What is really being discussed here is Rainwater Catchment or basically impermeable surface run-off. Over time any outdoor impermeable surface, be it a sidewalk, parking lot, patio or roof will accumulate various particulates from the surrounding atmosphere. Some of these are automotive exhausts, industrial pollutants, etc. These same pollutants are deposited on the surrounding soil. The difference is that when it rains, the soil deposited pollutants are, for the most part, absorbed into the soil itself whereas they are concentrated in the run-off from the impermeable surfaces. The express reason for constructing a small berm around a Garden Pond is to prevent any contamination from such run-off. Using untreated rain catchment water directly bypasses and is counter to the purpose of having a berm surrounding a pond. Any article on rain water catchment use will indicate that this water needs to, at least, be treated by the use of a series of settlement tanks/chambers. Rain water before it ever hits the ground has accumulated a certain amount of air-borne pollutants so it is anything but clean and in certain areas of the country would never be classified as potable without being treated.
