Deep Pool Effect And Experimental Manipulation Of Nitrogen And Phosphorus Retention In A Headwater Stream

As an indispensable part of the river system,headwater streams have the function of retention and reduction of nitrogen and phosphorus,which is of great significance to the regulation of water quality in the downstream.To investigate the characteristics of nitrogen and phosphate retention in natural deep pool and evaluate the effects of artificial regulation measures on the enhancement of nitrogen and phosphorus retention potential in streams,we selected Banqiao River,which is in the northern suburb of Hefei City,as the research object.Based on the field tracer experiments,the OTIS model was used to simulate hydrological parameters,and the retention potential and absorption kinetics of nitrogen and phosphorus in the deep pool were analyzed by estimating the nutrient nutrient spiraling metrics.According to the comparison of the changes of the transient storage potential and the nutrient spiraling metrics of the streams before and after the artificial regulation,the effectiveness of the artificial regulation measures in improving the retention potential of nitrogen and phosphorus in the streams was verified.The main results of this study are as follows:（1）Based on the OTIS model,the value of A_s/A in deep pools is relatively higher,and the exchange coefficient αbasically is at 10^-4 orders of magnitude,which is lower than that of a well-flowing stream.The mean values of S_w-N and S_w-Pin natural deep pools are 369m and 148m respectively,indicating that the retention capacity of nitrogen and phosphorus in natural deep pools is better,and that of PO₄^3-is slightly higher than that of NH₄⁺.（2）The results showed that the average S_w-amb of NH₄⁺and PO₄^3-in deep pool were 98.4m and 182.3m,respectively,which were significantly lower than the total length of the test stream.,and it means that the stream has a certain degree of retention capacity for nitrogen and phosphorus.The M-M equation simulation results shows that deep pools of U_max-N and U_max-P were 0.45、0.08 mg/（m²×s）,the K_m-Nand K_m-P average of 0.26、0.19 mg/L respectively.U_max-N>U_max-P indicates that the absorption rate of NH₄⁺is relatively stronger than that of PO₄^3-in deep pool.（3）By artificial regulations for stream found that the value of A_s/A of different level increase,the average growth of 47.9%and 20.3%respectively,and the exchange coefficientaalso increased significantly after being artificially regulations.Furthermore,after setting the Flooded bags,the hydraulic absorption length L_s of the stream becomes shorter and the hydraulic retention time R_h increases,indicating that artificial regulations are beneficial to improve the transient storage capacity of streams.（4）Compare to the control experiments,after setting diffirent distance Flooded bags,the average decrease of S_w-N value was 72.2%and 39.1%,respectively,and that of S_w-Pvalue was 70.1%and 48.8%,respectively.Similarly,the absorption rate U and mass transfer coefficient V_f also increased to varying degrees,indicating that the addition of Flooded bags was conducive to improving the retention of nitrogen and phosphorus nutrients in stream.（5）M-M equation can better show the dynamic absorption process of NH₄⁺and PO₄^3-after the artificial regulation of the stream,for 4 meters and 6 meters experiments,the average value of U_max-Nof NH₄⁺are 0.18 and 0.16 mg/（m²×s）,respectively,while U_max-P is not significantly different compare with the control experiment.In terms of K_m,the value of nitrogen and phosphorus nutrients in streams under artificial regulation decreased correspondently compared with that in the control experiment.