**5. Discussion**

#### *5.1. Regional Geochemical Distribution of Fe, Al, and PTE in Soils and Stream Sediments*

The geochemical signature of an element in a given sampling medium provides important information about external controlling conditions, either natural or anthropic. The geochemical maps for Fe, Al, and 20 PTE (Figures 2 and 3, Figures S1 and S2) in the twosampling media, as assessed by visual inspection, are notoriously similar. The composition of soils and stream sediments appears to be strongly controlled by bedrock lithologies in the area (Figure 1c), with evident differences between the north and south of the study

area. Higher concentrations for the majority of the elements in both sampling media are observed in the NPB in comparison to SPB (Figure 1b).

The spatial distribution of these elements is well-documented in previous studies of soil [24,44] and stream sediment [45,58]. For instance, the concentrations of Fe, V, As (Figure 2) ( ±Cd, Sb and Ag; Figure S1) and relative concentrations of Al, Ba, Mn, and Zn (Figures 2 and 3, Figure S2), along with two E-W corridors (tectonic trend of the region) in the NPB are controlled by the occurrences of Fe-Al-rich duricrusts, formed by the intense weathering of metamafic rocks and BIF of the Carajás Formation, which hosts the world-class Fe deposits (N4, N5, and S11D) of Carajás [61–63].

Similarly, the concentrations of Ni, Cr, Co, and ±V (Figure 3) are strongly controlled by the metavolcanic rocks of the Parauapebas Formation, generally intercalated with rocks from the Carajás Formation, and cross-cut by local mafic to ultramafic rocks of the Cateté Intrusive Suite (the Luanga Complex, located in the NE of the NPB; the Vermelho Complex, in the southern area of the NPB) [37,64]. Local anomalies also occur in the south of the SPB, due to the occurrences of Sapucaia greenstone belt [65].

The spatial distribution of Cu, Mo (Figure 2), and to a lesser extent Se (Figure S1) is a response of mineralization zones along two E-W corridors (northern and southern copper belts [27]) in the NPB, similarly observed for Fe and related elements. To some extent, higher concentrations of Se (Figure S1) were observed in stream sediments and, especially, in soils of the north of NPB, along the northern copper belt in comparison to the southern copper belt. This evidence indicates that these two corridors do not share common surface multi-element signatures, perhaps, due to different metallogenic evolution [27].

Uranium, Pb, and Sn tend to concentrate in quartz-feldspar rocks, such as granites and felsic granulites, particularly those with an alkaline tendency. In the NPB, the main sources for the enrichment of these elements are A-type granitoids (e.g., Estrela and Igarapé Gelado granites) and Paleoproterozoic anorogenic granites (e.g., Seringa and Serra dos Carajás granites) [38]. In addition, unusually high concentrations of Mn, Co, and Ba (Figure 3 and Figure S2) in soils and stream sediments and B in soils (Figure S1) were observed in the NW region of the SPB, under the influence of charnockitic rocks (Figure 1d) of the Diopside-Norite Pium unit [28].

Using integrated geochemical maps is important not only for understanding the regional distribution of a given element but also to compare two (or three) different sampling mediums in terms of concentration magnitude and source. Although, as described, the different lithologies of the area are the main source of enrichment for Fe, Al, and the 20 PTE. However, anthropogenic contributions cannot be disregarded and further studies should be conducted.

#### *5.2. The Geochemical Compartmentation of PB as Subsidy to Territorial and Watershed Management*

Watershed managemen<sup>t</sup> is essential for planning the sustainable use of natural resources. It supports many kinds of human needs in terms of water consumption, food production, recreation, and most importantly the maintenance of ecologic function [66]. Each watershed has its own particularities, for instance, climate conditions, land use, human occupation, and, for the purpose of the present study, geochemical background. All these multidisciplinary concerns have increased the need of developing robust watershedmanagemen<sup>t</sup> strategies aiming for realistic environmental policies [67].

As presented in previous studies, the chemical composition of soils [24,44] and stream sediments [45] changes with the geological domain across the IRW, including the study area (Figure 1b). Nevertheless, minor differences were also observed between the BD and CB in terms of spatial distribution for many elements, as discussed previously. The boxplots for the studied elements (Figure 4 and Figure S3) display the difference among the geological domains of the PB area (Figure 1d), suggesting that many elements actually present similar distribution (e.g., Al, As, Bi, and Cr; Figure 4). For this reason, the element concentration in soil and stream sediment samples from these two domains were statistically compared using the MWW test (Table 1). The obtained results sugges<sup>t</sup> that dividing the study area into NPB and SPB is statistically acceptable, at least for surface geochemical studies at a regional scale. However, the geochemical data of Cu in the PB represents a clear exception to this approach. Not only the MWW test revealed different statistical distribution between BD and CB (*p*-value < 0.05; cf. Table 1), but also the boxplots (Figure 4 and Figure S3) and the integrated geochemical map (Figure 2). The Cu signature in the PB is a clear response from highly mineralized areas that constitute a geochemical and metallogenic province [68], which cannot be generalized in terms of watershed management.

#### *5.3. The Relationship between Surface Soil and Stream Sediment Geochemistry*

A visual inspection of the boxplots (Figure 4 and Figure S3) and the comparison of descriptive statistics results (Table 2 and Table S2) revealed that element concentrations in soils are generally greater than in stream sediments of PB. This is particularly true for Fe, Al, Ag, As, B, Bi, Cd, Cr, Cu, Hg, Mo, Sb, V, Se, and Sn, and it is not observed for Ba, Co, Mn, Ni, Zn, Pb, and U. By using geoprocessing techniques, it was possible to identify the soil samples that are geographically located at a given catchment area, which is represented by a stream sediment sample collected at the outlet, and compare them. Scatter plots used to evaluate the relationship between surface soil and stream sediment samples revealed that these sampling media are strongly to moderately correlated for many elements (Fe, As, Bi, Cr, Cu, Hg, Mo, Ni, Sb, Sn, U, and V; Figure 5 and Figure S4), but weakly to other elements (Al, Ag, B, Ba, Cd, Co, Mn, Pb, Se, and Zn; Figure 5 and Figure S4). In terms of source apportionment, in the case of the elements with strong to moderate correlations the composition of soils and stream sediments seems to be both controlled by the bedrock lithologies or, alternatively, the soils themselves are the primary source for the constitution of stream sediment, driven by erosion processes that have been directly influenced by the deforestation in the area over the past decades [69]. The weak correlations may be due to three reasons: (i) Mineralogical sorting during hydrodynamic and sedimentation processes. For instance, Al is a major constituent of kaolinites, which is a naturally occurring mineral in the soils of the Amazon [70], but to a lesser extent in active river sediments; (ii) low sensitivity of the analytical method, which is clearly the case for Ag, B, Ba, Cd, and Se; (iii) Different sources contributing to the enrichment of an element in a given sampling media. For this case, Ba, Co, Mn, Pb, and Zn captured our attention because these elements are well-known for being part of important biogeochemical processes that take place in nature [71].

#### *5.4. Geochemical Threshold Variation in Soils and Stream Sediments vs. Environmental Guidelines*

The existence of solid legislations proposing reference-quality guideline values for different regions, demarked by considering a multidisciplinary approach, including the definition of geochemical compartments, instead of simply using political boundaries would be ideal. Under this context, the variation of the background concentrations in soils and stream sediments of the PB and the quality guideline values proposed by Brazilian environmental agencies [50,59,60] should be critically evaluated.

Firstly, the threshold values calculated by using statistical techniques widely applied in geochemical studies [72–74] pointed out different results among each statistical method (Table 3 and Table S3). These differences occur due to the statistical approach and the central criteria of the method, which is widely discussed in the literature [17,22,23,44,45]. In general, the highest background values were obtained by the TIF method, with several overestimated values (cf. Table 3 and Table S3). The P98 and P95 deliver threshold values by considering a fixed percentage of outliers, within the range of values in the data set, which is not entirely appropriate. Among the methods used herein to derive threshold concentration values, the MMAD appears to derive more consistent and realistic results. This conclusion has been also achieved in other studies [17,22,23]. For this reason, the following discussion will be addressed by using the MMAD results (Table 3 and Table S3).

The comparison of the threshold values, in both sampling media, between the NPB and the SPB regions revealed significant discrepancies. For instance, the threshold concentrations of Fe in soils of the NPB and SPB are 34.24 and 6.93 wt.%, respectively, and in stream sediment are 22.93 and 3.54 wt.%, respectively (Table 3). Similar behavior is observed for the majority of the studied elements (cf. Table 3 and Table S3). The differences observed in each region are a direct influence of the natural geological/geochemical variation, already described in this study, confirming the consistency of the results presented here.

The previous results demonstrated the existence of a large natural spatial variation of PTE in the territory of PB and indicated that establishing threshold values for the NPB and SPB regions is more adequate than assuming a uniform value for the entire PB. This conclusion is also relevant in terms of defining quality guideline values for large areas elsewhere. For instance, the comparison of threshold values in NPB and SPB (cf. Table 3 and Table S3) for some PTE [As (NPB = 2.3; SPB = 1.6), Cd (NPB = 0.16; SPB = 0.01), Pb (NPB = 24.4; SPB = 14.2), and Zn (NPB = 83; SPB = 47; all values in mg kg−1] contemplated in the Brazilian environmental resolution of stream sediments [60] shows that they have their threshold concentrations below the Level 1 (L1; also known as the Threshold Effect Level— TEL [75]) proposed by the National Council of the Environment (CONAMA) of Brazil [60] for the mentioned elements (cf. Table 3 and Table S3). Mercury exhibits a threshold value in NPB (0.23 mg kg−1) greater than the L1 (0.17 mg kg−1), but the referenced value in SPB (0.08 mg kg−1) is lower than L1. Nickel shows in NPB threshold value (56.4 mg kg−1) greater than the Level 2 (L2, also known as the Probable Effect Level—PEL [75]; L2 of Ni = 35.9 mg kg−1), but in the SPB the obtained value (12.6 mg kg−1) is lower than L1 (18 mg kg−1). The greatest differences were observed for Cu (482.4 mg kg−1) and Cr (160 mg kg−1), for which threshold values in the NPB are considerably greater than the L2 (Cu = 197; Cr = 90 mg kg−1). On the other hand, the threshold values in SPB for Cr (68 mg kg−1) are between the L2 and L1) (respectively, 90 mg kg−<sup>1</sup> and 37.3 mg kg−1), and for Cu (20 mg kg−1) below L1 (35.7 mg kg−1).

The quality guidelines of soils in Brazil are based on two resolutions, a Federal resolution [50] applicable for the entire Brazilian territory, which presents Prevention Guideline Values (PGV), and a State resolution for the definition of Quality Reference Value (QRV), which corresponds to the geochemical background. In the absence of QRV for the State of Pará (PA), where the PB is situated, the State Secretariat of the Environment and Sustainability (SEMAS-PA) adopted for the Pará territory the same QRV presented by the São Paulo Sanitation Technology Company (CETESB) [59], derived for the State of São Paulo.

Firstly, it should be emphasized that it is profoundly inadequate to establish geochemical background values for a given area (e.g., the PB situated in the State of Pará) based on values of another region (e.g., the State of São Paulo), with completely different environmental, geological and geochemical characteristics. This is clear when comparing the threshold values of the PTE with the QRV proposed by the State resolution [59]. Among the 14 PTE contemplated in the resolution, ten (As, Ba, Co, Cr, Cu, Hg, Mo, Ni, Pb, and Se) presented generally, in both NPB and SPB, higher threshold values in comparison to the QRV. From this group, when compared to the PGV, which values are somewhat higher than those of QRV (Table 3 and Table S3), the discrepancies with assumed background values of PB are reduced. In contrast, Ag, Cd, and Zn presented lower threshold values than the proposed QRV and PGV, whereas Se showed in NPB and SPB background values higher than QRV and lower than PGV (Table 3 and Table S3).

Secondly, instead of defining realistic threshold values for the entire State and some relevant areas, it appears that environmental agencies tend to define even more conservative QRV values, which is clearly seen by comparing the QRV and PGV values (Table 3 and Table S3). It is highly recommended that new QRV values should be established for the different large geological domains of the State of Pará, in particular for the Carajás region, by using soil samples from the area of investigation and considering the complexity of the geological setting, as similarly conducted in previous studies in the State of Pará [76,77].

The issues addressed above show how understanding the variation of the geochemical background is important for territorial and watershed management. Therefore, the source of the anomalies of the PTE needs to be carefully investigated on a case-by-case basis, considering the local scenario (geology, land use, possible anthropic interventions). It is not demonstrated that high concentrations of some PTE, above the threshold values, are indeed influenced by anthropogenic activities. In the case of PB, the majority of the soil and stream sediment samples with values above the threshold is actually due to bedrock lithologies and hydrothermal mineralized zones (e.g., northern and southern copper belts) that naturally occur in the area. This reinforces the need for new environmental resolutions, which consider the regional characteristics and can thus provide more realistic guideline values.
