The study looked at how pollution from rainfall runoff affects water quality in Dongguan City. Researchers analyzed runoff from different types of land and found that the pollution levels were similar to sewage, which is bad for the water in Tongsha Reservoir. They also found that the first part of runoff after rain, called the first flush, carries a lot of pollutants. Factors like heavy rain, dry days before the rain, and how much water can soak into the ground all affect how much pollution gets into the water. The study suggests that capturing the first 40% of runoff can help reduce pollution more effectively and economically than capturing it all.