| Nitrate Pollution is one of prevalent and serious dangerous Pollutants for water body, In recent years, there are continuous reports about rivers, lakes and city groundwater are polluted by nitrate, producing the eutrophication of water body, inducing esophageal cancer and causing serious harm to human health. In the karst area of Southwest China, there are 2836 subterranean rivers having been found with a total discharge of 1482 m3.s-1 and amount to the discharge of Yellow river. The karst groundwater resource that has an important role on people’s health and social and economic development are also polluted seriously with the intensification of industrial and agricultural production and the expansion of urbanization. Karst groundwater of Southwest China is facing the threat which becomes to a sewer, karst groundwater pollution in China has been so serious that the usable quantities of groundwater resources are very little and the protection of karst groundwater could not be delayed.Because the double-layer structure and the "three-water" fast switching in Karst regions of Southwest China, the karst groundwater response to environment sensitively. At the same time, because of the randomness of nitrate pollution, the complexity of the process of mechanism, the uncertainty of the ways and pollutants of discharging. Therefore, It is necessary to investigate the dynamic changes and influencing factors of nitrate from different spatial and temporal scales in the karst area, it can provide scientific knowledge to understand the dynamic changes of nitrate roundly and scientific reference for preventing nitrate pollution in the karst area. In this paper, we take the Guancun underground river of Liuzhou as an example and discuss the dynamic changes and influencing factors of nitrate between the outlet of the underground river and the surface stream contrastively in different temporal scales (season, rainfall, diel) basing on the outlet of Guancun underground river and the surface stream which is recharged by Guancun underground river. The following conclusions are obtained through this research.(1) NO3- ion of study area showed obvious seasonal variation, NO3- of underground river outlet was autumn>winter>spring>summer, NO3- of surface stream was autumn>winter>summer>spring, two monitoring points were higher during the dry season, and the highest value of NO3- in underground river outlet and surface stream were both in November, mainly due to the low flow in dry season,the minimum value were both in April and the NO3- ion in spring had a dramatic change, these may be affected by rainfall process.NO3- ion of the stream showed the process of nitrogen loss in September, October, November, December, February, March, August, the loss was 2.46kgN.d-1、1.77 kgN.d-1、3.01 kgN.d-1、2.09 kgN.d"1、2.08 kgN.d-1、17.34 kgN.d-1 respectively, and showed the process of nitrogen growth in January, April, June, July, the growth quantity was 1.06 kgN.d-1、6.84 kgN.d-1、5.92kgN.d-1、0.7 kgN.d-1 respectively. The ratio betweenδ15N-NO3- and NO3-18O-NO3- in the stream showed a positive correlation (R2=0.38, P<0.05) and most fell in the area of the ratio of 1:1, indicating that mainly occurred assimilation in the stream, and only a small part of the ratio fell in the vicinity of 2:1, indicating that denitrification affect little seasonal variation of NO3- ion in the atream. In addition, the loss of nitrogen was 28.75 kgN.d-1 (not including the loss of March and August) while the growth quantity was 14.52 kgN.d-1 in a year, indicating the process of nitrogen loss occurred mainly and was mainly affected by assimilation of aquatic plants. However the process of nitrogen loss did not occurred in January, April, June, July, mainly due to the effect of rainy eluviation, made the high concentrations of nitrate in the soil around the stream into the stream, then made the stream nitrate content increase and covered the effect of assimilation.(2) The response of nitrate showed a similar trend on the whole in the underground river outlet and surface stream after the rain, but the nitrate of surface stream also showed some different "unusual" changes with the nitrate of underground river outlet and influencing factors were more complicated than underground river outlet, the nitrate of surface stream were affected not only by recharged water of underground river outlet but also by the rain outside and soil water. And the outside rain and soil water made different effects on surface stream in different intensity and times of rainfall, for the first rainfall time, the high concentration of NO3- ion in soil water came into surface stream and improved the concentration of NO3- ion of surface stream, after some times of rainfall, the concentration of NO3- ion of soil water decreased gradually and played a dilution effect.In the two rainfall monitoring, the NO3- ion and electrical conductivity of the underground river outlet and surface stream showed more quickly response in R2 (the second rainfall monitoring in 2013 August) and rainfall process of 2014 May than in R1 (the first rainfall monitoring in 2013 August), it was mainly due to the larger intensity of rainfall in R2 and 2014 May and the more concentrated rainfall, that is to say rainfall intensity will affect response time and speed of NO3- ion and electrical conductivity. The greater the rainfall intensity has, the NO3- ion and electrical conductivity react more quickly.(3)The physical and chemical parameters (T、pH、DO、Spc、pC2) and NO3- ion of stream showed regular diurnal variations in 2013 July diel monitoring, the T, pH and DO showed increased during daytime but decreased during nighttime, conversely, Spc, pCO2 and NO3- ion showed decreased during daytime and increased during nighttime.The supply of underground river outlet affected little to the regular changes of physical and chemical parameters and NO3- ion in the stream, and the flow of stream had little change and showed no diurnal variations, indicating that the flow of stream was not factor which drived the diel changes of physical and chemical parameters and NO3- ion in the stream. DO and pCO2 of stream showed drastic negative correlation (R2=0.81, P<0.01), and the influence of water temperature on the rate of pCO2 was 27.48～54.88%, aquatic plants on water impact the PCO2 was 45.12-72.52%. To show that diurnal changes of DO and CO2 was mainly affected by the photosynthesis and respiration of aquatic plants, meanwhile DO and NO3- ion showed a negative correlation (R2=0.52,P<0.01), indicating that the diurnal changes of DO affected the diel changes of NO3- ion, and suggested that the regular changes of NO3- ion were mainly affected by the photosynthesis and respiration of aquatic plants.NO3- ion showed decreased trend as a whole while occurred increased trend and reached to maximum which was 3.72 mg.L-1 at 12:30, showed that changes of NO3- ion was not only affected by assimilation which was enhanced by photosynthesis but slao mainly affected by nitrification. The respiration of aquatic plants occupied leading role during nighttime, DO concentration decreased rapidly so assimilation and nitrification were restrained during nighttime and denitrification began to play a role, thus NO3- ion occurred reducing process. Diurnal changing quantity of nitrogen was 2.59 kgN.d-1, the increasing process was primary in afternoon and the other time was loss process primary, the increasing quantity was 0.99kgN.d-1,the loss was 0.8 kgN.d"1 during daytime and 0.8 kgN.d-1 during nighttime. Calculating the ratio of effects through the changing quantity of N, nitrification was primary in afternoon, the increasing quantity was 0.99kgN.d-1 and occupied 38 percent of diurnal changing quantity, denitrification was primary during nighttime, the loss content was 0.8 kgN.d-1 and occupied 31 percent of diurnal changing quantity, the loss quantity was 0.8 kgN.d-1 during daytime and was affected by assimilation. Therefore in the changing content was affected by biological process, nitrification occupied 38 percent, denitrification occupied 31 percent and assimilation occupied 31 percent.The physical and chemical parameters (T、pH、DO、Spc、PCO2) of stream showed regular diurnal variations in 2014 July diel monitoring, the T, pH and DO showed increased during daytime but decreased during nighttime, conversely, Spc and pCO2 showed decreased during daytime and increased during nighttime. However the NO3- ion of stream did not show the similar variation and correlation (R2=0.03) with DO,δ15N and δ18O did not show regular diurnal changes and correlation (R2=0.008,P=0.25), NO3- ion showed a weak correlation (R2=0.18,P=0.03) with 815N and no correlation (R2=0.01,P=0.25) with δ18O. The ratio of 815N and δ18O fell little in the ratio of 1:1 and some parts of the ratio fell in the vicinity of 2:1, indicating that assimilation and denitrification had some effects in changes of NO3- ion in stream. The losing process of nitrogen was primary in stream and the quantity was 2.71 kgN.d-1, may be affected by assimilation and denitrification in some extent, but δ15N-NO3- and δ18O-NO3- did not show correlation and mainly affected by nitrification and complex mixture of sources,meanwhile NO3-ion and flow showed a weak correlation (R2=0.25, P=0.03), suggested that physical processes related to the timing and sources of the upstream supply affected the diurnal changes of NO3- ion at the same time, meanwhile indicating that the diurnal changes of NO3- ion were both affected by biological prccess and physical processes related to the timing and sources of the upstream supply. |
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