| 其他摘要 | Numerical modeling is one of the main methods and techniques to evaluate the impact of human activities on groundwater quantity and quality, to assess groundwater resources, to predict the trend of groundwater contamination. Due to highly heterogeneous in the Karst aquifer systems, it is difficulty to numerically simulate groundwater flow and contaminant transport.
Zunyi city is located at the karst area of Southwest China and is the second large industrial city in Guizhou province. There is serious shortage of water. The exploitation of groundwater resources has alleviated the water shortage. Due to uncontrolled extraction and poor management for a long time, a series of environmental geologic problems such as groundwater depression, water quality worsening, subsidence and so on, have emerged in the course of exploiting groundwater. So, the study on numerical simulation of flow and contaminant transport at Zunyi city has the important theory significance and the practical application value.
Based on GIS, we establish hydrogeology information database. Groundwater level and quality regime are simply analyzed. We summarize the conditions of the natural geography, geology and hydrogeology. Then we present the concept model of the Gaoqiao-Hexiba area, and establish the two-dimensional steady-state model by Groundwater Vistas. The main conclusions of this study have been presented as follows:
1. The peak value of water level in fissure water is lag of precipitation about two to three months. The trends of karst water level, underground river discharge and spring discharge are consistent with precipitation intensity.
2. The main contaminants in study area are NH4+, NO3-, NO2-, SO42- and Mn. The annually polluting areas are not equal and change unregularly. These facts suggest that contaminants are possibly controlled by nonuniform of material input by human activity and rainfall. There are large area karst water being polluted, especially near the limestone caves, underground rivers and residents focus areas.
3. For management of groundwater resources, the karst groundwater system in Gaoqiao-Hexiba is conceptualized, and an equivalent porous media model is constructed. We use six MODFLOW package to simulate sources and sinks: Recharge package simulate rainfall percolation; Well package simulate pumping; GHB package simulate lateral groundwater recharge or discharge; Drain package represent the export of the underground river; River package simulate the exchange between river and groundwater; CHD package represent fixed-head. Calibration results not only match observed heads at measurement points but also match hydraulic gradients, which can meet the requirements of relevant national standard. It is showed that the equivalent porous media model is suitable for modeling the flow in the karst area.
4. Values of hydraulic conductivity were estimated by calibrating the steady-state model. Near the Gaoqiao and the Maocaopu, the values of hydraulic conductivity range from 100 to 400 m/d in dry season, whereas in wet season high conductivity reaches 3320 m/d. The values of hydraulic conductivity in the rest cell are below 100 m/d, and most of them are below 10 m/d. As a whole, the values of hydraulic conductivity in wet season are higher than in dry season.
5. By computing the mass balance, the recharge and discharge are quantified. In dry season, the lateral recharge from the east-north boundary, accounting for 70% of total recharge, is the most important recharge source. The maximum discharge is extraction. In wet season, recharge from fixed-head boundary, accounting for 49%, is the main recharge source. The lateral recharge from the east-north boundary, accounting for 39%, is the second recharge source. The discharge of underground river, accounting for 74% of total discharge, is the main discharge.
6. Sensitivity analysis is employed to evaluate the relative influences of different hydrogeological parameters and external stresses in groundwater flow. The results show that simulated water levels in the model are most sensitive to hydraulic conductivity and pumpage.
7. The velocity of groundwater flow is very speed, which indicates that solute transport in karst aquifer is predominantly advective, the effect of dispersion is relatively little. On the assumption that there is a point contaminant source in the Maocaopu, we use MT3DMS to simulate contaminant transport characteristics. The results show that the contaminant plume shape and position are mainly controlled by the groundwater flow field, and the concentration of pollutants is related with water volume. |
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