Determining natural background levels (NBLs) is a fundamental step in assessing the chemical status of groundwater bodies in the EU, as stipulated by the Water Framework and Groundwater Directives. The major challenges in deriving NBLs for trace metals are understanding the interaction of natural and anthropogenic processes and identifying the boundary between pristine and polluted groundwater. Thus, the purpose of this paper is to present a roadmap guiding the process of method selection for setting meaningful NBLs of trace metals in groundwater. To develop the roadmap, we compared and critically assessed how three methods for excluding polluted sampling points affect the NBLs for As, Cd, Cr, Cu, Ni, and Zn in Danish aquifers. These methods exclude sampling points based on (1) the primary use of the well (or sampling purpose), (2) the dominating anthropogenic pressure in the vicinity of the well, or (3) a combination of pollution indicators (NO₃, pesticides, organic micropollutants). Except for Ni, the NBLs derived from the three methods did not differ significantly, indicating that the data pre-selection based on the primary use of the wells is an important step in assuring the removal of anthropogenically influenced points. However, this pre-selection could limit the data representativity with respect to the different groundwater types. The roadmap (a step-by-step guideline) can be used at the national scale in countries with varying data availability.

中文翻译：

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确定地下水中痕量金属的自然本底水平的路线图

根据《水框架》和《地下水指令》的规定，确定自然背景水平（NBL）是评估欧盟地下水体化学状态的基本步骤。得出痕量金属的天然铅的主要挑战是了解自然过程和人为过程的相互作用，并确定原始与受污染的地下水之间的边界。因此，本文的目的是提出一个路线图，指导设置地下水中痕量金属的有意义的NBL的方法选择过程。为了制定路线图，我们比较并严格评估了三种排除污染采样点的方法如何影响丹麦含水层中As，Cd，Cr，Cu，Ni和Zn的NBL。这些方法基于以下因素排除采样点：（1）井的主要用途（或采样目的），₃，农药，有机微量污染物）。除了Ni之外，从这三种方法得出的NBL并无显着差异，这表明基于井的主要用途进行的数据预选是确保去除人为影响点的重要步骤。但是，这种预选择可能会限制有关不同地下水类型的数据表示性。该路线图（循序渐进的指南）可以在数据可用性各不相同的国家/地区在国家范围内使用。