Few things are cleaner than rain water, right?
Well, in its pure state, rain water is clean. But coming off your roof into your rain barrel, it collects several pollutants that could be harmful. Water in your rain barrel might contain heavy metals, polycyclic aromatic hydrocarbons (PAHs), microbes, pathogens, and pesticides. Think about the bird droppings on the roof and the gunk in your gutter!
I don’t mean to be alarmist, but I think the best use of rain barrel water is in the landscape, not the vegetable garden.