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 <δ(CCO2)-C-13<-3&PTSTHOUSND;, 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 C-13/C-12-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 δC-13(CO 2)-values of -10&PLUSMN;3 and +2&PLUSMN;2&PTSTHOUSND; 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 &AP;9&PTSTHOUSND;. C-13-depleted CO2 is liberated from the reservoir, whereas (CO2)-C-13 is accumulated in the residue.