Elçi Alper
Department of Environmental Engineering, Dokuz Eylül University, 35390 Buca/Izmir, Turkey.
J Contam Hydrol. 2017 Dec;207:39-49. doi: 10.1016/j.jconhyd.2017.10.008. Epub 2017 Nov 6.
Groundwater vulnerability assessment studies are essential in water resources management. Overlay-and-index methods such as DRASTIC are widely used for mapping of groundwater vulnerability, however, these methods mainly suffer from a subjective selection of model parameters. The objective of this study is to introduce a calibration procedure that results in a more accurate assessment of groundwater vulnerability. The improvement of the assessment is formulated as a parameter optimization problem using an objective function that is based on the correlation between actual groundwater contamination and vulnerability index values. The non-linear optimization problem is solved with the generalized-reduced-gradient (GRG) method, which is numerical algorithm based optimization method. To demonstrate the applicability of the procedure, a vulnerability map for the Tahtali stream basin is calibrated using nitrate concentration data. The calibration procedure is easy to implement and aims the maximization of correlation between observed pollutant concentrations and groundwater vulnerability index values. The influence of each vulnerability parameter in the calculation of the vulnerability index is assessed by performing a single-parameter sensitivity analysis. Results of the sensitivity analysis show that all factors are effective on the final vulnerability index. Calibration of the vulnerability map improves the correlation between index values and measured nitrate concentrations by 19%. The regression coefficient increases from 0.280 to 0.485. It is evident that the spatial distribution and the proportions of vulnerability class areas are significantly altered with the calibration process. Although the applicability of the calibration method is demonstrated on the DRASTIC model, the applicability of the approach is not specific to a certain model and can also be easily applied to other overlay-and-index methods.
地下水脆弱性评估研究在水资源管理中至关重要。诸如DRASTIC之类的叠加指数法被广泛用于绘制地下水脆弱性图，然而，这些方法主要存在模型参数主观选择的问题。本研究的目的是引入一种校准程序，以更准确地评估地下水脆弱性。评估的改进被表述为一个参数优化问题，使用基于实际地下水污染与脆弱性指数值之间相关性的目标函数。非线性优化问题采用广义简约梯度（GRG）法求解，这是一种基于数值算法的优化方法。为了证明该程序的适用性，利用硝酸盐浓度数据对塔赫塔利河流域的脆弱性图进行了校准。该校准程序易于实施，旨在使观测到的污染物浓度与地下水脆弱性指数值之间的相关性最大化。通过进行单参数敏感性分析，评估了每个脆弱性参数在脆弱性指数计算中的影响。敏感性分析结果表明，所有因素对最终的脆弱性指数都有影响。脆弱性图的校准使指数值与实测硝酸盐浓度之间的相关性提高了19%。回归系数从0.280增加到0.485。显然，校准过程显著改变了脆弱性等级区域的空间分布和比例。虽然在校准方法的适用性在DRASTIC模型上得到了证明，但该方法并不特定于某一模型，也可轻松应用于其他叠加指数法。
