Study On Nitrate Change Mechanism In Low Temperature Geothermal Water Under The Influence Of Physical And Chemical Factors

Based on the geothermal water data obtained from the field investigation and monitoring results,the"three nitrogen"concentration contour maps of the five aquifers in the area were drawn.The indoor soil column simulation experiments of nitrate transformation in the fine sand of the Neogene Minghuazhen Formation at 25?,35?and 45?were carried out.The main conclusions are listed below:?1?The geothermal wells with depths between 600 m and 1800 m were mainly polluted by nitrite;the pollution of ammonia nitrogen and nitrate was lighter.In addition,the plane distribution characteristics of three nitrogen compounds were different in different areas;the nitrite concentration in each aquifer exceeded government standard,mainly distributed in the region near the railway station,which was contributed to nitrification and denitrification to varying degree.However,ammonia nitrogen and nitrate concentration of each aquifer in the whole research area showed low value except individual sample points.?2?At the temperature of 25?,35?and 45?,the nitrate experienced the same three stages namely,slow rise,rapid rise and smooth fluctuations when transporting in simulated thermal reservoir,and the relative mass concentration C/C₀ of nitrate was very different at the time of penetrating.At this stage,convection and dispersion were the main migration process,and the nitrate reduction was weak.At the nitrate transformation phase,the transformation way of nitrate was the same,but the conversion intensity was different.The denitrification was stronger at 45?,and dissimilatory nitrate reduction to ammonium was stronger at 25?.At this phase,the main processes were solute transporting,denitrification and dissimilatory nitrate reduction to ammonium.Overall,the conversion intensity of nitrate was higher at 45?and 35?.?3?At 25?,35?and 45?,during the migration of nitrate in the simulated fine sand thermal reservoir,nitrate concentration decreased,and the pH value of the effluent solution at the bottom of the soil column increased with the temperature increasing.The ORP and nitrate concentration decreased with rise of the temperature,the ORP downtrend of the effluent samples at 45?was the quickest,and the corresponding nitrate concentration was the lowest,which may be related with the the great degree of denitrification and dissimilatory nitrate reduction to ammonium.The change trend of CON,TDS and nitrate concentration was basically the same,and all of them showed the trend of increasing first and then becoming stable and fluctuating in a small range.?4?The correlation coefficient of Na⁺and HCO₃^-in the geothermal water of the study area is 0.68,which is related to the HCO₃^--Na⁺type of geothermal water.The nitrite and nitrate concentration were significantly related,which may be related to the nitrification and denitrification reaction,and reduction reaction of the nitrate in the simulated experiment at three temperatures confirmed the results further.