A posted speed limit of 55 mph means that we can drive at a speed of 55 miles per hour(mph) under good conditions in that area. We cannot that speed limit in that area.
Groundwater is all the water that has penetrated the earth's surface and is found in one of two soil layers. The one nearest the surface is the "zone of aeration", where gaps between soil are filled with both air and water. Below this layer is the "zone of saturation", where the gaps are filled with water.