Mechanisms controlling spatial variability of geogenic ammonium in coastal aquifers: Insights from Holocene sedimentary evolution.

The contamination of groundwater with geogenic ammonium (NH) across various geological backgrounds has garnered significant attention, particularly in coastal aquifer systems. However, there remains a gap in our understanding of the mechanisms governing the spatial variability of NH in coastal groundwater at a macroscopic scale. In this study, we collected the sediment samples from two boreholes corresponding to high-NH-N and low-NH-N groundwater. We analyzed the age, physicochemical properties, and soluble organic matter (SOM) characteristics of these sediments. The aim was to reconstruct the sedimentary evolutionary history of the Pearl River Delta and establish a link between sedimentary evolutionary processes and organic matter (OM) to further elucidate the mechanism underlying the formation of spatial heterogeneity of NH in groundwater. The results suggested that the studied Quaternary shallow confined porous aquifer system was predominantly formed during the Holocene and comprised three depositional stages, including fluvial facies, estuarine-tidal flat facies, and deltaic plain facies. The depositional environment significantly controlled the physicochemical and SOM characteristics of sediments. In the paleo-channel area, the aquifer was covered by estuarine-tidal flat facies sediments abundant in OM and exhibited considerable SOM degradation. Consequently, a substantial amount of ion-exchange form NH-N (IEF) was liberated through compaction and diffused into the aquifer. In the paleo-interfluve area, the aquifer was covered by fluvial sediments characterized by extensive weathering and low OM content, resulting in the limited production of significant amounts of IEF that could infiltrate into the aquifer. This study provided an inaugural elucidation of the control mechanism of sedimentary evolution on the spatial variability of NH in coastal groundwater at a macroscale, thereby enhancing the scientific substantiation for both the exploitation and protection of groundwater resources.