Estimation of nitrogen transformation in Cau River basin, Vietnam using a material flow analysis model

Excess nitrogen causes harmful algal blooms and pollution of surface water and groundwater, and has become an increasingly urgent issue in Cau River Basin, Vietnam. However, due to a lack of information, researchers struggle to find suitable tools for analysis of the problem. In order to overcome this, we introduce a material flow analysis (MFA) model to assess the environmental impact of human activities on nutrient flows at the river basin scale. The MFA model was used to estimate nitrogen transformation and identify the main source of nitrogen in Hai Duong province in Cau River Basin. The dynamic transformation of nitrogen was quantified using results from a survey and previous studies. All nitrogen inputs such as from agriculture, aquaculture, and domestic sources, as well as nitrogen outputs including to the atmosphere, surface water, and soil and groundwater were considered and estimated by mass balance and network equations. The rate of nitrogen transformation in each process and parameters of the MFA model were then determined and verified by measured data. The MFA model results indicate that nitrogen loads to the air accounted for 64.2% whereas nitrogen loads to surface water and soil or groundwater accounted for 23.9% and 11.9% respectively. Nitrogen loads from rice paddies were the main contributors to surface water (55.7%). Other contributions to surface water were from aquaculture (20.6%) and waste water (23.7%). Using the defined parameters and input data, nitrogen loads in Hai Duong province were estimated for a scenario in 2020. The MFA model is a useful tool that will support managers and authorities in assessing the economic development plan of Hai Duong province. It will also provide researchers with useful input data and information for dealing with environmental issues in Cau River Basin.