I recently read in a book on rainwater collection that rainwater has only 1/20th (5%) of the contaminants that groundwater has.

The main problem is preventing contaminants from getting into the rainwater to begin with.

One way to keep birds from perching on the ridgeline of a building is to stretch a stainless steel wire about an inch or two above it.

One good book on rainwater collection is Water Storage: Tanks, Cisterns, Aquifers and Ponds by Art Ludwig. (Who also has a few good books on greywater).

Another is Rainwater Harvesting for Drylands: Vol. 1 Guiding Principles by Brad Lancaster, first in a trilogy. Volume 2 is Water-Harvesting Earthworks, and Vol 3 is Roof Catchment and Cistern Systems, which may not be out yet.

Sue