Investigation of water-rock interactions in Cambrian black shale via a flow-through experiment

FeS2+7/2O2(aq)+H2O→Fe2++2SO42-+2H+,where the rate law for the reaction at pH = 4.2-5.6 and T = 25 °C was determined to be Rsp = 10−8.42±0.06 [O2]0.5 (mol m−2 s−1). This result is consistent with the reaction rate of pyrite grains with dissolved oxygen reported by previous researchers. Hydraulic conductivity decreased by approximately 12.5% during the nitrogen-inflow stage and by 69% during the oxygen-inflow stage. This change can be attributed to the oxidizing reaction within the rock and to the probable clogging of fine weathering products in the pore space. The geochemical modeling of the experimental outflowing water and seepage samples from the field using the PHREEQC program suggested that pore water develops into an acidic solution rich in iron and sulfate due to long-term pyrite oxidation and evaporation.