Tracing groundwater recharge conditions based on environmental isotopes and noble gases, Lom depression, Bulgaria

Study region: Lom depression, Bulgaria. Study focus: A multi-tracer investigation was applied to identify the recharge conditions and isotope hydrological character of four aquifers in the Lom depression (Northwest Bulgaria) using environmental isotopes (delta O-18, delta H-2, delta C-13, H-3, C-14) and noble gases. The radiocarbon age model of Ingerson and Pearson was used to estimate the mean residence time of groundwater samples from four aquifers (Dacian, Pontian, Meotian, Sarmatian). Our study focuses on the study of recharge conditions and provides an additional information to hydrodynamic understanding of the four aquifers. New hydrological insights for the region: The mean residence time of groundwater samples represent the last twelve thousand years. In addition to a recently recharged groundwater sample, some samples represent the early Holocene and samples closely correspond to the late Pleistocene or the transition time between the early Holocene - late Pleistocene. The lower noble-gas recharge temperature values may indicate that the recharge occurred during the Late Glacial-Holocene climatic transition. These data can also be used to verify the existence of paleowater from the Pontian and Sarmatian aquifers on a trend of the mean residence time of older water heading north away from the study area. Besides the paleoclimatological investigation, it was determined that the Pontian aquifer has no hydraulic connection with the Danube River. The sandy layers and lenses forming the Dacian aquifer are apparently hydraulically isolated and therefore have high degree of heterogeneity.