Vinegar is a mild acid. Limestone is alkaline. If your gravel is mostly limestone it will keep your water high in p.h. with alkaline. So when you put high alkaline in acid it bubbles. Put a small amount of baking soda in a small amount of vinegar and watch it foam up. put baking soda and water on your car's battery terminal and watch it foam up. They tend to nutralize each other. Good hard flint river rock shouldn't bubble in vinegar.