Allochthonous or Autochthonous? Origins of Berlin’s Groundwater DOM

Dissolved organic matter (DOM) plays a pivotal role in aquatic ecosystems, influencing water quality, microbial activity, and carbon cycling. This study investigates the composition, sources, and dynamics of DOM in Berlin’s urban groundwater, focusing on its variability across aquifer layers and the implications for water quality and ecosystem health. Groundwater samples collected over three years (2021–2023) were analyzed using fluorescence spectroscopy and excitation-emission matrices (EEMs). The primary objectives were to identify DOM sources, assess surface water infiltration risks, and explore dynamic changes in DOM composition. PARAFAC analysis, performed on fluorescence EEMs, revealed eight components (UC1–UC8). Four components were characterized as terrestrial humic (UC1, UC3, UC7, UC8), two were microbial humic (UC2, UC6), one was anthropogenic humic (UC4), and one was a protein-like component (UC5). Component distribution varied across aquifers, reflecting differences in DOM sources and transformations. Shallow aquifers contained higher dissolved organic carbon (DOC) concentrations and microbial humic components (e.g., UC2), while deeper aquifers exhibited recalcitrant terrestrial humic components (e.g., UC7, UC8), potentially stored over long time scales due to anoxic conditions and slow degradation. Protein-like DOM (UC5) was restricted to shallow aquifers, indicating recent surface water inputs.

Overall, these findings underscore the heterogeneity of DOM sources and transformations within Berlin’s groundwater system. The dominance of recalcitrant humic components in deeper aquifers suggests long-term DOM storage, whereas shallow aquifers reflect active surface-water interactions. Anthropogenic influences were most pronounced in shallow and unconfined aquifers, emphasizing the importance of protecting groundwater from urban pollution. Our findings provide valuable insights into the ecological and biogeochemical roles of groundwater DOM and its implications for water management in urban systems.