Study On Process Based Assessment Method For The Confined Aquifer Vulnerability In Leakage Area

With the rapid development of industry and agriculture, the increasing expansion of population, and the acceleration of urbanization process, the confined groundwater exploitation is steadily increasing, resulting in the decline of groundwater head. When the confined groundwater head is lower than that of the shallow aquifer, the polluted shallow groundwater will recharge the confined aquifer through aquitard, which could lead to the pollution of confined aquifer. Hence, it is very urgent to map the degree of confined aquifer vulnerability to contaminants leaking from shallow aquifer due to confined groundwater pumping in order to realize the confined groundwater protection and sustainable development.In this thesis, based on the review and summarization of the concept, classification and assessment methods of groundwater vulnerability, the cumulative time for the pollutant concentration at the coping of confined aquifer exceeding the maximum allowable level is defined as the confined aquifer vulnerability index. Then the assessment of confined aquifer vulnerability induced by a single pumping well is used to illustrate the rationality of vulnerability index. The confined aquifer vulnerability index has explicit physical meaning and has a detailed description of the process for the pollutant to enter the confined aquifer. The index can clearly express the probability that the confined aquifer is polluted and diminish the effect of the human factor to the assessment of the confined aquifer vulnerability.Shallow groundwater quality in Jining city is evaluated by the set pair analysis model based on entropy weight. Then the representative pollutant is chosen. Three dimensional numerical simulation models of groundwater flow and solute transport are established by MODFLOW and MT3D respectively. Then the confined aquifer vulnerability index is obtained. The results are reasonably consistent with the change of total hardness concentration after 2 years. Through comparison with the results from overlay and index methods, the advantages of the proposed method employing process simulation are analyzed. By analyzing the characteristic of vulnerability assessment and physical process of pollutant penetrating the aquitard, the simplified vulnerability assessment method through the establishment of one dimensional groundwater flow and solute transport models in the aquitard is constructed. Compared by the assessment results of the three dimensional groundwater flow and solute transport models, the simplified method is verified to be rational and especially suitable for the lack data area.As there are some uncertainties in confined aquifer vulnerability assessment, which affect the reliability of assessment results, the reliability of confined aquifer vulnerability assessment results induced by a single pumping well is quantitatively analyzed by randomly generating the most sensitive parameters using Monte Carlo method. The local sensitivity analysis and global sensitivity analysis-Morris screening method are used to analyze the sensitivity of parameters in regional confined aquifer vulnerability assessment.