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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 '=±o'~="=," "'I , r, , , I . t ! i f ,., -~{ ~,/l, '\ -""= .J '; 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 3JS