Total Phosphorus in Reservoirs: What Is the Normal Standard?

Reservoirs are classified as lake-type water bodies under China's Environmental Quality Standards for Surface Water (GB 3838-2002), which sets stricter total phosphorus limits for lakes and reservoirs than for rivers because their slower flow makes them far more vulnerable to eutrophication.

The standard defines five water quality classes. For lakes and reservoirs, the Class I limit for total phosphorus is 0.01 mg/L, suitable for headwater sources and national nature reserves. Class II, applying to first-grade protected drinking water sources, allows up to 0.025 mg/L. 

Class III, covering second-grade drinking water source protection zones, sets a limit of 0.05 mg/L. Class IV, for general industrial and non-contact recreational use, permits up to 0.1 mg/L, while Class V, for agricultural and general landscape use, allows up to 0.2 mg/L.

The contrast with rivers is striking. For Class III water, rivers may contain up to 0.2 mg/L of total phosphorus, four times the reservoir limit of 0.05 mg/L. This difference reflects the fact that phosphorus entering a reservoir tends to remain in the system, accumulating in sediments and fuelling algal blooms, whereas flowing rivers can flush excess nutrients downstream.

In practice, a reservoir meeting Class III—the minimum standard for a drinking water source—should keep total phosphorus at or below 0.05 mg/L. Many well-managed reservoirs perform better: monitoring reports from Chinese reservoirs routinely show concentrations of 0.01 to 0.03 mg/L, comfortably within Class II limits. 

Maintaining these levels requires controlling agricultural runoff, wastewater discharge, and internal nutrient loading, since once phosphorus accumulates in reservoir sediments, it can be released back into the water column for years.