Study On Pollutant Migration In Soil Under Fluctuating Groundwater Table

The migration of pollutants in soil has become one of the research emphasis inenvironmental geotechnical engineering. Groundwater table is not stable, especially in thecase of geotechnical engineering construction which usually cause groundwater table risingor falling, such as water table falling causes by foundation pit dewatering, water tablerising caused by the construction of underground tunnel. It is inevitable that groundwatertable fluctuation could lead to the change of moisture and solute in soil. So it is hasimportant theoretical value and practical significance to study the effect of groundwatertable fluctuation on migration law in soil and build reasonable contaminant transportmodels considering groundwater table fluctuation.In the study, two typical pollutants, nitrate and admium, were selected as study object.Based on the domestic and foreign research results and development trend of migration ofpollutants in soil, a laboratory experiment was conducted to simulate groundwater tablefluctuation. And in this experiment, the migration law of the two contaminants insaturated-unsaturated soil layer was studied under the condition of water table fluctuation.At last, we considered LuoYang as an study object-taking real situation into consideration,collecting hydrogeological data and groundwater dynamic index-and analysed the effect onnitrate migration law caused by groundwater table fluctuation. The laboratory experimentconsisted of two parts: part one, the migration law of heavy metal cadmium under thecondition of water table fluctuation and the effect of soil particle size distribution oncadmium adsorption-desorption law; part two, considering the water table rising andfalling, rainfall and different soil media as affecting factors, studied nitrate migration lawin soil. From the laboratory experiment and field analysis, the conclusions were drew that:(1)The particle size distribution has a significant influence on cadmiumadsorption-desorption law. When there is a small amount of cadmium ion in soil,adsorption is the main process, and water table fluctuation has small effect on cadmium ionmigration; However, when there is lots of cadmium ion in soil, water table fluctuation havean impact on cadmium ion migration by convection.(2)Water table fluctuation can affect nitrate migration in soil without rainfall. Whenwater table falling, the mass concentration of nitrate nitrogen increased and the massconcentration of ammonium nitrogen decreased in saturated and unsaturated soil layers.When water table rising, the mass concentration of nitrate nitrogen decreased and the massconcentration of ammonium nitrogen increased. And the law of nitrite nitrogenmass concentration is always not abvious. Mean while, water table fluctuation is conducive tonitrate vertical migration which could increase nitrate pollution of groundwater, howevercould reduce ammonium nitrogen in soil. And the effect on nitrate pollution caused bywater table fluctuation should not be ignored. Compared with the Medium sand soil layer,silt soil layer is more likely to cause nitrate pollution under the condition of water tablefluctuation. When taking rainfall into consideration, stable water table is more likely tocause nitrate pollution compared with rising water table, while rising water table is easilyto cause ammonium nitrogen pollution.(3)In field situation, water table fluctuation has a certain effect on the index ofgroundwater characteristics, and has a significant effect on mass concentration ofammonium nitrogen and nitrate nitrogen in saturated soil. So in the field research ofgroundwater study, it is important to take the groundwater table fluctuation intoconsideration. Moreover, it is necessary to consider the groundwater table fluctuation ingroundwater pollution risk assessment, control and repair of nitrate pollution.