Removal of useful components of the soil and addition of the substances that affect the soil quality adversely and kills the diversity of the organisms living in the soil is called as soil erosion. 

The following are the factors or processes result in the formation of the soil:

(i) The sun. The sun causes heating of rocks during the day. As a result they expand. The rocks cool down at night and contract. Since, all parts of the rock do not expand and contract at the same rate, it results in the formation of cracks. Finally huge rocks break up into smaller pieces which forms the soil. .

(ii) Water. It acts in two ways :

(a) It gets into cracks of rock formed due to uneven heating by the sun. In winter, this water freezes and cause widening of cracks. It is because water expands on freezing (ice). This leads to the breaking of the roacks.

(b) Running water wears away hard rocks over a long period. Water  flowing at high speed carries rock pieces alongwith it and cause these rock pieces to rub against each other. As a result the rocks wear down to smaller particles. The running water carries these particles far away from their parent rock and deposits them down its path.

(c) Wind. Strong winds and storms also erode rocks down. The strong wind carries small rock pieces and sand from one place to the other like water.

(d) Living organisms. The Lichens (symbiotic association of fungi and algae) grow on the surface of rock. They release certain substances that cause the rock surface to powder down to form a thin layer of soil.

On this thin layer of soil small plants like moss grow. They cause the rock to break up further. The roots of big plants/trees sometimes go into cracks in the rock. As these roots grow bigger the cracks are widen.

All these processes lead to the formation of soil. 

The activity which pollutes the water resource are:

River. The sewage from the town and the cities are dumped into the water bodies and lead to its pollution.

Well. Washing and bathing near the well leads to the pollution of water.  

River/well.

Organisms need water for

(i) Cellular processes - as all the processes take place in a water medium.

(ii) Transportation of substances - water is used for the transportation of substances from one part of the body to the other.

(iii) Reactions - All the reactions that occur in the cell or body occur between substances dissolved in water.