Groundwater pH buffering in carbonate aquifers exposed to high-calcium ash backfill

High-calcium ashes are known to produce highly alkaline leachates in laboratory settings, which has raised concerns regarding potential groundwater impacts when such ashes are used in mine backfills. In this study, we evaluated the effect of a high-calcium oil-shale ash backfill on groundwater pH in carbonate aquifers using an integrated approach combining geochemical modelling with a two-year field experiment. The backfill was injected into an abandoned mine drift in northeastern Estonia, and groundwater was monitored using continuous automatic sensors complemented by periodic sampling. Laboratory leaching tests confirmed the ash backfill has the potential to increase leachate pH values to above 12.5. However, geochemical modelling predicted rapid attenuation of alkalinity through buffering by bicarbonate in the carbonate aquifer, resulting in only minor pH changes restricted to the immediate vicinity of the backfill. Field observations corroborated the model predictions, showing no significant increase in pH in monitoring wells located 10 m from the backfill body. Instead of pH elevation, the most notable geochemical response in groundwater was an increase in sulphate concentrations. The results demonstrate that carbonate aquifers possess a strong natural buffering capacity that effectively mitigates the high alkalinity introduced by high-calcium ash backfill leachate. Although based on a single field site and therefore not directly generalisable to all carbonate mining settings, our results indicate that high-calcium industrial byproducts may be reused for mine reclamation with limited pH effects, while SO 4 2− enrichment emerged as the dominant groundwater response. Future backfilling operations should account for the potential increase in dissolved SO 4 2− in adjacent groundwater and incorporate sulphate-focused monitoring and mitigation measures into the design.