Establishing reference nutrient conditions using improved statistical methods in a river network with typical monsoon climatic pattern

Establishing reference conditions for water quality variables is a critical step towards developing pollutant reduction goals for river pollution control. The term reference condition is used to describe the naturalness condition of the water quality, and helps us identify the best water quality that can be obtainable if human impacts are absolutely controlled. The percentile of reference streams approach (PRSA) and the percentile of general streams approach (PGSA) are two methods recommended by the U.S. Environmental Protection Agency (U.S. EPA) for developing nutrient reference conditions. The PGSA is a surrogate for the PRSA to establish nutrient conditions when reference data are unavailable. Many studies had examined the matching relationship between the PRSA and the PGSA for developing nutrient reference conditions. However, there is little information on whether there were differences on the matching relationships between the PRSA and the PGSA for different nutrients. In addition, it is unknown whether very high seasonal variations in precipitation and river runoff in monsoon climatic region could affect the well-known PRSA-PGSA relationship. In this study, we determined the total nitrogen (TN) and total phosphorus (TP) reference conditions in the dry and rainy seasons using the PRSA, the PGSA and regression approach based on hydro-chemical data in the Luan River with typical monsoon climatic pattern during 2006-2013. The results showed that the matching relationships between the PRSA and the PGSA were different for TN and TP, and were consistent in dry and rainy seasons. The regression approach results verified the different matching relationships. The reference nutrient concentrations established by the three methods all had seasonal variations. The recommended reference concentrations of the Luan River for TN were 1.642 mg/L and 1.297 mg/L respectively in dry and rainy season, while the reference concentrations for TP were 0.041 mg/L and 0.049 mg/L, respectively. These results imply that a few reference sites should be located to develop the reference-to-general river sites relationship for different nutrient species, and the seasonal variation of the reference nutrient concentrations should be considered for better water quality management.