09/23/2016

Factors Controlling the Migration of Tailings-Derived Arsenic: A Case Study at the Yara Siilinjärvi Site

Text:
Kaisa Turunen, Planner

Arsenic (As) is a ubiquitous metalloid that is found in low concentrations in the atmosphere, in aquatic environments, in soils and sediments, and in organisms. Its presence in the environment can be due to natural processes, such as weathering, biological, and volcanic activity, or anthropogenic activities such as mining. As is problematic because of its relatively high mobility over a wide range of redox conditions and its toxicity to humans, animals, and plants. The behaviour of arsenic (As) derived from tailings was investigated at the Yara Siilinjärvi apatite mine and industrial site in eastern Finland. The study assessed factors controlling the migration and fate of As and compared the anthropogenic As load to the natural geogenic background. Environmental risks related to As were assessed by examining the As concentrations in humus, glacial till, aquatic sediments, groundwater, and surface water. The occurrence and fractionation of As and the presence of secondary precipitates and geochemical transformations in the tailings and in the ambient soil and sediment were evaluated by selective extraction. The water-derived emissions were evaluated by field measurements, hydrogeochemical analysis, and modelling.

In natural conditions, As speciation is controlled by chemical properties such as pH and redox conditions, along with absorption, desorption, and ion exchange reactions. The results of this study indicate elevated environmental risks due to dust and seepage emissions from the tailings since the concentrations and mobility of As and other potentially harmful elements (PHEs) such as Co, Ni, and Zn were elevated relative to the geogenic background. These elements were mainly associated with Fe (oxy)hydroxides in the soil and so their mobility was closely linked to Fe biogeochemistry. In acidic aquatic environments, even a slight increase in pH may lead to adsorption of As onto Fe (oxy)hydroxide particles, decreasing the amount and migration of dissolved As in surface water. Thus, although the concentrations of As and PHEs were high in the tailings pond and seepage water, they decreased in ambient groundwater and surface water, indicating Fe (oxy)hydroxide stability in current chemical conditions. This was supported also by the hydrogeochemical modelling, which indicated precipitation of Fe oxides and hydroxides. Since different As species exhibit wide-ranging levels of toxicity to various organisms, it is important to analyse As speciation and fractionation, not just total concentration. According to speciation modelling, As was mainly present in groundwater as the toxic trivalent arsenious acid (H3AsO3) at pH 6, and mainly as the less toxic pentavalent As acids (H2AsO4- and HAsO42-) at pH 6 and in surface water.

Although this study showed that As is mainly adsorbed to precipitates and present as less toxic pentavalent As acids, if the physico-chemical conditions in the vicinity of tailings change, As adsorbed to Fe(oxy)hydroxides may be mobilized, and As speciation may change into the more toxic form. Since, the ecosystem has developed in the presence of naturally occurring low background concentrations of As, elevated concentrations may pose a risk to the ambient environment. Furthermore, because the concentrations of As and other PHEs were elevated in groundwater pipes near the tailings, it seems that despite of the preventive pumping, the seepage water is able to migrate from the tailings or the tailings ponds to the environment. Thus it is essential to control the environmental effects by minimizing the dust emissions from the tailings area and pumping the tailings seepage water from the bedrock fracture zone.

See full article:

Turunen, K., Backnäs, S., Neitola, R., & Pasanen, A. (2016). Factors controlling the migration of tailings-derived arsenic: A case study at the Yara Siilinjärvi site. Mine Water and the Environment, 2016, 1-14. http://link.springer.com/article/10.1007%2Fs10230-016-0393-5

Kaisa Turunen

Text: Kaisa Turunen

MSc Kaisa Turunen works as a planner at the Industrial Environments and Recycling Unit of the Geological Survey of Finland. Her research topics are mining environment, environmental geochemistry and environmental geology. She is the coordinator of the Finnish Mine Water Network