WATER QUALITY MANAGEMENT IN CONNECTED CATCHMENT- SHALLOW LAKE SYSTEMS - Integrating GIS, Remote Sensing, and Mathematical Modelling for Surface Water Quality Management - page 17

activities, which will, in turn, modify (or affect) the natural composition of the water (Peters and
Meybeck., 2000).
The chemical composition of water varies depending on the nature of the solids, liquids, and gases that
are either generated internally (in situ) or with which the water interacts. Furthermore, the chemical
composition depends on the type of interaction. At the mostly pristine part of the hydrologic cycle,
precipitation quality is derived from interactions with gases, aerosols, and particles in the atmosphere.
Evaporation purifies water as vapor but concentrates the chemical content of the water from which it
evaporated. Condensation begins the process of imparting chemical quality to atmospheric moisture
by the inclusion of chemical substances through the dissolution of condensation nuclei. The
complexity of the water-material interaction increases as precipitation falls on the land. The physical
characteristics and mineralogical composition of soil and bedrock, topography, and biology
substantially affect water quality. Most freshwater is a mixture of water derived from several
hydrologic pathways. Figure (2-1) shows the hydrologic cycle and its components.
Figure (2-1): Hydrologic Cycle and its components
Furthermore, the stream water composition may change in situ due to biological reactions or due to the
interactions with the streambed and adjacent riparian zone. Even the groundwater component of
stream water is a mixture of water derived from different hydrologic pathways that vary in their
composition due to the residence time of the water, the length of the hydrologic pathway, biological
reactions, and the nature of the materials with which the water interacts. Temperature is another
important variable that affects physical characteristics (e.g., transfer of gases), state changes (vapor,
water, and ice), and chemical and biological reaction rates of the water. Natural water quality varies
markedly and is affected by the geology, biology, and hydro-climatic characteristics of an area (Hem,
1985).
The quality of water changes as it moves through the hydrologic cycle from the sea, lakes, and rivers
to its hydrosphere and then back to earth as precipitation, where the water again percolates through the
soil or falls on the ocean, rivers, or lakes. (Changes that occur are physical, chemical, and biological.)
These changes are either natural changes or changes due to human impacts and interventions. It is
important to note for understanding how contamination of groundwater and surface water occurs, that