Intertidal aquifers are hotspots of biogeochemical cycling where nutrients and contaminants are processed prior to discharge to the ocean. The nature of the dynamic subsurface mixing zone is a critical control on mitigating reactions. Simulation of density‐dependent, variably saturated flow and salt transport incorporating realistic representations of aquifer heterogeneity was conducted within a Monte Carlo framework to investigate influence of nonuniform permeability on intertidal groundwater flow and salt transport dynamics. Results show that heterogeneity coupled with tides creates transient preferential flow paths within the intertidal zone, evolving multiple circulation cells and fingering‐type salinity distributions. Due to heterogeneity, strain‐dominated (intense mixing) and vorticity‐dominated (low mixing) flow regions coexist at small spatial scales, and their spatial extent reaches peaks at high tide and low tide. Such topological characteristics reveal complex tempo‐spatial mixing patterns for intertidal flow with localized areas of high and low mixing intensities, which have implications for intertidal biogeochemical processing.

Most existing numerical research on tide‐induced groundwater dynamics assumes a constant surface water salinity on the seaward boundary (constant salinity case). Few studies have investigated the influence of tidally varying salinity on shallow groundwater dynamics in coastal aquifers (tidal salinity case). We compiled field observations of tidally varying salinity in multiple estuaries across the eastern coast of China and a tidal creek in North Inlet‐Winyah Bay, United States. Numerical simulations were then conducted to explore the effect of tidally varying salinity on groundwater flow and salt transport in an idealized creek marsh aquifer. Results showed that the upper saline plume and classical saltwater wedge appeared in all cases, but the salinity in the saltwater wedge was diluted in the tidal salinity cases. Notably, groundwater transit times were shorter in the tidal salinity case than in the constant salinity case, especially under the creek bottom. Quantitative analyses indicated that tidally varying salinity significantly enhanced surface water‐groundwater exchange, increasing submarine groundwater discharge by 10% and the total inflow of surface water across the water‐sediment interface by 7%. As the density of groundwater differs from that of the overlying surface water, fingered saltwater flow formed in sediments under the creek bottom, leading to some small local water circulation cells. These small cells reduced groundwater transit times and almost doubled the water exchange rate. Coupling the density‐dependent flow to a simplified nitrogen reaction network revealed that the tidally varying salinity may have the potential to influence nitrogen biogeochemical transformations that modify nitrogen loads prior to discharge.

Preferential flow can result in rapid contamination of groundwater resources. This is particularly true in aquifers with connected, high permeability geologic structures and in coastal systems where the oceanic source of contamination is ubiquitous. We consider saltwater intrusion due to pumping in volcanic aquifers with lava tubes represented as connected high‐K structures and compare salinization responses to those of heterogeneous aquifers with different structure and equivalent homogeneous systems. Three‐dimensional simulations of variable‐density groundwater flow and salt transport show that conduits formed by lava flows create preferential groundwater flow in volcanic aquifers. These conduits allow fresh groundwater to extend further offshore than in other systems. However, onshore pumping causes saltwater to migrate landward quickly through the conduits relative to the other models, resulting in more severe saltwater intrusion, particularly at shallow depths. The geometry of geologic heterogeneity in volcanic aquifers leads to increased risk of salinization of fresh groundwater as well as substantial uncertainty due to significant spatial variation in saltwater intrusion. The findings illustrate the importance of considering geologic heterogeneity in assessing the vulnerability of coastal freshwater resources in volcanic and other aquifers with connected high‐permeability geologic structures.

A numerical study, based on a variably saturated groundwater flow model within a Monto Carlo framework, was conducted to investigate flow and solute transport in a heterogeneous beach aquifer subjected to tides. The numerical simulations were conducted based on our previous tracer experiments performed in a laboratory beach. Heterogeneity is assumed to be multifractal generated using the Universal Multifractal model. Our results show that heterogeneity greatly alters temporal and spatial evolution of the tracer plume migrating in the beach. The spreading coefficient of the plume shows very dynamic response to tides; it increases as the tidally driven recirculation cell overlaps with the plume and decreases as the recirculation cell moves far from the plume with tides. Descriptive statistics suggests that heterogeneity enhances spreading of the plume in the beach in an ensemble sense along with significant spatial and temporal variation. Due to heterogeneity, high spots of the pore water velocity are formed within the recirculation cell, creating transient preferential flow paths in the beach in response to tides. Contours of the Okubo‐Weiss (OW) parameter show that coupling with tides, heterogeneity creates vorticity‐dominated flow regions and also expands strain‐dominated flow regions more downward, indicating complex local‐scale mixing in the beach, compared to corresponding homogeneous case. Geologic heterogeneity also alters the spatial extent of the recirculating cell and induces highly variable transit time along the recirculating flow paths. The results provide insights into effects of geologic heterogeneity on seawater‐groundwater mixing and associated solute transport processes in tidally influenced coastal aquifers.

The intertidal zone of beach aquifers hosts biogeochemical transformations of terrestrially derived nutrients that are mediated by reactive organic carbon from seawater infiltration. While dissolved organic carbon is often assumed the sole reactive organic carbon component, advected and entrapped particulate organic carbon (POC) is also capable of supporting chemical reactions. Retarded advection of POC relative to groundwater flow forms pools of reactive carbon within beach sediments that support biogeochemical reactions as dissolved solutes move across them due to transient groundwater flow. In this work, we simulate the contribution of POC to beach reactions and identify parameters that control its relative contribution using a groundwater flow model (SEAWAT) and reactive transport model (PHT3D). Results show transient contributions of POC to denitrification, as the spatial extent of the saline circulation cell varies over time due to changing hydrologic factors. A decrease in POC retardation and an increase in tidal amplitude during POC deposition resulted in POC expansion, which increased the relative contributions of POC to beach reactivity. Decreased hydraulic conductivity and increased tidal amplitude post‐deposition decreased the utilization of POC for denitrification by allowing the oxic, saline water to completely encompass the pool of POC. Results highlight that POC is an intermittently utilized source of carbon that displays complex spatial relationships with groundwater flow conditions and overall beach biogeochemistry. This work demonstrates that POC may be a periodically important but overlooked contributor to biogeochemical reactions in carbon‐poor beach aquifers.