POLLUTION ASSESSMENT INTRH9UTARY WATERS OF MAHAWELI RIVER AROl;JND KANDY CITY

Abstract

In most part of the world surface water satisfies majority of water requirements of humancommunity. In this regard. rivers have been the most significant contributor of surface wateramong the limited water resources available. In most of the developing countries. frequently theriver pollution can be mainly attributed to untreated waste discharges from commercial andindustrial establishments. The danger is that in a situation where water pollution may be steadilyincreasing, unless an effort is made to build up records, and understanding of the processes andtrends. the changeover from tolerable condition to intolerable conditions may come as a suddenevent

Among the rivers. "Mahaweli" is the largest river basin found in Sri Lanka estimated in draining16 percent of Sri Lanka's land surface (10327 km-). In recent times with increase in populationand urbanization, settling around townships especially on the banks of "Mahaweli" has shown amarked increase. Among these, Kandy the second largest city in Sri Lanka is believed tocontribute most of pollution into the headwaters through a number of its tributaries around thecity.

In this study, three major tributaries namely Pinga Oya, Nanu Oya and Maha Oya around Kandyarea are selected for monitoring the water quality. Out of these tributaries, two flows throughhighly populated areas with concentrated dwellings and industrial establishments especially bythe side of the river. In an initial survey, highly polluted zones were identified and samplingpoints were selected from head waters to the downstream based on the findings. At each crosssection, samples were taken at various depths across the river twice a month from August 2000.According to the obtained data, temperatures were recorded in the range of 23°C and 31°C andpH almost in neutral conditions creating ideal condition for bacterial growth. The recorded DOconcentration was in the range of 6.5-2mg/1 in most occasions. Comparatively higher DOconcentration was recorded at the upstream than the downstream. Conversely. due to large wasteloads and stagnant water conditions. very low DO concentrations were observed at thedownstream of Pinga Oya. Furthermore, depletion of DO values from the beginning to the endof the sampling indicates the deteriorating water environment of the Pinga Oya. Though thephosphate variation is not much significant, nitrate level has increased by three to four timesduring the sampling period. The low level of nutrients and the BOD5 values (BODs less than5mg/1) indicates effects of good dilution and high assimilation. However, higher COD values'recorded at some locations implying that the organic matter making up the COD may includesubstances that are slowly biodegradable that can be harmful to aquatic life. Further, the mostalarming circumstance was the high levels of fecal coliform detected in all three tributariesthroughout the study period (in the order of 500 to 7000 cfullOOml). Hence, from the obtainedresults, especially low DO concentrations and high coliform counts show high pollution levels inthe tributaries and indicate a possible limiting state of their assimilative capacity in some riverstretches.