Mixing Model Analysis for Ion Source Identification and Quantification of Geogenic and Anthropogenic Inputs during baseflow in São Francisco River, Brazil

Abstract

Chemical weathering represents a fundamental process within the global carbon cycle, serving as an important mechanism for atmospheric CO₂ sequestration through rock-atmosphere interactions. The quantitative assessment of CO₂ consumption during chemical weathering reactions requires comprehensive mass balance analyses as a foundational approach for understanding carbon fluxes at regional and global scales. This study employs a mass balance mixing model to evaluate the relative contributions of natural and anthropogenic sources to dissolved solute fluxes across 1,418 microbasins within the São Francisco River Basin, Brazil. The results indicate that silicate weathering processes dominate in 66% of the studied basins, whereas carbonate weathering exhibits greater prevalence in the karstic terrains of the northern regions. Atmospheric and anthropogenic inputs contribute an average of 2% to total solute loads, with diffuse anthropogenic influences being most pronounced in urbanized catchments. These findings underscore the critical importance of accurate source apportionment in hydrogeochemical studies and highlight the role of chemical weathering as a key component in understanding global carbon dynamics within the context of climate change mitigation and adaptation strategies.

Introduction

The accumulation of atmospheric CO₂ represents one of the principal drivers of climate change (IPCC, 2023). Within the global carbon cycle framework, continental chemical weathering may serve as a critical long-term sink for atmospheric CO₂ by converting carbonic acid (H₂CO₃) into bicarbonate ion (HCO₃^-) which is subsequently transported via fluvial networks to marine environments where it may be permanently sequestered as carbonate sediments (X. Li et al., 2019). This biogeochemical mechanism plays a fundamental role in regulating atmospheric CO₂ concentrations over geological timescales and exhibits complex interdependencies with surface denudation processes, bedrock lithology, climatic variables, and hydrological regimes.

The hydrogeochemical signatures of surface water provides an integrated proxy for weathering processes and associated solute fluxes across drainage basins, enabling indirect estimations of chemical weathering rates (Chai et al., 2024). However, quantifying these processes at various spatial scales remains a significant challenge due to the complex interactions between natural geochemical processes and anthropogenic sources (S. Li et al., 2014). The quantification of the sources is a preliminary step and prerequisite before deriving reliable estimates of chemical weathering rates and associated CO₂ consumption at the catchment.

The primary objective of this investigation was to quantify the relative contributions of natural and anthropogenic sources to riverine solute loads across small catchments exhibiting contrasting lithological characteristics, land use and climatic gradients. This assessment was accomplished through the implementation of mixing model approach based on the hydrochemical composition of surface waters.

Experimental

This hydrogeochemical study analyzed 1,418 microbasins tributary to the São Francisco River in Minas Gerais, Brazil, using the Geological Survey of Brazil (SGB/CPRM) database (Figure 1-2). Surface water samples were collected during the baseflow period (Marques et al., 2023). Major anions (Cl⁻, SO₄²⁻) were quantified by ion chromatography (Metrohm 881 Compact IC Pro), while HCO₃⁻ was determined by titrimetric analysis (Metrohm 905 Titrando). Cation samples were acidified (pH < 2) with HNO₃ and analyzed by ICP-OES (PerkinElmer AVIO 500) for Ca²⁺, Mg²⁺, Na⁺, K⁺, and Si concentrations. 

This approach is based on the concentration of major ions in river water which is predominantly attributable to atmospheric deposition (via precipitation), anthropogenic inputs, and chemical weathering of minerals (carbonate dissolution, silicate, evaporite, and oxidative weathering of sulfide minerals) (Equation 1) (Gaillardet et al., 1997; Komba et al., 2024; Moore et al., 2025; Xu et al., 2024).

The contribution of evaporite was negligible based on the geological context. All SiO₂ was attributed to silicate. The atmospheric input was corrected by ion:Cl^- ratio in seawater (Gaillardet et al., 1999) and remaining Cl^- considered to come from anthropogenic input. The baseline ion:HCO₃^- ratio was used to differentiate between natural and anthropogenic (Roy et al., 1999). Then, residual SO₄^2- was designated as sulfide input and the Na⁺ and K⁺ as silicate. Ca²⁺, Mg²⁺ and HCO₃^- originate from both silicate and carbonate. To differentiate, the silicate ratios of the average silicate rock, (Ca/Na) and (Mg/Na), were used (X. Li et al., 2019). The results are expressed as percentages.

Results and Discussion

The mean total dissolved solid (TDS) concentration in the microbasin is 85 mg L^-1, comparable to the global mean of 100 mg L^-1 (Liu et al., 2025). Approximately 360 basins exceed this limit. The ion composition is predominantly Ca²⁺, Na⁺, and HCO₃^- ions, with mixed natural and diffusion-derived sources observed in over 100 microbasins (Machado et al., 2024). Approximately 50% of the study area is affected by anthropogenic activity including mining, agriculture, pasturelands, and urban areas.

Solute contribution quantification was achieved through mass budget equations considering four end members: atmospheric precipitation, anthropogenic input, carbonate and silicate rock weathering, and sulfide mineral weathering (Figure 3). Among 1,418 microbasins examined, atmospheric deposition and anthropogenic sources contribute averages of 2 ± 2% and 2 ± 5%, respectively. Silicate weathering accounts for approximately 60 ± 25% of total dissolved load, while carbonate weathering contributes 36 ± 27%. Sulfate-related inputs manifest locally with 0.5 ± 2% average contribution and pronounced spatial variability (CV = 398%).

Eight microbasins exhibited sulfide contributions >10%, predominantly in sub-basin VG (n = 6). Highest individual contributions were found in RV and VG (28% and 22%, respectively), while ASF showed the lowest maximum contribution (0.8%). Atmospheric input decreases from south to north, aligning with climatic gradients. The southern basin experiences higher rainfall and temperature in Atlantic Forest and Cerrado biomes, while the northern sector drains semi-arid Caatinga biome. Seventeen microbasins showed atmospheric contributions >10%, concentrated in the southern sector.

Only 42% (n = 602) of microbasins exhibited no anthropogenic influence. Among remaining basins, 6% showed diffuse anthropogenic inputs exceeding 10% (49 basins at 10-20%, 29 basins >20%). Highest inputs are associated with urban centers, highlighting the land use-water chemistry relationship.

In 66% of microbasins, silicate contributed more than carbonate dissolution. Sub-basins RTM and MSF showed comparable contributions from both lithologies, while carbonate dominance is prominent in the karst zone. Distinguishing lithologic contributions is critical for carbon cycle understanding. Silicate weathering provides long-term CO₂ sequestration by transforming H₂CO₃ into HCO₃^-. Carbonate weathering is carbon-neutral over geological timescales but important for gross CO₂ uptake over human timescales. In climate change scenarios, high atmospheric sulfur levels could make H₂SO₄ a relevant weathering agent, altering carbonate dissolution and potentially causing net CO₂ emissions (Chai et al., 2024). These dynamics highlight the need to account for acid-specific weathering pathways in carbon budget models.

Conclusions

This study demonstrates that silicate weathering is the primary geochemical process contributor to riverine solute fluxes in the São Francisco River Basin. While anthropogenic contributions remain relatively minor at the regional scale, their localized impacts emphasize the critical importance of sustainable watershed management. These findings highlight the necessity to differentiate between sources when estimating weathering-derived CO₂ fluxes and underscore the role of silicate weathering as a long-term atmospheric CO₂ sink under natural conditions. However, future changes in weathering agents due to climate change, such as an increase in H₂SO₄, could significantly alter the carbon neutrality of carbonate dissolution, necessitating continued monitoring of these biogeochemical dynamics in future climate scenarios.

Acknowledgements

This research was supported by FAPERJ - Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, under SEI Process - 260003/001202/2025.