Well, you probably ended up with some precipitate in the bottom of the jug, right? That's likely calcium carbonate, and it's now inert.
Your alk solution won't be as potent as it could be, so if you're dosing to maintain a certain alk, it will take more of this tainted solution.
It won't hurt anything, but if you've carefully tested to determine the optimum dose needed to keep your alk in line, you'll have to throw out that calculation and start over.
Were it me, and I was just using baked baking soda, I'd pitch the gallon and start over. Baking soda is cheap.