The researchers studied the water quality of several rivers in Tianjin at different times of the year. They used a method called Principal Component Analysis to evaluate the water quality. The results showed that the water quality was worst in the dry season, with over half of the rivers having poor quality. However, the water quality improved in the wet season. The rivers south of the Haihe River were more polluted, especially with ammonia nitrogen, BOD5, CODMn, and TP, indicating organic and eutrophic pollution.