Group of Hydrogeology and Environmental Geology, Geo³, ArGEnCo, Aquapole, University of Liège, Building B52/3, B-4000 Liège, Belgium.
J Contam Hydrol. 2010 Oct 21;118(1-2):79-93. doi: 10.1016/j.jconhyd.2010.08.008. Epub 2010 Aug 31.
Regional degradation of groundwater resources by nitrate has become one of the main challenges for water managers worldwide. Regulations have been defined to reverse observed nitrate trends in groundwater bodies, such as the Water Framework Directive and the Groundwater Daughter Directive in the European Union. In such a context, one of the main challenges remains to develop efficient approaches for groundwater quality assessment at regional scale, including quantitative numerical modelling, as a decision support for groundwater management. A new approach combining the use of environmental tracers and the innovative 'Hybrid Finite Element Mixing Cell' (HFEMC) modelling technique is developed to study and forecast the groundwater quality at the regional scale, with an application to a regional chalk aquifer in the Geer basin in Belgium. Tritium data and nitrate time series are used to produce a conceptual model for regional groundwater flow and contaminant transport in the combined unsaturated and saturated zones of the chalk aquifer. This shows that the spatial distribution of the contamination in the Geer basin is essentially linked to the hydrodynamic conditions prevailing in the basin, more precisely to groundwater age and mixing and not to the spatial patterns of land use or local hydrodispersive processes. A three-dimensional regional scale groundwater flow and solute transport model is developed. It is able to reproduce the spatial patterns of tritium and nitrate and the observed nitrate trends in the chalk aquifer and it is used to predict the evolution of nitrate concentrations in the basin. The modelling application shows that the global inertia of groundwater quality is strong in the basin and trend reversal is not expected to occur before the 2015 deadline fixed by the European Water Framework Directive. The expected time required for trend reversal ranges between 5 and more than 50 years, depending on the location in the basin and the expected reduction in nitrate application. To reach a good chemical status, nitrate concentrations in the infiltrating water should be reduced as soon as possible below 50mg/l; however, even in that case, more than 50 years is needed to fully reverse upward trends.
地下水硝酸盐区域性退化已成为全球水资源管理者面临的主要挑战之一。为扭转地下水硝酸盐的观测趋势，如欧盟的《水框架指令》和《地下水指令》，已经制定了相关法规。在这种背景下，如何在区域尺度上开发地下水质量评估的有效方法，包括定量数值模拟，以作为地下水管理的决策支持，仍然是一个主要挑战。本研究提出了一种将环境示踪剂与创新的“混合有限元混合单元”（HFEMC）建模技术相结合的新方法，用于研究和预测比利时 Geer 盆地区域含水层的地下水质量，并将其应用于白垩含水层的区域尺度。氚数据和硝酸盐时间序列用于生成白垩含水层包气带和饱和带区域地下水流动和污染物运移的概念模型。这表明，Geer 盆地的污染空间分布主要与盆地内的水动力条件有关，更确切地说与地下水年龄、混合有关，而与土地利用的空间模式或局部水力弥散过程无关。建立了一个三维区域尺度地下水流动和溶质运移模型。该模型能够再现氚和硝酸盐的空间分布以及白垩含水层中观测到的硝酸盐趋势，并用于预测盆地中硝酸盐浓度的演变。模型应用表明，盆地内地下水质量的整体惯性很强，在 2015 年欧洲水框架指令规定的最后期限之前，预计不会出现趋势逆转。根据盆地位置和预期硝酸盐施用量的减少，预计趋势逆转所需的时间在 5 年至 50 年以上不等。为了达到良好的化学状态，应尽快将入渗水的硝酸盐浓度降低到 50mg/L 以下；然而，即使在这种情况下，也需要 50 多年的时间才能完全扭转上升趋势。
