Derivation of natural background levels and threshold values for groundwater bodies in the Upper Rhine Valley (France, Switzerland and Germany)

Abstract

According to the requirements of the EU Water Framework Directive (EU-WFD, article 17.2a) criteria for the assessment of the chemical status of groundwater have to be developed, which may serve as starting points for a trend reversal (article 17.2b) in case the good status is failed. Against this background an EU-wide applicable approach to assess natural background levels (NBLs) and threshold values (TVs) for the definition of the groundwater chemical status has been developed. NBLs are derived based on a preselection method using aquifer typologies as natural reference systems. Samples with purely anthropogenic substances (e.g. PAC, pesticides) as well as samples, in which indicator substances for anthropogenic inputs (e.g. nitrate) are exceeding a certain value, are excluded. The NBLs are defined for the remaining groundwater samples of the aquifer typology under concern as the concentration range between the 10% and 90% percentiles. The threshold values (TVs) get established with reference to the NBLs and a “not acceptable reference value” (REF), e.g. drinking water standards or toxicological values, guaranteeing that the TVs are higher than the NBLs, but lower than the REF values. The applicability of the methodology for NBL and TV derivation has been applied to the transboundary groundwater body of the Upper Rhine Valley in Germany, Switzerland and France.