Abstract
Abstract. Isotopes of dissolved inorganic carbon (DIC) are used to indicate both transit times and biogeochemical evolution of groundwaters. These signals can be complicated in carbonate aquifers, as both abiotic (i.e., carbonate equilibria) and biotic factors influence the δ13C and 14C of DIC. We applied a novel graphical method for tracking changes in the δ13C and 14C of DIC in two distinct aquifer complexes identified in the Hainich Critical Zone Exploratory (CZE), a platform to study how water transport links surface and shallow groundwaters in limestone and marlstone rocks in central Germany. For more quantitative estimates of contributions of different biotic and abiotic carbon sources to the DIC pool, we used the NETPATH geochemical modeling program, which accounts for changes in dissolved ions in addition to C isotopes. Although water residence times in the Hainich CZE aquifers based on hydrogeology are relatively short (years or less), DIC isotopes in the shallow, mostly anoxic, aquifer assemblage (HTU) were depleted in 14C compared to a deeper, oxic, aquifer complex (HTL). Carbon isotopes and chemical changes in the deeper HTL wells could be explained by interaction of recharge waters equilibrated with post-bomb 14C sources with carbonates. However, oxygen depletion and δ13C and 14C values of DIC below those expected from the processes of carbonate equilibrium alone indicate considerably different biogeochemical evolution of waters in the upper aquifer assemblage (HTU wells). Changes in 14C and 13C in the upper aquifer complexes result from a number of biotic and abiotic processes, including oxidation of 14C-depleted OM derived from recycled microbial carbon and sedimentary organic matter as well as water–rock interactions. The microbial pathways inferred from DIC isotope shifts and changes in water chemistry in the HTU wells were supported by comparison with in situ microbial community structure based on 16S rRNA analyses. Our findings demonstrate the large variation in the importance of biotic as well as abiotic controls on 13C and 14C of DIC in closely related aquifer assemblages. Further, they support the importance of subsurface-derived carbon sources like DIC for chemolithoautotrophic microorganisms as well as rock-derived organic matter for supporting heterotrophic groundwater microbial communities and indicate that even shallow aquifers have microbial communities that use a variety of subsurface-derived carbon sources.
Highlights
Groundwater is the most important freshwater reserve on earth and a crucial part of the global hydrological cycle
Mg2+ concentrations are higher in HTU compared to HTL and highest at wells H-52 and H-53
The combination of the Han–Plummer plot and geochemical modeling yielded consistent results and allowed us to identify and quantify the different processes contributing to these large variations in dissolved inorganic carbon (DIC) within a single sub-catchment
Summary
Groundwater is the most important freshwater reserve on earth and a crucial part of the global hydrological cycle. According to the Intergovernmental Panel on Climate Change (IPCC), groundwater demand by humans is likely to increase in future, due to a general increase in global water use and a decline in surface water availability caused by higher precipitation variability (Parry et al, 2007). The critical zone (CZ) is defined as the space ranging from the outer extent of vegetation through soils, down to the saturated and unsaturated bedrock (NRC, 2001). It is the crucial connection between groundwater and surface conditions and the space where fundamental physical, chemical, and biological processes act that are of high importance for sustaining soil and groundwater quality for agricultural and groundwater use (Akob and Küsel, 2011). Assessments of groundwater vulnerability and sustainable groundwater management require a sound knowledge of water movement and carbon transport through the CZ (Küsel et al, 2016)
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