Effect of organic pollution on some hydrographic features of Cochin backwaters

196-200Some hydrographic features of Cochin backwaters were studied from September 1974 to August 1975 in 7 stations of which 6 receive sewage discharge in varying quantities and one is a 'clean' station near the barmouth. Thermal gradient of the waters was more largely dependent on the depth of the station than on the sewage discharge. Seasonal and tidal variations in salinity were more evident during monsoon than in pre - or postmonsoon periods. No significant change in salinity was noticed due to discharge of sewage. Significant seasonal and tidal fluctuations in the values of oxygen were observed. Lowest oxygen values were seen during premonsoon periods when temperature was 32.6° C at surface and 32° C at bottom. Dissolved oxygen content varied proportionately with sewage discharge. High temperature and rapid decomposition of organic waste during premonsoon accelerated pollution effect. BOD5 varied with stations, season and tide. Oxygen consumption of the bottom waters of polluted areas was 12 to 13 times as high as that in barmouth. In polluted areas fluctuating increase of BOD5 values was noticed with the commencement of premonsoon while a decreasingly fluctuating trend prevailed with monsoon. BOD5 at low tide was higher than at high tide in the polluted area. High sulphide content reaching to a maximum of 4.92 mg/litre was observed at the bottom of polluted stations especially during low tide of premonsoon periods