Study On Riverine Phosphorus Pollution Characteristics And Quantitative Identification Of Phosphorus Source In Shuanggangxi Watershed

With the strengthening of point source pollution control,non-point source pollution has become the main reason for water pollution.Surface runoff and subsurface flows（base flow）are the key drivers of non-point source pollution.Due to the weak mobility of phosphorus（P）in soil,many previous studies considered that surface runoff was the main pathway for P non-point source pollution.While only very limited studies explored the P contribution from subsurface flows.Previous studies have reported considerable P leaching from soils with high P accumulation and high organic matter content,which will lead to groundwater P pollution.Therefore,the contribution from groundwater/subsurface flows to riverine P pollution should not be ignored.Combining watershed characteristics,meteorology,land-use and monthly monitoring water quality and hydrological data（from Oct.2017 to Dec.2020）in Shuangangxi River Watershed（a small typical agricultural watershed）in Zhejiang Province,this study explored the spatio-temporal dynamics of riverine P pollution with several conventional statistical analysis methods.By adopting water stable isotopes based sine-wave fitting method and nonlinear reservoir method,we estimated the contribution of surface runoff and subsurface flows to river runoff.By developing P load apportionment model,this study then quantified the P contributions from point source,surface runoff and subsurface flows.These results could provide crucial basis for effective riverine P pollution control.The main results are as follows:（1）During the study period,the average concentration of total phosphorus（TP）in Shuangangxi River was 0.12±0.10 mg L^-1.The water quality of the river reached the standard of classⅢ,which was mild to moderate eutrophication.The main form of riverine P pollution was dissolved phosphorus（DP,accounting for 76%）.Generally,the temporal variability of riverine P pollution was weak,while the spatial variation was significant between the main stream and tributaries.（2）In terms of temporal variation,rainfall showed a significant positive correlation to TP concentrations and particulate phosphorus（PP）concentrations（p<0.05）.There were significant positive correlations between DP concentrations and conductivity（p<0.01）,evaporation（p=0.019）and chloride ion concentrations（p=0.012）.In terms of spatial variation,there was a significant correlation between riverine PP concentrations and the percentages of farmland forest area.While there was no significant correlation between P concentrations in different forms and temperature,p H,dissolved oxygen and discharge.In terms of spatio-temporal variation,there was a significant correlation between DP concentrations in river water and groundwater（p=0.003）.The average DP concentration of groundwater in monitoring site was 0.08 mg L^-1(with the highest concentration reached 0.32 mg L^-1),which was significantly higher than that observed in river water.（3）The results of nonlinear reservoir method showed that the subsurface flows were the main source of the runoff for the main stream（average contribution:68%,range:28%-96%）.The results of sine-wave fitting method showed that young water（<47 days）fractions ranged from 26-29%（subsurface flows contributed 71-74%）,which indirectly supported the results estimated by the nonlinear reservoir method.Subsurface flows were mainly affected by temperature and rainfall,but less affected by human activities.（4）According to the load apportionment model,the riverine P pollution load contributed by non-point source pollution was further divided into surface runoff and subsurface flows contributions.The improved load apportionment model for Shuangangxi River showed high accuracy for TP（calibration period R²=0.73-0.79,NSE=0.61-0.77;verification period:R²=0.88-0.92,NSE=0.54-0.64）and DP（calibration period:R²=0.78-0.84,NSE=0.67-0.72;verification period:R²=0.92-0.98,NSE=0.56-0.83）.（5）Results based on the improved load apportionment model showed that the TP load and DP load were mainly from non-point source pollution,with the average contribution proportion were 77%and 88%,respectively.Compared with surface runoff contribution,subsurface flows contributed 63%and 73%of the TP load and DP load respectively.In summary,the subsurface flows contributions were the main cause of riverine P pollution,which might be mainly related to the long-term excessive phosphate fertilizer input,the acidity of the soil,and the considerable subsurface flows contribution to river runoff in Shuanggangxi River Watershed.Therefore,the key to control P pollution in Shuanggangxi River is to mitigate DP pollution in subsurface runoff effectively.