Have you measured voltage across the open connection? In my experience, it's fairly common to have several amps of current flowing in a ground wire, but when it is disconnected, the voltage across it is low.
Just because it sparks doesn't mean there's dangerous voltage or current. There's a reason the code requires ground wires like this to be bigger than a lamp cord. There's more stray current flowing through the earth than you can imagine, and there's a good chance that in your case, even if you completely disconnect the electrical system from the building there'd still be enough current to cause a spark.
If it's less than 20 volts, and less than 5 amps, I wouldn't be any too concerned.