SPRINKLER TALK FORUM - You Got Questions, We've Got Answers

Gravity. AS valves, and vacuum breakers, work on gravity. Higher than anything they feed, with no shutoff valves downstream, and open to the atmosphere when there is no pressure in the device, the downstream water would have to defy gravity, and climb uphill in an open pipe, in order to cause contamination.

It would depend on what equipment you're using. Do you have a brand name you can google and look for information with? Actual 'sprinkler heads' would be above grade. Even drip emitters are meant to be above grade. They make little tube supports for positioning, and even 'bug caps' to keep critters out of the tubing.

And you could have an antisyphon valve feeding your drip. It just takes some plumbing skills to connect one.

Is the irrigation system on a hill such that once the system is turned off, the water in the line drains out of an emitter at the low end, sucking in air (and possibly some mulch) from the high end?

The other possibility is if your hydrant is lower than the emitter, the same thing could be happening when you disconnect the 1/2" line, or the hydrant might have a PVB (Pressure Vacume Breaker) that is working sort of like an anti-siphon valve. But in that situation, the PVB is just there to insure water doesn't flow back into the hydrant. If there is a hose connected to the hydrant, and the water in the hose is higher than the hydrant (causing the water to TRY to flow back into the hydrant, a PVB would protect the hydrant by discharging the water. Basically, the way the PVB works is that when there is water pressure from behind (i.e. the hydrant), it pushes a stop forward that covers drain hole and all the water goes in the hose. But when the water is shut off (i.e. no more pressure behind it) the excess pressure on the front side opens the drain holes and the water drains out. Now if that is happening, then the emitters should be sucking in air (and mulch) as the water drains.

By the way, do you have a filter on the water supply. Even if you have city water, there is still plenty of "stuff" that could clog drip irrigation. That is why ALL drip irrigation needs a FINE filter (150 mesh). If you go to one of the big box retailer and purchase a kit made to connect to a hose spicot, the kit will contain a backflow preventer (basically an extreamly simple form of PVB), a filter, and some connection to the 1/2" tubing usually used for drip irrigation.