Evaluation of the migration and environmental effects of metal elements within cementitious gangue-fly ash backfill in underground coal mines

Cementitious gangue-fly ash backfill (CGB) is used as a green mining technology worldwide. However, under the coupled effects of geological stress and groundwater, the metal elements in the CGB tend to migrate into nearby strata, which can consequently result in pollution of the groundwater environment. In this paper, the influence of initial pH and stress damage on the migration behavior of metal elements in CGB is quantitatively studied through the multi-physical field coupling model of stress-permeability-concentration. The enhanced Nemerow index evaluation method is used to comprehensively evaluate the impact of these metal elements migration behaviors on the groundwater environment. The research results show that: (1) When the stress damage of the CGB increases from 0.76 to 0.95, the Darcy velocity at the bottom of the CGB first increases, then decreases, and finally stabilizes at 2.01×10⁻⁷ m/s. The longest time to reach the maximum Darcy velocity is 3 a. (2) When the damage of the CGB is 0.95, the farthest migration distances of Al, Cr, Mn, Fe, Ba, and Pb are 40.5, 34.0, 29.8, 32.9, 38.8 and 32.1 m, respectively. (3) The alkaline environment stimulates the migration of Al, Cr, Fe, Mn, and Pb, whereas Ba migrates farther under acidic conditions. The farthest migration distance of Ba is 31.6 m under pH 3. (4) The enhanced Nemerow index indicates that when stress damage increases from 0.76 to 0.95, the areas with poor water quality increase from 0 to 1.71%, and no area is classified as very poor grade. When the initial pH changes from 3 to 11, 100% of the region is classified as fair or above. The initial pH of the CGB has a relatively slight influence on the groundwater environment. This study provides experimental data and theoretical basis for the environmental evaluation of CGB.