Degradation of phosphorus-containing natural organic matter facilitates enrichment ofgeogenic phosphorus in Quaternary aquifer systems: A molecular perspective

High levels of geogenic phosphorus (P) in groundwater have been widely found worldwide, posing a potential threat to aquatic environment. Although degradation of P-containing natural organic matter (NOM) is an important process driving the enrichment of geogenic P, the detailed mechanism underlying P enrichment based on dissolved organic matter (DOM) characterization remains unclear. Herein, we chose high-P Quaternary aquifer systems in the central Yangtze River Basin, and used molecular characteristics of P-containing DOM coupled with hydrogeochemistry and carbon isotopes to unravel the detailed mechanisms responsible for the enrichment of geogenic P. The results indicate that P-containing NOM is the most critical factor controlling P enrichment in groundwater. The molecular characterization via Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) reveals that a total of 208–1534 P-containing compounds were detected in 10 groundwater samples, and predominantly consisted of one-P-atom (1P) and two-P-atom (2P) compounds. Compared to 1P compounds, 2P compounds have greater numbers of N/S-containing compounds; smaller proportions of highly unsaturated and aliphatic compounds (considered as intermediates or end-products of biodegradation); larger proportions of polyphenols and polycyclic aromatics (considered as sedimentary inputs from terrestrial vascular plants); lower H/C and nominal oxidation state of carbon (NOSC) values; and higher m/z, O/C, P/C, N/C, double bond equivalents (DBE), and aromaticity index (AI) values. We find that, at the molecular level, the degradation of P-containing DOM overall results in an increase in H/C and a decrease in O/C, and a processing gradient is observed from 2P to 1P compounds. To our knowledge, this is the first study to reveal the underlying mechanism for the enrichment of geogenic P from a molecular perspective in alluvial-lacustrine aquifer systems worldwide, which improve our understanding of biogeochemical behavior of P in subsurface environment.