I'm guessing here, but if your home has low humidity it could be static electricity.
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Agree you have some kind of serious problem, within either the house wiring or something plugged into your load. Water pipes are a dangerous place from which to take a safety ground, although many older homes were done this way. Telco used to do that too but they know better now. A local lightning strike could light up someone washing their hands, in the shower, or doing dishes!
Although ground rods are not a bad idea, many (I would say most, but I don't really know the statistics) local codes allow grounding to the copper water supply pipe. Many feet of copper tubing 3 - 4 ft under ground certainly contacts more earth than a 4 -5 foot rod. In very dry areas, deeper might be better for the moisture content. You do not need earth ground to have a complete circuit. The current flows through wires from the power company to your house, not through the ground. Static electricity is a possibility. Lightning rods discharge current to ground even when there is not a flash and boom. Other possibilities are socket(s) or light fixture(s) wired in reverse, or significant leakage to ground in some load. You could unplug and turn off everything and connect/turn on one at a time while measuring voltage from your ground buss to the pipe (or rod). GFCI outlets and breakers protect against the case where there is sufficient current flowing to earth ground that there is a possibility of dangerous voltage on the supposedly grounded (zero potential)housing of the appliances/tools. BTW - a spark tends to indicate that the pipe is indeed acting as earth ground. In any case, you should consult with your power company and/or electrician and be safe.
The only way there could be a spark is A) Static Electricity, B) A potential difference between the pipes (maybe earth ground - maybe not) and House ground. So measure the difference. Disconnect the water pipe and measure the voltage drop between the two. Since the house ground is not connected to the pipes and all wires have resistance there will be a potential difference – hence a spark. Good advise is to check with an electrician and do what your local code allows. You might also want to look into an isolated ground circuit. There are some good discussions here about isolated grounds, if you do a search in the discussion forum and you will find them. – Dan