I have some amount of experience with tingle voltages on plumbing and properly grounded appliances. In 1 case a 40 foot long or so underground conduit had rotted to the point where it was leaking 7 amps at 120 volts during a drought. The current flow was energizing the drainpipes and basement floor.
To get rid of the nasty tingle voltage at the bathtub I had to bond the cold water pipe, the hot water pipe, AND the drain pipe to the grounding electrode conductor. Since Article 680 of the National Electrical Code allows homemade ground clamps that are made out of stainless steel I used stainless steel hose clamps and washers to bond to the cast iron drain pipe. I also had to file a bare spot on the cast iron and then corrosion proof it with Ilsco Deox electrical grease. Some utilities require this when you have an electric water heater - good idea even if you do not have one.
To get rid of the tingle voltage at the washing machine I had to just simply cut the power to the detached garage.
If you are really worried about the resistance of your substation ground mat you should think about sinking some copper or galvanized steel pipes very deep into the ground. If your soil is amenable to it you can use a Borit or Borzit (2 different companies) directional drilling tool to sink the pipes into the ground. This is a rather cheap tool that is used for directional drilling underneath driveways and sidewalks and to build small diameter shallow wells. Plan on having to leave the drilling bit at the bottom of the hole. You could also drill small holes ( 1/4 inch 6 millimeter) holes along the pipe so that you can attach a water hose to the top to beef up soil conductivity. You may even be able to configure some as shallow wells for watering the others.
Or, have a well driller sink some metal well casings well below the water table. You could also do a combination system that uses deep wells to irrigate your ground mat.
