Experimental And Mathematical Simulation Of Groundwater And Its Nutrients From Unconfined Aquifer Into Lakes

Human actives like agricultural fertilization polluted local shallow groundwater quality. The eutrophication process of lake water (especially lakeshore water) would be accelerated because of the great deal of nutrients from shallow aquifers. Based on the investigations about groundwater flow to lake, soil tank experiments and mathematical simulation were done for studying groundwater and its nutrients from two kind typical aquifers into shallow lake. The distribution of seepage and nutrient concentration along transversal profile of lakeshore and lake bottom were discussed. The fluxes of groundwater and nutrients from eastern and northern aquifers entering into Dianchi Lake were obtained.Following results were obtained by experimental and mathematical simulation.(1) Based on the investigations about groundwater flowing into lake, two kinds typical system of aquifer and lake were conceptually summarized, namely the thin aquifer (on the assumption that the thickness of aquifer approximates to the depth of lake) and thick aquifer with lake (on the assumption that the thickness of aquifer is bigger than the depth of lake). The exchange mechanisms of water flow and solute between aquifer and lake were detailed.(2) The seepage velocity through lakeshore and lake bottom interfaces under different lakeshore inclinations were discussed by experiments in soil tank and model simulations using MODFLOW. The seepage flow from thin aquifer caused on a narrow littoral zone on lakeshore interface, and the seepage velocity decreased as a descending exponential function along the transversal profile with offshore distance. However, the seepage flow from thick aquifer caused synchronously on two interfaces of lakeshore and lake bottom, and concentrated at two zones: littoral zone and the joint of lakeshore and lake bottom. The seepage velocity on the two interfaces still satisfied with descending exponential function along the transversal profile. With the increasing of lakeshore inclination, the seepage from lakeshore interface decreased gradually, but increased from lake bottom interface. The seepage from lake bottom interface maybe ignored when the inclination was little.(3) The distribution of chloride concentration on exchange interfaces was studied. When the distribution of chloride in upstream groundwater was uniform, the concentrations on interface were also even if the transport time enough. However, the concentrations weredecreased with offshore distance before being even. This reduction was due primarily to dispersion effect acting in the final portion of the flow path. Solute concentrations were also more likely to be attenuated with distanced from a shoreline due to the longer flow path.(4) The transport of nitrogen seeped from surface soil to groundwater and lake was experimented in soil column and tank equipments. And the transport of nitrate nitrogen in groundwater entering into lake was simulated by using MODFLOW. Results show that the surface soil pollution maybe regarded as a continued constant pollution source and that the unsaturated soil zone delineated as pollution enriched zone (or layer) and pollution transfer zone (or layer) was reasonable. The nutrient from surface soil with precipitation was resorted in upper aquifer and transport longitudinally with groundwater to lake. As a result, the high concentration nutrient concentrated at littoral zone, and the distribution of nitrate concentration satisfied with a decline exponential function.(5) As an example, the groundwater and its nutrients discharged into Dianchi Lake were calculated by using MODFLOW model. The available evidence indicates that groundwater discharge to the lake occurs primarily along the northern and eastern shorelines, with limited additional discharge to the western shoreline. Groundwater discharge to the lake from the adjacent aquifer system is probably concentrated in the near-shore areas of the lake bottom. The groundwater discharge into Dianchi Lake from northern and eastern aquifers is about 6.0 × 10~6m~3/a, and the nitrate nitrogen and total phosphorus are 87. It/a and 1.7t/a respectively. The half of groundwater and nutrient (50%) discharge into Dianchi Lake concentrated in 8.0m and 4.5m width narrow zone along transversal profile of lakeshore. These nutrients load may accelerate the eutrophication of the lakeshore water in Dianchi Lake.