Geological sources of and hydrogeochemical controls on naturally high nutrient concentrations in groundwater of the coastal lowlands of the western Netherlands

Study region The origin of nutrients in groundwater in the coastal lowlands of the Netherlands is poorly understood, despite naturally elevated levels that raise concern. This study focuses on the densely populated Western Netherlands to address these concerns. Study focus To identify potential sources of ammonium (NH 4 ) and phosphate (PO 4 ), results from approximately 3000 groundwater analyses conducted after 1970 from 1850 wells were evaluated. Spatial and statistical analyses were applied by a priori classifying data sets by landscape type or geological unit. Furthermore, thermodynamic modelling was utilised to assess the complex paleohydrological, sedimentological, and hydrogeochemical processes influencing the observed patterns of nutrient concentrations. New hydrological insights for the region Geology is the key factor controlling natural nutrient occurrences, with concentrations significantly higher in Holocene and Pleistocene marine and fluvial formations, marine clay, and deep polders. Findings indicate that sedimentary organic matter (SOM) decomposition, rather than peat decomposition, primarily drives nutrient release. Additional mobilisation mechanisms include desorption from and reductive dissolution of Fe-oxides for PO 4 , and cation exchange for NH 4 . Modelling reveals that OH-apatite and vivianite act as PO 4 sinks, except under sulfate-reducing conditions where Fe(II) sequestration into iron sulfides limits vivianite precipitation, and local pH shifts induce OH-apatite dissolution. Consequently, geogenic factors, not anthropogenic activities, predominantly influence groundwater nutrient concentrations below the groundwater table in Dutch coastal lowlands. The methodology applied can determine nutrient origins in other low-lying deltas worldwide.