Trace metals associated with organic colloids in AMD-influenced groundwater

Mobile natural nanoparticles (NPs) can facilitate heavy metal (HM) and radionuclide (RN) transport in soils and groundwater systems (1). Organic compounds such as biopolymers and humic substances are known to show colloidal characteristics (1). In the framework of the BMBF-funded project USER-II (2), groundwater (GW) samples were obtained over two years at the test field “Gessenwiese” (3) in the former Uranium mining area near Ronneburg, Thuringia, Germany. Here, bioenergy production using short-rotation forestry is combined with several bioremediation strategies. Planting of fast-growing trees, the addition of calciferous soil, and inoculation with mycorrhiza and streptomyces have led to an increased soil pH and a decreased fraction of mobile HM/RN in the soil material. Still, the capillary rise of the shallow, acid mine drainage (AMD) influenced low-pH GW has the potential to transport HM/RN into the soil. Organic (colloidal) compounds in the <0.45 µm fraction of groundwater samples were characterized and quantified using a Liquid Chromatography - Organic Carbon Detection - (Organic) Nitrogen Detection system (LC-OCD-OND). In addition, operational fractions derived from LC-OCD-OND were analyzed with ICP‑MS to elucidate specific HM or RN associations. Total NP concentration and size distribution were analyzed using Nanoparticle Tracking Analysis (NTA). Conventionally, the fraction <0.45 µm is considered dissolved, but NP concentrations of 4.2E+07 ± 3E+07 particles ml^-1 with a median hydrodynamic diameter of 135 nm and an interquartile range (IQR) of 111-161 nm in the GW highlight the importance to investigate NP-associated HM/RN transport in this area. Dissolved organic carbon concentrations of up to 5.4 mg/L additionally emphasize the potential importance of colloidal/nanoparticulate organic compounds for soil transport processes. The influence of the determined metal speciation on root and biomass uptake will be discussed as well.