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 wel l water samples (<0.22 <mu>m) and one spring located along two transects se t up in a catchment from Western Europe (Kervidy/Coet-Dan catchment, France ). Previous hydrological and hydrochemical (NO3-, SO42-) investigations dem onstrated that three chemically and spatially distinct groundwater types ar e present in this catchment, which is fully confirmed by the REE and DOC re sults. 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 g roundwater type, characterized by the development of temporary reducing con ditions, records high and variable REE contents (2 < Sigma REE < 16 ppb) an d displays slight or no negative Ce anomaly (Ce/Ce* = 0.8-1.05); (ii) a sha llow, organic-poor (DOC < 3 mg/l), NO3--rich groundwater type (86.8 < NO3- < 155 mg/l) located in the weathered schists, below the hillslope domains. This second type corresponds to recently recharged, oxidized water and disp lays also high and variable REE concentrations (2 ppb < Sigma REE < 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 (NO3- close to 0.2 mg/l; detection limit) groundwa ter. This third type corresponds to reduced water flowing into the deep fre sh schists and yields low to very low REE contents (Sigma REE < 0.15 ppb) a s well as slight negative Ce anomaly (Ce/Ce* = 0.18 to 0.9). Temporal REE concentration variations were assessed using samples regularly collected over a six month period. Results show that the spatially distrib uted Ce anomaly and REE pattern signatures are preserved throughout the stu died period. By contrast, REE concentrations are quite variable through tim e, especially in the wetland waters where the REE concentrations are seen t o vary in phase with both redox changes and DOC, Fe, U and Th content varia tions. Three REE-rich water samples tone DOC-rich and Two DOC-poor) were also filt ered 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 th e shallow, DOC-poor groundwater samples are controlled by the colloidal fra ction, which is likely to consist in these inorganic waters of a mixture of mineral phases. In the wetland groundwaters, the fraction of REE controlle d 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 s how that the deep Ce anomalies found in the upper non organic part of the a quifer are probably not source-rock inherited features: most likely, these anomalies arise from the oxidative precipitation of Ce. The very low REE co ntent displayed by waters flooding the deep fresh schists is interpreted as due to the combined effects of (i) pH variation, (ii) secondary sulfate mi neral precipitation and (iii) the trapping of colloids-borne REE by the aqu ifer-rock pores. Data from wetland groundwaters show that the REE, Fe, U an d Th budgets of these waters are mainly controlled by seasonal changes in r edox conditions and organic matter content. However, unlike organic-poor wa ters, it appears difficult to relate the Ce behaviour in these organic-rich waters solely to redox conditions. It is likely that the complexation of C e by organic colloids in the organic-rich waters may mask redox changes by inhibiting the development of negative Ce anomalies. Copyright (C) 2000 Els evier Science Ltd.