Carbon chemistry and groundwater dynamics at natural analogue site Ruprechtov, Czech Republic: Insights from environmental isotopes

Hydrological, geochemical and environmental isotope data from groundwater wells at Ruprechtov natural analogue site were evaluated to characterize the flow pattern and C chemistry in the system. The results show that water flow in the Tertiary sediments is restricted to 1–2 m thick zones in the so-called clay/lignite horizon with two different infiltration areas in the outcropping granites in the western and south-western part of the investigated area. Generally the flow system is separated from the underlying granite by a kaolin layer up to several tens of metres thick. Differences in stable isotope signatures in the northern part of the site indicate very local connections of both flow systems via fault zones. The observed increase of δ¹³C values and decrease of ¹⁴C activities in dissolved inorganic C during evolution of the groundwater from the infiltration area to the clay/lignite horizon was modelled using simple open- and closed-system models as well as an inverse geochemical model (NETPATH), which included changes in other geochemical parameters. The results from both types of models provided some insights into timescales of groundwater flow, but mainly revealed that additional sources of C are active in the system. These are very likely biodegradation of dissolved and sedimentary organic C (DOC and SOC) as well as the influx of endogenous CO₂. The occurrence of microbial degradation of SOC in the clay/lignite layers is indicated by the increase of biogenic DIC, DOC, and phosphate concentrations. The impact of microbial SO₄ reduction on this process is confirmed by an increase of δ³⁴S values in dissolved ${\text{SO}}_4^{2-}$ from the infiltration area to the clay/lignite horizon with an enrichment factor of 11‰. The very low DOC concentrations of 1–5 mg C/L in the clay/lignite horizon with an organic matter content up to 50% C is probably caused by very low availability of organic matter to the processes of degradation and DOC release, demonstrated by extraction experiments with SOC.