Groundwater-surface water interactions as controls on hydrochemical transfer and nitrogen availability in a nitrate-enriched calcareous fen

Agredazywczuk, P., Robinson, S. ORCID: https://orcid.org/0000-0003-1045-4412, Wade, A. ORCID: https://orcid.org/0000-0002-5296-8350, Wilkinson, D. and Earl, G. (2026) Groundwater-surface water interactions as controls on hydrochemical transfer and nitrogen availability in a nitrate-enriched calcareous fen. Wetlands. ISSN 1943-6246 (In Press)

Abstract/Summary

Calcareous fens are groundwater-dependent peatlands that regulate nitrogen (N) availability between aquifers and surface waters, yet their functioning under legacy nitrate (NO3--N) groundwater contamination remains poorly understood. This study investigated spatial and temporal patterns of NO3--N at Greywell Fen, a chalk aquifer-fed system in southeast England subject to decades of agricultural N enrichment and groundwater abstraction. Two to four week hydrochemical sampling at 45 locations revealed spatial NO3--N heterogeneity controlled by groundwater discharge zones. Hierarchical Cluster Analysis identified three hydrochemical signatures: 78% of pore waters exhibited depleted NO3--N (< 1 mg NO3--N L⁻¹) with elevated dissolved organic carbon (DOC) and iron (Fe2+/3+), while 22% maintained high NO3--N (1.37-6.86 mg NO3--N L⁻¹) and Ca2+ concentrations. Principal Component Analysis confirmed transport-dominated zones (high NO3--N, Ca2+) versus reaction-dominated zones (high Fe2+/3+, DOC). Strong negative correlations between NO3--N and the Mg2+:Ca2+ ratio (ρ = -0.69) revealed residence time controls on N transformation. Regression and relative importance analysis identified DOC and Fe2+/3+ as dominant controls on pore water NO-3-N (91.4% of variance), with Mg2+:Ca2+ contributing only 8.6%, demonstrating that groundwater discharge influences pore water NO₃-N indirectly through redox conditions. A heterogeneous clay layer likely explains spatial distribution by partitioning the fen from NO-3-N-enriched groundwater; high NO3--N areas emerge at discrete discharge zones, where the clay is thinner. Temporal concentration stability, maintained by consistent groundwater inflows and floating fen characteristics, confirms that hydrogeomorphic controls dominate over seasonal conditions. These findings challenge hydrological connectivity assumptions in groundwater-dependent ecosystems and suggest zone-based management approaches over uniform strategies.

Item Type	Article
URI	https://centaur.reading.ac.uk/id/eprint/129153
Refereed	Yes
Divisions	Science > School of Archaeology, Geography and Environmental Science > Department of Geography and Environmental Science
Publisher	Springer
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