The effects of waterlogging on the solubility of arsenic and ecotoxicity of soil pore water in non-fertilized and fertilized soils in historical mining sites.

Soil contamination with arsenic in historical mining sites is a matter of considerable environmental concern, as the concentrations of As in those soils are locally as high as thousands mg/kg. Some of soils, particularly those affected in the past by tailings that were released from impoundments, are located in floodplains and used as grasslands. Those lands are periodically flooded, and the frequency and duration of flooding will probably increase in the future with changing climatic conditions. Reducing environment that develops upon soil flooding can cause a release of As from soil solid phase. This is an inherent effect of reductive dissolution of amorphous and crystalline iron hydroxides that are the main hosting components for metalloids. Changing redox conditions affect also the speciation of As in pore water, influencing its toxicity to soil biota. Moreover, soil fertilization with inorganic fertilizers that contain phosphates, or with organic fertilizers such as cattle manure, can accelerate As release from iron hydroxides, mainly via competitive desorption. The effects of all those processes are highly dependent on soil properties and still require a close examination.

Three kinds of soil material, containing up to 8000 mg/kg As, were collected from the tailings-affected floodplain of the Tująca river in Złoty Stok, a historical As mining centre. A laboratory incubation experiment with fertilized and non-fertilized soils was carried out to examine the changes in As concentrations in soil pore water, as well as to assess pore water ecotoxicity, determined in standard bioassays, including Microtox and Phytotox. Soil flooding resulted in a rapid release of As from soil solid phase. As concentrations in soil pore water in all samples exceeded 10 mg/L after a 2-day incubation, and tended to increase slowly with time. In some cases, after the 270-day incubation, As concentrations in pore water reached several hundred mg/L. Those effects resulted in a very high ecotoxicity of pore water, caused lethal effects to bacteria and springtails, and impeded plant germination. Soil amendment with manure was a factor that significantly enhanced those effects. The factors responsible for various effects that were reported from three soils were discussed.