Here are some other terms that can be used, depending on the context:

* Impervious: Similar to impermeable, but can also refer to things that are not affected by something.

* Dense: Describes soil or rock that is tightly packed, making it difficult for water to pass through.

* Compacted: Similar to dense, referring to soil or rock that has been pressed together, making it less porous.

* Non-porous: This describes a material with no spaces for water to flow through.

The best word to use will depend on the specific situation and the level of formality required.