Quantification of local groundwater inflow into the Spree River and its relevance to the iron precipitation problem in the Lusatian mining area (Germany).

Groundwater inflow into the Spree River and its tributaries is an important factor for the iron precipitation problem of the Spree in the Lusatian mining district (Eastern Germany). The input of dissolved iron into the Spree is difficult to estimate mainly because of unknown groundwater inflow. As part of this study, the radio-active isotope 222-Radon (²²²Rn) was used as a natural tracer to localize and quantify groundwater inflow into the Spree River and one of its tributaries ( Kleine Spree). Based on two ²²²Rn monitoring campaigns in the catchment and by applying the ²²²Rn mass balance model FINIFLUX, we were able to quantify local groundwater inflow for a 20 km long river section of the Kleine Spree and a 34 km long section for the Spree River. For the first campaign in May 2018 total groundwater inflow was estimated with ~3,000 m³/d for the Kleine Spree and ~20,000 m³/d for the Spree River. For the second campaign in August 2018 estimated total groundwater inflows were significantly higher with ~7,000 m³ d⁻¹ (Kleine Spree) and ~38,000 m³ d⁻¹ (Spree). Preferential groundwater inflow areas were identified (with up to 70% of total inflow) along the Spreewitzer Rinne, a local high permeable aquifer consisting of excavated mining materials. Based on a stoichiometric ratio calculation and by measuring instream sulfate and dissolved iron loadings, we additionally were able to estimate iron precipitation rates for the entire catchment of the Spree in the Lusatian mining area. According to our calculations, for the entire catchment of the Spree River in the Lusatian mining district total iron precipitation rates reach values as high as 120 tons/day; large quantities of iron (oxy)-hydroxides that are retained within the catchment as iron precipitates.