DON’T CALL Nick Robbins a Doomsday prepper. Sure, Mr. Robbins, 50, a technology lawyer and general counsel for the Health Coach Institute, produces his own drinking water at his Phoenix home -- a move often associated with people who hoard canned goods and watch YouTube tutorials on how to field-dress a wound. But, for the most part, it was health considerations rather than apocalyptic fears that inspired him and his fiancée to go off the water grid.

With two small children, the couple was worried about pollutants like heavy...