If a rock has high porosity but low permeability, what does this imply about groundwater movement?

• A. It can hold a lot of water but transmits it slowly
• B. It transmits water quickly but holds little water
• C. It transmits water rapidly due to high porosity
• D. It neither stores nor transmits water effectively

Answer: (A)

Porosity determines how much water a rock can store, while permeability determines how easily water can move through interconnected pore spaces. When a rock has high porosity, it can hold a large amount of water in its pore spaces. If the permeability is low, those pore spaces aren’t well connected or the channels are very narrow, so water moves through the rock only very slowly. So groundwater movement through such a rock is slow, even though it can store a lot of water. In short, it’s a good reservoir for water but a poor conduit for rapid flow.