As long as you don't let it stand for more than a couple of weeks you should be just fine. There are products you can get that are used in small ponds to kill the mosquito larva. They are also used by people who collect and store rain water for their gardens. I don't know what it is called, but it comes in table form and can be purchased a just about any garden center. As for purifying the water, letting it set for 24 hours is enough to lose the chlorine, but outside plants rarely suffer from chlorine like indoor plants because the soil filters out most of it.
Good Luck