Hydrogeochemistry in soils and sediments in the area of the Lagoa Campestre lake (Salitre, MG, Brazil): chemical balances of major and trace elements and dynamics of rare earth elements

Abstract

Study of the sediments filling the Lagoa Campestre peaty lake (Minas Gerais, Brazil) showed a high rare earth elements (REE) content, especially in phosphatic bottom beds (authigenic phosphates), whose fluctuations could be related to variations in the erosion conditions and the nature of the plant cover. In order to understand better the mode of complexation and transport of trace elements in this environment, a geochemical survey was undertaken of the waters of the area, both in surface streams and at the water table. The concentration of the main cations decreases from the top of the slope down towards the lake. This is especially true during the rainy season and could be related to a more active leaching in this period. The chondrite-normalized REE pattern in the soil sequence is very similar to the pattern found for the local bedrock and is characterized by a depletion in heavy REE (HREE). For the groundwaters, the REE abundance normalized to average REE values of soils shows an enrichment in HREE as well as downslope, near the lake, and in the upper part of the sequence. At the outlet of the lake, the water of the stream has an alkaline pH and high values for numerous elements, contrasting with the low contents in the water table and in the lake. Thus it seems that the water of the outlet could be at least partly related to an apatite rich and carbonatitic formation, more abundant in this area. This is confirmed by positive Sm and Eu anomalies. Waters were also collected in 2 types of lysimeters. The cylindrical collectors gave higher REE contents than the plate collectors, a feature that might result from adsorption by the metabolic activity at the rhyzosphere. The geochemical data acquired in the groundwaters, in the lake and at the outlet of the Lagoa Campestre allow a better understanding of the dynamics of the basin. During the rainy season, leaching is active on the slopes; it enhances dissolution of residual minerals in the latosolic cover and induces mobilization of relatively high quantities of REE in the groundwaters. The stream at the outlet is probably mainly fed by deeper waters circulating through apatite rich rocks, except during the rainy season when the lake also partly contributes to its supply. This means that most of the material leached on the slopes of the basin accumulates in the central part of the depression.