Phosphorus is a big problem in freshwater, causing eutrophication. To fix this, we need to control where the phosphorus is coming from. In Xi'an's water source, they used math to figure out how much phosphorus is in the water. Rain brings in a lot of phosphorus, which is bad for the water quality. Some villages are big sources of phosphorus pollution. To fix this, we should turn farmland into forests, plant more vegetation, and reduce farming.