4.0 RESULTS AND DISCUSSION

Transcription

1 4. RESULTS AND DSCUSSON The purpose of this chapter is to discuss some of the prominent outcomes of applying the tool to the Colombo Canal Data and also some shortcomings of the tool itself Water Quality Grid - BOD t is evident that in the Water Quality Grid for BOD, the cell bounded by the areas Meetotamulla. Wellampitiya, Megoda Kolonnawa, Gotatuwa, Madinnagoda, Cotta Road. Borella and Kuppiyawatta (or the cell including Mahawatte Eta, Serpentine Canal. southern part of Dematagoda Canal and Northern part of Kolonnawa Eta) has the worst quality with respect to organic pollution (BOD). t has an average monthly BOD level of 52 mg/1 for the period from January 1997 to December 24 with a coefficient of variation of.28. The fiftieth percentile BOD level is 34 mg/1, while it exceeds 22 mg/1 seventy percent of the time and 4 mg/1 OO%.of the time. Thus it seems unrealistic to expect the BOD level to be maintained at 4 mg/1 (present ambient standard) % of the time, in the short or medium term. t would be more realistic, for example, to aim at maintaining the BOD at below 34 mg/ 1, wh ich is the fiftieth percentile value, over the entire period considered. The grid cell bounded by the areas Pita Kotte, Etul Kotte, Madinnagoda, Kalapaluwawa, Koswatta, Talangama South and Madiwela (or the cell including Parliament Lake, Korte Eta North and the southernmost part of Kolonnawa Ela) has the highest quality when considering organic pollution. t has an average monthly BOD level of for the period considered with a coefficient of variation of.21. The fiftieth percentile BOD level is 8, whi le it exceeds 4, ninety five percent of the time. The BOD level of this cell is largely influenced by that of monitoring point 3 (Parliament Road Bridge on Kotte North Canal), since data for the points 18 (Diyawanna Oya, Kimbulawala, Madiwela), 19 (Diyawanna Oya, Battaramulla South, Pela,,atte ). and 2 (Diyawanna Oya Outlet On Sri Jayawardanepura Mawatha} are a\ ailable for only the last seven months of 24. f the data for points 18, 19 and 2 were available for a longer period, the BOD level of this cell would have been even lower. since the area adjacent to points 18, 19 and 2 has a relatively lower degree of pollution when compared to that of point 3. This is apparent when considering the data for points 18, 19 and 2 from June - December 24, where the 4 mg/1 limit is exceeded only 6% of the time compared to point 3 where the same limit is exceeded 95% of the time. When comparing the two main sections of the canal system, the cells in the Northern Canal System have an average BOD level of 39 for the period considered, while that of the cells in the Southern System is 21. Therefore, undoubtedly the Northern System is more polluted than the Southern System. This may be explained by the fact that the first three canals (especially Serpentine Canal) included in the most polluted cell above (which is in the northern system) are exposed to many industrial and domestic sources having high levels of pollution (Nippon Koei, 2 ). On the other hand, the Southern System has the benefit of being connected to many large water bodies and marshes (Parliament Lake, Korte Marsh, Heen Marsh and Weras Ganga) \\ hich help to intercept and dilute the pollution entering canals. The Kalapaluwawa 6

2 Marsh is the only Marsh area connected to the Northern System. Therefore, the application also highlights the role that water bodies and wetlands play as natural buffers of pollution. Another advantage that the Southern System has over the Northern System is being connected to two sea outlets. The tidal action helps to flush out the canals by reducing the retention time of pollutants inside canals. Sea water which comes into canals during high tide, flows back to the sea during low tide, flushing away pollutants along with it. The first priority in a long term solution to the problem of pollution of Colombo Canals should be to disconnect direct pollution sources within the most polluted cell in the Water Quality grid by introducing wastewater treatment, solid waste management and proper sewage treatment facilities. f the level of pollution in this area is lowered, the water quality of the rest of the Northern System will also be dramatically improved. 4.2 Monitoring Points- BOD 4.2. Average BOD Levels Monitoring point 1 on Serpentine Canal has the highest average BOD level (93 mg/1) for the period with a coefficient of variation of.16. Monitoring Point 2 (Diyawanna Oya Outlet On Sri Jayawardanepura Mawatha) has the lowest average BOD level of 4.5 mg/1 with a coefficient of variation of.29. This result again demonstrates the high level of pollution entering the Serpentine Canal, and the low level of pollution surrounding Parliament lake. The average BOD levels of the two inlet points of Parliament lake, points 18 and 19 (Diyawanna Oya, Kimbulawala, Madiwcla and Diyawanna Oya, Battaramulla South, Pelawatte) have average BOD levels of 6 mg/1 and 5 mg/1 respectively. The lower average BOD level at the outlet of Parliament Lake (point 2) of 4.5 mg/1 points to the conclusion that Parliament lake does play an important role in intercepting pollutants entering the canal system from the surrounding watershed. However, measurements were taken at monitoring points 18, 19 and 2 for a period of only seven months compared to 96 months at monitoring point. Therefore records for a longer period at monitoring points 18, 19 and 2 are necessary to reinforce the above results Quality Criteria The Water Quality Monitor also demonstrates that at present, it is not realistic to reach the ambient water quality standard (proposed by CEA) of 4 mg/1 BOD, hundred percent of the time, except in the case of Parliament Lake and its immediate downstream Canal segments. This is further illustrated in Table 4.1, where the BOD value at each percentile point on the empirical probability distribution is tabulated for 61

3 all monitoring points. Therefore, setting more realistic quality criteria for the rest of the canals may be considered. Table 4.1 BOD Value on the Empirical Probability Distribution (mg/1) - Percentage of Time Value is Exceeded Station No. Percentile Position o.o1 1 2 T 3o \ l \31 (>(,) )1 l\().1 <\) \ 1\1;) 1S.'6 )\) 11.'6 \) s S \OS \ Monitoring Points - COD Average COD Levels Once again monitoring point on Serpentine Canal has the highest average COD level of 134 mg/1 with a coefficient of variation of.24. ts fiftieth percentile level is 15 mg/1 and the seventieth percentile level is 81 mg/1, while it exceeds 2 mg/1 almost 95% of the time. Monitoring points 19 and 2 both have the same lowest average COD of mg/1 with a coefficient of variation of Quality Criteria As in the case of BOD, here too, hundred percent achievement of the ambient water quality standard of 2 mg/1 is not realistic at present, except at Parliament Lake and canal segments immediately downstream. Other more realistic quality criteria may be contemplated for the rest of the canals. Table 4.2 lists out the COD value at each percentile point on the empirical probability distribution for each monitoring point. 62

4 Table 4.2 \ Station \ \ COD Value on the Empirical P robability Distribution (mg/1) - Percentage of T ime Value is Exceeded Percentile Position o.ot to \ \ \.9 1() \ \6 \ H \\.\ \ \\),, 281 4\ () \5 \1 \(.) \ \ \ so \ \ too so S Relationship of BOD and COD with Water Level a nd Discharge Although generally the BOD and COD concentrations of monitoring points demonstrate inverse relationships with water level, none of them are statistically significant. The best r 2 value obtained for BOD is.3469, for Station 4 (Railway Bridge on Torrington Canal), \\hile that for COD is.4115 for Station (Baseline Road Bridge on Dematagoda Canal), after removing outliers following the procedure set out in section 2.4. The graphs for stations, 4, and 13 are shown in Figures 4.1 and 4.2 as examples. t is to be noted that a greater percentage of data on all these graphs lie below.5 m MSL, and almost all points are below. m MSL. Wide scatter in the data is evident especially in the region below.5 m MSL. This scatter occurs mainly due to three reasons as explained hereafter \ \ \ \3.9 No.

5 15-1 g m...5 water level (m M SL) (a) Variation of BOD with Canal Water Le\'el for Station m V\ ater leve1 / (m M SL) (c) Variation of BOD with Canal Water Level for Station J 75.5 water Level (m M SL) (b) Variation of BOD with Canal Water Level for Station 4' -5 g.. '" Wiler Level/ (m M SL) (d) Variation of BOD with Canal Water Level for Station 13 Figure 4.1 Variation of BOD with Canal Water Level for Stations 1, 4, 1 and 13 Canals in Colombo have near zero bed slope. When the canal water level is below or about.5 m MSL, water is almost stagnant. n other words, there is zero discharge. Therefore according to the Advection Dispersion equation (equation, section ). the advection component is weak in this region compared to the dispersion component and the sources and sinks terms. Therefore, the BOD and COD concentrations are governed by demand components such as point and non-point sources, organic carbon in dead organisms and excreted materials, microbial 64 ' g y = e 1 OOSh. R 2 = y = 119.9eoso. 75 r- 15 T y = e R2 =.3469 r 5!il y = e.o 3237x R 2 =

6 degradation, benthic release of reduced minerals, scour and leaching of organic carbon and settling from water column etc. ( Jammer and Hammer Jr., 1998 and nc. Metcalf & Eddy, 22), rather than hydraulics of the flow, while these demands do not show a direct correlation with hydraulics ;.. R Level for Station water Levell (m M SL).5 Le' el for Station water Level / (m M SL) R2 t 75 (.). ol.5 water Level/ (m M SL) (b) Variation of COD with Canal Water Level for Station t\1 25 ', :;- 1., water Level (m M SL) (d) Variation of COD with Canal Water Level tor Station 13 Figure 4.2 Variation of COD with Canal Water Level for Stations 1, 4, 1 and 13 rhc second reason for scatter in data is the effect of tidal variations on canal discharge. Due to zero bed slope, discharge occurs according to the gradient in 65 (<.:) Variation of COD with Canal Water E 75 /...., : 8 51 : y = e.C 356x R2 = y = e-c mx 1 R2 =.352 (a) Variation of COD with Canal Water :.,.. 5., 25 : l 8 5 i y = 12.83e 1 722b = y e 2 s =.4115

7 energy head. This gradient depends on the tidal level at the sea outlets, so that even if the canal water level does not change, a change in gradient, hence in discharge and pollute concentrations may occur due to a change in the tidal elevation. This effect is clearly evident below 1. m MSL region in Figure 4.3, which shows a graph of measured discharges versus canal water level for Station 4 (Ghanapala, 2). According to Ghanapala (2), whenever the tidal height is above.3 m MSL, canal discharge is affected by the tide. The third reason is the backwater effect on canal discharge by other connected canals due to near zero bed slope (Ghanapala, 2). This effect can produce significant variations in pollute concentrations even at the same water level, whenever rainfall is not uniformly spatially distributed across all canals. Considering the arguments presented above, one would expect the graphs of pollute concentration versus discharge to show better correlation (Figure 4.4). However, these graphs for Station 4 again show wide scatter in the region below.5 m 3 /s. This is again due to two reasons. The first is, as explained earlier, the governing factor for pollute concentration in low discharge regions is not advection: but other biological, physical and chemical parameters which affect BOD and COD production, while not being directly related to canal discharge. The second reason is the presence of two peak domestic discharges, rich in organic pollution, during any day, one in the morning and one in the evening (Ghanapala, 2). During low flow regimes, the domestic component of the flow dominates the pollute concentration in canals rather than groundwater base flow. Therefore, the BOD and COD concentrations depend on the time of measurement during the day, since measurements nearer the domestic peak will yield higher BOD, even at a higher discharge. Figure 4.3 ::J en :E.s -Q) 1 "' en ''lf: as charge (m3/s) Measured Discharges Vs Canal Water Level for Station 4- Railway Bridge on Torrington Canal (Source: Glmanapala, 2) 66

8 . 1, D : Doscharge (m3/s) 1 J 25 l y = e.o.s.<J6x R 2 =.175 (/t Figure 4.4 Graphs of Discha rges Vs BOD and COD for Station 4 - Railway Bridge on Torrington Canal 4.5 Relationship of BOD and COD with Average Daily Rainfall Although the BOD and COD generally show an inverse relationship with average daily rainfall, again none of the relationships are statistically significant. The best r 2 for BOD,.586. was obtained for Station 18 (Diyawanna Oya, Kimbulawala, Madiwela), while that for COD,.1456, was obtained for Station 4 (Railway Bridge on Torrington Canal). The graphs for stations, 4, 13 and 18 are shown in figures 4.5 and 4.6 as examples. Again wide scatter is observed in the data in all graphs. One reason for this scatter is the use of average daily rainfall for each month as the independent variable. since it does not directly represent the conditions in the canals on the day of sampling. For example, the first nush of rainfall will carry a higher load of pollutants in canals while after one or tv.o days of rain, the pollutant concentrations \\ill reduce, although the average daily rainfall for that particular month may remain lo''. Another reason is the insignificance of advection of pollutants during very low discharges as explained in section Ooscharge (m3/s) 125., =-=;!... 1 *!: y 5 t..; 8 -:-! :.... ' + = e.o 275 R2 =.227

9 o Averageoa,lyRa,nfaJ/(mm) J (a) Variation of BOD \\ith Average Dail) Rainfall for Station Average Da.ly Rainfall/ (mm) (b) Variation of BOD,.,.ith Average Dai ly Rainfall for Statign 18 Figure 4.5 Variation of BOD with Average Daily Rainfall for Stations 4 a nd E :;: y =.95x x R2;;.1389 ol Average 311yRaonfall/ (mm) (c) Vari ation of COD with Average Daily Rainfall for Station 'J 1 25 y =.434x x R2= Average Daoly Rainfall/ (mm) (d) Variation of COD with Average Daily Rainfall for Station 13 Figure 4.6 Variation of COD with Average Daily Rainfall for Stations 1 and , ll 5 1 :? _. _:_,1 r- :.!! :? 5 E' 1 R2 = o.586 R 2 =.2131 y = -.1x x y =.126x x

10 4.6 Some shortcomings of Water Q uality Monitor. Almost all the water level data input to WQM is in the range.- 1. m MSL, whereas the water level can go up to even 1.8 m MSL (Ghnanapala, 2). A statistically significant relationship between water level and pollute concentration may exist in the region from m MSL since the advection component in the governing equation is more prominent than the dispersion and source and sink terms in this region. However, with the existing data there is no way to verify whether this holds true. 2. Similarly to water level, almost all discharge data for station 4 is below 1. m 3 /s. Therefore, the behaviour of pollute concentration with discharge cannot be established in the region beyond 1. m 3 /s. According to Ghnanapala (2) canal discharge can even go up to m 3 /s. 3. Although WQM demonstrates that generally with increasing water level, discharge and rainfall pollute concentrations decrease, it is not possible to obtain any specific water level, discharge or rainfall values corresponding to a particular pollute concentration with this model due to lack of statistically significant relationships 4. At present, the facility for a user to add additional data for the Colombo Canal system through the user interface does not exist. Currently the only way to add new data is to modify the underlying Microsoft Access Database itself, which is cumbersome. 5. When using the User Defined File Option, presently WQM does not have the facility to generate the relationships between pollute concentration, water level, discharge and rainfall. 4.7 Some Possible nterventions for the Reduction of Pollution in the Colombo Canal System Based on the results of Water Quality Monitor, it is generally observed that as the canal water level moves towards 1. m MSL, there is a general reduction in pollution concentrations although there is no statistically significant relationship between canal pollution and water level in the region between -. m MSL. Similarly, this trend is observed as the canal discharge moves towards 1. m 3 /s (water level of.58 according to rating curve for Station 4 developed by Ghnanapala, 2) and beyond, while generally pollution concentrations reduce with increasing average daily rainfall. rhis general reduction in pollution is due to: 69

11 . ncrease in canal discharge at higher water levels resulting in reduced retention time of pollutants within the canal system. 2. The effect of dilution due to addition of less polluted water during rainfall. Considering the above, the following interventions can be recommended for reduction of pollution in the Colombo Canal System:. Construction of tidal outlet gates at the outlets of the canal system including at Mutwal Tunnel which is presently blocked in order to artificially increase canal discharge. 2. Construction of a pumping station to pump in less polluted water from river Kelani into the Northern System both to increase discharge as well as dilute pollutant concentrations. 3. Make arrangements to store water during the wet season in the Parliament lake and release into the canal system during dry weather. 4. Provision of constructed wetlands within Kalapaluwawa marsh and Kotte Marsh in order to absorb pollutants.. 5. Dredging of canals just before the start of the rainy season in order to remove benthic pollutants. n addition to the above, the following measures can be adopted to reduce pollution entering into the canals:. Disconnect as many as possible direct sources of pollution within the most polluted cell in the northern system (the cell including Mahawatte Eta, Serpentine Canal, southern part of Dcmatagoda Canal and Northern part of Kolonnawa Ela) on a priority basis by providing waste water bins and proper wastewater and sewage treatment facilities. 2. Extend the above procedure to cover the whole canal system in the long term. 3. Commence a program to raise public awareness on reducing pollution entering canals and the importance of reduction of such pollution. The Water Quality Map presented in this study can be used for knowledge dissemination on where urgent action is necessary. 4. mprove regulatory functions of relevant authorities, such as possible fines to offenders including the general public. 7