I have a 2007 Buick LaCrosse. Automatic transmission. 150,000 miles.

I have had a coolant leak since I bought the car. The car has not run hot with me because I keep a constant check to make sure that the reservoir has water.

I can drive the car about a hundred miles and then I have to put 3/4 gallon of water in the plastic reservoir.

I have replaced the water pump, radiator and thermostat and it has not stopped the water leak. 

The water is leaking from beside the front passenger wheel below the reservoir. You can see it puddled on the ground. I have wondered if there could simply be a hole in the plastic reservoir.

I have taken this car to three different mechanics and get different answers from front engine cover or seal, cracked block, water pump, radiator and thermostat.

I don't notice that the oil looks weird in color. And I don't notice any excessive exhaust.

Incidentally, there is a PO300 random misfire code which I don't think has anything to do with this problem that they can't seem to fix either.

What could be causing this leak?

Thank you very much for your help or for any insight that you might have.