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

Although degradation of phosphorus-containing natural organic matter (NOM) is an important process associ-ated with geogenic phosphorus (P) enrichment, the detailed mechanism underlying the association remains unclear. Herein, we used molecular characteristics of P-containing dissolved organic matter (DOM) coupled with hydrogeochemistry and carbon isotopes to unravel this controlling mechanism. Results indicate that P-containing DOM is indeed an important factor controlling groundwater P enrichment. Further, one-P-atom (1P) and two-P -atom (2P) compounds were widely detected. Compared to 1P compounds, 2P compounds have greater numbers of N/S-containing compounds; smaller proportions of highly unsaturated and aliphatic compounds; larger pro-portions of polyphenols and polycyclic aromatics; 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 modified aromaticity index (AImod) values. At the molecular level, P-containing DOM degradation 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, during which dissolved inorganic phosphorus (DIP) was gradually accumulated and retained, eventually becoming the predominating P pool in groundwater. To our knowledge, this is the first study to reveal geogenic P enrichment from a molecular perspective in groundwater systems worldwide.

Automated summary

This study reveals that phosphorus-containing dissolved organic matter (DOM) is an important factor controlling groundwater phosphorus enrichment. Through the degradation of DOM, an accumulation and retention process of phosphorus is observed at the molecular level, eventually becoming the predominant phosphorus pool in groundwater.