Characterization of the Natural Organic Matter (NOM) in groundwater contaminated with ⁶⁰Co and ¹³⁷Cs using ultrafiltration, Solid Phase Extraction and fluorescence analysis

Spot samples of shallow groundwaters have been taken between the years 2004 and 2010 near a site formerly used for the dispersal of radioactive liquid wastes. Three sampling points, one clean (upstream), and two downstream of the contamination source, were processed by ultrafiltration (5000 Da cut-off) and Solid Phase Extraction (SPE) to determine the association of selected artificial radionuclides (⁶⁰Co, ¹³⁷Cs) with Natural Organic Matter (NOM). The last two sampling episodes (2008 and 2010) also benefited from fluorescence analysis to determine the major character of the NOM. The fluorescence signals are reported as humic-like, fulvic-like and protein-like, which are used to characterize the different NOM types. The NOM from the clean site comprised mostly fine material, whereas the colloidal content (retained by ultrafiltration) was higher (e.g., 15–40% of the Total Organic Carbon – TOC). Most of the 137Cs was present in the colloidal fraction, whereas ⁶⁰Co was found in the filtered fraction. Fluorescence analysis, on the other hand, indicated a contrasting behavior between the clean and contaminated sites, with a dominance of protein-like material, a feature usually associated with human impacts. Finally, SPE removed almost quantitatively the protein-like material (>90%), whereas it removed a much smaller fraction of the ¹³⁷Cs (<28%). This finding indicates that the ¹³⁷Cs preferential binding occurs with a fraction other than the protein-like NOM, likely the fulvic-like or humic-like portion. - Highlights: • The major constituent of the colloidal material is Natural Organic Matter (NOM). • Cesium-137 is associated with colloidal NOM downstream from the wastes site. • The NOM in the contaminated site was dominated by protein-like material. • The protein-like NOM was mostly hydrophobic. • The humic- and fulvic-like NOM were mostly hydrophilic