Monitoring of shallow groundwater in Lower Saxony, Germany—spatial variability of redox conditions and benefit of the redox proxy ∆Mn-Fe

Surveillance monitoring of shallow groundwater revealed that redox conditions can vary on a small scale. Therefore, the aim of this study was to categorize redox conditions in the groundwater of Lower Saxony, Germany, and to analyze the spatial distribution and trends of parameters related to redox conditions during surveillance monitoring from 1957 to 2015 in Lower Saxony, Germany. Methodically, trends were considered by applying the Mann-Kendall test and redox conditions of groundwater were classified according to the scheme of Jurgens et al. (2009). While the porous aquifers were dominated by anoxic redox conditions, the karst and joint aquifers presented a high percentage of the oxic redox category. A third of the monitoring wells showed trends with respect to redox conditions. Positive ΔMn-Fe values, indicating manganese-reducing conditions, were observed in 17% of the samples, which were mostly taken in areas of high groundwater recharge. Remarkably, different redox regimes were sampled in close neighborhoods within areas that were assumed to be homogeneous with respect to recharge rate, usage, and hydrogeology. In conclusion, parameters that are sensitive to redox conditions should be investigated stepwise; first, monitoring wells should be categorized according to the redox conditions and second, only wells with the same redox conditions should be compared. Third, the parameter ∆Mn-Fe should serve as a proxy for potential changes of the redoxcline, e.g., due to nitrate emissions to shallow groundwater. This stepwise procedure allows the assessment of groundwater and can serve as a basis for the design of measures to reach environmental aims according to the European Water Framework Directive.