The word “infiltration” has to do with spies. But it also has to do with stormwater.
When spies infiltrate a dangerous place, that means they sneak into it. When rainwater infiltrates the ground, it moves down through the soil. It “sneaks” into the ground.
In a city, water cannot infiltrate, or move down into, the ground when there is a lot of pavement. This can cause problems. The water has to go somewhere. So it flows off of the pavement and into the storm sewers. Unfortunately, rainwater picks up pollutants along the way, carrying them into the sewers.
Cities are loaded with pavement. When there is a lot of pavement, water cannot infiltrate, or move down into, the ground.