Combined Use Of Groundwater Dating And Stable Isotopes To Resolve The Changes Of Nitrate Sources

Groundwater nitrate contamination is an environmental problem worldwide,threatening the human health severely.It is necessary to identify the sources and variation trends of nitrate accurately so as to achieve the effective prevention and control of groundwater nitrate pollution.Under the background of industrialization and urbanization,the source of nitrate in high-intensity exploited aquifer is particularly complex and changeable,which is the difficulty in the analysis of nitrate source in groundwater.The Hutuo River catchment area was selected to reveal the variation of nitrate sources in the over-exploited aquifer and to identify the main controlling factors of nitrate pollution in the context of urbanization,by combined use of chronologic,isotopic,and statistical methods.And then the conceptual pattern of the nitrate sources evolution was established.The study mainly emphasizes on the several aspects as follows:(1)The content of nitrate varies significantly in time and space:The sites with high content of nitrate(>15mg/L in nitrogen)were expanded from scattered point distribution to plane distribution horizontally and extended from above 50m to 60-120m vertically;The high content of nitrate are mainly located in the urban,areas near the Hutuo River and sewage irrigation area in Luancheng.(2)Groundwater ages were determined by combi1ing rH-He method and rH series dating method.The nitrate time series in recharge was established according to the groundwater age and sampling time.(3)The groundwater age,nitrogen and oxygen isotopes were used to identify the sources and their changes of nitrate qualitatively and quantitatively.The sources of farmland irrigated with clear water were changed from manure(51.2%)to mixture of manure and chemical fertilizer(33.9 and 32.7%respectively).The sources of sewage irrigation area were changed from industrial sewage to domestic wastewater mainly.The sources of nitrate in residential area are derived from manure and sewage(58.1-64.7%),with no significant changes along with the recharge time.The sources of nitrate were transformed from chemical fertilizer and manure to sewage and manure in the areas where the land use has changed from the farmland to the urban.(4)The evolution process of nitrate contamination with the recharge time was analyzed and its controlling factors were investigated under different land-use.The nitrate in groundwater increased with recharge time on the whole.The agriCLiltural development is the main factor related to the nitrate contamination in the farmland,and rapid urbanization,industrial development,and over-exploitation of groundwater are the main factors influencing the nitrate content in urban area.(5)Based on the analysis of the change of groundwater nitrate sources and driving mechanism,the evolution process of nitrate sources in the study area is divided into three stages:the primary agricultural development stage,the large-scale agricultural development stage and the rapid urbanization development stage.According to the above research results,the distinct prevention and control measures were proposed to mitigate the groundwater nitrate pollution based on the land use types and groundwater age.