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Understanding subsurface heterogeneity is critical for predicting groundwater flow, pollutant transport, and managing water resources. While traditional methods often rely on sparse borehole or geophysical data, this study explores a spectral analysis approach to infer aquifer structure from groundwater level fluctuations. We use a coupled surface–subsurface flow model to simulate hydraulic head time series in synthetic aquifers with bimodal hydraulic conductivity distributions. The frequency characteristics of these head fluctuations are analyzed to compute the scaling exponent (defined as the slope of the log-power spectral density of head fluctuations versus log-frequency) and its spatial gradient magnitude. Results show that areas with significant heterogeneity, such as transitions between high- and low-permeability zones, exhibit strong spatial gradients in the scaling exponent. These features can be used to delineate unsaturated zones, groundwater flow systems, and aquifer heterogeneity. By testing four scenarios with different hydraulic conductivity contrasts, we demonstrate that this method is sensitive to aquifer configuration. Our findings suggest that the gradient magnitude of the scaling exponent may serve as a diagnostic tool for characterizing heterogeneity in groundwater models and has the potential for future applications in estimating permeability distributions from monitored groundwater level data. 

Abstract The interactions between the atmosphere, ocean, and beach in the swash zone are dynamic, influencing water flux and solute exchange across the land‐sea interface. This study employs groundwater simulations to examine the combined effects of waves and evaporation on subsurface flow and salinity dynamics in a shallow beach environment. Our simulations reveal that wave motion generates a saline plume beneath the swash zone, where evaporation induces hypersalinity near the sand surface. This leads to the formation of a hypersaline plume beneath the swash zone during periods of wave recession, which extends vertically downward to a maximum depth of 30 cm, driven by the resulting vertical density gradients. This hypersaline plume moves approximately 2 m landward to the top of the swash zone and down the beachface due to wave‐induced seawater infiltration and is subsequently diluted by the surrounding saline groundwater. Furthermore, swash motion increases near‐surface moisture, leading to an elevated evaporation rate, with dynamic fluctuations in both moisture and evaporation rate due to high‐frequency surface inundation caused by individual waves. Notably, the highest evaporation rates on the swash zone surface do not always correspond to the greatest elevations of salt concentration within the swash zone. This is because optimal moisture is also required—neither too low to impede evaporation nor too high to dilute accumulated salt near the surface. These insights are crucial for enhancing our understanding of coastal groundwater flow, biogeochemical conditions, and the subsequent nutrient cycling and contaminant transport in coastal zones. 

Abstract Groundwater mixing dynamics play a crucial role in the biogeochemical cycling of shallow wetlands. In this paper, we conducted groundwater simulations to investigate the combined effects of evaporation and local heterogeneity on mixing dynamics in shallow wetland sediments. The results show that evaporation causes groundwater and solutes to upwell from deep sediments to the surface. As the solute reaches the surface, evaporation enhances the accumulation of the solute near the surface, resulting in a higher solute concentration than in deep sediments. Mapping of flow topology reveals that local heterogeneity generates spatially varied mixing patterns mainly along preferential flow pathways. The upwelling of groundwater induced by surface evaporation through heterogeneous sediments is likely to create distinct mixing hotspots that differ spatially from those generated by lateral preferential flows driven by large‐scale hydraulic gradients, which enhances the overall mixing in the subsurface. These findings have strong implications for biogeochemical processing in wetlands. 

Abstract Terrestrial groundwater travels through subterranean estuaries before reaching the sea. Groundwater‐derived nutrients drive coastal water quality, primary production, and eutrophication. We determined how dissolved inorganic nitrogen (DIN), dissolved inorganic phosphorus (DIP), and dissolved organic nitrogen (DON) are transformed within subterranean estuaries and estimated submarine groundwater discharge (SGD) nutrient loads compiling > 10,000 groundwater samples from 216 sites worldwide. Nutrients exhibited complex, nonconservative behavior in subterranean estuaries. Fresh groundwater DIN and DIP are usually produced, and DON is consumed during transport. Median total SGD (saline and fresh) fluxes globally were 5.4, 2.6, and 0.18 Tmol yr⁻¹for DIN, DON, and DIP, respectively. Despite large natural variability, total SGD fluxes likely exceed global riverine nutrient export. Fresh SGD is a small source of new nutrients, but saline SGD is an important source of mostly recycled nutrients. Nutrients exported via SGD via subterranean estuaries are critical to coastal biogeochemistry and a significant nutrient source to the oceans.