A Study Of The Relationship Between Shallow Groundwater And Lake Nitrogen Pollution In Taihu Lake

With rapid socio-economic development in the Taihu Lake basin, water pollution, especially eutrophication in Taihu Lake, has become a serious problem. A large number of surveys and studies indicate that agricultural non-point nitrogen source is one of the main causes of eutrophication in Taihu Lake. Nitrogen from agriculture chemicals not only was carried into rivers, lakes or coastal waters with runoff to contaminate surface water but also seeps into soils or aquifers to contaminate groundwater which eventually discharges to rivers, lakes or coastal waters to contaminate surface water. Most existing studies were mainly focused on the non-point nitrogen source brought into rivers and lakes with surface runoff and little attention was paid to nitrogen transported into surface water through subsurface. It is thus important to study the transport of nitrogen pollutants in groundwater in the Taihu Lake basin and to evaluate groundwater contribution to surface water nitrogen pollution. The results of this study will provide a scientific basis for water pollution control in Taihu Lake basin and for management of the lake eutrophication.Changxing county-a typical agricultural region in the Taihu Lake basin was selected as the study area. First of all, a field survey was conducted and groundwater levels and samples were collected. Results show that the shallow groundwater in the Changxing county is seriously polluted, the nitrate concentrations in 7 out 43 (16.3%) groundwater samples collected are larger than 10mg/L with the maximum of 22mg/L. Secondly, a groundwater flow model was build using MODFLOW based on the hydrogeological conditions of the study area. After calibrating the model with the observed water levels, the groundwater flow in the area was simulated. The flow results show that the groundwater in the study area is recharged by rainfall as well as the seepage from rivers in part of the area and discharged into rivers in other part of the area. The direct discharge from the aquifer to the lake is relatively small. Thirdly, based on the groundwater flow model a nitrate transport model in the shallow aquifer was built and calibrated with the observed groundwater nitrate concentrations. The results show that about 539 tons nitrate-nitrogen (NO3-N) of shallow groundwater were discharged to surface water during one year period from September of 2011 to September of 2012, among which 134 tons NO3-N were discharged directly into Taihu lake and 405tons were discharged NO3-N into the rivers. Finally, it is estimated that a total of NO3-N discharged into the Taihu Lake by groundwater is about 14245 tons/year, which is 27.8% of the total nitrogen carried into the lake by all the rivers in the basin.