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Are there any plumbing fixtures along that wall inside the house, or an outside water spigot above the leak?
Based on what you have told me it appears that there must be a water line that has a leak in that wall. I would get the water main to the house shut off ASAP. Call the City water dept. for assistance if needed. you will most likely have to open up the wall for the repair. Gaining access from the inside if possible should be the best route but I don't know what your house is like.
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