Microbial Iron Retention in the Groundwater upstream to a River

摘要

High iron and sulphate concentrations are observed in several surface waters of the Lusatian mining district. Especially iron affects the freshwater fauna (fish, insect larvae) and aquatic plants by lowered pH or high turbidity. Both iron and sulphate are mobilized from iron sulfides like pyrite and marcasite, which are decomposed under oxic conditions. These conditions appear in the underground of large areas after lowering the groundwater table to enable the opencast lignite mining. To develop a treatment technology, we built a pilot plant at the location "Ruhlmuhle" (north Saxony) in 2014, where an iron hot spot (about 400 to 550 mg/L Fe) in groundwater was detected. At the pilot plant, groundwater is extracted by 3 wells, mixed with glycerin and re-infiltrated. The infiltration line consists of 30 lances and has a width of about 100 m. The infiltrated glycerin serves as a carbon source for sulphate-reducing bacteria (SRB). Although the microbes operated under the difficult starting conditions (pH-value of about four), it took a longer time than expected to rise the pH above five by bacterial metabolism. Under these conditions, the iron could precipitate as iron sulfide, generating additional alkalinity. Up to now, the iron concentration has been reduced to about 200 to 300 mg/L. Sulphate and electrical conductivity have decreased (SO_4 from about 1100 mg/L to about 500 mg/L) too, and the neutralization potential has risen from between -15 to -20 mmol/L to above -10 mmol/L. We expect a further decrease of the iron concentrations down to 30 mg/L during the continued operation.