Over previous years, increasing amounts of pollutants carried by rivers entering Lake Erhai are suspected to have affected its water quality.The long-term effects of human life and industry on the rivers include deterioration of upstream and river vegetation and serious soil erosion, which are main contributory sources of sediment and nutrients flowing into Lake Erhai. Industrial and human wastes delivered directly into streams obviously affect the water quality of Lake Erhai. Consequently, the study of the water quality of the main lowland river entering Lake Erhai and analysis of the current pollution conditions of Lake Erhai will allow appropriate countermeasures to be taken and, correspondingly allow the nitrogen and phosphorus pollution load to be reduced. This has practical significance in managing the water environment of Lake Erhai and mitigating environmental stress arising from unreasonable human development and utilization of natural resources.
A survey and analysis were undertaken of the major nutrient distribution in Miju River, the largest river flowing into Lake Erhai, and the results contrasted with those of the water of the Lake. The purpose was to provide data for the planned construction of a wetland in the estuarine region of the Miju River, to mitigate the pollutant loading. Thirteen sampling points were located in the lower reaches of the Miju River, an area seriously impacted by human activities. The water quality of the Miju River is between III and V, based on The People's Republic of China surface water environment quality standards. The two major pollution sources are total nitrogen (TN) and total phosphorus (TP), with mean concentrations of 1.17 mg/L and 0.06 mg/L, respectively. The maximum TN concentration exceeded 2.00 mg/L, and concentrations varied monthly. NO^-₃-N concentrations varied from 0.21 to 1.42 mg/L, and NH₃-N concentrations ranged from 0.08 to 1.23 mg/L. Mean concentrations of NO^-₃-N and NH₃-N were 0.56 mg/L and 0.39 mg/L, respectively. Concentrations of both N and P in the Miju River varied seasonally, being much higher in the wet season than in the dry season. The maximum concentration of NO^-₃-N was in the dry season, while for NH₃-N it was in the early wet season. Contents of N and P in the Miju River were 2.10 and 2.90 times higher, respectively, than in Lake Erhai. Mean annual concentrations of TN and TP in Lake Erhai between 1990 and 2000 were 0.286 mg/L and 0.018 mg/L, respectively, and were 0.55 mg/L and 0.024 mg/L between 2004 and 2008. Annual concentrations of TN and TP in the dock of Erhai were 0.91 mg/L and 0.03 mg/L, respectively, which far exceeded the recorded water quality data. With an increase of rainfall, N concentrations were first positively correlated and then negatively related later, TP concentrations were more affected by the rainfall which is a principal characteristic of non-point source polluted rivers. Nitrogen and phosphorus, derived mainly from irrigated farming and cultivated land fertilization, sewage, trash, livestock manures and plant waste from cultivated land, were carried by rainfall runoff. In a broad sense, non-point pollution played a leading role in the pollutant concentrations observed in the Miju River.