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Abstract [Objectives] This study was conducted to systematically monitor the contents of "three nitrogen"， including ammonia nitrogen （NH+ 4 N）， nitrate nitrogen （NO- 3 N） and nitrite nitrogen （NO- 2 N） in groundwater in Ledong and analyze the reasons.
[Methods] A total of 30 sampling points were set up in Ledong， including 22 ground waters and 8 surface waters. NO- 3 N， NH+ 4 N， NO- 2 N and pH indicators were detected.
[Results] ① There is a problem of excessive nitrogen in the groundwater of Ledong， Hainan. NH+ 4 N and NO- 3 N exceeded the standards severely， and the over standard rates were 27.27% and 13.64%， respectively. NO- 2 N didnt exceed the standard. Among them， the highest concentration of NH+ 4 N was up to 2.692 mg/L; and the highest concentration of NO- 3 N reached 24.071 mg/L. ② The pollution trends of NH+ 4 N and NO- 2 N in Ledong County were similar， and NO- 3 N had no similarity. ③ There was a regional difference in the groundwater three nitrogen in Ledong， Hainan. The pollution level near the farmland was greater than that far from the farmland. The pollution level near the farm area was greater than that far from the farm area. In addition， in combination with the layout of the sampling points and the test results， it was not found that the area far from the coastline would be lower than the area near the coastline in the pollution level. ④ The main reasons for the excessive nitrogen in the groundwater of Ledong， Hainan are farmland fertilization， random discharge of domestic sewage， discharge of aquaculture wastewater， and discharge of industrial water at will.
[Conclusions] This study is of great significance to safe prevention and control of groundwater in Ledong.
Key words Groundwater; Three nitrogen; Pollution; Survey
In recent years， the quality of groundwater has deteriorated， and the three nitrogen pollution has become one of the most common and prominent water pollution problems. In the groundwater environment， the main nitrogen compounds are ionic NH+ 4 N， NO- 3 N and NO- 2 N， i.e.， often referred to as "three nitrogen". The main causes of three nitrogen pollution in groundwater are farmland fertilization， domestic sewage， and random discharge of nitrogenous industrial wastewater， leaching and infiltration of solid waste， sewage recharge， atmospheric deposition， etc.[1]. Groundwater three nitrogen pollution exists in many cities at home and abroad [2-6]. According to the statistical analysis of groundwater quality in 130 cities and regions in China， the overall quality of groundwater in the country is good， but most cities still receive a certain degree of point and non point source pollution， resulting in the condition that some elements exceed the standard in local areas. The NO- 3 N， total hardness and salinity of groundwater in cities such as Beijing， Tianjin， Shanghai and Shijiazhuang increase generally[7]. The nitrogen pollution of groundwater in Tongliao City is getting more and more serious. The over standard rates of NH+ 4 N and NO- 2 N are 78% and 24.0%， respectively， and the maximum concentrations are 4.78 and 0.627 mg/L， respectively[8]; and NH+ 4 N exceeds the standard by 19.82% in Kunming[9]. From this point of view， the investigation and detection of groundwater three nitrogen pollution are of great significance to the control of nitrogen pollution. Ledong Li Autonomous County of Hainan Province is located in the southwest of Hainan Province and southwest of the South China Sea. It is known as the "natural greenhouse"， "tropical corp treasure land"， "green treasure house" and "hometown of cashew nuts". It is the largest cashew base in China and winter cucurbit and vegetable production base， higher place pond shrimp base， banana production base and grain， oil， mango， betel nut and rubber production bases. The agriculture in Ledong area of Hainan is relatively developed， and the groundwater may have problems such as agricultural non point source pollution. Therefore， sampling points in Ledong County were set to systematically monitor three nitrogen contents in groundwater and analyze the causes of pollution， which will provide certain support and advices on safety control for Ledong groundwater.
Materials and Methods
Arrangement of sampling point
Drinking water sources and pollution in rural areas of Ledong were preliminarily investigated， and according to the sources of groundwater and different land use types， 30 sampling points were set in Foluo Town， Yinggehai Town， Huangliu Town， Liguo Town and Jiuzhen Town， including 22 groundwater points and 8 surface water points. The detection indicators included pH value， NO- 3 N， NO- 2 N and NH+ 4 N. The water samples were mainly taken from the existing water wells. Water was directly sampled at the point with an electric pump or a hand pump， and at other points， a glass water collector was used to collect the samples. After the water sample was collected， it was stored at 4 ℃ and brought back to the laboratory for monitoring. The sampling diagram is shown in Fig. 1. The numbering of the points was from top to bottom and from left to right.
Experimental instruments and methods
According to the "Monitoring and Analysis Methods for Water and Wastewater" （the 4 th edition）， NH+ 4 N， NO- 3 N and NO- 2 N were determined by Nesslers reagent spectrophotometry （722 visible spectrophotometer， Shanghai Opler Instrument Co.， Ltd.） ， phenol disulfonic acid spectrophotometry and N （1 naphthyl） ethylenediamine dihydrochloride spectrophotometry， respectively. The pH value was measured directly by a pH meter （METTLER DELTA 320）.
Results and Discussion
Analysis of the over standard situation of groundwater three nitrogen
The concentration of NH+ 4 N in groundwater in Ledong， Hainan Province ranged from 0.002 to 2.691 mg/L， with an average concentration of 0.578 mg/L （Table 1）; the concentration of NO- 2 N ranged from 0.002 to 0.194 mg/L， with an average concentration of 0.044 mg/L; and the concentration of NO- 3 N ranged from 0.170 to 24.071 mg/L， with an average concentration of 7.550 mg/L. According to Chinas groundwater quality standard （GB/T 14848 2017）， NH+ 4 N and NO- 3 N exceeded the standards severely， and the over standard rates were 27.27% and 13.64% ， respectively. NO- 2 N didnt exceed the standard. The pH range was 7.03-7.85， and the water was alkaline.
Characteristics and causes of three nitrogen pollution
As shown in Fig. 2， the concentration of NH+ 4 N in such nine samples as DX7， DX8， DX9， DX10， DX13， DX14， DX15， DX16 and DX20 exceeded the standard value. In combination with the sampling point layout （Fig. 1）， it was found that the four sampling points， DX7， DX8， DX9 and DX10， were distributed around DB6 which was fish pond culture water， and combined with surface water NH+ 4 N concentration （Fig. 3）， it was estimated that the reason for excessive NH+ 4 N at DX7， DX8， DX9 and DX10 was the discharge of aquaculture water. In addition， DX7 and DX8 were in the vicinity of Yingge Sea Saltern， and it was speculated that the concentrations of NH+ 4 N at these two sampling points exceeding the standard was also related to the discharge of salt field wastewater. The four sampling points， DX13， DX14， DX15 and DX16， were distributed around DB8 which was rice field water sample， so the NH+ 4 N concentrations at DX13， DX14， DX15 and DX16 exceeding the standard might be caused by farmland fertilization. DX20 was on the edge of a river， and it was speculated that the excessive NH+ 4 N was because that the local residents domestic wastewater was discharged into the river and then infiltrated into the groundwater. In addition， from the pH value， it could be known that the water samples were all alkaline， which was not conducive to the progress of the nitrification reaction， and also caused the NH+ 4 N to accumulate continuously， which led to the high concentration of NH+ 4 N. The four sampling points， DX7， DX8， DX9 and DX10， were obviously higher than DX11 and DX12， and it can be inferred that the concentration of NH+ 4 N will decrease slowly with the distance from the farm increasing. The concentrations of NH+ 4 N at such four sampling points as DX13， DX14， DX15 and DX16 were higher than that at DX18， and it can be inferred that the concentration of NH+ 4 N pollution will gradually decrease when getting away from farmland. It was further concluded that the over standard concentrations of NH+ 4 N at such four sampling points as DX7， DX8， DX9 and DX10 might be caused by aquaculture water discharge， and the over standard concentrations of NH+ 4 N at such four sampling points as DX13， DX14， DX15 and DX16 might be caused by farmland fertilization.
As an intermediate， NO- 2 N is mainly converted from NH+ 4 N and NO- 3 N. The trends of NO- 2 N and NH+ 4 N concentrations in the groundwater samples collected were similar， indicating that they might originate from a unified source of pollution. The concentrations of NO- 2 N at the 10 sample points， DX3， DX7， DX8， DX10， DX13， DX14， DX15， DX16， DX19 and DX20， exceeded the standard value （Fig. 4）. The surface water NO- 2 N is shown in Fig. 5. DX3 was taken from the well water near the surface water DB3 which was the fish pond culture water， and the NO- 2 N concentration exceeding the standard at DX3 might be caused by the discharge of aquaculture wastewater. Such three sampling points as DX7， DX8 and DX10 were distributed around DB6 which was aquaculture wastewater， so it was estimated that the excessive NO- 2 N at the three sampling points was caused by the discharge of aquaculture wastewater. In addition， DX7 and DX8 were located near Yingge Sea Salt Saltworks， and it was speculated that the over standard concentrations of NO- 2 N at these two sampling points is also related to the discharge of salt field wastewater. Such four sampling points as DX13， DX14， DX15 and DX16 were distributed around DB which was a rice field water sample， so it was speculated that the over standard concentrations of NO- 2 N at the four sampling points were caused by fertilization in farmland. At the two sampling points， DX19 and DX20， the pollution was caused by the discharge of wastewater from local residents into the river and then into the groundwater. The highest concentration of NO- 3 N is the main form of three nitrogen in groundwater. This is because NO- 3 N cannot be easily adsorbed by soil and is highly soluble in water， so it is easy to accumulate in groundwater （Fig. 6）， The concentrations of NO- 3 N at the three sampling points， DX10， DX19 and DX22， exceeded the standard. The over standard concentrations of NO- 3 N at DX10 and DX19 were caused by the discharge of domestic wastewater mainly， and also partially affected by fertilization in farmland.
Conclusions
In this study， the groundwater in Ledong， Hainan was taken as the research object. A total of 30 sampling points were arranged in Ledong， including 22 groundwater points and 8 surface water points. Such four indices as NO- 3 N， NH+ 4 N， NO- 2 N and pH value were detected. The following conclusions are drawn：
（1） There is a problem of excessive nitrogen in the groundwater of Ledong， Hainan. NH+ 4 N and NO- 3 N exceeded the standards severely， and the over standard rates were 27.27% and 13.64% ， respectively. NO- 2 N didnt exceed the standard. Among them， the highest concentration of NH+ 4 N was up to 2.692 mg/L; and the highest concentration of NO- 3 N reached 24.071 mg/L.
（2） The pollution trends of NH+ 4 N and NO- 2 N in Ledong County were similar， and NO- 3 N had no similarity.
（3） There was a regional difference in the groundwater three nitrogen in Ledong， Hainan. The pollution level near the farmland was greater than that far from the farmland. The pollution level near the farm area was greater than that far from the farm area. In addition， in combination with the layout of the sampling points and the test results， it was not found that the area far from the coastline would be lower than the area near the coastline in the pollution level.
（4） The main reasons for the excessive nitrogen in the groundwater of Ledong， Hainan are farmland fertilization， random discharge of domestic sewage， discharge of aquaculture wastewater， and discharge of industrial water at will.
References
[1] CHEN X. Removal of nitrate in source water by biological denitrification with solid carbon source[D]. Nanjing： Nanjing Forestry University， 2008. （in Chinese）
[2] YAN LJ. Present situation， harm and removal technology of nitrate pollution in water[J]. Energy Environmental Protection， 2013， 27： 39-42. （in Chinese）
[3] ERNESTO PASTN ZAPATA， ROGELIO LEDESMA RUIZ， THOMAS HARTER， et al. Assessment of sources and fate of nitrate in shallow groundwater of an agricultural area by using a multi tracer approach[J]. Science of the Total Environment， 2014， 470（43）： 855-864. [4] KUMARI RINA， DATTA P S， CHANDER KUMAR SINGH， et al. Determining the genetic origin of nitrate contamination in aquifers of Northern Gujarat， India[J]. Environmental Earth Sciences， 2014， 71（4）： 1711-1719.
[5] NECAT ACA， SEMA KARANLIK， BERKANT DEMI
瘙 塁 . Assessment of ammonium， nitrate， phosphate， and heavy metal pollution in groundwater from Amik Plain， southern Turkey[J]. Environmental Monitoring and Assessment， 2014， 186（9）： 5921-5934.
[6] SACCHI E， ACUTIS M， BARTOLI M， et al. Origin and fate of nitrates in groundwater from the central Po plain： insights from isotopic investigations[J]. Appl Geochem， 2013， 34： 164-180.
[7] DING KN， HAO AB， WANG MK. Characteristics and mechanism of groundwater pollution in Shijiazhuang City[J]. Hydrogeology and Engineering Geology， 1996， （6）： 29. （in Chinese）
[8] FANG SY， LEI GF， BAO MR， et al. Investigation on the status of three nitrogen pollution in groundwater in Tongliao City[J]. Journal of Environment and Health， 2002， 19 （2）： 121. （in Chinese）
[9] HAN RP， WANG YW， YANG ZL. Hygienic status and analysis of self provided deep well water in Kunming City[J]. Yunnan Preventive Medicine， 2001， 6（2）： 58. （in Chinese）
[Methods] A total of 30 sampling points were set up in Ledong， including 22 ground waters and 8 surface waters. NO- 3 N， NH+ 4 N， NO- 2 N and pH indicators were detected.
[Results] ① There is a problem of excessive nitrogen in the groundwater of Ledong， Hainan. NH+ 4 N and NO- 3 N exceeded the standards severely， and the over standard rates were 27.27% and 13.64%， respectively. NO- 2 N didnt exceed the standard. Among them， the highest concentration of NH+ 4 N was up to 2.692 mg/L; and the highest concentration of NO- 3 N reached 24.071 mg/L. ② The pollution trends of NH+ 4 N and NO- 2 N in Ledong County were similar， and NO- 3 N had no similarity. ③ There was a regional difference in the groundwater three nitrogen in Ledong， Hainan. The pollution level near the farmland was greater than that far from the farmland. The pollution level near the farm area was greater than that far from the farm area. In addition， in combination with the layout of the sampling points and the test results， it was not found that the area far from the coastline would be lower than the area near the coastline in the pollution level. ④ The main reasons for the excessive nitrogen in the groundwater of Ledong， Hainan are farmland fertilization， random discharge of domestic sewage， discharge of aquaculture wastewater， and discharge of industrial water at will.
[Conclusions] This study is of great significance to safe prevention and control of groundwater in Ledong.
Key words Groundwater; Three nitrogen; Pollution; Survey
In recent years， the quality of groundwater has deteriorated， and the three nitrogen pollution has become one of the most common and prominent water pollution problems. In the groundwater environment， the main nitrogen compounds are ionic NH+ 4 N， NO- 3 N and NO- 2 N， i.e.， often referred to as "three nitrogen". The main causes of three nitrogen pollution in groundwater are farmland fertilization， domestic sewage， and random discharge of nitrogenous industrial wastewater， leaching and infiltration of solid waste， sewage recharge， atmospheric deposition， etc.[1]. Groundwater three nitrogen pollution exists in many cities at home and abroad [2-6]. According to the statistical analysis of groundwater quality in 130 cities and regions in China， the overall quality of groundwater in the country is good， but most cities still receive a certain degree of point and non point source pollution， resulting in the condition that some elements exceed the standard in local areas. The NO- 3 N， total hardness and salinity of groundwater in cities such as Beijing， Tianjin， Shanghai and Shijiazhuang increase generally[7]. The nitrogen pollution of groundwater in Tongliao City is getting more and more serious. The over standard rates of NH+ 4 N and NO- 2 N are 78% and 24.0%， respectively， and the maximum concentrations are 4.78 and 0.627 mg/L， respectively[8]; and NH+ 4 N exceeds the standard by 19.82% in Kunming[9]. From this point of view， the investigation and detection of groundwater three nitrogen pollution are of great significance to the control of nitrogen pollution. Ledong Li Autonomous County of Hainan Province is located in the southwest of Hainan Province and southwest of the South China Sea. It is known as the "natural greenhouse"， "tropical corp treasure land"， "green treasure house" and "hometown of cashew nuts". It is the largest cashew base in China and winter cucurbit and vegetable production base， higher place pond shrimp base， banana production base and grain， oil， mango， betel nut and rubber production bases. The agriculture in Ledong area of Hainan is relatively developed， and the groundwater may have problems such as agricultural non point source pollution. Therefore， sampling points in Ledong County were set to systematically monitor three nitrogen contents in groundwater and analyze the causes of pollution， which will provide certain support and advices on safety control for Ledong groundwater.
Materials and Methods
Arrangement of sampling point
Drinking water sources and pollution in rural areas of Ledong were preliminarily investigated， and according to the sources of groundwater and different land use types， 30 sampling points were set in Foluo Town， Yinggehai Town， Huangliu Town， Liguo Town and Jiuzhen Town， including 22 groundwater points and 8 surface water points. The detection indicators included pH value， NO- 3 N， NO- 2 N and NH+ 4 N. The water samples were mainly taken from the existing water wells. Water was directly sampled at the point with an electric pump or a hand pump， and at other points， a glass water collector was used to collect the samples. After the water sample was collected， it was stored at 4 ℃ and brought back to the laboratory for monitoring. The sampling diagram is shown in Fig. 1. The numbering of the points was from top to bottom and from left to right.
Experimental instruments and methods
According to the "Monitoring and Analysis Methods for Water and Wastewater" （the 4 th edition）， NH+ 4 N， NO- 3 N and NO- 2 N were determined by Nesslers reagent spectrophotometry （722 visible spectrophotometer， Shanghai Opler Instrument Co.， Ltd.） ， phenol disulfonic acid spectrophotometry and N （1 naphthyl） ethylenediamine dihydrochloride spectrophotometry， respectively. The pH value was measured directly by a pH meter （METTLER DELTA 320）.
Results and Discussion
Analysis of the over standard situation of groundwater three nitrogen
The concentration of NH+ 4 N in groundwater in Ledong， Hainan Province ranged from 0.002 to 2.691 mg/L， with an average concentration of 0.578 mg/L （Table 1）; the concentration of NO- 2 N ranged from 0.002 to 0.194 mg/L， with an average concentration of 0.044 mg/L; and the concentration of NO- 3 N ranged from 0.170 to 24.071 mg/L， with an average concentration of 7.550 mg/L. According to Chinas groundwater quality standard （GB/T 14848 2017）， NH+ 4 N and NO- 3 N exceeded the standards severely， and the over standard rates were 27.27% and 13.64% ， respectively. NO- 2 N didnt exceed the standard. The pH range was 7.03-7.85， and the water was alkaline.
Characteristics and causes of three nitrogen pollution
As shown in Fig. 2， the concentration of NH+ 4 N in such nine samples as DX7， DX8， DX9， DX10， DX13， DX14， DX15， DX16 and DX20 exceeded the standard value. In combination with the sampling point layout （Fig. 1）， it was found that the four sampling points， DX7， DX8， DX9 and DX10， were distributed around DB6 which was fish pond culture water， and combined with surface water NH+ 4 N concentration （Fig. 3）， it was estimated that the reason for excessive NH+ 4 N at DX7， DX8， DX9 and DX10 was the discharge of aquaculture water. In addition， DX7 and DX8 were in the vicinity of Yingge Sea Saltern， and it was speculated that the concentrations of NH+ 4 N at these two sampling points exceeding the standard was also related to the discharge of salt field wastewater. The four sampling points， DX13， DX14， DX15 and DX16， were distributed around DB8 which was rice field water sample， so the NH+ 4 N concentrations at DX13， DX14， DX15 and DX16 exceeding the standard might be caused by farmland fertilization. DX20 was on the edge of a river， and it was speculated that the excessive NH+ 4 N was because that the local residents domestic wastewater was discharged into the river and then infiltrated into the groundwater. In addition， from the pH value， it could be known that the water samples were all alkaline， which was not conducive to the progress of the nitrification reaction， and also caused the NH+ 4 N to accumulate continuously， which led to the high concentration of NH+ 4 N. The four sampling points， DX7， DX8， DX9 and DX10， were obviously higher than DX11 and DX12， and it can be inferred that the concentration of NH+ 4 N will decrease slowly with the distance from the farm increasing. The concentrations of NH+ 4 N at such four sampling points as DX13， DX14， DX15 and DX16 were higher than that at DX18， and it can be inferred that the concentration of NH+ 4 N pollution will gradually decrease when getting away from farmland. It was further concluded that the over standard concentrations of NH+ 4 N at such four sampling points as DX7， DX8， DX9 and DX10 might be caused by aquaculture water discharge， and the over standard concentrations of NH+ 4 N at such four sampling points as DX13， DX14， DX15 and DX16 might be caused by farmland fertilization.
As an intermediate， NO- 2 N is mainly converted from NH+ 4 N and NO- 3 N. The trends of NO- 2 N and NH+ 4 N concentrations in the groundwater samples collected were similar， indicating that they might originate from a unified source of pollution. The concentrations of NO- 2 N at the 10 sample points， DX3， DX7， DX8， DX10， DX13， DX14， DX15， DX16， DX19 and DX20， exceeded the standard value （Fig. 4）. The surface water NO- 2 N is shown in Fig. 5. DX3 was taken from the well water near the surface water DB3 which was the fish pond culture water， and the NO- 2 N concentration exceeding the standard at DX3 might be caused by the discharge of aquaculture wastewater. Such three sampling points as DX7， DX8 and DX10 were distributed around DB6 which was aquaculture wastewater， so it was estimated that the excessive NO- 2 N at the three sampling points was caused by the discharge of aquaculture wastewater. In addition， DX7 and DX8 were located near Yingge Sea Salt Saltworks， and it was speculated that the over standard concentrations of NO- 2 N at these two sampling points is also related to the discharge of salt field wastewater. Such four sampling points as DX13， DX14， DX15 and DX16 were distributed around DB which was a rice field water sample， so it was speculated that the over standard concentrations of NO- 2 N at the four sampling points were caused by fertilization in farmland. At the two sampling points， DX19 and DX20， the pollution was caused by the discharge of wastewater from local residents into the river and then into the groundwater. The highest concentration of NO- 3 N is the main form of three nitrogen in groundwater. This is because NO- 3 N cannot be easily adsorbed by soil and is highly soluble in water， so it is easy to accumulate in groundwater （Fig. 6）， The concentrations of NO- 3 N at the three sampling points， DX10， DX19 and DX22， exceeded the standard. The over standard concentrations of NO- 3 N at DX10 and DX19 were caused by the discharge of domestic wastewater mainly， and also partially affected by fertilization in farmland.
Conclusions
In this study， the groundwater in Ledong， Hainan was taken as the research object. A total of 30 sampling points were arranged in Ledong， including 22 groundwater points and 8 surface water points. Such four indices as NO- 3 N， NH+ 4 N， NO- 2 N and pH value were detected. The following conclusions are drawn：
（1） There is a problem of excessive nitrogen in the groundwater of Ledong， Hainan. NH+ 4 N and NO- 3 N exceeded the standards severely， and the over standard rates were 27.27% and 13.64% ， respectively. NO- 2 N didnt exceed the standard. Among them， the highest concentration of NH+ 4 N was up to 2.692 mg/L; and the highest concentration of NO- 3 N reached 24.071 mg/L.
（2） The pollution trends of NH+ 4 N and NO- 2 N in Ledong County were similar， and NO- 3 N had no similarity.
（3） There was a regional difference in the groundwater three nitrogen in Ledong， Hainan. The pollution level near the farmland was greater than that far from the farmland. The pollution level near the farm area was greater than that far from the farm area. In addition， in combination with the layout of the sampling points and the test results， it was not found that the area far from the coastline would be lower than the area near the coastline in the pollution level.
（4） The main reasons for the excessive nitrogen in the groundwater of Ledong， Hainan are farmland fertilization， random discharge of domestic sewage， discharge of aquaculture wastewater， and discharge of industrial water at will.
References
[1] CHEN X. Removal of nitrate in source water by biological denitrification with solid carbon source[D]. Nanjing： Nanjing Forestry University， 2008. （in Chinese）
[2] YAN LJ. Present situation， harm and removal technology of nitrate pollution in water[J]. Energy Environmental Protection， 2013， 27： 39-42. （in Chinese）
[3] ERNESTO PASTN ZAPATA， ROGELIO LEDESMA RUIZ， THOMAS HARTER， et al. Assessment of sources and fate of nitrate in shallow groundwater of an agricultural area by using a multi tracer approach[J]. Science of the Total Environment， 2014， 470（43）： 855-864. [4] KUMARI RINA， DATTA P S， CHANDER KUMAR SINGH， et al. Determining the genetic origin of nitrate contamination in aquifers of Northern Gujarat， India[J]. Environmental Earth Sciences， 2014， 71（4）： 1711-1719.
[5] NECAT ACA， SEMA KARANLIK， BERKANT DEMI
瘙 塁 . Assessment of ammonium， nitrate， phosphate， and heavy metal pollution in groundwater from Amik Plain， southern Turkey[J]. Environmental Monitoring and Assessment， 2014， 186（9）： 5921-5934.
[6] SACCHI E， ACUTIS M， BARTOLI M， et al. Origin and fate of nitrates in groundwater from the central Po plain： insights from isotopic investigations[J]. Appl Geochem， 2013， 34： 164-180.
[7] DING KN， HAO AB， WANG MK. Characteristics and mechanism of groundwater pollution in Shijiazhuang City[J]. Hydrogeology and Engineering Geology， 1996， （6）： 29. （in Chinese）
[8] FANG SY， LEI GF， BAO MR， et al. Investigation on the status of three nitrogen pollution in groundwater in Tongliao City[J]. Journal of Environment and Health， 2002， 19 （2）： 121. （in Chinese）
[9] HAN RP， WANG YW， YANG ZL. Hygienic status and analysis of self provided deep well water in Kunming City[J]. Yunnan Preventive Medicine， 2001， 6（2）： 58. （in Chinese）