Backflow Prevention from Yard Hydrants and Cattle Waterers

Hello,

I am a couple days away from the backhoe arriving to help me bury piping for (2) heated livestock waterers and (3) yard hydrants.
I live in Zone 3b, so the recommended water line bury depth is 8ft to prevent freezing.

I am just now starting to worry about the possibility of backflow/siphoning in the event that I lose water pressure.

For anyone not familiar with a yard hydrant, there is a drain in the bottom of it and when the flow is shut off, the water in the vertical pipe is supposed to drain out below ground.
Additionally, I am installing (2) curb stop valves so that I can turn off water to the hydrants or to the cattle waterers separately. Since the cattle waterers don't have drain-to-waste holes like the yard hydrants, I am using a drain-to-waste curb stove valve so that it will drain when I turned it off and vented the waterers.

I am NOT worried about water siphoning in from hoses connected to the yard hydrants (I can easily install vacuum breaks).
I AM worried about water siphoning in from underground, where the drain ports are.

In theory, if I lost water pressure (lets say my pressure switch failed and shut off my well pump), then a vacuum could be created which could siphon dirty underground water (from my cattle area, i.e., not clean) in to my drinking water.

The simple answer is to install a "backflow preventer".
But, I need my entire plumbing system to be 8ft underground to prevent freezing, and I cannot seem to find any reliable underground backflow prevention units that I could use.
They all seem to need to break to atmosphere and typically are installed above ground.

Any suggestions on preventing the possibility of backflow while keeping my plumbing underground and reliable (i.e., backflow device that doesn't require routine maintenance)?

Thanks,

Could only be an issue if you left a hose connected to the hydrant put in a water tank  with the hydrant handle left in the on position  and even then it's a far stretch at that I've been using hydrants the water livestock all my life I've never heard of that happening however people that use hydrants to fill up chemical tanks with hoses need to use backflow devices  I don't think there's any dangerous bacteria that comes out of the Cattles mouths into the water cup of the automatic water that could be dangerous to humans anyway

You didn't say what size line, or what it's made of. If it's only something like 3/4" PEX you can install one of these deep in the ground. Just remember that one day it'll go bad and you'll be digging it up.
http://amzn.to/2dycsni

Personally, I'd do without, and I'm a plumber.

Primates have been drinking dirty water since before we were primates. We were built to easily handle that sort of thing. Besides, whatever, if dirty water gets into the pipes, just run clean water through them when the water pressure comes back on. How often does the water pressure go off anyway? Once a year?