What is the best way to ground a DC solar system with no way to ground in the area but run a wire 600ft?

Solarenergyeffects:

If you are grounding for lightning protection, you could do two things, both would result in a very low impedance path to ground:

1. Install a marine grounding plate (Bronze) on the bottom of the dock
2. Install some length of solid copper #4 or bigger bare wire and attach to the mooring buoy of the dock.

Check out the article posted on lightning protection for more good info:

https://www.greentechrenewables.com/resource/solar-arrays-and-lightning-protection