Hydrogeochemical modelling of groundwater in a fractured Carboniferous sandstone aquifer

Deep groundwater monitoring in tight bedrock is not commonly performed and rarely documented. For a case study in Bochum, NRW, deep monitoring wells allow groundwater investigation in the outcropping Carboniferous down to a depth of 186 m. Groundwater monitoring was carried out by combining sampling of the monitoring wells and local springs at the site. Inverse models were developed using the hydrogeochemical code PHREEQC to understand the natural geochemical processes of deep groundwater in the coal-bearing sandstone. The identified hydrogeochemical processes are pyrite oxidation in shallow fracture systems and Na-Ca/Mg cation exchange in deep fracture systems. Core samples reveal fractures containing Fe-dolomite and calcite cements. A freshening process dominates the deep fractured rocks, where fresh water is flushing a saline aquifer. The data presented here support the planning of further monitoring wells and help to improve future modelling approaches.

Automated summary

Deep groundwater monitoring in tight bedrock is not commonly performed and rarely documented. A case study in Bochum, NRW utilized deep monitoring wells to investigate groundwater in the Carboniferous down to 186 m depth. Groundwater monitoring involved sampling from monitoring wells and local springs, and inverse models were developed using PHREEQC to understand the geochemical processes. The identified processes include pyrite oxidation in shallow fractures and cation exchange in deep fractures. The findings support future planning of monitoring wells and improve modeling approaches.