Long-term redox conditions in a landfill-leachate-contaminated groundwater

Highlights: • Municipal landfill leachate contamination of a confined aquifer in Southeast Norway • Contaminants impaired water quality, hindering exploitation of the groundwater. • Redox conditions that developed are similar to those reported from phreatic aquifers. • Apparent breakdown in long-term redox conditions as landfill attained phase IV. • Redox conditions control spread of pollutant and minimise degradation of ecosystems. Indicators of redox conditions; oxygen, sulphate, nitrate, ammonium, iron and manganese, and in addition, bicarbonate and total organic carbon were studied in groundwater samples contaminated by leachate emanating from Revdalen Landfill (Norway). Based on these variables, the study aimed to deduce the redox conditions in the aquifer. Literature on landfill leachate contamination of confined aquifers is scarce and to the best of our knowledge, this study, which describes long-term analysis of redox chemistry, is the first of its kind in such an environment. Groundwater samples were monitored for a period of 24 years, enabling us to describe redox conditions on both short-term and long-term bases. Levels of measured parameters in the contaminated aquifer varied spatially and with time, but were generally elevated except oxygen; pH (4.9–8.8), oxygen (0–11.3 mg/L), sulphate (0–28 mg/L), nitrate (0–16 mg N/L), ammonium (0.02–40 mg/L), iron (0–99 mg/L), manganese (0.06–16 mg/L), bicarbonate (22–616 mg/L) and total organic carbon (1.3–47 mg/L). From the result, levels of iron, manganese, nitrate and ammonium violated the Norwegian drinking water norms. However, iron, ammonium, total organic carbon and bicarbonate showed strong attenuation along the groundwater flow path. By contrast, oxygen, nitrate and sulphate increased farther out in the plume. The redox conditions that developed in the aquifer were similar to those previously reported for phreatic aquifers, structuring by proximity to the landfill as sulphate-reducing, iron-reducing, manganese-reducing, nitrate-reducing, and finally aerobic condition. Eventually, there was an apparent breakdown of this system due to ecosystem shift in the landfill when leachable reduced ions were depleted and the landfill became aerobic. Overall, the redox framework provided remarkable attenuation to contaminants, and thus prevented potential degradation of ecological health due to the landfill leachate.