Is there an RFID tag that can identify water piping located 3 or 4 feet underground?
Yes. Berntsen makes a product called InfraMarker, which is designed for precisely this application. Here are a few articles we have written about this solution:
You can watch videos that the company presented at past RFID Journal LIVE! events here: Most Innovative Use of RFID, Award Finalist for Best Use of RFID to Enhance a Product or Service: Berntsen International and RFID Infrastructure Marking for a Connected World.
A decade ago, 3M also introduced a system for tracking underground pipes (see 3M Upgrades Dynatel Locators), but I do not know if they are still selling it.
—Mark Roberti, Founder and Editor, RFID Journal