Because of our clay soils, we can't just count on storm water percolating through the earth to clean itself. So when we build or improve roads, we often incorporate water quality bioswales. These swales are designed and built with a thin layer of gravel as ballast rock which then develops a nice coat of slime. The slime means living organisms are at work to clean the water which flows off the roadway. Planted with rushes and sedges right through the gravel, this engineered green infrastructure will be invisibly doing its work for decades on end.