| Riverside well field is widely used at home and abroad because of its abundant water,good water quality,easy to centralized mining and management.However,due to the difference of physical,chemical and biological properties between surface water and groundwater,nitrate nitrogen infiltration into groundwater may turn into harmful ammonia nitrogen,which will threaten the safety of water supply.Based on this,this paper relies on the project"Key Technologies and Engineering Demonstration of Water Quality Safety Guarantee for Songhua River Intake"(2014ZX07201-010)of the 12th Five-Year Plan of the Ministry of Environmental Protection of China and the project"Biogeochemical Process of Iron and Manganese in River Infiltration Zone Affected by Sediment Sedimentation and Erosion"(41877178)of the National Natural Science Foundation of China.By means of field monitoring,laboratory experiments and numerical simulation,the migration and transformation of ammonia nitrogen in groundwater during river water infiltration driven by riverside exploitation were studied,and the influence of nitrate nitrogen concentration on groundwater ammonia nitrogen concentration in the second Songhua River was predicted.The results are as follows:(1)The contribution of river water to groundwater in the study area decreases with the increase of the distance from the river bank,and the greater the hydraulic gradient of the nearshore zone,the greater the contribution of river water.The vertical infiltration recharge rate of river water is 0.04~0.34 m/d,and the infiltration recharge rate of river water increases when groundwater is exploited in winter.(2)According to the field monitoring and analysis,it is concluded that ammonia nitrogen migration and transformation occur in the process of river water infiltration to recharge groundwater.In the process of river water infiltration,nitrate nitrogen in water decreases and ammonia nitrogen increases.The concentration of ammonia nitrogen in groundwater is related to hydraulic gradient,nitrate nitrogen and total dissolved organic carbon in river water.(3)The results of dynamic soil column simulation experiment show that the nitrate nitrogen concentration in the bottom outlet water decreases by 1.10,1.76,1.17and 1.12 mg/L under the conditions of high hydraulic gradient,high nitrate nitrogen concentration,high organic carbon concentration and control experiment,respectively.The reduction rates are 80.88%,77.24%and 86.15%,82.00%,and the ammonia nitrogen concentration is 0.5,1.0,0.8 and 0.4 mg/L,respectively.The faster the flow rate of groundwater recharged by river water infiltration,the longer the migration distance of ammonia nitrogen;the higher the concentration of nitrate nitrogen and organic carbon in river water,the more ammonia nitrogen generated during river water infiltration.(4)The simulation results of solute transport show that the concentration of ammonia nitrogen in groundwater decreases slowly in the area receiving river water infiltration and recharge,increases slowly in the area receiving groundwater lateral runoff recharge,and ammonia nitrogen enriches in groundwater near mining wells;the concentration of ammonia nitrogen in groundwater generally increases when the nitrate nitrogen concentration in river water increases by 50%.In January 2024 and January 2029,the concentration of ammonia nitrogen in groundwater increased by13.83%and 36.36%respectively at 700m away from the riverbank(funnel center),and the area of ammonia nitrogen concentration in groundwater above 3.00 mg/L was0.9 km~2 and 1.6km~2,respectively.When the concentration of nitrate nitrogen in river water was reduced by 50%,the concentration of ammonia nitrogen in groundwater generally decreased.In January 2024 and January 2029,the concentration of ammonia nitrogen in groundwater decreased by 13.04%and 43.87%respectively at700m away from the riverbank(funnel center),and the area of ammonia nitrogen concentration in groundwater less than 1.50 mg/L was 1.39 km~2 and 2.01 km~2,respectively. |
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