Computation of Dilution Discharge and Mean Concentration of Effluents Within a Tidal Estuary

lJEP ]0 ( 5 ) : 333 - 335

Computation of Dilution Discharge and Mean Concentration of Effluents within a Tidal Estuary

P. K. Dinesh Kumar Nationalln5lituu ofOceanogrJphy, Regional Centre, Versova , 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 Amba river (Figure I) which is narrow, broadens in that of lhe effective disposal of effluents. A given load to a wide e,tnary with its width exceeding 260 m in the of effiuent i, to be discharged at a given point and a mouth region during spring tides. Thi' width decrea,es prediction is needed for the concentration at other to about 65 m at 40 km inland. The estuary is shallow points up and downstream. A common method availa· with the channel depth generally less than 5 m during ble is to use the one· dimensional analysis and to use low tides. The lower reaches of the river often refe the distribution of ambient ..linity as a guide ( Fisher, rred to as Dharamtar creek, is navigable upto Ohara· 1979). Amba River which originates in the Western mtar jetty for medinm sized crafts under all tidal con Gh.ts follows a narrow and meandering course along ditions, A Konkan type Bandhara constructed across its length of over 140 km before opening into the Bom· the river at Nagothana ( about 50 km upstream from bay Harbour. An extensive environmental survey was the month) impounds the riverine flow. Though the carried out in these waters in 1986 in connection with region downstream of Bandhara is known to experi the wastewater disposal from a petrochemicals complex ence significant seawater incursion during dry season, (NIO Report, 1987). Although at present the estuary no information is available about the hydrography. does not receive any pollutant through direct disch .rges, the .'tuary which is predominantly tide domi MATERIAL AND METHOD nant during non· monsoon months opens into Bom bay Harbour. Its water quality is expected to be lar Amba river is considered long enough, narrow eno gely influenced by the Harbour water which receive. ugh, and sufficiently unstratified to he analyzed as an estimated 4 x 10 5 mS/day of domestic wastewater thoullh it is one - dimensional. For the sake of visua and over 2 x 10' m"/day of industrial effluent, ( Zin lization it is assumed that the ,alinity at any point in gde, 1989). In addition, Patalganga river which rece the estnary results from the mixing of two flow" the ives a variety of pollutants through industrial inputs freshwater tribntary flow at the upstream and a flow di,cbarges in the mouth region of Amba river and can of ocean water which circulates in and out. The oce also influence the water quality. anic water as it moves upstream from the mouth of the estuary is progressively diluted, but for the pur The present study illustrates the e'timation of the mean pose of the pre,ent analy,is, a flow of pure ocean flow available for dilnting the effluent in the vicinity of water coming from the effluent discharge point, mixing the propo,ed effluent discharge point near Mankule with the effluent, and tributary discharges and return Bunder by one· dimensional analysis. Although the inll to the ocean i' assumed. Let Qo be the circulatinll WlSte from the petrochemicals complex will be well trea flow of the ocean water, Q, be the tributary diseharge '. ted before release into the estuary, an attempt i' made from all tribntaries upstream of the effluent discharge to estimate the average concentration of the toxic mate- point, Q. be the efllnent Dow. The salt balance in the rial for an a"umed discharge rate. estuary requires that:

JNOIAN J. ENVIRONMENTAL PROTECTION, VOL. 10, NO.5, MAY 1990 333 © 1990 • Xalp..a Corporati.. RESULT AND DISCUSSION r, '=±o'~="=," "'I , The tides experienced in the region were generally mi· , , xed semi ~ diurnal type with two high and low waters I . t occuring each tidal day with varying tidal amplitudes. ! f i 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 eno -~{ ~,/l, ugh 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 dissol ved substance are assumed to depend only on the dis tance (rom the estuarine mouth. The effiuent is consi '\ dered as conservative, -""= .J '; 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 Neep Avg. time diffe· Q.S,=( Q.+Q.+Q, ) S. renee- where So is the ocean salinity solving for QQ gives Rewas -0.03 -0.14 - 2 to - 3 Q.=( Q.+Q, ) SIS, -S Mankule -0,11 -0.01 - 2 to+3 Dharamtar 0.09 -0.02 The total flow available for diluting the effluent is : +5 Khar Jui -0.33 0.21 +15 Q.=Q.+Q,+Q, = ( Q.;Q'd s. Gandhe -1.08 0.45 -1-45 , Bendse -1.39 0.32 +60 The mean concentration of effluent near the point of Nagothane 1.76 0.73 +65 to 70 discharge can be estimated from: 'Add these values to Apolio Bandar hiah water to get C.~M Q. the approximate tidal heights and time of occurrence of high water at respeclive locetion where, M is the discharge ratc of material in units of mals per unit time. Table 2. Dilution discharge and mean concentration of Two stations, namely the proposed effluent discharge effluent near the discharge point point near Mankule Bunder and Bombay Harbour Date Q", m 3 l Q" rn'l SO' % S, % Q",m', Cd·' ( eastern region) are selected for the present study sec sec sec ppt ( Figure 1 ). The effluent discharge point is situated 23.7.86 0.21 121.8 21.4 6.7 177.62 0.01 about 5 km from the estuary mouth. Spring tide ob 22.8.86 O.~ I 59.7 34.9 11.7 90.12 0.Q2 Icrvations of salinity of surface. middle depth and I m 35. J 27.5 56.43 0.03 above the sediment level were made usin. a Hydrolab 19.9.86 0.17 12.3 during July to October 1986 over a complete tidal 4.10.86 0.17 5.6 36.7 29.4 2901 0,06 cycle. The river flow data was obtained from the gau 'Assumed discharge rate of the material (M)=IO ppl ging station near Nagothana. Wastewater discharge ( for Q.. Q" s.. S, Q" and C" refer text) ( Q.) during peak wet - weather is estimated at 0.21 3 S m /sec and the flow is expected to be around 0.17 m / The average salinity at both the stations shows an In' sec durin, dry weather ( NIO Report. 1987). TIle dis creasing trend from July to October. In July, the ave charge rate of the effluent material (M) is assumed rage Bombay Harbour salinity i. observed as 21.4 x as 10 ppt. 10-' and the corresponding salinity at the discharge

334 INDIAN J. ENVIRONMENTAL PROTECTION, VOL. 10. NO.5, MAY 1990 © 1990 - Kalpana Corporation nal mixing takes place. poiot is 6.7 x 10-'. In October, the Bombay Harbour ..linity was 36.7 x 10-' and at the discharge point it ACKNOWLEDGEMENT is 29.4 x 10-'. From the river flow data obtained from The authors wish to express their deep gratitude to the gauging station near Nagothana, it is evident that Dr B.N. Desai, Director, NIO for keen interest and the tributary discharge ( Q, ) is considerably low dur providing facilities. Sincere thanks arc due to Dr. ing September and October and this period is consi M.D. Zingde, Scientist - in - charge, Re, NIO, Bom dered as dry weather aod the wastewater discharge bay, for guidance and constant inspiration. (Q.) during this period is taken as 0.17 m"/sec. Total flow available for diluting the effluent ( Q. ) is maxi REFERENCE mum during July (177.62 m"/sec ) and minimum du Fischer. H.B. 1979. Mixing in inland and coastal ring October (29.01 m'/sec). The corresponding mean waters. Academic Press, N.Y. pp 266 - 268. concentrations of effluent near the discharge point is NIO Report. 1987. Hydrographic studies in Amba 0.01 and 006 ppt. However, it should be recalled that rjver estuary for wastewater disposal from a petro only the average concentrations of the effluents are chemicals complex ( Part I ). National Institute of computed here. Peak concentrations in the effiuent Oceanography. plume near the discharge point may be higher. Allow Zingde, M.D. 1989. Environmental status of the coa ance is made in the analysis form the higher concen stal marine environment of India. 10 Managemont trations expected near the source before cross - seelio- of aquatic ecosystems. pp 37 - 57. ( Received OD JDDe 5. 1990 )