Fluxo de Th e identificação de fontes de sedimentos na bacia hidrográfica do rio Ipojuca

Abstract

The Ipojuca River is the third most polluted river system in Brazil. High concentrations and fluxes of metals in sediments indicate the contamination pattern, being compared with values from catchments impacted by mining. Recent hydrological and morphological changes in the estuarine-marine zone increased the susceptibility of fine particle and metal sedimentation in these environments. It is assumed here that other elements may be being contributed, such as radionuclides, and that the identification of sources and controlling soil erosion and sediment transport can reduce the distribution of contaminants to the outlet and the local coastal environment. Accordingly, the objectives of this study were (1) to evaluate the natural contents in soils and the distribution of Th in suspended (SS) and bed (BS) sediments, (2) to evaluate the patterns of delivery of sediments based on in the combination of different classifications of potential sources of the fingerprinting method and (3) to evaluate the dynamics of delivery of different sediment sources from the semiarid portion of the Ipojuca River. (1) The mean concentration of Th in the soils of the catchment was 28.6 mg kg-1. The estimated Quality Reference Values were at 21 mg kg-1 and 86.3 Bq kg-1. The concentrations in the bed and in the suspended sediment ranged from 2.8 to 32.9 mg kg-1. The suspended sediments transported 3.42 t year-1 of Th, equivalent to more than 99% of the flux of this element. In the downstream cross section, suspended sediment samples exhibited Th concentrations similar to those observed in rivers impacted by mining activities. However, there was no evidence of anthropogenic impacts on Th concentrations. The flux of sediments from the study area to the ocean is mainly triggered by soil erosion processes in the mid-lower course region. (2) The downstream region (SS = 80.5% and BS = 86.7%), the Oxisols (SS = 65% and BS = 30.8%) and sugarcane cultivation (SS = 62% and BS = 63.6%) were the dominant sediment sources. Combined, the Oxisol and sugarcane areas controlled about 63.5% of the SS and 47.2% of the BS transported, i e., less than 8% or approximately 280 km² of the catchment. Approaches with combinations of potential sources, based on robust discriminations, tend to provide greater detail of areas under dominant erosion and reduce the scale of containment of these processes. (3) The semiarid lower catchment presented significantly higher contributions than the other regional sources (middle and upper), approximately 69% and 56% of the SS and BS, respectively. There was no dominant source of land use. The Caatinga (SS = 49%; BS = 47%) expressed a mild superiority in relation to the channel bank (SS = 35%; BS = 39%). Therefore, the recovery and conservation of vegetation in the Caatinga and the stabilization of channel banks, especially in the lower stretch, are fundamental for controlling the transport of river sediments in the outlet of this semi-arid catchment. More studies are needed to better assess the conservative patterns of color parameters in semiarid environments. Here, we provide some of the first information on sediment contributions from the main land uses and land cover in the semiarid region of northeastern Brazil.