Salinity Intrusion and Seasonal Water Quality Variations in the Tidal ...

violently multiplies wherever the current is not inclement. Eichornia and other fresh
water floating plants mask the actual pattern of dissolved nutrients and dissolved
oxygen. When they grow up explosively, phosphate and nitrate content of water will
diminish to misguiding levels. When they wilt and wither, DO is used up leading to
hypoxia, nitrate is reduced to ammonia and as a result phosphate outdistances nitrate.
Sometimes the profiles show a marked deviation from the predicted and
expected patterns because the effect of the tide-generating forces, the phase and
magnitude to the tides, rainfall, temperature, influx ofpollutants and degree of mixing
cannot always be the same. The chemistry of the water sample is dependent on these
physical factors. BOO, COD, phosphate, total hardness, sulphate and chloride peak
twice every year in the portions where tidal action is effective. The first peak is in
late December or early January and the second is in late April as a general rule. After
the initial jump, the dissolved solids subside to a plateau during the course of the dry
season and again peaks when the first rains enter the scene. In most of the tidal
canals, contrary to the expectation that alkalinity would rise by oceanic influence; it
actuallyshows a declining tendency across the lean months.
BOD and COD have, more or less, the same profile. And the revealing pattern
is that at the onset of the dry season, when the pioneering quanta of salinity appear,
COD as well as BOO take a slump because of the shift in biota, especially micro
organisms and large-scale precipitation. Throughout the dry spell, both BOO and
COD are on the high, as sewage influx is not effectively flushed out. With the first
serious rains of the year, COD and BOO reach a low and salinity levels collapse. By
and by, fresh water organisms get established. Here also the canals of West Cochin
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