WATER QUALITY MANAGEMENT IN CONNECTED CATCHMENT- SHALLOW LAKE SYSTEMS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 24

biochemical oxygen demand (BOD). In the practice of surface water resources engineering and water
quality modeling, the following concentrations or variables are of importance according to Rafailidis
(1994).
The Carbonaceous Biochemical Oxygen Demand (BOD) . This is an indicator of the overall loading
of the aquatic system due to the oxidation needs of organic pollutants. It also includes the respiration
demand of aquatic microbes, which metabolize organic and fix inorganic matter (e.g. nitrates,
inorganic phosphates, etc.).
The Dissolved Oxygen content (DO) . Dissolved oxygen is a very important water quality parameter
because it is essential for the respiration of the aquatic life. The maximum amount of oxygen that can
be dissolved in water is called the oxygen solubility or dissolved oxygen saturation concentration
(DOS), which decreases with an increase in temperature. Dissolved oxygen is utilised by dumped
wastes in chemical oxidation reactions, or in biochemical reactions where organic material is
biologically decomposed by aquatic microorganisms.This parameter is more critical because it shows
whether there is sufficient oxygen in the water for aquatic life to survive. The actual DO content
reflects the equilibrium between re-aeration at the surface added to photosynthetic oxygen generation
by CHL-aorophyll in the water body, minus the biological and any chemical oxygen demand.
The concentrations of nutrients (ammonia, nitrates, phosphates, inorganic nitrogen or phosphorous),
these are linked directly to non-point sources of agricultural watersheds, as a consequence of soil
fertilization, insecticides or pesticide use, etc. Aquatic microorganisms metabolize nutrients and the
inorganic elements are fixed to more complex compounds. Algae play a very important role in these
processes, enhancing water denitrification (release of N 2 to the atmosphere) or nitrification (capture of
N 2 from air). On the other hand, simultaneous presence of phosphorous enhances algal growth, leading
to eutrophication, i.e., abnormal growth of algae and aquatic flora. This is particularly troublesome in
enclosed waters (e.g., lakes, lagoons) but also occurs in coastal areas suffering from large pollution
inflows and suppressed natural circulation flushing.
The coliform bacteria concentration . Although these micro-organisms are not pathogenic and exist
naturally in the intestine of humans, their presence indicates pollution due to urban sewage effluents.
Upon discharge into the water body environmental conditions such as temperature and sunlight
determine the eventual fate of coliform bacteria through a multitude of processes (e.g., photo-
oxidation, sedimentation, ph, algae, etc.).
Apart from the above pollutants, sediments in the water column may also cause environmental
problems as they bury benthic flora and choke the gills of aquatic animals. Therefore, it is concluded
that BOD in surface waters indicates the overall organic pollution of the water, and DO shows whether
the aquatic life may be sustained there, whereas nutrient concentration gives the potential for
eutrophication. Coliform counts indicate the danger of disease for humans using the water for any
purpose of use.
Other very important water quality indicators are the flora and fauna . The growth of aquatic plants,
both rooted and phytoplankton (mainly algae) is controlled by light, nutrients and animal grazing.
Light enables photosynthesis, which produces oxygen. The most important nutrients as mentioned
earlier are nitrogen , carbon and phosphorous . Nitrogen and phosphorous are both considered most
important pollutants in non-point sources in agricultural watersheds. Therefore, the coming section
explains in details the nitrogen and phosphorous cycles.
Nitrogen Cycle
Brown and Jhonson (1991) give details of the effect of nitrogen on water quality. The
nitrogen cycle is a representation of the various forms of N and how they relate to one another
through many complex interactions. Figure (2-4) , a simplified version of the nitrogen cycle,
illustrates many of the complex interactions of various forms of nitrogen, including: