Response of suspended sediment particle size distributions to changes in water chemistry at an Andean mountain stream confluence receiving arsenic rich acid drainage

Abstract

Acid drainage is an important water quality issue in Andean watersheds, affecting the sustainability of urban, agricultural, and industrial activities. Mixing zones receiving acid drainage are critical sites where changes in pH and chemical environment promote the formation and dissolution of iron and aluminum oxy/hydroxides. These particles can significantly change the speciation of toxic metals and metalloids throughout drainage networks via sorption, desorption, and settling processes. However, little is known about the behavior of particle size distributions (PSDs) in streams affected by acid drainage and their relationship to metal speciation. This work studied: (a) the PSDs for a wide range of mixing ratios found at a fluvial confluence affected by acid drainage, and (b) the response of PSDs and arsenic speciation to environmental changes found when the particles approach complete mixing conditions. The confluence between the Azufre River (pH ~ 2, high concentration of dissolved metals) and Caracarani River (pH = 8.6, low concentration of dissolved metals) was used as a representative model for study. Field measurements show a bimodal PSD with modal diameters of ~50 and 300 μm. At shorter distances from the junction, the smaller modes with smaller particle volumes were dominant across the stream cross‐sections. A systematic shift towards larger particle sizes and larger particle volumes occurred downstream. The analysis of laboratory PSDs for Azufre/Caracarani mixing ratios between 0.01 and 0.5 (pH from 6.2 to 2.3) showed a bimodal trend with ~15 and 50 μm characteristic diameters; larger particles formed at pH>4. When particle suspensions were transferred in laboratory experiments from very low pH to full mixing conditions (pH ~ 2.8 and mixing ratio ~ 0.25) particle sizes varied, and the dissolved arsenic concentration decreased. The observed reaction kinetics were slow compared to the time scale of advective transport, creating opportunities for engineered controls for arsenic. This work contributes to a better understanding of the chemical‐hydrodynamic interactions in watersheds affected by mining, and identifying opportunities to improve water quality at points of use.

Occurrence and Removal of Copper and Aluminum in a Stream Confluence Affected by Acid Mine Drainage

“…In the case of change in the flow ratio between both flows, for example due to seasonal variation of the flow of the Yerba Loca Creek, a dilution effect could be generated and a consequent impact on the pH and concentration values after the junction. Therefore, as a final consequence, the removal of Al and Cu due to precipitation processes could be generated, as has been done for other confluences [14][15][16]. Abarca et al [14] show that after the Azufre River-Caracarani River confluence, particle sizes varied, and the dissolved As concentration decreased.…”

“…In addition, high Al concentrations, low pH and high levels of sulfate generate toxicity in the aquatic environment, which can be fatal for animal and plant life [13].Several studies have been carried out about the effects of the confluence of water courses related to pollution by AMD. The Azufre River-Caracarani River Confluence (Chile) presents a change in particle sizes and a decrease in As concentration after the confluent point [14]. Agrio River-Odiel River Confluence (Spain) shows a strong seasonal variation in water composition [15].…”

“…The complete mixing results in a circumneutral pH and metals in solution exhibit transformations to colloidal forms [16]. The mixing zone receiving acid drainage is a reactive zone where changes in pH and the chemical environment promotes the formation and precipitation of minerals [14]. Metal partitioning between the solid phase and solution can be modified along the flow path by changes in pH, redox and biotic factors downstream of the mixing zone [17,18].…”

Field and Numerical Investigation of Transport Mechanisms in a Surface Storage Zone

“…In recent years, we have investigated in detail the effects of legacy mining in the high-altitude environment of the Lluta River in northern Chile, focusing on the coupling of physical and biogeochemical processes in the system (Abarca et al, 2017;Arce et al, 2017;Contreras et al, 2015;Guerra, Gonzalez, et al, 2016;Guerra, Simonson, et al, 2016;Leiva et al, 2014). Acid drainage in this watershed has increased the concentration of elements and compounds such as arsenic, aluminum, iron, chlorides, and sulfates in the river.…”

“…These highly reactive particles have a strong binding capacity toward metals and metalloids, actively changing their chemical speciation from the dissolved phase to the solid phase. The speciation processes in the river and their implications have been widely discussed in our previous investigations (for details, the reader is referred to Abarca et al, 2017;Arce et al, 2017;Guerra, Gonzalez, et al, 2016;Guerra, Simonson, et al, 2016).…”