That double wall thing is a red herring. The water system already protects against intrusion because the water itself exerts pressure on the pipes. Thus leaks a

That double wall thing is a red herring. The water system already protects against intrusion because the water itself exerts pressure on the pipes. Thus leaks are typically of the water getting out and not contamination of the water (most of the time, anyway.) Contamination rarely happens outside of the source of supply, and not somewhere along the pipeline.