Effects of Small-Scale Gold Mining Tailings on the Underwater Light Field in the Tapajós River Basin, Brazilian Amazon

Abstract

Artisanal and Small-scale Gold Mining (ASGM) within the Amazon region has created several environmental impacts, such as mercury contamination and changes in water quality due to increased siltation. This paper describes the effects of water siltation on the underwater light environment of rivers under different levels of gold mining activities in the Tapajós River Basin. Furthermore, it investigates possible impacts on the phytoplankton community. Two field campaigns were conducted in the Tapajós River Basin, during high water level and during low water level seasons, to measure Inherent and Apparent Optical Properties (IOPs, AOPs), including scattering (b) and absorption (a) coefficients and biogeochemical data (sediment content, pigments, and phytoplankton quantification). The biogeochemical data was separated into five classes according to the concentration of total suspended solids (TSS) ranging from 1.8 mg·L −1 to 113.6 mg·L −1 . The in-water light environment varied among those classes due to a wide range of concentrations of inorganic TSS originated from different levels of mining activities. For tributaries with low or no influence of mining tailings (TSS up to 6.8 mg·L −1 ), waters are relatively more absorbent with b:a ratio of 0.8 at 440 nm and b 660 magnitude of 2.1 m −1 . With increased TSS loadings from mining operations (TSS over 100 mg·L −1 ), the scattering process prevails over absorption (b:a ratio of 10.0 at 440 nm), and b 660 increases to 20.8 m −1 . Non-impacted tributaries presented a critical depth for phytoplankton productivity of up to 6.0 m with available light evenly distributed throughout the spectra. Whereas for greatly impacted waters, attenuation of light was faster, reducing the critical depth to about 1.7 m, with most of the available light comprising of red wavelengths. Overall, a dominance of diatoms was observed for the upstream rivers, whereas cyanobacteria prevailed in the low section of the Tapajós River. The results suggest that the spatial and temporal distribution of phytoplankton in the Tapajós River Basin is not only a function of light availability, but rather depends on the interplay of factors, including flood pulse, water velocity, nutrient availability, and seasonal variation of incoming irradiance. Ongoing research indicates that the effects of mining tailings on the aquatic environment, described here, are occurring in several rivers within the Amazon River Basin.

Distribution of Artisanal and Small-Scale Gold Mining in the Tapajós River Basin (Brazilian Amazon) over the Past 40 Years and Relationship with Water Siltation

“…Increased water siltation has a strong impact and direct consequences for how light is attenuated in the water and consequently the depth of the euphotic zone. For instance, Lobo et al [41] have defined that in the Tapajós River basin, the euphotic zone can decrease from 6.0 m to 1.0 m in non-impacted and ASGM impacted rivers, respectively. These changes can potentially change the phytoplankton community [41], and primary production [42].…”

“…With regards to ASGM, the consequences of water siltation to the aquatic environment, more specifically, to underwater light field and phytoplankton production, have been evaluated by [41]. In short, strong light attenuation reduces euphotic zone from 6 m to, approximately, 1 m in non-impacted and impacted water bodies, which can potentially change the phytoplankton community and decrease the photic zone and, thus, primary production.…”

Monitoring Water Siltation Caused by Small-Scale Gold Mining in Amazonian Rivers Using Multi-Satellite Images

“…In addition to the direct use for SSGM controlling and monitoring by these national agencies, the applicability of these products for aquatic research is enormous. In fact, evaluation of light attenuation caused by mining tailing has shown that, in general, the euphotic zone in impacted rivers has decreased to at least half of nonimpacted rivers [36]. Several studies can use TSS distribution as an input for hydrological and sediment transportation models; studies on light attenuation and consequences to the biota, as well as socioeconomic studies, will benefit from this research.…”

Mapping Mining Areas in the Brazilian Amazon Using MSI/Sentinel-2 Imagery (2017)

“…Industrial iron mining in Carajás, for example, applies high-end technology throughout the whole process of surveying, exploiting, and recovering the area, resulting in high yields with minimum environmental impacts. On the other hand, small-scale gold mining in the Tapajós River, for example, uses low-end technology in ore processing (water jets and dredges) with low yields and high impact levels on land degradation and water contamination (sediment and mercury) [14][15][16][17].…”