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

ocearnc ongm, But Phosphate level recovers from the initial slump and rises in line
with chloride and peaks along with chloride, and then curiously enough, falls in phase
with chloride. Phosphate level rises because the sewage input is not flushed out as
residual flow becomes too weak to take care of the nutrient load. Anoxic or near
anoxic conditions prevailing during the dry months corroborate this fact. But P
should fall when Cl rises. It does not happen so because the expected nse In
alkalinity and pH does not happen and also because of the accumulation of sewage.
Thus phosphate remains in suspension all along the dry months. The reason why
alkalinity does not rise with the intrusion of cations is to be explained in terms of
carbonate-bicarbonate balance.
The governing equation is CO2+H 20 H2CO 3 H+ +HC03
When H+ ions are added to the system bicarbonates and protons are consumed to keep
the proton concentration constant producing H 2C03. When OH ions are added to the
system, H+ ions are consumed forming water more H+ ions are produced by the
dissociation of carbonic acid to form protons and bicarbonate. Insoluble calcium
carbonate molecules play a crucial role in this reversible buffering reaction.
But it is to be assumed that intrusion of marine cations does not reach
significant levels to initiate a perceptible change in the local aquatic chemistry. It is
interesting that total hardness and sulfate ephemerally fall sharply in February after
the initial rise, which coincides with the fall in phosphate; and alkalinity temporarily
rises at this stage. The plant nutrients N03-N and P04 -P are never above the
dangerous levels in the water bodies. Total iron, for one, is in most cases above the
recommended levels. But iron is not a grave aquatic pollutant; it is more of an
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