Urbanisation has a great effect on the run off pattern by contributing to the existence of building surfaces. These surfaces already inhibit layers of pollutants that have blanketed upon each other over a period of time. The first flush of stormwater runoff washes away the pollutants on the citified base and further gets accumulated which is the pollution which has either suspended solids or dissolved ones. The moment the rainwater falls on it, it's not “just” water anymore. There are a lot of things that the water interacts with and takes them with itself as it flows.
This website zooms into these unnoticed tiny day to day pollutants and our contribution to its existence.
Danica Liongson.,Polly Sinclair, 2020.Thinking Through Soil:Wastewater agriculture soils. Harvard University.