Betsy, sounds like low oxygen could be the culprit. Our shallow groundwater also has a swamp smell (high iron) before its treated for drinking.
You're right Tabor. Las Vegas gets 4 inches of rain "per year!" If I recall correctly they get their water from the ground and the nearby Lake Mead reservoir. That reservoir is down around 100 feet from its 1999 level. Perhaps the El Nino winter will replenish it.