Degradation coefficient of water quality in Dan River based on water mass tracking method
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Abstract:
Degradation coefficients are important parameters in water quality models and are commonly used to describe pollutant contents attenuation rate. The research on degradation coefficients primarily focuses on large rivers and plain river networks. There are few quantitative studies on the variability of degradation coefficients in mountain rivers and the calculation errors caused by excessively low pollutant contents. The Dan River, part of the Qin River system in the Yellow River basin, is mountainous and, its watershed is dotted with numerous villages and towns, and the disordered discharge of agricultural and domestic sewage has caused severe non-Point source pollution, creating an urgent need for research on water quality degradation coefficients in mountain rivers. To elucidate the variability of pollutant degradation coefficients in mountain rivers, in-situ water mass tracking was used for field monitoring of the Niucun section of the Dan River to elucidate the variability of pollutant degradation coefficients in mountain rivers. Based on this, the "two-Point method" and "multi-Point method" derived from the first-order reaction kinetics model were used to calculate the degradation coefficients of different pollutants. A one-dimensional advection-diffusion model was used to calculate pollutant prediction values, and the relative error of prediction values was quantitatively studied to evaluate the errors in the calculation process of degradation coefficients. To analyze the impact mechanisms of degradation coefficients in the environment, and to explore the differences in the response mechanisms of mountain river water quality degradation coefficients to different environmental factors compared to plain areas regression and correlation analyses were conducted to understand the relationship between the degradation coefficients and environmental factors in the study area. The results showed that the degradation coefficients calculated for ammonia nitrogen (NH+4-N) in the Niucun section of the Dan River were (0.684±0.486) d?1, for total nitrogen (TN) were (0.518±0.411 d?1, for nitrate nitrogen (NO-3-N) were (0.444±0.280) d?1, for nitrite nitrogen (NO-2-N) were (0.628±0.350) d?1, for chemical oxygen demand (COD) were (0.482±0.343) d?1, for five-day biochemical oxygen demand (BOD5) were (0.491±0.430) d?1, for total phosphorus (TP) were (0.905±0.461) d?1, and for orthophosphate (PO3-4-P) were (1.226±0.612) d?1. Threshold analysis revealed that when the mass concentration of NH+4-N ≥ 0.6 mg/L, and the mass concentrations of TP and PO3-4-P ≥ 0.04 mg/L, the prediction value mass concentration calculation error tends to slow down and remains within a reasonable range. Correlation and regression analyses showed that the main influencing factors for the degradation coefficients were water temperature, potential of hydrogen (pH), oxidation-reduction potential (ORP), and dissolved oxygen (DO). By analyzing the response relationship of hydrological factors to degradation coefficients, it was found that the contact area between the pollutants and the riverbank increased when the flow rate was less than 2.726 m/s, causing the TP degradation coefficient to increase with flow rate. When the flow speed exceeded 0.459 m/s, the residence time of the pollutants on the river surface decreased, causing the TP degradation coefficient to decrease with increasing flow speed. It is concluded that the greater vertical gradient in mountainous areas compresses the horizontal ecological characteristics of rivers, causing the ecological processes in mountain rivers to respond more intensely to hydrological conditions and temperature changes than in flat areas. Compared to plain areas, the water quality degradation coefficients in mountain rivers are higher. Through quantitative analysis of the relative error of prediction values, it was found that below a certain threshold mass concentration, the first-order kinetics model could not simulate molecular diffusion effectively, leading to increased error in the calculation of degradation coefficients. Above a certain mass concentration threshold, the error fluctuations in the calculation results of the degradation coefficients by the first-order kinetics model tended to stabilize and were acceptable. In the Dan River basin, the degradation effect of total phosphorus (TP) was best under moderate flow speeds. The research results are of great significance for the water quality management of the Dan River, providing a reference for understanding the degradation process of pollutants in mountainous rivers and the ecological regulation of dams and gates.
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