The distribution of rare earth elements in groundwaters: assessing the role of source-rock composition, redox changes and colloidal particles
Rare earth element (REE), dissolved organic carbon (DOC) and trace-element (Al, Mn, Fe, Sr, Ba, U and Th) concentrations were measured in fourteen well water samples (<0.22 μm) and one spring located along two transects set up in a catchment from Western Europe (Kervidy/Coët-Dan catchment, France). Previous hydrological and hydrochemical (NO 3-, SO 42-) investigations demonstrated that three chemically and spatially distinct groundwater types are present in this catchment, which is fully confirmed by the REE and DOC results. These include: (i) a shallow, organic-rich groundwater (4.4 < DOC < 34.6 mg/l) from the wetland areas, close to the river network. This first groundwater type, characterized by the development of temporary reducing conditions, records high and variable REE contents (2 < ΣREE < 16 ppb) and displays slight or no negative Ce anomaly (Ce/Ce∗ = 0.8-1.05); (ii) a shallow, organic-poor (DOC < 3 mg/l), NO 3--rich groundwater type (86.8 < NO 3- < 155 mg/l) located in the weathered schists, below the hillslope domains. This second type corresponds to recently recharged, oxidized water and displays also high and variable REE concentrations (2 ppb < ΣREE < 15 ppb), but distinguish from the former by the occurrence of very strong negative Ce anomalies (Ce/Ce∗ = 0.05-0.10); finally (iii) a deep, organic-poor (DOC < 1 mg/l), nitrate-poor (NO 3- close to 0.2 mg/l; detection limit) groundwater. This third type corresponds to reduced water flowing into the deep fresh schists and yields low to very low REE contents (ΣREE < 0.15 ppb) as well as slight negative Ce anomaly (Ce/Ce∗ = 0.8 to 0.9). Temporal REE concentration variations were assessed using samples regularly collected over a six month period. Results show that the spatially distributed Ce anomaly and REE pattern signatures are preserved throughout the studied period. By contrast, REE concentrations are quite variable through time, especially in the wetland waters where the REE concentrations are seen to vary in phase with both redox changes and DOC, Fe, U and Th content variations. Three REE-rich water samples (one DOC-rich and two DOC-poor) were also filtered through membranes of decreasing pore size (100,000 D, 30,000 D, 5,000 D). The results show that between about 40% to 65% of the REE present in the shallow, DOC-poor groundwater samples are controlled by the colloidal fraction, which is likely to consist in these inorganic waters of a mixture of mineral phases. In the wetland groundwaters, the fraction of REE controlled by microparticles is higher than 65%, which confirms the predominant role of organic colloids as major REE carriers in wetland waters. Using the above data set in conjunction with analyses of soil samples, we show that the deep Ce anomalies found in the upper non organic part of the aquifer are probably not source-rock inherited features: most likely, these anomalies arise from the oxidative precipitation of Ce. The very low REE content displayed by waters flooding the deep fresh schists is interpreted as due to the combined effects of (i) pH variation, (ii) secondary sulfate mineral precipitation and (iii) the trapping of colloids-borne REE by the aquifer-rock pores. Data from wetland groundwaters show that the REE, Fe, U and Th budgets of these waters are mainly controlled by seasonal changes in redox conditions and organic matter content. However, unlike organic-poor waters, it appears difficult to relate the Ce behaviour in these organic-rich waters solely to redox conditions. It is likely that the complexation of Ce by organic colloids in the organic-rich waters may mask redox changes by inhibiting the development of negative Ce anomalies.