Computed tomography assessment of soil and sediment porosity modifications from exposure to an acid copper sulfate solution

Abstract

Mine spills, like acidified copper sulfate solutions, are known for the contaminating effects in soils and sediments, chemically altering them. However, little is known about the physical effects of this process. Therefore, we investigated the response of the soil and sediments porosity to the infiltration of an acid copper sulfate solution (ACSS) generated for mining purposes. The samples used consisted of intact/undisturbed soil and sediment columns extracted in the upper Rio Sonora basin, located in northern Mexico in the state of Sonora, where extensive mining activity has been carried out since 1880's. Three infiltration and solute transport experiments were conducted to monitor the changes in pH, electrical conductivity (EC) and flow velocity in the collected leachate. Computed X-Ray Tomography was used to microscopically analyze a series of soil porosity images before and after each irrigation with ACSS. We observed that the interactions of soil/sediment matrices with ACSS, promoted changes in the flow velocity, which can be related to the modification of the original porosity. The computed X-Ray Tomography showed that the passage of the ACSS through the soil and sediment caused different effects in both matrices: (1) a decrease in the number and size of pores in the soil column and (2) an increase in the number and size of pores in the sediment, showing that the passage of the solution in both matrices had opposite effects, causing a decrease in soil porosity as a possible response to the presence of suspended particles related to the ACSS, and the formation of new minerals that caused the sealing of pores in the soil and the erosion of the walls of the pores in the sediment creating preferential flows. This work shows that soils and sediments that interact with the ACSS are not only affected chemically, but also that there may appear modifications in porosity, leading to study also the physical effects that occur in sites affected by this type of events.