Possible Clues:

• Harsh fall that may damage trees
• What's ecologically damaging a Channel Island watercourse
• Environmental problem
• Environmentalist's concern
• Dangerous precipitation - would it do for a sewer in Guernsey?
• Pollution during a downpour perhaps
• It'll account for all corrosion initially found on one sewer
• It's polluted in a Channel Islands sewer
• Threat to alfresco artwork
• Environmental awareness topic