Yena Shen

A Bayesian approach for calculating variable total maximum daily loads and uncertainty assessment

To account for both variability and uncertainty in nonpoint source pollution, one dimensional water quality model was integrated with Bayesian statistics and load duration curve methods to develop a variable total maximum daily load (TMDL) for total nitrogen (TN). Bayesian statistics was adopted to inversely calibrate the unknown parameters in the model, i.e., area-specific export rate (E) and in-stream loss rate coefficient (K) for TN, from the stream monitoring data. Prior distributions for E and K based on published measurements were developed to support Bayesian parameter calibration. Then the resulting E and K values were used in water quality model for simulation of catchment TN export load, TMDL and required load reduction along with their uncertainties in the ChangLe River agricultural watershed in eastern China. Results indicated that the export load, TMDL and required load reduction for TN synchronously increased with increasing stream water discharge. The uncertainties associated with these estimates also presented temporal variability with higher uncertainties for the high flow regime and lower uncertainties for the low flow regime. To assure 90% compliance with the targeted in-stream TN concentration of 2.0mgL(-1), the required load reduction was determined to be 1.7 × 10(3), 4.6 × 10(3), and 14.6 × 10(3)kg TNd (-1) for low, median and high flow regimes, respectively. The integrated modeling approach developed in this study allows decision makers to determine the required load reduction for different TN compliance levels while incorporating both flow-dependent variability and uncertainty assessment to support practical adaptive implementation of TMDL programs.

Spatio-temporal variations of nitrogen in an agricultural watershed in eastern China: Catchment export, stream attenuation and discharge

Using the monthly hydrogeochemical data of ChangLe River system from 2004 to 2008, total nitrogen (TN) export load (S(n)) from nonpoint sources (NPS) to stream and in-stream attenuation load (A(L)) was estimated by the inverse and forward format of an existing in-stream nutrient transport equation, respectively. Estimated S(n) contributed 96 ± 2% of TN entering the river system, while A(L) reduced the input TN by 23 ± 14% in average. In-stream TN attenuation efficiency in high flow periods (10 ± 5% in average for the entire river system) was much lower than that in low flow periods (39 ± 17%). TN attenuation efficiency in tributaries (28 ± 16% in average) was much higher than that in mainstream (11 ± 8%). Hydrological conditions are important in determining the spatio-temporal distributions of NPS TN export, stream attenuation and discharge. Increasing the water residence time might be a practical method for mitigating stream TN.

Response of Stream Pollution Characteristics to Catchment Land Cover in Cao-E River Basin, China

Abstract This study addressed the relationship of river water pollution characteristics to land covers and human activities in the catchments in a complete river system named Cao-E River in eastern China. Based on the hydrogeochemical data collected monthly over a period of 3 years, cluster analysis (CA) and principal component analysis (PCA) were adopted to categorize the river reaches and reveal their pollution characteristics. According to the differences of water quality in the river reaches and land use patterns and average population densities in their catchments, the whole river system could be categorized into three groups of river reaches, i.e. , non-point sources pollution reaches (NPSPR), urban reaches (UR) and mixed sources pollution reaches (MSPR). In UR and MSPR, the water quality was mainly impacted by nutrient and organic pollution, while in NPSPR nutrient pollution was the main cause. The nitrate was the main nitrogen form in NPSPR and particulate phosphorus was the main phosphorus form in MSPR. There were no apparent trends for the variations of pollutant concentrations with increasing river flows in NPSPR and MSPR, while in UR the pollutant concentrations decreased with increasing river flows. Thus dry season was the critical period for water pollution control in UR. Therefore, catchment land covers and human activities had significant impact on river reach water pollution type, nutrient forms and water quality responses to hydrological conditions, which might be crucial for developing strategies to combat water pollution in watershed scale.

Relationship between catchment characteristics and nitrogen forms in Cao-E River Basin, Eastern China

The distribution of different nitrogen forms and their spatial and temporal variations in different pollution types of tributaries or reaches were investigated. Based on the catchments characteristics the tributaries or reaches can be classified into 4 types, including headwater in mountainous areas (type I), agricultural non-point source (NPS) pollution in rural areas (type II), municipal and industrial pollution in urban areas (type III), and combined pollution in main stream (type IV). Water samples were collected monthly from July 2003 to June 2006 in the Cao-E River Basin in Zhejiang, eastern China. The concentrations of NO3(-)-N, NH4(+)-N, and total nitrogen (TN) were measured. The mean concentrations of NO3(-)-N were decreased in the sequence type IV > type II > type I > type I, whereas, NH4(+)-N, total organic nitrogen (TON), and TN were in the sequence: type III > type IV > type II > type I. In headwater and rural reaches, CNO3(-)-N was much higher than CNH4(+)-N. In urban reaches, TON and NH4(+)-N were the main forms, accounting for 54.7% and 32.1% of TN, respectively. In the whole river system, CNH4(+)-N decreased with increasing distance from cities, and CNO3(-)-N increased with the increasing area of farmland in the catchments. With increased river flow, CNO3(-)-N increased and CNH4(+)-N decreased in all types of reaches, while the variations of CTON and CTN were different. For TN, the concentration may be decreased with the increase of river flow, but the export load always increased.