Loading Mechanisms And Environmental Effects Of Groundwater Nitrate In A Agricultural Catchment Of Central Subtropical

In last decades,the rapid development of agriculture has induced a series of environmental problems in China,for example,mass discharge of rural domestic sewage and untreated aquaculture wastewater;Improper application of chemical fertilizers and pesticides;Irrational landscape layout;Unreasonable land development and utilization and so on.These problems make the ecological environment gradually deteriorate,which also seriously affects the survival and health of the local people and the sustainable development of agriculture in China.In this paper,we selected Jinjing River catchment as a typical middle subtropics agricultural catchment,which is located in Changsha County,Hunan Province,China.In the four seasons of 2013-2014,we randomly sampled 120 catchment of water wells,to study the effect of landscape pattern on groundwater nitrate nitrogen?NO₃^--N?concentration.The spatial distribution of groundwater pollution and the effects of different land use patterns on groundwater pollution intensity were studied by using the method of spatial statistics.The purpose of this study is to clarify the temporal and spatial variation of NO₃^--N pollution in catchment groundwater,to understand the effect of land use on nitrogen pollution of groundwater,to quantify the contribution of basin groundwater runoff process to NO₃^--N output,to understand the effect of land use on groundwater NO₃^--N output.The main results of this research are listed as follows:?1?groundwater NO₃^--N pollution occurred in the catchment,with 4.9%-17.5%samples exceeding the WHO drinking water standards(>10 mg N L^-1),and particularly higher pollution frequencies in summer and winter than those in spring and autumn seasons;?2?groundwater NO₃^--N concentrations showed obvious spatial auto-correlation mainly due to the NO₃^--N transport through groundwater flow,and the Moran's global indices were greater in summer and autumn?0.254-0.277?than in spring and winter?0.152-0.170?at the catchment scale;?3?groundwater NO₃^--N concentrations significantly related to the areal proportion of agriculture,forest,and residential area when fitted by the spatial lag regression model?P<0.05?,and their determine coefficients of fitting?R²?strongly varied with seasons and the radius from the wells.Therefore,the results could provide technical basis for reasonable landscape planning to protect the groundwater environment and drinking water safety in subtropical hilly agricultural catchments.?4?Tuojia River watershed?727 millimeter?which represents high strength rice cultivation generates more baseflow than Jianshan River watershed?426.5 millimeter?which represents low strength rice cultivation.Moreever,Tuojia River watershed has a higher contribution rate of base flow than Jianshan?Tuojia 41.9%,Jianshan 28.4%?.However,rice farming may result in significant loss of NO₃^--N in groundwater systems.The average concentration of NO₃^--N in Tuojia based flow was higher than Jianshan(1.43 vs.1.07 mg N L^-1).In addition,the NO3-N output of the Tuojia based flow is greater than Jianshan,especially in the fallow season.In conclusion,the results of this paper could provide basic technical basis by protecting the groundwater environment and ensuring the safety of drinking water in the middle subtropical hilly region for the agricultural watershed.