Legacy nitrate and trace metal (Mn, Ni, As, Cd, U) pollution in anaerobic groundwater: Quantifying potential health risk from “the other nitrate problem”

• Published in: Applied geochemistry Vol. 139; p. 105254
• Main Authors: Riedel, Thomas, Kübeck, Christine, Quirin, Markus
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01.04.2022
• Subjects: biogeochemistry; Denitrification; Dissolved organic carbon; ecosystem services; Germany; groundwater; Groundwater pollution; iron; nitrates; nitrogen; oxygen; pollution; pyrite; risk; Trace metals; water quality; water treatment; Germany; Dissolved organic carbon; Denitrification; Groundwater pollution; Trace metals
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/j.apgeochem.2022.105254

Denitrification in soils and aquifers sustains low nitrate concentrations in many anaerobic groundwaters despite massive inputs of N from agriculture. However, this ecosystem service sometimes comes at the cost of trace metal mobilization and concerns have been raised that denitrification in anaerobic groundwater may lead to trace metal contamination. But it remains unclear, if denitrification must necessarily result in trace metal concentrations that are potentially harmful for humans. For example, formation of iron(oxy)hydroxides after the reaction of nitrate with pyrite may reduce rather than increase the mobility of certain trace metals in aquifers. We quantified the potential health risk resulting from denitrification-associated trace metal pollution (Mn, Ni, As, Cd, U) in anaerobic groundwater with different degrees of nitrate pollution in >800 wells located in Northern Germany. Overall, observed rates of violations of legal quality standards for U, As and Cd are moderate in the study area (<10% of all wells) but elevated for Mn (>50%), which is a common contaminant under the often anaerobic conditions in the study area. However, in groundwater where denitrification had partially proceeded, the risk for drinking water standard violations was higher for Ni, Cd and U (up to a factor of 4 for Ni) as compared to anaerobic groundwater without denitrification, but lower for Mn and As. Especially poorly buffered groundwaters with pH < 5.5 are at risk of Ni and Cd contamination resulting from denitrification, while the opposite is true for Mn and U. Thus, we establish a clear linkage between nitrogen biogeochemistry and trace metal mobility and show how perturbations of groundwater redox and pH conditions by nitrate can further deteriorate groundwater quality. In addition, we find that the combination of oxygen concentrations of lower than 1 mg L−1, and nitrate concentrations above 1 mg L−1 allows for identification of denitrification (as evidenced by excess N2 in groundwater) with 90% accuracy. The methodology developed herein can be used to inform water treatment planning about potential trace metal contamination when drinking water should be produced from anaerobic aquifers beneath agricultural landscapes. [Display omitted] •Soil derived dissolved organic carbon drives denitrification when groundwater is close to the surface.•Nitrate contamination in anaerobic groundwater mobilizes Ni, Cd and U in well water to a level relevant for human health risk.•When nitrate is present, concentrations of Mn and As in anaerobic groundwater are lower as compared to nitrate-free groundwater.•Pyrite oxidation may be more common at greater depth.