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Tidal-driven hydrochemical evolution in coastal shallow groundwater: Implications for short-term pollutant dynamics
Guangyang Zhou1, Pengpeng Zhou1, Suna Li1
1MOE Key Laboratory of Groundwater Circulation and Environmental Evolution, China University of Geosciences (Beijing), Beijing, 100083, PR China; School of Water Resources & Environment, China University of Geosciences (Beijing), Beijing, 100083, PR China.
Abstract:
Tidal forcing drives rapid hydrochemical changes in coastal shallow groundwater. However, its role in modulating pollutant delivery to nearshore waters over spring-neap cycles remains insufficiently understood. This study investigated hydrochemical evolution patterns by integrating ionic ratios, ionic deltas, and three endmember mixing cases, including a principal component analysis-based model, a Bayesian mixing model, and a support vector regression-constrained mixing Bayesian model. Results demonstrate that tidal forcing induced pronounced spatiotemporal heterogeneity in hydrochemistry. Spatially, the aquifer exhibits a regime shift from seawater infiltration dominance (nearshore) to water-rock interaction dominance (landward). Temporally, the proportion of cation exchange decreased from 80.95% during spring tide to 64.00% during neap tide. Notably, endmember mixing model successfully quantified that the landward zone remained consistently dominated by the terrestrial groundwater endmember, with contributions exceeding 92% throughout all tidal stages. Within the transition zone, the peak proportion of seawater endmember exhibited a seaward migration pattern as the tidal amplitude declined from spring to mean tide. In the nearshore zone, the seawater fraction decreased during the neap tide, leading to a regime co-dominated by terrestrial groundwater and the tidally modified endmember known to carry dissolved inorganic carbon, nutrients, and redox-sensitive metals, the latter reaching a peak contribution of 94.80%. These tidal-modulated shifts imply that even short (spring-neap) tidal cycles can create pulsed deliveries of land-derived solutes to coastal waters, with potential implications for eutrophication and water quality. Our findings provide a mechanistic framework linking tidal amplitude to hydrochemical transformations, supporting tide-aware management of coastal groundwater resources and nearshore pollution.
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