Mapping and quantifying groundwater inflow to the Spree River (Lusatia) and its role in Fe fluxes, precipitation and coating of the river bed.

The spatial distribution and temporal dynamics of groundwater inflow to rivers is often poorly defined but central to understanding water and matter fluxes. This is especially true for the Spree River which drains the Lusatia mining district, Brandenburg Germany. In the Spree catchment iron and sulphate fluxes to the river stem from the pyrite rich groundwater system, and the area’s history of open-pit lignite mining and re-flooding of many of these mines at the end of their lifetime. This iron flux threatens the river ecosystem, tourism in downstream communities (Spreewald) and the drinking water of Berlin. Iron is often observed as precipitates along the river bed, as well as colouring the river water yellow-brown, indicating the presence of iron (oxy)hydroxides such as ferrihydrite and goethite. In this work we have used radon as a natural groundwater tracer to delimited areas of active groundwater discharge to both the main Spree River and the Kleine Spree River to better understand the spatial destitution of groundwater input to the system. This was combined with mass-balance modelling to quantify the groundwater flux along the river using the FINIFLUX model. This was complemented by measurement of iron and sulphate concentrations in the steam and stream-near groundwater. During two measurement campaigns during 2018 the total groundwater inflow for a 20 km long reach of the Kleine Spree and a 34 km long reach of the Spree ranged between ~3,000 and ~7,000 m³ d^-1 (Kleine Spree) and ~20,000 and ~38,000 m³ d^-1 (Spree). Particularly high groundwater inflow was identified (up to 70% of total inflow) along the Spreewitzer Rinne, a local aquifer consisting of excavated mining materials. For the Kleine Spree the dominant groundwater and Fe flux occurred shortly before the confluence with the Spree. For these river reaches large amounts of dissolved iron and sulphate enters the rivers with inflowing groundwater as calculated from the radon data. Using the measured iron and sulphate loadings we calculated that up to 120 tons/day of iron (oxy)hydroxide was retained in the combined Spree and Klein Spree catchments, a large amount of which remains in the mining lakes.