Download Computation of Dilution Discharge and Mean Concentration of

lJEP ]0 ( 5 ) : 333 - 335
Computation of Dilution Discharge and Mean Concentration of
Effluents within a Tidal Estuary
P. K. Dinesh Kumar
Nationalln5lituu of OceanogrJphy, Regional Centre, Verso va , Bombay - 400 061
Tide dominated coastal plaiu Ambo rinr e.tuary alone the ...estcoaat of Iadia " . . .tudied dnring July - Octoher
1986 to e.timatc Ibe mean flo... avail.ble for dilnting the cou.enalin effluents from a ptlrochemic.ll complex,
based on observed .alinity at the proposed discharge point and Bomhay Harbour and the tributary illfln" data. The
nerage concentration of the toxic material in the ,icinity of the
di~ch.rle
point is estimated for ao assumed dis-
charge rare of the material.
INTRODUCTION
GENERAL DESCRIPTION OF THE AREA
A commonly posed problem in estuarine analysis is
that of lhe effective disposal of effluents. A given load
of effiuent i, to be discharged at a given point and a
prediction is needed for the concentration at other
Amba river (Figure I ) which is narrow, broadens into a wide e,tnary with its width exceeding 260 m in the
points up and downstream. A common method availa·
ble is to use the one· dimensional analysis and to use
the distribution of ambient ..linity as a guide ( Fisher,
1979). Amba River which originates in the Western
Gh.ts follows a narrow and meandering course along
its length of over 140 km before opening into the Bom·
bay Harbour. An extensive environmental survey was
carried out in these waters in 1986 in connection with
the wastewater disposal from a petrochemicals complex
(NIO Report, 1987). Although at present the estuary
does not receive any pollutant through direct disch.rges, the .'tuary which is predominantly tide domi-
nant during non· monsoon months opens into Bombay Harbour. Its water quality is expected to be largely influenced by the Harbour water which receive.
an estimated 4 x 10 5 mS/day of domestic wastewater
and over 2 x 10' m"/day of industrial effluent, ( Zingde, 1989). In addition, Patalganga river which receives a variety of pollutants through industrial inputs
di,cbarges in the mouth region of Amba river and can
also influence the water quality.
The present study illustrates the e'timation of the mean
flow available for dilnting the effluent in the vicinity of
the propo,ed effluent discharge point near Mankule
Bunder by one· dimensional analysis. Although the
WlSte from the petrochemicals complex will be well treated before release into the estuary, an attempt i' made
'. to estimate the average concentration of the toxic material for an a"umed discharge rate.
mouth region during spring tides. Thi' width decrea,es
to about 65 m at 40 km inland. The estuary is shallow
with the channel depth generally less than 5 m during
low tides. The lower reaches of the river often referred to as Dharamtar creek, is navigable upto Ohara·
mtar jetty for medinm sized crafts under all tidal conditions, A Konkan type Bandhara constructed across
the river at Nagothana ( about 50 km upstream from
the month) impounds the riverine flow. Though the
region downstream of Bandhara is known to experience significant seawater incursion during dry season,
no information is available about the hydrography.
MATERIAL AND METHOD
Amba river is considered long enough, narrow enough, and sufficiently unstratified to he analyzed as
thoullh it is one - dimensional. For the sake of visualization it is assumed that the ,alinity at any point in
the estnary results from the mixing of two flow" the
freshwater tribntary flow at the upstream and a flow
of ocean water which circulates in and out. The oceanic water as it moves upstream from the mouth of
the estuary is progressively diluted, but for the purpose of the pre,ent analy,is, a flow of pure ocean
water coming from the effluent discharge point, mixing
with the effluent, and tributary discharges and returninll to the ocean i' assumed. Let Qo be the circulatinll
flow of the ocean water, Q, be the tributary diseharge
from all tribntaries upstream of the effluent discharge
point, Q. be the efllnent Dow. The salt balance in the
estuary requires that:
JNOIAN J. ENVIRONMENTAL PROTECTION, VOL. 10, NO.5, MAY 1990
© 1990 •
Xalp. .a Corporati. .
333
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r,
,
,
I
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i
f
,.,
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-""=
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RESULT AND DISCUSSION
The tides experienced in the region were generally mi·
xed semi ~ diurnal type with two high and low waters
occuring each tidal day with varying tidal amplitudes.
Table I gives the differences in high water levels along
the estuary as compared to the predicted tide at AP9110
Bander (NIO Report. 1987). The one - dimensional
,.nalysis for the dilution discharge of the Amba river
(:stuary during July to October 19'1'\6 is gilr'en in table
2. The estuary is consider.d long .nough, narrow enough and sufficiently well mixed vertically so that it can
be analysed as though it is one - dimensional. The
flow velocity, salinity and concentration of any dissolved substance are assumed to depend only on the distance (rom the estuarine mouth. The effiuent is considered as conservative,
Table 1. AYerage dlfferencBJ between the high water
height ( m ) and time of occurrence ( min) at
.ariou. stations as compartd to Apollo Bondar
Figure 1. Location map
Location
Spring
Rewas
Mankule
Dharamtar
Khar Jui
Gandhe
Bendse
Nagothane
-0.03
-0,11
0.09
-0.33
-1.08
-1.39
1.76
Neep
Q.S,=( Q.+Q.+Q, ) S.
where So is the ocean salinity solving for QQ gives
Q.=( Q.+Q, ) SIS, - S
The total flow available for diluting the effluent is :
Q.=Q.+Q,+Q, = ( Q.;Q'd
,
s.
The mean concentration of effluent near the point of
discharge can be estimated from:
- 2 to - 3
- 2 to+3
+5
+15
-1-45
+60
+65 to 70
'Add these values to Apolio Bandar hiah water to get
the approximate tidal heights and time of occurrence
of high water at respeclive locetion
M
C.~­
Q.
where, M is the discharge ratc of material in units of
mals per unit time.
Two stations, namely the proposed effluent discharge
point near Mankule Bunder and Bombay Harbour
( eastern region) are selected for the present study
( Figure 1 ). The effluent discharge point is situated
about 5 km from the estuary mouth. Spring tide obIcrvations of salinity of surface. middle depth and I m
above the sediment level were made usin. a Hydrolab
during July to October 1986 over a complete tidal
cycle. The river flow data was obtained from the gauging station near Nagothana. Wastewater discharge
( Q.) during peak wet - weather is estimated at 0.21
m 3 /sec and the flow is expected to be around 0.17 m S /
sec durin, dry weather ( NIO Report. 1987). TIle discharge rate of the effluent material (M) is assumed
as 10 ppt.
334
-0.14
-0.01
-0.02
0.21
0.45
0.32
0.73
Avg. time diffe·
renee-
Table 2. Dilution discharge and mean concentration of
effluent near the discharge point
Date
23.7.86
22.8.86
19.9.86
4.10.86
3
Q", m l Q" rn'l SO' % S, % Q",m', Cd·'
sec
sec
0.21
O.~ I
0.17
0.17
121.8
59.7
12.3
5.6
sec
21.4
34.9
35. J
36.7
6.7
11.7
27.5
29.4
177.62
90.12
56.43
2901
ppt
0.01
0.Q2
0.03
0,06
'Assumed discharge rate of the material (M)=IO ppl
( for Q .. Q" s.. S, Q" and C" refer text)
The average salinity at both the stations shows an In'
creasing trend from July to October. In July, the average Bombay Harbour salinity i. observed as 21.4 x
10-' and the corresponding salinity at the discharge
INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 10. NO.5, MAY 1990
©
1990 - Kalpana Corporation
poiot is 6.7 x 10-'. In October, the Bombay Harbour
..linity was 36.7 x 10-' and at the discharge point it
is 29.4 x 10-'. From the river flow data obtained from
the gauging station near Nagothana, it is evident that
the tributary discharge ( Q, ) is considerably low during September and October and this period is considered as dry weather aod the wastewater discharge
(Q.) during this period is taken as 0.17 m"/sec. Total
flow available for diluting the effluent ( Q. ) is maximum during July (177.62 m"/sec ) and minimum during October (29.01 m'/sec). The corresponding mean
concentrations of effluent near the discharge point is
0.01 and 006 ppt. However, it should be recalled that
only the average concentrations of the effluents are
computed here. Peak concentrations in the effiuent
plume near the discharge point may be higher. Allowance is made in the analysis form the higher concentrations expected near the source before cross - seelio-
nal mixing takes place.
ACKNOWLEDGEMENT
The authors wish to express their deep gratitude to
Dr B.N. Desai, Director, NIO for keen interest and
providing facilities. Sincere thanks arc due to Dr.
M.D. Zingde, Scientist - in - charge, Re, NIO, Bombay, for guidance and constant inspiration.
REFERENCE
Fischer. H.B. 1979. Mixing in inland and coastal
waters. Academic Press, N.Y. pp 266 - 268.
NIO Report. 1987. Hydrographic studies in Amba
rjver estuary for wastewater disposal from a petrochemicals complex ( Part I ). National Institute of
Oceanography.
Zingde, M.D. 1989. Environmental status of the coastal marine environment of India. 10 Managemont
of aquatic ecosystems. pp 37 - 57.
( Received OD JDDe 5. 1990 )
INDIAN J. ENVIRONMENTAL PROTECTION. YOLo 10, NO. S. MAY 1990
©
1990 - Jl:alpaDo Cor,oratioo
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