I have power at the corner of my dam and am looking at running a 5 diffuser system. The longest run will be a double diffuser almost 500ft away. It will go down to a depth of 20ft before rising across a point to settle at 12-15ft deep. Would it be cost effective to run pvc for majority of the run? How is everyone running air that far from the pump?

There shouldn't be any reason you can't use pvc. I know there are some people that have used Pex. The issue with both of those is keeping them on the bottom.

The concern to me would be going down to 20' and then back up to a shallower depth. The air is coming from the ambient temperature, which in summer could be pretty high, and then cooling. In this case it is going to cool quite a bit down to 20'. The air has water vapor in it and as it cools the water will condense out - the same process as dew showing up in the morning when temps drop overnight. That happens to everybody but In your case the condensed water is going to settle in your low spot and 500' is a lot of line. The result is to increase the pressure loss and depending on your compressors cfm output and discharge pressure you might not be able to push the water out. It will definitely raise your discharge pressure. If you aren't on a tight budget the best way to overcome the issue is to get a compressor with high volume and high discharge pressure. You already have a 500' run and are planning on running others so you are already going to be dealing with a lot of pressure drop just from line loss. No matter what compressor you get be sure you have your lines plumbed so that you can isolate each line. That way if you start seeing increasing pump pressures you can isolate each line for a few hours and try to blow it out that way. The other thing that would help is to run a smaller diameter line so that when you try to blow the line clean or even under normal operation the velocity of the air is greater and has more ability to push water through the line. HOWEVER, running small diameter line is going to significantly increase your line loss and is not what you would typically want to do. Normally, you would want to run larger diameter line not smaller.

Sorry to take your time if you already knew all this but your system is going to have some issues that most people probably don't see. If you are on a budget and trying to size the pump as accurately as possible the low spot is going to require some of your discharge pressure but it is hard to quantify how much. High humidity is going to make it worse and I assume it gets pretty humid in Kentucky.

Is this going to be run on dry land, or under water?

If under water, just use the weighted airline going to each diffuser assembly, any water in the line will not be a problem and neither will be the dip. If on land, then the dip will cause problems with water condensing and collecting in the dip. You'd have to run some type of anti-freeze system in the winter to prevent the line from freezing, unless you can bury it all below the frost line.

We have done single runs of over 800 feet to a 5 diffuser airstation, all under water using BottomLine weighted airline from Vertex. We have also run black poly line over 2,000 feet under ground from the compressor to the pond without any issues.