In our "Ask the Experts" series from the WASRAG Newsletter here is a response to a question about "How long can rainwater be stored?"
"How long can rainwater be stored? What can be done to preserve it in a 40,000 gallon concrete reservoir?"
Professional Services Team leader Ron Denham's response highlights how you often need a lot more information before a question can be answered. For example: 
- Is the water collected from a roof?
- Do you divert the water from the first rainfall away from the tank, so that only clean water goes into the tank?
- Are you using chlorine or other purification?
- Is the reservoir open or enclosed?
- What is the rainfall pattern - frequent small showers or irregular heavy downpours
- What is the temperature inside the reservoir - or - what are the ambient conditions in the location of the reservoir?