Assessment of Seasonal Changes on the Carbon Cycle in the Critical Zone of a Surface Water (SW)-Groundwater (GW) System

Groundwater is a valuable source of water for human consumption, and its quality is a current issue worldwide. Understanding carbon and water cycling presents the basis of biogeochemical reactions occurring in the aquifer; therefore, understanding their interaction is imperative for sustainable water management. In the paper, this interaction was investigated within the complex surface water (SW)-groundwater (GW) system in the Varazdin region (Croatia) by using a multi-parameter approach: delta C-13(DIC) values, carbon species (DIC, DOC), delta O-18 and delta H-2 values, geochemical indicators (T, pH, DO, EC), and delta C-13 measurements in solids. Both delta O-18/delta H-2 and delta C-13(DIC) were recognized as good indicators to differentiate shallow and deep GW. Transit time of water (TT) was evaluated as an important parameter in controlling carbon cycling within the SW-GW system. Shallow GW is characterized by shorter TT, seasonal changes in carbon species and delta C-13(DIC), and lower possibility of carbon capture in the system. Deep GW has longer TT without pronounced seasonal changes in carbon species and delta C-13(DIC). The conceptual model of the carbon cycle revealed major sources and sinks of CO2 in the study area. Our results suggest that GW acts as both source and sink for CO2, depending on the prevailing geochemical process. Surface waters are primarily a source of CO2, excluding the gravel pit, which acts primarily as a sink for CO2. Our study shows that the current SW-GW dynamics regulate carbon balance without having negative impacts on groundwater quality but also demonstrates that implementing carbon cycle in water management studies is of vital importance for sustainable use of groundwater.