How to preserve and purify well water and for agriculture and drinking purpose?

We have dug well in Purandar taluka, Pune District where generally we get inadequate rains. Hence we need to store water and use it judiciously for agriculture as well as drinking . We need your advice as to how we should preserve it by adding any oil/chemical/any material so that it can be used for agriculture purpose. We also have the facility to store 20,000 litres in addition to well water.Please also let us know how should we purify it and use for human and animal consumption.