First Response:

If it is true that you have a flow rate of 50 gpm, it is possible to use more than 660000 gallons of water in 30 days.

Since 30 days = 720 hours = 43200 minutes

50 gpm x 43200 minutes = 2160000 gallons

You are right to question how much water can pass through a 1.5 inch pipe in 30 days. I did some research about flow rate and as far as I can tell 10 gpm is reasonable for a home. This would mean

10 gpm x 43200 minutes = 432000 gallons

So even if you were running your water for the entire 30 days, it would not even be possible to use that much water. I definitely would be asking some questions to local officials, plumbers or someone who would be more qualified to evaluate your situation.

Hope this helps,

Janice

Second Response:

You are right that flow depends on pressure and width of pipe; it also depends on how long a run of that pipe you are dealing with. That last you possibly do not know.

However, the figure you give, 50GPM, is not a pressure but a flow rate, so if it's accurate we are past that hurdle. 50GPM is 3000 gallons per hour, 72,000 gallons per day, or 2,160,000 gallons per month. Working backwards, 660,000 gallons is only about a third of that, which would come out as about sixteen gallons per minute.

However, that does sound like a lot of water. You may be able to demonstrate meter failure or reader error by looking at other evidence.

* Can you put that much water through your taps? See how long it takes to fill a gallon jug at each tap, when all the other taps are turned on. Divide 1 by each of those numbers to get gallons per minute for each tap [eg, three minutes -> 0.33 gal/min.) If those add to significantly under sixteen, you cannot be using this much water inside.
* I don't know where you live in the US, but in many places you cannot easily use water outside at this time of year.
* A US gallon of water is a little under 4 liters. 660,000 gallons is about 2.5 million liters, or 2500 cubic meters of water. That's enough to fill an Olympic-sized swimming pool. If that water were not put continuously into a drain, where would it go? Much less than that amount of water would turn a typical backyard to mud. You cannot have used that much water in casual watering. Irrigating a major field, possibly.
* I'm assuming your water is metered at the house. If there is any way you could be losing this water BETWEEN the meter and the house, you would have a major break AND a washout allowing it to drain away. This would be very serious of course, but seems unlikely.
Good hunting!
RD