The water is polluted by a number of anthropogenic activities. These include discharge of waste water directly into water bodies, discharge of solid waste into water, chemical runoff (containing fertilizers, pesticides, etc.) from agricultural fields, industrial discharges, etc. There are a number of ways an individual can contribute to protection of water. One of the very first things that we can do is to reduce our water consumption (if possible). Even though the wastewater from our homes is treated, not all the impurities are completely removed (regulations vary from state to state and country to country). The less we generate, the lesser the chances of impurities reaching the water. One can also minimize the use of non-organic food products to reduce the consumption of chemicals in agriculture. An individual can also minimize the amount of solid waste he/she generates, by reducing the use of packing material and using cloth bags (instead of paper or plastic bags), etc. Liquids leach from solid waste in landfill sites and contaminate our groundwater. One can also motivate others to participate in similar activities within his/her friend circle, using social media, etc.
hope this helps.