Gravel pathways are a great alternative to concrete as they provide a permeable alternative. When there are too many impermeable surfaces such as concrete, rain water washes over these surfaces accumulating chemicals and other toxic substances (such as oil). This water, also referred to as run-off is collected by storm drains and is then delivered to waterways, polluting these streams and oceans. Since gravel pathways are permeable the rainwater is able to percolate through the rock and return to the ground as part of the water cycle.