Spatiotemporal Distribution And Source Analysis Of Phosphates In Karst Basin Based On Phosphate Oxygen Isotopes

Phosphorus（P）is a fundamental nutrient element for organisms,but excessive input of phosphorus into aquatic systems can lead to eutrophication and affect water quality.Due to their unique geological and hydrological characteristics,karst basin are particularly vulnerable to phosphorus pollution.Therefore,understanding the temporal and spatial variations and sources of phosphorus in karst basin is of great significance for water resource management and protection.This study aims to explore the temporal and spatial distribution and source apportionment of phosphorus in surface water and groundwater in the Gaoping Basin,and to analyze the causes of phosphorus pollution in water bodies and its influencing factors.This article mainly focuses on the surface water and groundwater in the Gaoping Basin.Water samples were collected and analyzed during four different hydrological periods,including the physicochemical indicators,total phosphorus（TP）,dissolved total phosphorus（TDP）,soluble reactive phosphorus（SRP）,particulate phosphorus（PP）concentration,and the values of phosphate oxygen isotopes(δ¹⁸O_P),The aim is to explore their spatial and temporal distribution and the sources of phosphorus.The research results show that:（1）Characteristics of physicochemical parameters of water:the temperature of groundwater was relatively stable during the four periods,and the p H was neutral.The dissolved oxygen（DO）content was lower during the baseflow period（October）.The temperature of surface water fluctuated greatly,and the p H was weakly alkaline.The DO content was higher during the baseflow period（October）.The electrical conductivity（EC）and ORP of groundwater were higher than those of surface water.（2）Spatiotemporal variation characteristics of phosphorus in groundwater and surface water:in the Gaoping basin,the TP concentration of groundwater ranged from0.02 to 0.20 mg/L,with a mean of 0.06（±0.05）mg/L,which met the standard of Class II surface water environmental quality;the TP concentration of surface water ranged from 0.03 to 0.44 mg/L,with a mean of 0.15（±0.11）mg/L,which met the standard of Class III surface water environmental quality.Phosphorus in groundwater and surface water mainly existed in the form of TDP,followed by SRP,while the PP content was relatively low.The concentrations of phosphorus in groundwater and surface water differed significantly at different periods,with the overall trend showing that the phosphorus concentration in groundwater was highest during the baseflow period（May）,followed by the wet season,and lowest during the dry season.The phosphorus concentration in surface water was highest during the dry season,with the lowest TDP concentration in the baseflow period（May）,and the lowest PP and SRP concentrations during the wet season.Uneven distribution of annual rainfall is a factor affecting the seasonal variation of P concentration in groundwater and surface water.The P concentration in surface water is low during the rainy season and high during the dry season,while the P concentration in groundwater is high during the rainy season and low during the dry season,opposite to that of surface water.Meteorological conditions,land use types,domestic sewage,and agricultural non-point sources all contribute to the pollution of groundwater and surface water bodies.The spatial distribution of P is influenced by point and non-point sources,and shows significant heterogeneity in spatial distribution.（3）Analysis of the oxygen isotope composition and sources of dissolved phosphate in groundwater and surface water:the Eδ¹⁸O_P values of groundwater in the Gaoping basin ranged from 13.2‰to 19.2‰,while those of surface water ranged from 11.3‰to 19.4‰.Theδ¹⁸O_P values for groundwater samples ranged from 8.2 to 18.3‰,and for surface water samples from 8.8 to 18.3‰.Theδ¹⁸O_P values for all three periods deviate from the Eδ¹⁸O_P values.Theδ¹⁸O_P values at the same point varied considerably in time,with the lowest groundwaterδ¹⁸O_P values occurring during the baseflow period and the low surface waterδ¹⁸O_P values occurring during the abundant water period.The composition of theδ¹⁸O_P values differed significantly between phosphorus sources.By comparing theδ¹⁸O_P values with those of different phosphorus sources,P in groundwater may originate from fertilizer and farm irrigation as well as domestic sewage,and P in surface water may originate from migration of soil phosphorus and domestic sewage.This study explores the spatial and temporal distribution of phosphorus in karst basin waters and its source analysis based on phosphate oxygen isotopes.Through the analysis of water samples,the sources of phosphorus in water bodies and its spatial and temporal distribution are effectively analyzed,and the results can provide theoretical support for the prevention and management of water pollution in small karst agricultural watersheds,which can help to formulate reasonable water management strategies and reduce water pollution and ecological environment damage.