Gravel, composed of sand, silt or clay and varying sizes of broken limestone, quartzite and granite, is dumped then flattened onto a pathway to create a road. The good news is gravel is permeable, so water and pollutants drain downward. The bad news is that the bottom layers of the gravel wind up packed so tightly water can't flow through them. Polluted waters pool atop those impermeable spots way beneath the gravel's surface, making it difficult to identify those pools and dashing hopes of routing the pooled water through purifying pathways.
Gravel is dusty, too, but gravel roads are less traveled than paved roads and therefore accumulate and relay fewer pollutants.