Stable isotope ratios and the evolution of acidulous ground water

Ground waters in North Hesse (Germany) are conspicuous by high amounts of dissolved inorganic carbon (DIC) at low pH. The DIC is received from the uptake of soil CO₂ and CO₂ of volcanic origin and the subsequent dissolution of Triassic and Permian limestone and dolomites. The volcanic CO₂ is related to Miocene basaltic magma which has liberated gaseous CO₂ during the breakthrough to Triassic and Permian sediments. The volcanic CO₂ (-6 < δ¹³ CCO₂ < -3‰, PDB) was trapped within pore spaces and intra- and intergranulares of Permian evaporites and Triassic sandstones and was stored within such reservoirs until recent times. The uptake of volcanic CO₂ occurs as ground water migrates through such reservoirs. The ¹³C/¹²C-signatures of the DIC indicate mixture of soil-CO₂ and CO₂ of volcanic origin for the dissolution of marine limestone and dolomites. The obtained two types for CO₂ of volcanic origin with δ¹³C_CO2-values of - 10 ± 3 and +2 ± 2‰ can be explained by diffusion of CO₂ through micropores, faults, and interfaces of solids. This mobilisation of CO₂ is accompanied with a kinetic fractionation of ≈ -9‰. ¹³C-depleted CO₂ is liberated from the reservoir, whereas ¹³CO₂ is accumulated in the residue.