There are many causes for water pollution but two general categories exist: direct and indirect contaminant sources.
Direct sources include effluent outfalls from factories, refineries, waste treatment plants etc.. that emit fluids of varying quality directly into urban water supplies. In the United States and other countries, these practices are regulated, although this doesn't mean that pollutants can't be found in these waters.
Indirect sources include contaminants that enter the water supply from soils/groundwater systems and from the atmosphere via rain water. Soils and groundwaters contain the residue of human agricultural practices (fertilizers, pesticides, etc..) and improperly disposed of industrial wastes. Atmospheric contaminants are also derived from human practices (such as gaseous emissions from automobiles, factories and even bakeries).
Contaminants can be broadly classified into organic, inorganic, radioactive and acid/base. Examples from each class and their potential sources are too numerous to discuss here.
b)What are the effects of water pollution?
The effects of water pollution are varied. They include poisonous drinking water, poisionous food animals (due to these organisms having bioaccumulated toxins from the environment over their life spans), unbalanced river and lake ecosystems that can no longer support full biological diversity, deforestation from acid rain, and many other effects. These effects are, of course, specific to the various contaminants.
c)What are the ways we can take to decrease those problem?
Science provides many practical solutions to minimizing the present level at which pollutants are introduced into the environment and for remediating (cleaning up) past problems. All of these solutions come with some cost (both societal and monetary). In our everyday lives, a great deal can be done to minimize pollution if we take care to recycle materials whose production creates pollution and if we act responsibly with household chemicals and their disposal. Additionally, there are choices we make each day that also can affect the quantity of pollutants our actions will introduce into the environment. Heavily packaged foods, for instance, contain boxes, cartons, bottles etc.. made with polluting dyes, many of which are released from groundwater at municipal land fills. Whether we choose to drive to the corner store rather than walk or ride a bicycle will determine how much we personally contribute to acid and hydrocarbon emissions to the atmosphere (and ultimately to global fresh water supplies).
In the end, there are many choices on the personal and societal level that we must make (consciously or not) that affect the amount of pollution our town or country will be forced to live with. Our standard of living and very way of life is based upon practices which are inherently "dirtier" than those of our distant ancestors, although they too polluted their environment to some extent. Without taking a step backward in terms of our standards of living, the answer seems to lie in a combination of many small changes in our daily practices and paying more for goods and services, so that manufacturers of various materials and drivers of automobiles (for instance) will have cleaner devices with which to conduct their activities.