Study On The Change Characteristics And Regulation Of "Three Nitrogen" In Shallow Groundwater In Linqing City

Linqing city is located in the downstream of the Weishan irrigation area,with poor river diversion conditions and insufficient surface water resources.In order to meet the needs of residents and economic and social development,long-term exploitation and use of groundwater has broken the natural circulation state of groundwater system,and shallow groundwater has become an important part of local water resources.Long-term exploitation has resulted in the formation of a groundwater funnel zone in Linqing city,and has also affected the groundwater quality.In order to study the "three nitrogen" distribution of shallow groundwater,to protect the local groundwater resources and water environment,choosing the Linqing city as the research area,this paper analyzed the embedded depth of groundwater,water level,the distribution features of the "three nitrogen",and the correlation between other components and "three nitrogen".The groundwater flow model and the nitrate and ammonia migration model of groundwater was established in the study area.Scenario analysis method based on established models was adopted,which was under different nitrogen application modes in each typical year(2013,2015 and 2018)to simulate the nitrate and ammonia migration in groundwater in the study area.The results showed:(1)The annual mean value of groundwater depth and annual mean value of water level at the states in the study area from 2008 to 2018 did not change much.The annual mean value of groundwater depth in each year was around 2~18m,and the annual mean value of water level was around 16~31m.From 2008 to 2018,the maximum groundwater depth and the lowest groundwater level in the study area occurred in the central urban area and its surrounding areas every year,followed by Yandian town and Panzhuang town in the west,and Liugaizi town,Daiwan town,Kangzhuang town,Laozhaozhuang town and Jinhaozhuang town in the east.(2)During the sampling period,the concentration of nitrate in the groundwater in the study area ranged from 0 to 12.00 mg/L,the concentration of nitrite ranged from 0 to 0.080mg/L,and the concentration of ammonia nitrogen ranged from 0 to 7.00 mg/L.The spatial and temporal variability of "three nitrogen" distribution were all large.The concentration of ammonia nitrogen in the water samples at the monitoring well of Daxinzhuang station had been significantly higher than that at other sampling points.In all groundwater samples,the concentration of nitrate was higher in April and gradually decreased after that.Nitrate concentrations in May,June and July were all lower than 2 mg/L.With the passage of time,the concentration of nitrite did not show a strong regularity,fluctuating within a small range of 0~0.060 mg/L.Ammonia nitrogen levels peaked in early May.The Pearson correlation coefficient between each reference index and "three nitrogen" showed that the correlation degree varied with time and place.(3)Based on the MODFLOW module and MT3 D module of Visual MODFLOW,the groundwater flow model and the nitrate and ammonia migration model of groundwater in the study area were successively established.After parameter identification and verification,the majority of the normalized RMS of the models were less than 10%,with relatively accurate accuracy.(4)Overall,the groundwater level in the study area was the highest in 2013,followed by2018,and the lowest in 2015.Regardless of annual rainfall,groundwater levels were higher in the western,south-central and eastern parts of the study area,and lower in the north-central(urban and surrounding)areas.In fact,the level of groundwater was the external representation of the water equilibrium state of the groundwater system where the study area was located.(5)Under the condition of the same nitrogenous fertilizer application mode,in terms of three typical years,the groundwater nitrogenous concentration of the study area was higher in2013,followed by 2018 and lowest in 2015;In the same level year,the maximum concentration value of nitrate and ammonia in the simulation period were the highest under the condition of mode 1,followed by mode 2 and mode 3,and mode 4 was lowest.The influence of groundwater runoff on nitrate nitrogen and ammonia nitrogen migration in the shallow groundwater was greater than that of groundwater depth.Along the direction ofgroundwater runoff,the mass concentrations of nitrate and ammonia nitrogen showed an increasing trend and had a cumulative effect,which was especially significant in dry years.(6)The protection of groundwater environment still depended on protection.For farmland groundwater system,corresponding protective measures should be taken in the process of source,destination and migration of nitrogen.The research content of this paper could provide reference for the regulation of local shallow groundwater "three nitrogen",groundwater resources and groundwater environment protection.