to Thiruvananthapuram city, is facing
enormous load of untreated domestic wastes
emanating
from
the
fastly
growing
Thiruvananthapuram city over the last few years. The
city sewage farm is located at Muttathara on the bank
of Parvathy-Puthanar canal which joins the estuary
about 2.4 km down to Muttathara and a huge quantity
of untreated sewage from the disposing area leaches
into the estuary through the canal. The seriousness of
this waste disposal problem has been realized lately
but no steps have been taken to study the effect of
Fig. 1—Map of the Poonthura estuary, with study sites (St. I Karimbuvila, II - Moonnattumukku, III - Edayar, IV - Panathura,
V - Poonthura)
INDIAN J. MAR. SCI., VOL. 30, JUNE 2001
76
pollution except for the fragmentary work of Anila
Kumary & Abdul Azis9. The present paper deals with
the sediment characteristics such as texture, redox
potential and organic carbon of the sediments in
Poonthura estuary and discusses the extent of
pollution in the estuary.
Materials and Methods
Five stations, almost equidistant from one another
which representing different ecological conditions,
were selected in the estuary for regular monitoring
(Fig. 1). Station I (Krimbuvila) representing a riverine
station is comparatively free from sewage pollution
and is exposed to freshwater influx. Station II
(Moonnattumukku) is located in the western segment
of the estuary where the sewage drains open directly
into the estuary and is heavily polluted. Station III
(Karimbuvila) represents the interior segment of the
estuary where the sewage is comparatively diluted.
Station IV (Panathura) lies in the estuary near the
estuary mouth into the Arabian Sea and station V is
a typical beach station located outside the estuary
mouth. The depth of these stations varied
from 0.5 to 3.5 m.
Sediment samples (triplicate) were collected from
all these stations at monthly intervals for a period of
one year (February 1992-January 1993) using a metal
corer (5.5 cm internal diameter and 45 cm long).
These samples were transferred separately to clean,
dry polythene containers, homogenized well and were
brought to the laboratory for analysis. Readings of
redox potential were taken as soon as the samples
were brought to the laboratory using inert platinum
electrode connected top pH-Eh meter (Pearson &
Stanley)10. A portion of each of these samples was
dried at 60-70°C and pulverised to estimate the
organic carbon11. The oven-dried samples were
subjected to textural study by sieving and pipette
method following Krumbein & Pettijohn12.
Results and Discussion
The textural study revealed that, silt and clay were
the predominant fractions at stations I and II, whereas
the sand fraction was predominant at stations III, IV
and V (Table 1). The percentage of fine fraction was
maximum (65%) at station II. Further, the sand
fraction shows an increasing trend and the fine
fraction shows decreasing trend from stations III to V.
In estuaries generally fine mud get deposited near the
head and other sheltered regions, while along the
main channel and at the mouth coarser sediments are
deposited. Weak tidal currents in the upstream and the
heavy land drainage could be the reason for the
dominance of silt at station I. However, the mixing up
Table 1—Monthly variation in percentage eight of sediments grain size composition at different stations
St no. Sediment
type
Feb
Mar
Apr
May
June
July
Aug
Sept
Oct
Nov
Dec
Jan
I
Sand
Silt
Clay
29.25
65.46
5.29
32.50
61.35
6.15
41.21
56.85
1.94
27.85
64.75
7.40
51.55
39.28
9.17
42.35
46.83
10.82
22.30
76.52
1.18
31.65
62.25
6.10
28.73
55.25
16.02
36.78
60.95
2.27
52.25
44.72
3.03
46.85
45.85
7.30
II
Sand
Silt
Clay
22.10
70.67
7.23
38.65
42.88
18.47
26.38
38.31
35.31
32.40
29.55
38.05
41.25
52.35
6.40
36.12
58.75
5.13
38.15
55.29
6.56
37.85
40.55
21.60
35.28
52.25
12.47
39.85
50.95
9.20
38.13
48.64
13.23
33.58
50.38
16.04
III
Sand
Silt
Clay
48.50
41.95
9.55
56.78
40.65
2.57
42.15
34.83
23.02
57.54
31.87
10.59
49.88
47.74
2.38
79.37
18.55
2.08
58.45
38.15
3.40
60.15
27.43
12.42
52.83
44.54
2.63
48.15
44.94
6.91
59.35
39.18
1.47
44.18
46.30
9.52
IV
Sand
Silt
Clay
63.35
30.63
6.02
55.95
40.35
3.70
59.29
34.24
6.47
74.75
24.25
1.00
93.25
6.75
0.00
100
0.00
0.00
81.58
10.20
8.22
80.15
17.45
2.40
62.75
36.85
0.40
76.52
23.20
0.28
53.43
40.58
5.99
41.75
53.25
5.00
V
Sand
Silt
Clay
72.00
10.80
17.20
84.15
12.35
3.50
59.90
28.50
11.60
95.00
5.00
0.00
100
0.00
0.00
100
0.00
0.00
95.00
3.50
1.50
90.80
7.55
1.65
100
0.00
0.00
96.00
2.80
1.20
100
0.00
0.00
90.50
2.50
7.00
ANILA KUMARY et al.: SEDIMENT CHARACTERISTICS OF POONTHURA ESTUARY
of great amount of sewage and the low energy
condition allowing the settling of fine particles
favored the enrichment of silt at station II.
Oxidation-reduction potential (Eh) of sediments is
a convenient index as to whether or not the sediment
is reducing in reaction. A positive Eh reading result
from a state tending towards oxidation, and a negative
Eh indicates a system causing reduction. The
predominance of positive values at stations I, IV and
V, and negative values at stations II and III (Fig. 2),
suggests that the marine and riverine sediments are
less reduced than interior estuarine sediments. The
monthly values varied between −211 (station II) and
+156 mv (station V). Table 2 also shows that most
reduced sediments were present at station II and most
oxidised conditions at station V. Variations between
stations were significant at 1% level (ANOVA:
DF = 4, F = 26.6201).
77
Redox potential on one hand is related with the
distribution of oxygen and on the other hand, with the
cycles of most of the important elements occurring in
water. Hence the redox state in a given place is
influenced to a great extent by hydrodynamical and
sedimentological conditions13. Riverine zone was
characterised by strong water flow and high
concentration of dissolved oxygen in the overlying
water, which could lead to the strongly oxygenated
conditions. The oxygenated conditions at the marine
zone can be attributed to the highly porous medium of
coarse sediment, which made a high oxygen diffusion
rate possible. The rearrangement of sediment by
strong hydrodynamic action could also be
contributing to healthier sediment at the marine
zone14.
A distinct seasonal influence was discernible on the
variation in the oxidation-reduction potential. All the
Fig. 2—Monthly variations of redox potential at the 5 stations
INDIAN J. MAR. SCI., VOL. 30, JUNE 2001
78
stations showed least values of the potential during
the premonsoon period (February-May) and the
highest values during the monsoon period (JuneSeptember) at stations I, II and III and during the
postmonsoon period (October-January) at stations IV
and V (Table 2). Statistical analysis also proves this
by showing significant variation between seasons
(ANOVA: DF = 2, F = 9.5159) and period within
seasons (ANOVA: DF = 11, F = 3.9417). Stations II
and III were characterised by reduced sediments,
especially during the premonsoon period. Degradation
of the natural environment due to discharge of sewage
waste is the noticeable aspect at these stations. During
summer the fresh water influx was at its minimum
and hence the chances of dilution of wastes in the
water column was also less. The most surprising
situation was observed at station II which being
located near the entrance of sewage fed water.
Anaerobic condition with high hydrogen sulphide in
the overlying water was reported from this station9.
The presence of large quantities of hydrogen sulphide
as well as the observed Eh value gives indirect
evidence of sulphide reduction and consequently of
bacterial process occurring within the sediment13.
Aiyer & Rajendran15 also have reported very low
values of Eh when high concentration of hydrogen
sulphide would be produced with total depletion of
oxygen.
The organic carbon values of the sediments were
higher at station II compared to other stations (Fig. 3).
At station I, it varied from 8.4 to 26.8 mg/g in
December to March; at station II from 13.4 to 84.3
mg/g in December to February; at station III, from 6.7
to 37.5 mg/g in November to February; at station IV,
from 7.8 to 30.8 mg/g in November to August and at
station V, from 2.5 to 15.4 mg/g in January to April.
Variations in organic carbon between stations were
significant at 1% level (ANOVA: DF = 4, F=
19.8005). Annual mean values were minimum at
stations I and V and maximum at the polluted
Table 2—Seasonal (n=4) and annual (n=12)mean valus of sediment Eh at different stations
Premonsoon
Monsoon
Postmonsoon
Annual
Station I
Station II
Station III
Station IV
Station V
62.50± 6.96
73.75± 7.89
69.75± 13.76
68.67± 5.18
-136.75± 23.07
20.50± 20.12
-66.25± 57.80
-60.83± 26.48
-80.25± 11.04
-22.25± 20.41
-49.75± 50.49
-50.75± 17.44
73.25 15.42
78.25± 8.38
84.25± 34.34
78.58± 11.21
110.00± 17.42
130.25± 11.25
116.00± 28.01
118.75± 10.36
Fig. 3—Monthly variation of organic carbon at the 5 stations
ANILA KUMARY et al.: SEDIMENT CHARACTERISTICS OF POONTHURA ESTUARY
79
Table 3—Seasonal (n=4) and annual (n=12) mean values of sediment organic carbon(mg/g) at different stations
Premonsoon
Monsoon
Postmonsoon
Annual
Station I
Station II
Station III
Station IV
Station V
18.60± 3.3
12.90± 1.7
12.80± 2.5
14.80± 1.5
62.40± 8.8
35.80± 3.2
20.10± 3.3
39.40± 5.8
28.6± 4.8
16.3± 2.6
9.5± 1.2
18.1± 2.8
24.1± 2.1
17.5± 4.6
9.6± 0.9
17.1± 2.3
7.30± 2.8
7.3± 1.5
5.60± 1.3
6.1± 1.0
estuarine stations (Table 3). The upstream fresh water
station is mainly characterised by weak tidal currents
and high oxygen. These factors are detrimental to the
ample supply and accumulation of organic carbon in
sediments16. The low organic carbon content in the
beach (station V) can be ascribed to the sandy nature
of the substratum and strong tidal currents, which
continually drain out available suspended organic
matter to the sea. Association of organic matter with
fine-grained sediment is well established4,8,17-19. Finer
particles may provide increased surface area per unit
weight for absorption of organic matter. The effect of
land drainage and the possible organic supply from
tributaries in the upstream region accounts for the
comparatively higher percentage at the upstream
region when compared to the beach station. The high
values of stations II, III and IV can be ascribed to the
heavy sewage discharges and the absorption of
organic matter by the increased finer fractions of the
sediment. Similar input of wastes and enrichment of
organic matter was earlier recorded from the Mandovi
estuary16 and from the Cochin harbour20.
Changes in organic carbon content of sediment
with respect to seasons were observed in all the five
stations (Table 3). Organic carbon was highest during
the premonsoon period at all the stations whereas the
seasonal minimum at all the stations was during the
postmonsoon period. Variations in the organic carbon
between seasons were statistically significant at 1%
level
(ANOVA:
DF=2,
F=23.7213).
The
postmonsoon period followed by monsoon was also
marked with heavy rain and river discharge and the
estuary was heavily flooded and the conditions were
not favorable for the accumulation of organic matter.
The low rainfall, reduced river discharge and the
closure of the mouth of the estuary during the
premonsoon resulted in the development of a stagnant
condition in the estuary leading to a sharp rise in the
organic carbon content at all stations.
Mixing of sewage is one of the principal sources of
organic pollution in estuaries. In most unpolluted
estuaries organic carbon content of the bottom
sediment is < 5% whereas in areas where organic
pollutants are high16, organic carbon level often
exceeds 5%. Organic carbon in sediments in the
present study exceeded this proposed limit during
summer months at station II with strong negative Eh
values. The accumulation of organic compound in the
form of sewage contributed to the high percentage of
organic carbon at station II where the sewage drains
open into the estuary whereas the dilution of sewage
in the downstream stations of the estuary is
responsible for the gradual decrease in organic carbon
content at stations III and IV.
The organic matter in the Poonthura estuary is
higher than the reported values from some other
estuaries and coastal waters of Kerala6,7,17. Apart from
the autonomous contribution sewage drain into the
estuary contributes to the greater enhancement of
organic matter. The low level of tidal mixing and
seasonal monsoon dilution at the polluted stations are
not adequate to enable self-purifying mechanisms to
operate successfully. However, the horizontal gradient
in organic load noted indicates that better dilution can
reduce the high organic load to a considerable extent.
Acknowledgement
The first author is grateful to University of Kerala
for the award of a Research Fellowship during the
period of work.
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