From the "Electrical Grounding" book by NTT,Inc.:
There is a system known as an "Electrolytic Ground Rod" in which a copper-alloy tube/electrode, approx 2" diameter is inserted into a 6" diameter hole in the ground and then filled with a mixture of calcium chloride and sodium chloride. Then a slurry of bentonite is packed around the tube in the remaining void in the ground. Moisture from the air is pulled into the tube and accumulates with the salts and forms an low resistance electrolyte, which seeps into the surrounding bentonite, lowering the resistance of immediate area surrounding the tube/electrode. (Or something like that). Approx life - 25 years.