I would probably consider the water dirty. Just be sure to check the tank to see the water level just incase if it hasn't rained for awhile. Make sure you get a good filter installed to besure that you don't have leaves and other debris in your tank. Clean it a couple times in the year, so you make sure that you don't have the gravel from the shingles on your roof clogging your nozzles on your sprinkler heads.
You can use a sprinkler pump, but don't forget to install your filter BEFORE your sprinkler pump. There is no need for backflow prevention because I am assuming that you are not drinking this water.
