Low levels of persistent organic pollutants in sediments of the Doce River mouth, South Atlantic, before the Fundão dam failure

Highlights

• The first evaluation of PCBs and OCPs distribution in the DRM was presented.

• No adverse biological effect by POPs was identified in sediments before dam rupture.

• The river flow influences the POPs distribution mainly in the summer (wet) period.

• In the winter (dry) period occur a POPs northward transport.

• The baseline data allows identifying unexpected impacts of the Fundão dam failure.

Abstract

The Doce River mouth (DRM) was severely impacted by the rupture of the Fundão Dam in 2015, considered the greatest Brazilian environmental tragedy in terms of tailings volume released (>40 million m³) and traveled distance (~600 km until the Atlantic Ocean). Environmental monitoring has been performed since then, but background levels are scarce or absent to Persistent Organic Pollutants (POPs), making impact assessments difficult. In the current study, we presented the baseline levels, inventories, and risk assessment of the POPs polychlorinated biphenyl (PCBs) and organochlorinated pesticides (OCPs), in surface sediment of the DRM. Samples were collected in December 2010 and July 2011, i.e., four years before the Fundão dam failure. The total PCBs and the OCPs (Aldrin, HCHs, and Chlordanes) were detected in both sampling campaigns, with levels up to 9.50 and 1.64, 0.28, and 0.63 ng g^-1, respectively. The decrease of the Doce River flow was the main factor contributing to seasonal variations in the spatial distribution, and to a slight decline in the levels and frequency of the analyzed POPs in sediments collected in the dry season (July 2011). Environmental risk assessment, inventories, and total mass results suggest a low potential of PCBs and OCPs accumulation before the dam failure. This is the first POPs assessment in the study area that helped identify some unexpected impacts of the Fundão dam failure and contributed to the understanding of POPs cycles in the Southern Atlantic, data that are still scarce in the region.

Introduction

Persistent organic pollutants (POPs) are a diverse group of compounds used worldwide in domestic, agricultural, and industrial applications. Due to their chemical stability, toxicity potential, persistence, long-range transport, bioaccumulation, and biomagnification capacity, POPs were globally banned at the Stockholm Convention, in 2001 (UNEP, 2001). Although primary emissions have been regulated, some studies suggest that a reduction of POPs levels is unlikely in the next years (Breivik et al., 2007; Combi et al., 2017). Incineration of chlorinated products as plastics (e.g., PVC), inappropriate disposal of products containing POPs, and reemission through the soil and water volatilization or by remobilization of sediments previously contaminated are sources of these compounds (Bigus et al., 2014; Souza et al., 2018a).

In current studies, POPs are still detected in marine environments (e.g., Zhao et al., 2019; Combi et al., 2020; Duarte-Restrepo et al., 2020; Santos et al., 2020; Girones et al., 2021), and their main actual sources are atmospheric precipitation, river discharges, and continental runoff (Ma et al., 2016; Girones et al., 2021). Once in the water column, these hydrophobic compounds tend to be adsorbed by suspended particulate matter and accumulate in marine sediments (Lohmann et al., 2007; Ma et al., 2016). Therefore, in a transition environment as coastal areas, sediments play a key role in the transport and storage of anthropogenic substances in the land-to-ocean system, acting as sinks and secondary sources of contaminants. However, few studies have been developed to evaluate the presence of organic pollutants in the western South Atlantic continental shelf (e.g., Mahiques et al., 2016; Souza et al., 2018b; Santos et al., 2020).

The Doce River basin is 853 km long, and has 83,400 km² with approximately 3.1 million inhabitants (CPRM, 2015), and hosts the largest ore complex in Latin America. The region has received greater attention since November 2015, after the environmental disaster in which an ore tailings dam (Fundão dam) collapsed. The event released more than 40 million m³ of mining tailings, and part of this material reached the coast 600 km far (Gomes et al., 2017). Besides this tragedy, the Doce River watershed has been historically altered by the cumulative effects of the disordered population occupation and a variety of anthropogenic activities including mining, irrigation, water catchment for industrial activities and human consumption, and development of the industrial and agriculture sectors (Grilo et al., 2013; Oliveira and Quaresma, 2017; Hobbs et al., 2020). Despite this variety of potential sources of contaminants, scientific studies about chemical contamination in the region are scarce (Smith and Val, 2020). This lack of information has been partially filled with some studies showing the impact of the Fundão dam disaster in this environment (e.g., Marta-Almeida et al., 2016; Gomes et al., 2017; Bernardino et al., 2019; PMBA, 2019; Fernandes et al., 2020; Gabriel et al., 2020; Orlando et al., 2020; Sá et al., 2021).

A comprehensive monitoring research of the Fundão dam failure impacts recorded POPs in the continental shelf adjacent to the Doce River mouth (DRM) (PMBA, 2019). Polychlorinated biphenyl (PCBs) and dichloro-diphenyl-trichloroethane (DDTs) were the most frequent POPs detected in sediments (PMBA, 2019). High values of other contaminants, such as metals and hydrocarbons, have been related to the Fundão dam failure (Bernardino et al., 2019; Sá et al., 2021; Longhini et al., 2021) suggesting the influence of this event on the recent contribution of a series of contaminants to the marine environment. However, different from metals, there is no baseline information regarding POPs levels in the region, which difficulties the assessment of the dam failure influence on the recent POPs contamination.

The knowledge of anthropogenic sources and the load of organic substances that reach the environment is of great importance to environmental management of the Doce river basin, as well as to assess the impact that events like Fundão dam failure may have promoted to this environment and adjacent coastal area. Thus, this study aimed to determine the levels, inventories, and total mass of the PCBs and organochlorinated pesticides (OCPs) in the DRM before the Fundão dam disaster. In addition, temporal variation of PCBs and OCPs between the two different periods and climatic features (summer/wet and winter/dry period) are presented. The results of this study are compared with the recent POPs results from the monitoring of this area, performed three years after the dam failure. This study may be used as baseline data for future evaluations, and to contribute to the global inventory of POPs in the inner continental shelves of the planet.

The Doce River catchment basin has an area of 83,400 km² and is situated in the Brazilian States of Minas Gerais and Espírito Santo. Originating in the Mantiqueira Mountain Range, it flows for 853 km to the mouth at the SW Atlantic (19°45′S, 39°48′W; Fig. 1a-b). The sedimentation pattern of the DRM is directly influenced by the river sediment supply and seasonal variations (Quaresma et al., 2015, Quaresma et al., 2020). Climatologically, around 94% of the suspended sediment load transported to this ocean zone occurs during the ‘wet’ period (November to March – austral summer season), characterized by higher median streamflow (≈ 965 m³ s^-1) and sediment loads (≈ 133 × 10⁶ tons) (Oliveira and Quaresma, 2017). Meanwhile, in the ‘dry’ period (April to October – austral winter season), the estimated streamflow and the suspended sediment load are approximately 368 m³ s^-1 and 10 × 10⁶ tons, respectively (Oliveira and Quaresma, 2017). The wind pattern is characterized by a higher frequency of north-northeasterly winds, influencing an immediate fine material deposition at the south of the river mouth (Quaresma et al., 2015; Oliveira and Quaresma, 2018); south-southeasterly winds are associated with cold fronts and storms, most common in winter, and higher wave energy and orbital velocities that remobilize and transport fine sediments northward (Quaresma et al., 2015; Oliveira and Quaresma, 2018).