Abstract
Manganese (Mn) concentrations in approximately 32,000 groundwater analyses from more than 4800 monitoring wells in northern Germany were evaluated. This region was considered well suited to study Mn in shallow groundwater in unconsolidated sediments. Spearman rank correlation was used to correlate between redox-sensitive parameters and the Mann–Kendall test for an evaluation of temporal trends. Manganese concentrations varied over two orders of magnitude and more than 40% of the wells had concentrations above 0.3 mg/L. Median Mn concentrations in the major hydrogeological units, the Geesten, tidal wetlands, and fluviatile lowlands were 0.12 mg/L, 0.46 mg/L, and 0.27 mg/L, respectively. Separating the data by land use, the median concentrations were 0.20 mg/L for arable land, 0.15 mg/L for forests, and 0.24 for grassland. Calculated background concentrations of Mn varied from <0.25 mg/L to 4.79 mg/L. A new parameter, ∆Mn-Fe, defined as the concentration difference between Mn and Fe in mg/L together with nitrate concentrations exceeding 50 mg/L was used to identify the fertilizer-borne input of nitrate. However, the factor controlling Mn occurrence seemingly was the depth of monitoring wells and the screen-length. Elevated concentrations of Mn and a high ∆Mn-Fe were generally found in shallow wells and wells with short screen-lengths.
Highlights
The two redox-sensitive transition metals manganese (Mn) and iron (Fe) are included in most groundwater monitoring programs and their concentration is directly related to redox reactions
In this study background values for Mn were determined for the hydrogeological subunits that comprise the three major hydrogeological units: Geesten, tidal flats and fluviatile lowlands
Based on the analyses of properly characterized wells an investigation on spatial extend of trends would be a separate and follow-up step. This and numerous other studies have demonstrated that redox processes are a major influence controlling the chemical composition of groundwater. This complicates groundwater monitoring, since redox processes vary on scales of centimeters to meters, while well screens are often larger than 2 m and horizontally, the wells in a monitoring net are kilometers apart
Summary
The two redox-sensitive transition metals manganese (Mn) and iron (Fe) are included in most groundwater monitoring programs and their concentration is directly related to redox reactions. In addition to the aquifer matrix of a hydrogeological subunit other factors influence the background concentrations of a given element, such as, the groundwater recharge rate [45], land use [46], redox conditions [38], depth of the groundwater surface [40], geological heterogeneity [45] and screen length [41,46]. Of those factors, the screen length and screen depth are complicating factors since they directly affect which redox zone is sampled and the degree of mixing between different redox zones. It would be necessary to determine background values for a multitude of redox conditions using monitoring wells with short screens to prevent mixing of groundwater from different redox zones
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