Unravelling Coupled Hydrological and Geochemical Controls on Long-Term Nitrogen Enrichment in a Large River Basin

Abstract

Many groundwater and surface water bodies around the world show a puzzling and often steady increase in nitrogen (N) concentrations, despite a significant decline of agricultural N inputs. This study uses a combination of long-term hydrogeochemical and hydraulic monitoring, molecular characterization of dissolved organic matter (DOM), column experiment, and reactive transport modeling to unravel the processes controlling N-reactive transport and mass budgets under the impacts of dynamic hydrologic conditions at a field site in the central Yangtze River Basin. Our analysis shows that the desorption of ammonium (NH₄⁺) from sediments via cation exchange reactions dominates N mobilization and aqueous N concentrations, while the mineralization of organic N compounds plays only a minor role. The reactive transport modeling results illustrate the important role of cation exchange reactions that are induced by temporary NH₄⁺ input and cation concentration changes under the impact of both seasonal and long-term hydrologic variations. Historically, cation exchangers have acted as efficient storage devices and mitigated the impacts of high levels of NH₄⁺ input. The NH₄⁺ residing on cation exchanger sites later acts as a long-term N source to waters with the delayed desorption of sediment-bound NH₄⁺ induced by the change of hydrologic conditions. Our results highlight the complex linkages between highly variable hydrologic conditions and NH₄⁺ partitioning in near-surface, river-derived sediments.

Keywords: cation exchange; groundwater surface water interaction; nitrogen cycling; reactive transport.

Figure 1

(A) Location of JHP and regional groundwater flow, (B) the Shahu monitoring site, and (C) hydrogeological cross section in which groundwater flow paths are indicated as dotted lines for wet season conditions and as solid lines for conditions during the dry season. TSR and DJR refer to the Tongshun River and the Dongjing River, respectively.

Figure 2

(A) Groundwater levels and river stages, (B) NH₄⁺ concentration, and (C) NO₃^– concentrations change along with time at the Shahu site in the discharge area of JHP. Two whisker ends represent minimum and maximum, and solid squares show the mean values.

Figure 3

Vertical distribution of (A) DON and exchangeable NH₄⁺ concentrations, (B) measured cation exchange capacity (CEC) and specific surface area (SSA), and (C) relative abundance of DOM formulas extracted from sediment collected from the Shahu site located in the JHP discharge area. Clay-rich sediments at depth from 4.5 to 20 m showed 1 order of magnitude higher CEC and SSA values than the silty and sandy sediments.

Figure 4

Breakthrough curves of Br, NH₄⁺, NO₃^–, Ca, and Mg in columns A, B, and C, as well as exchangeable NH₄⁺ concentrations in the sediments at the end of the experiments.

Figure 5

Comparison of results from different reactive/nonreactive model variants with observation data for major ions, N species (NH₄⁺ and NO₃^–), and pH at 10 m depth. The observed data shown represent the averaged values from 13 monitoring wells.

Figure 6

Comparison of the concentrations of groundwater NH₄⁺, NO₃^–, and DO, as well as sediment-bound NH₄⁺ (NH₄X) between models with different hydrologic conditions.