Partitioning of copper at the confluences of Andean rivers.

The fate of copper (Cu) in rivers impacted by acid drainage remains poorly studied in waters with comparatively low Al and Fe concentrations. This work addresses the role of confluences in controlling the physical and chemical fate of Cu in a system with total molar ratio Cu/Al > 0.2 and Cu/Fe > 0.15. Two consecutive confluences were studied in the upper Mapocho watershed, a densely populated basin with intensive mining located in the Chilean Andes. The inflow had acidic conditions with seasonal variations and Cu up to 9 mg L-1. Lability measurements with diffusive gradient in thin films showed that Cu entered as a dissolved labile form. However, downstream from the confluences a higher pH shifted Cu toward nonlabile compounds and solid phases enriched with Cu. Measurements of x-ray absorption spectroscopy of freshly formed particles showed that composition was dominated by sorbed Cu and Cu(OH)2(s) precipitates, with a higher proportion of sorbed Cu downstream from confluences when pH < 5. Particle size distributions (PSD) measured in field showed that downstream from the confluences the total volume and average diameter of the suspended particles grew progressively, with estimated mean settling velocities increasing from 0.3 to 4.2 cm s-1. As a result, 7-30% of the influent Cu was removed from the river flow. These results highlight that shifts in chemical partition and PSDs in river confluences and the hydrodynamic environments at the river reach level control the mobility of Cu in systems with high Cu/Al and Cu/Fe.

Rusty river: Effects of tufa precipitation on sediment entrainment in the Estero Morales in the central Chilean Andes.

Rivers and streams continuously shape and reform their channels through the transport of sediment. One of the most important parameter used to assess this transformation is the threshold for incipient grain motion. To date, limited studies have reported that several biotic and abiotic factors can affect this parameter. However, the effects of tufa precipitation on sediment entrainment and dynamics are still unexplored. The Estero Morales is an Andean stream in Central Chile affected by the phenomenon of tufa precipitation during the winter. Along the wetted channels, tufa precipitate creates a thin solid layer that covers the sediments. A series of field surveys and flume experiments were conducted to analyze the effect of tufa precipitation on the initiation of motion and sediment dynamics. Along the wetted areas of the river, a portable dynamometer was used to explore the force needed to dislocate the grains affected by tufa precipitation from the surrounding sediments. Flume experiments were conducted to compare the incipient motion of sediment covered by tufa precipitation with unaffected sediment. Geochemical analyses were conducted to study the precipitate chemistry, mineralogy and texture. The results demonstrate that greater force is needed to move sediment particles affected by tufa precipitation compared to unaffected ones. In addition, lower sediment transport rates were measured on sediment affected by tufa precipitation, especially for the largest sediment size. These results could have important implications for studies concerning sediment dynamics and contaminant fate in the environment. Moreover, the results allow us to make some assumptions regarding the long-term role that tufa precipitation can play in rivers. Such analysis can help us to better understand and predict the changes in sediment transport rates due to tufa precipitation.

Enhancement of particle aggregation in the presence of organic matter during neutralization of acid drainage in a stream confluence and its effect on arsenic immobilization.

Acid drainage (AD) is an important environmental concern that impacts water quality. The formation of reactive Fe and Al oxyhydroxides during the neutralization of AD at river confluences is a natural attenuation process. Although it is known that organic matter (OM) can affect the aggregation of Fe and Al oxyhydroxides and the sorption of As onto their surfaces, the role of OM during the neutralization of AD at river confluences has not been studied. Field and experimental approaches were used to understand this role, using the Azufre River (pH 2) - Caracarani River (pH 8.6) confluence (northern Chile) as model system. Field measurements of organic carbon revealed a 10-15% loss of OM downstream the confluence, which was attributed to associations with Fe and Al oxyhydroxides that settle in the river bed. Laboratory mixtures of AD water with synthetic Caracarani waters under varying conditions of pH, concentration and type of OM revealed that OM promoted the aggregation of Fe oxyhydroxides without reducing As sorption, enhancing the removal of As at slightly acidic conditions (pH ∼4.5). At acidic conditions (pH ∼3), aggregation of OM - metal complexes at high OM concentrations could become the main removal mechanism. One type of OM promoted bimodal particle size distributions with larger mean sizes, possibly increasing the settling velocity of aggregates. This work contributes to a better understanding of the role of OM in AD affected basins, showing that the presence of OM during processes of neutralization of AD can enhance the removal of toxic elements.