Hydrological and soil geochemical controls of the abundance and fractionation patterns of rare earth elements in a periodically acidic Boreal stream

Abstract

The concentrations and shale-normalized patterns of rare earth elements (REEs) were determined along the main stem of a medium-sized Boreal stream (catchment area, 107 km 2 ), to which areas covered with acid sulphate soils, developed in sulphide-bearing marine sediments, drain. In the upper reaches (headwater), where there are no acid sulphate soils, the REE concentrations were low at all sampling events (e.g. Nd=1.0 to 1.3 μg l −1 ) and the shale-normalized profiles were characterized by peaked middle-REE enrichment (centred on gadolinium) with a secondary maximum at ytterbium (Yb)/lutetium (Lu). Downstream there was an increase in the REE concentrations caused by leaching of acid sulphate soils which increase in abundance towards the basin outlet. The strongest downstream increase occurred at high-water flow in autumn (Nd concentrations up to 53 μg l −1 ) and the weakest during baseflow conditions (Nd=2–7 μg l −1 ). Downstream, at elevated REE concentrations, there was a depletion of the heavy REEs caused by: preferential scavenging (and thus removal) of the heavy REEs by oxy-hydroxides contained in the source soil; preferential complexation of the light and middle REEs by the dissolved SO 4 2− ; and/or the existence of a source mineral (in the acid sulphate soils) depleted in the heavy REEs. In two tributaries and a first-order stream sampled on a single occasion, the REE features were consistent with those along the main stem. In a subsurface drainpipe, also sampled once, there was, however, a convex REE-fractionation pattern characterized by a stronger depletion from gadolinium (Gd) to lanthanum (La)- than from Gd to Lu. The origin of this pattern, which seems to be of minor importance in the REE-enriched waters in the catchment, remains unexplained.