Nearshore Aquifer Research Articles

Field and numerical investigation of groundwater flow and dissolved inorganic nitrogen (DIN) dynamics in a sandy nearshore aquifer

Field and numerical investigation of groundwater flow and dissolved inorganic nitrogen (DIN) dynamics in a sandy nearshore aquifer

Perturbations of heterotrophs and autotrophs in tidally dynamic coastal sediments: Development of a scalable model

AbstractTidal wetlands undergoing regular tidal inundation contribute more than 60% CO2 uptake of the coastal ocean. However, a comprehensive understanding of tidal microbial spatial distribution is poorly known, giving rise to previously ignored perturbations of microbial biomass in heterogeneous nearshore aquifers. In most traditional reactive models of nearshore aquifers until now, the spatio‐temporal production of microbial biomass was not considered, and reaction rates depended only on the first‐order decay of nutrient inputs. Here, using a biomass‐based model with tidal fluctuations, we identify autotrophic and heterotrophic high biomass zones that can dictate the hot spots of aerobic respiration, nitrification, and denitrification. Crucially, the accretive microbial biomass in eutrophic environments may minimize the hydraulic conductivity and weaken the tide‐driven upper saline plume, but slightly enhance seawater wedge intrusion. In addition, our simulations recreated empirical patterns of N removal in contaminated subterranean estuaries and led to a description of general rules of autotrophic and heterotrophic processes in sediments. Our findings point to previously ignored perturbations of microbial biomass to reactions in nearshore aquifers and underscore that the microbial distribution in coastal sediments has its inherent regularity to environmental alteration.

Freshwater cooling injection to mitigate saltwater intrusion and support sustainable groundwater management

This study examined the influence of thermal forcing on saltwater intrusion into nearshore aquifers (SWI) in the context of sea level rise due to climate change and continued groundwater withdrawal due to increasing demand. The study used the SEAWAT code to test two case studies: the Henry problem as a benchmark and the Biscayne Aquifer, Florida, USA, as a real case. Simulations were performed for the base case, sea level rise (SLR), and inland groundwater recharge reductions. A new management method was proposed for mitigating SWI in nearshore aquifers using the Abstraction of saline water, Desalination, Cooling the desalinated water, and Injection into the aquifer (ADCI) strategy. The method was tested with different freshwater temperatures of 25, 20, 15, 10, and 5 °C. The results showed that freshwater recharge in coastal regions is sensitive to the thermal regime, resulting in the attenuation of SWI. The results of this study are important for the sustainable management of groundwater resources in coastal areas to meet increasing water demands. The Biscayne salt repulsion reached +9.80 %, +10 %, +10.20 %, +10.30 %, and + 10.50 % for changing the injection recharge by 25, 20, 15, 10, and 5 °C. Future planning, design, and development of SWI mitigation measures should consider thermal regime effects. In addition, a cost–benefit analysis for this method using 3D models is needed to determine the feasibility and economic aspect of cooling water supply compared to current techniques.

Potential for Shoreline Recession to Accelerate Discharge of Groundwater Pollutants to Coastal Waters

AbstractDischarge of groundwater‐derived pollutants to inland and marine coastal waters is influenced by the transport and reactive processes occurring in nearshore aquifers. The effect of shoreline change on these processes and subsequent discharge of pollutants to coastal waters is unclear. The objective of this study was to evaluate the impact of shoreline recession (landward movement of the mean shoreline) on the transport of nitrogen (N) and phosphorus (P) in a nearshore aquifer and their discharge to coastal waters. Field investigations were conducted on a permeable unconfined nearshore aquifer on Lake Huron, Canada, in years coinciding with historically low and high lake water levels. At the site, a septic system‐derived nutrient‐rich (N and P) groundwater plume is moving toward the lake and the mean shoreline position moved ∼30 m landward between sampling years due coastal erosion and mean lake water level increase. Data indicate PO4‐P fluxes to the lake were higher following shoreline recession due to shortened travel pathways. In contrast, NO3‐N fluxes were governed by the specific geochemical conditions near the sediment‐water interface, which are not only a function of the shoreline position. Further, findings show shoreline recession may modify mineral phases that tend to sequester pollutants (e.g., iron oxides) near the sediment‐water interface and this may possibly mediate release of sediment‐bound pollutants. The findings provide new insights into potential impacts of shoreline change on chemical discharge to coastal waters as needed to inform long‐term water quality predictions and management.

Effects of Unsaturated Flow on Salt Distributions in Tidally Influenced Coastal Unconfined Aquifers

AbstractUnsaturated flow influences both the seawater extent under steady‐state conditions and the propagation of tides in coastal aquifers. However, its effects on salt distributions in tidally influenced coastal aquifers are little investigated. The present study used numerical simulations and data from laboratory experiments to analyze the effects of unsaturated flow on density‐dependent solute transport in coastal unconfined aquifers. The effects of the inland boundary condition (i.e., constant‐head or constant‐flux) were tested. Compared to a stable sea level, the results show that unsaturated flow has a more pronounced influence on salt distributions in coastal unconfined aquifers when tides are considered, regardless of the type of inland boundary condition. Neglect of unsaturated flow effects leads to expansion of the upper saline plume (USP), shrinkage of the saltwater wedge (seaward movement of saltwater wedge), and overestimation of water and salt exchange across the aquifer‐ocean interface. This is caused by a lower head in the nearshore area during high‐tide periods with the unsaturated zone effects removed. Thus, without the unsaturated zone, stronger head gradients within the nearshore aquifer occur at high tide, leading to stronger tidally driven seawater infiltration and hence a larger USP. Counterintuitively, ignoring unsaturated flow effects leads to greater average inland head over a tidal period, which shifts the saltwater wedge seaward. It is concluded that unsaturated zone effects should not be neglected for modeling tide‐affected seawater intrusion, especially if quantification of near‐shore conditions is important.

Dynamics of upstream saltwater intrusion driven by tidal river in coastal aquifers

For the coastal aquifers, recent research have shown that the tidal has a significant effect on saltwater intrusion in the near-shore aquifer. However, it is currently unclear how the tidal river contributes to the groundwater flow and salinity distribution in the upstream aquifer of the estuary. This study examined the effects of a tidal river on the dynamic characteristics of groundwater flow and salt transport in a tidal river-coastal aquifer system using field monitoring data and numerical simulations. It was found that changes in tidal-river level led to the reversal of groundwater flow. For a tidal cycle, the maximum area of seawater intrusion is about 41.16 km2 at the end of the high tide stage. Then the area gradually decreased to 39.02 km2 at the end of the low tide stage. More than 2 km2 area variation can be observed in a tidal cycle. Compared to the low tide stage, the area of SWI increased by 5 % at high tide stage. The SWI region was also spreading landward from the tidal river. In addition, we quantified the water exchange and salt flux between the tidal river and aquifer. When the tidal fell below the level of the riverbed, the water exchange rate was stabilized at about −1.6 m/h. The negative value indicated that the river was recharged by the groundwater. With the increasing of tidal water level, the water exchange rate gradually changes from negative to positive and reached the maximum value of 3.2 m/h at the beginning of the falling tide stage. The presence of a physical river dam can amplify the difference in water level between high and low tides, thereby enhancing the influence of a tidal river on water exchange and salt flux. The findings lay the foundation for gaining a comprehensive understanding of the tidal river on groundwater flow and salt transport in upstream aquifers.

Macropores and burial of dissolved organic matter affect nitrate removal in intertidal aquifers

Coastal ecosystems are sensitive to the input of chemicals originating from submarine groundwater discharge (SGD). The mixing zone between freshwater and saltwater in beach aquifers promotes biogeochemical transformations affecting nutrient fluxes into the coastal ocean. Macropores affect groundwater flow, leading to a more complicated mixing process and greatly promoting nitrogen transformations. Laboratory-scale experiments and numerical modeling were employed in this study to investigate how macropores affect freshwater-saltwater mixing and terrestrial solute transport. A numerical investigation is presented demonstrating the influence of macropores on the transport and transformation of nitrate (NO3−) in a homogeneous unconfined nearshore aquifer under spring-neap tide action using COMSOL. The results indicate that macropores modify the groundwater discharge pathway of terrestrial NO3− by changing nearshore groundwater flow dynamics. The denitrification zone expanded from the low tide mark to the high tide mark, extended downward to the base of the upper saline plume (USP), and expanded with increasing macropore depth and quantity. Macropores increased NO3− removal by creating larger mixing zones, which increased with the macropore depth and quantity. Lower freshwater levels enhanced denitrification and weakened the impact of macropores on denitrification. The deep burial dissolved organic matter (DB-DOM) simulation results demonstrated that dissolved organic matter (DOM) could stimulate denitrifiers to increase the denitrification zone by fully consuming O2 along longer flow paths, resulting in up to 85% higher NO3− removal. The findings provide notable implications for management design engineering to protect coastal ecosystems and decrease NO3− fluxes into the ocean.

Changes in the salt concentration distribution in surface water (seawater) due to seawater recirculation in a porous medium under tidal fluctuations

Many studies have used numerical simulations and experiments to simulate saltwater intrusion in a nearshore aquifer. Thus, it is known that seawater recirculation occurs in a porous medium just below a sloping beach, and a saltwater wedge occurs in the medium below where the slope intersects the water level at low tide. This study investigated the salt distribution in the surface water when subterranean recirculation of seawater is occurring and there is a saltwater wedge in an underlying porous medium, along with the relationships between the seawater recirculation and the tidal amplitude and beach gradient. The salt distribution in a subterranean aquifer and in surface seawater where the shore sloped gently seaward with a constant slope of 1/10 or 1/5 under tidal amplitudes of 0.5, 1.0, or 2.0 m was numerically simulated by the ASGMF method, which couples water pressure and water flow in a porous medium with those in the overlying surface water and can simulate variable-density flows and the salt concentration distribution in both the porous medium and the surface water. The results showed that the recirculation of seawater depended on the tidal amplitude, being greater when the amplitude exceeded 1.0 m, but that it was unlikely to occur when the beach gradient was steep. Thus, the aspect ratio (width to depth) of the seawater recirculation decreased as the tidal amplitude increased above 1.0 m. Furthermore, the simulated surface water level was often lower than the tidal water level; thus, the surface water level was not consistent with the tidal water level imposed at the right boundary of the simulation domain but varied slightly under the influence of water flow in the surface water and gas flow in the atmosphere. The salt concentration in the surface water was not always the same as the seawater salt concentration because a thin freshwater layer and a brackish water layer of mixed freshwater and saltwater overlay the seawater.

Effect of time-varying wave conditions on the fate of nitrogen in a freshwater unconfined nearshore aquifer

Coastal forcing including waves lead to complex and dynamic mixing between discharging groundwater and recirculating coastal water in permeable unconfined nearshore aquifers. This mixing can drive biogeochemical transformations that affect the fate of pollutants in nearshore aquifers and their ultimate discharge to coastal waters. The objective of this study was to examine the effect of time-varying wave conditions on the fate of nitrogen in a freshwater unconfined nearshore aquifer. This was achieved by conducting numerical groundwater flow and reactive transport simulations using MODFLOW-NWT 2005 with a new modified wave-PBC package together with PHT3D v2.10. The simulations demonstrate that time-varying wave conditions enhance mixing in the nearshore aquifer compared to simulations that only consider constant wave conditions, and in doing so impact nitrogen transformations. For the conditions and reaction network simulated, time-varying wave conditions resulted in up to 99% of nitrogen removal in the nearshore aquifer due to the intensified mixing and increased delivery of organic matter into the mixing zone. Sensitivity analysis shows that the abundance and reactivity of coastal-derived organic matter plays a key role in nitrogen removal in aquifers along wave-influenced shorelines. The simulations highlight the importance of considering time-varying wave conditions when designing field investigations and analysing field data. The findings have implications for prediction and estimation of chemical fluxes to the coastal waters and also suggest that increasingly frequent storm events and higher wave conditions as the climate changes may modify pollutant fluxes from nearshore aquifers to coastal waters.

Factors controlling phosphorus mobility in nearshore aquifers adjacent to large lakes

Internal P stores in offshore lakebed sediments play an important role in lake nutrient dynamics. While P stores in nearshore aquifer sediments may also be important for nutrient dynamics, it is unclear whether P accumulates in these sediments, and if so, what factors control P accumulation and its potential later release from the sediments to nearshore waters. This knowledge gap was addressed by conducting field investigations at seven nearshore sites located along the shores of Lake Erie, Lake Huron and Lake Ontario, Canada, with more detailed dissolved and sediment phase characterization completed for two nearshore sites. PO4 concentrations were observed to be higher (>50 μg/L) in the more reducing nearshore aquifers compared to more oxidizing nearshore aquifers (<20 μg/L), despite similar total solid phase P concentrations at the sites. PO4 mobility in the nearshore aquifers was found to be closely linked to redox-driven Fe cycling. In the more reducing aquifers, dissolved PO4 was highest near the redox boundary present in the shallow sediments where oxic infiltrating surface water mixes with reducing groundwater. In the more oxidizing aquifers, solid phase characterization indicated that PO4 is sequestered to Fe oxide mineral phases throughout the nearshore aquifer which explains the low dissolved PO4. While pH was not found to be important for PO4 mobility at the study sites, batch laboratory experiments indicate that increased infiltration of more alkaline surface water into nearshore aquifers may promote PO4 release from the sediments. The study findings demonstrate that while internal P storage mechanisms in nearshore aquifer sediments may currently be limiting P loads to lakes, it is possible that P stores that build up over time may result in increased P loads to lakes in the future.

Oxygen‐controlled recirculating seepage meter reveals extent of nitrogen transformation in discharging coastal groundwater at the aquifer–estuary interface

AbstractNutrient loads delivered to estuaries via submarine groundwater discharge (SGD) play an important role in the nitrogen (N) budget and eutrophication status. However, accurate and reliable quantification of the chemical flux across the final decimeters and centimeters at the sediment–estuary interface remains a challenge, because there is significant potential for biogeochemical alteration due to contrasting conditions in the coastal aquifer and surface sediment. Here, a novel, oxygen‐ and light‐regulated ultrasonic seepage meter, and a standard seepage meter, were used to measure SGD and calculate N species fluxes across the sediment–estuary interface. Coupling the measurements to an endmember approach based on subsurface N concentrations and an assumption of conservative transport enabled estimation of the extent of transformation occurring in discharging groundwater within the benthic zone. Biogeochemical transformation within reactive estuarine surface sediment was a dominant driver in modifying the N flux carried upward by SGD, and resulted in a similar percentage of N removal (~ 42–52%) as did transformations occurring deeper within the coastal aquifer salinity mixing zone (~ 42–47%). Seasonal shifts in the relative importance of biogeochemical processes including denitrification, nitrification, dissimilatory nitrate reduction, and assimilation altered the composition of the flux to estuarine surface water, which was dominated by ammonium in June and by nitrate in August, despite the endmember‐based observation that fixed N in discharging groundwater was strongly dominated by nitrate. This may have important ramifications for the ecology and management of estuaries, since past N loading estimates have generally assumed conservative transport from the nearshore aquifer to estuary.

Occurrence of Arsenic in Nearshore Aquifers Adjacent to Large Inland Lakes.

Metal oxides that form near sediment-water interfaces in marine and riverine settings are known to act as a sediment trap for pollutants of environmental concern (e.g., arsenic and mercury). The occurrence of these pollutant traps near sediment-water interfaces in nearshore lake environments is unclear yet important to understand because they may accumulate pollutants that may be later released as environmental conditions change. This study evaluates the prevalence of pollutant sediment traps in nearshore aquifers adjacent to large lakes and the factors that affect the accumulation and release of pollutants, specifically arsenic. Field data from six sites along the Laurentian Great Lakes indicate widespread enrichment of arsenic in nearshore aquifers with arsenic sequestered to iron oxide phases. Arsenic enrichment at all sites (solid-phase arsenic >2 μg/g) suggests that this is a naturally occurring phenomenon. Arsenic was more mobile in reducing aquifers with elevated dissolved arsenic (up to 60 μg/L) observed, where reducing groundwater mixes with infiltrating oxic lake water. Dissolved arsenic was low (<3 μg/L) in all oxic nearshore aquifers studied despite high solid-phase arsenic concentrations. The findings have broad implications for understanding the widespread accumulation of reactive pollutants in nearshore aquifers and factors that affect their release to large lakes.

Spatiotemporal controls on septic system derived nutrients in a nearshore aquifer and their discharge to a large lake

This study evaluates spatiotemporal variability in the behavior of septic system derived nutrients in a sandy nearshore aquifer and their discharge to a large lake. A groundwater nutrient-rich plume was monitored over a two-year period with the septic system origin of the plume confirmed using artificial sweeteners. High temporal variability in NO3-N attenuation in the nearshore aquifer prior to discharge to the lake (42–96%) reveals the complex behavior of NO3-N and potential importance of changing hydrological and geochemical conditions in controlling NO3-N discharge to the lake. While PO4-P was retarded in the nearshore aquifer, the PO4-P plume extended over 90 m downgradient of the septic system. It was estimated that the PO4-P plume may reach the lake within 10 years and represents a legacy issue whereby PO4-P loads to the lake may increase over time. To provide broader assessment of the contribution of septic systems to P and N loads to a large lake, a regional scale geospatial model was developed that considers the locations of individual septic systems along the Canadian Lake Erie shoreline. The estimated P and N loads indicate that septic systems along the shoreline are only a minor contributor to the annual P and N loads to Lake Erie. However, it is possible that nutrients from septic systems may contribute to localized algal blooms in shoreline areas with high septic system density. In addition, disproportionate P and N loads in discharging groundwater may change the N:P ratio in nearshore waters and promote growth of harmful cyanobacteria. The study provides new insights into factors controlling the function of the reaction zone near the groundwater-lake interface including its impact on groundwater-derived nutrient inputs to large lakes. Further, the study findings are needed to inform septic system and nutrient management programs aimed at reducing lake eutrophication.

Effect of Transient Wave Forcing on the Behavior of Arsenic in a Nearshore Aquifer.

Groundwater-coastal water interactions influence the fate of inorganic chemicals in nearshore aquifers and their flux to receiving coastal waters. This study evaluates the impact of variable wave conditions on the geochemical changes and distribution of mobile arsenic (As) in a nearshore aquifer. Field measurements in a sandy nearshore aquifer on Lake Erie, Canada, are presented with geochemical changes analyzed over a period of high waves. A numerical model of wave-induced groundwater flow dynamics, validated against field data, is used to provide insight into the physical flow processes underlying the observed geochemical changes. Rapid changes in dissolved As, Fe, Mn, and S demonstrate the importance of reactions as well as dynamic transport in controlling the behavior of reactive species, especially those that are redox sensitive. Field data suggest the presence of sediment traps, which under certain hydrological and geochemical conditions may result in a "hot moment" with episodic release of As. The study provides new insight into factors controlling the fate of reactive species in dynamic coastal environments as required to better predict chemical fluxes to coastal waters. Additionally, it highlights the need to pay particular attention to "hot moments" for reaction and transport caused by storms and waves.

A Transient ElectroMagnetic (TEM) Method Survey in North-Central Coast of Crete, Greece: Evidence of Seawater Intrusion

Seawater intrusion into near-shore aquifers is one of the main environmental problems that affect Mediterranean islands. Crete is the biggest and most populated island of Greece, characterized by limited surface waters and strong dependence on groundwater sources as the primary source of natural water supply for extensive agricultural activity and human use. Freshwater demand in Crete has increased notably the last decades. The Geropotamos aquifer is located on the north-central coast of Crete and freshwater management is in a delicate balance with saltwater at coastal areas of the aquifer due to the scarce precipitation and high evaporation as well as the intense over exploitation of the groundwater resources. The geological setting of the study area is considered complex and the local tectonic regime is characterized by two sets of faults orientated NW-SE and NE-SW. Investigation of the aquifer using a survey grid of 1179 Transient ElectroMagnetic soundings (TEM) in 372 sites, has resulted in 1D models, and 2D/3D visualization of geoelectric structures, depicting the zones of salination of groundwater in the aquifer. Geological mapping, hydro-lithological data and geochemical analysis of 24 water samples (22 boreholes and 2 springs) are in agreement with results obtained from TEM soundings, supporting our interpretation that the aquifer is degraded by saline intrusion which likely occurs along fractures in a fault zone, emphasising the critical role of fracture pathways in salination problems of coastal aquifers.

Tracing submarine groundwater discharge flux in Tolo Harbor, Hong Kong (China)

Submarine groundwater discharge (SGD) is an important pathway for groundwater and associated chemicals to discharge to the sea. Groundwater levels monitored along a transect perpendicular to the shoreline are used to calculate SGD flux from the nearshore aquifer to Tolo Harbor, Hong Kong (China). The calculated SGD flux—recharge/discharge measured with Darcy’s Law methods—agrees well with estimates based on geo-tracer techniques and seepage meter in Tolo Harbor during previous studies. The estimated freshwater SGD is 1.69–2.0 m2/d at the study site and 0.3 ± 0.04 cm/d for the whole of Tolo Harbor, which is comparable to the river discharge (0.25 ± 0.07 cm/d) and precipitation (0.45 ± 0.15 cm/d). The tide-driven SGD in the intertidal zone is 13.98–17.59 m2/d at the study site and 2.42 ± 0.56 cm/d for the whole of Tolo Harbor. The SGD occurring in the subtidal zone and the bottom of Tolo Harbor is 3.12 ± 4.63 cm/d. Fresh SGD accounts for ~5% of the total SGD, while the rest (~95%) is contributed by saline SGD driven by various forces. About 96% of the tide-driven SGD in the intertidal zone occurs in the ebbing tide period because the head difference between the groundwater level and sea level is great during this period. Tide-driven SGD in the spring tide is ~1.2 times that during neap tide. The tidal fluctuation amplitude and tide-driven SGD in the intertidal zone are positively correlated to each other; thus, a spring neap variation of the tide-driven SGD is observed.

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

Abstract Sustainable coastal resource management requires sound understanding of interactions between coastal unconfined aquifers and the ocean as these interactions influence the flux of chemicals to the coastal ocean and the availability of fresh groundwater resources. The importance of submarine groundwater discharge in delivering chemical fluxes to the coastal ocean and the critical role of the subterranean estuary (STE) in regulating these fluxes is well recognized. STEs are complex and dynamic systems exposed to various physical, hydrological, geological, and chemical conditions that act on disparate spatial and temporal scales. This paper provides a review of the effect of factors that influence flow and salt transport in STEs, evaluates current understanding on the interactions between these influences, and synthesizes understanding of drivers of nutrient, carbon, greenhouse gas, metal and organic contaminant fluxes to the ocean. Based on this review, key research needs are identified. While the effects of density and tides are well understood, episodic and longer-period forces as well as the interactions between multiple influences remain poorly understood. Many studies continue to focus on idealized nearshore aquifer systems and future work needs to consider real world complexities such as geological heterogeneities, and non-uniform and evolving alongshore and cross-shore morphology. There is also a significant need for multidisciplinary research to unravel the interactions between physical and biogeochemical processes in STEs, as most existing studies treat these processes in isolation. Better understanding of this complex and dynamic system can improve sustainable management of coastal water resources under the influence of anthropogenic pressures and climate change.

Effects of beach slope breaks on nearshore groundwater dynamics

AbstractInteractions between fresh groundwater and seawater affect significantly the nearshore pore water flow, which in turn influences the fate of nutrients and contaminants in coastal aquifers prior to discharge to the marine environment. Field investigations and numerical simulations were carried out to examine the groundwater dynamics in the intertidal zone of a carbonate sandy aquifer on the tropical island of Rarotonga, Cook Islands. The study site was featured by distinct cross‐shore slope breaks on the beach surface. Measured pore water salinities revealed different distributions under the influences of different beach profiles, inland heads, and tidal oscillations. Fresh groundwater was found to discharge around a beach slope break located in the middle area of the intertidal zone. The results indicate a strong interplay between the slope break beach morphology and tidal force in controlling the nearshore groundwater flow and solute transport. The fresh groundwater discharge location was largely determined by the beach morphology in combination with the tidal force. The nearshore groundwater flow can be very sensitive to beach slope breaks, which induce local circulation and flow instabilities. As slope breaks are a common feature of beaches around the world, these results have important, general implications for future studies of nutrients transport and transformations in nearshore aquifers and associated fluxes via submarine groundwater discharge.

Dynamic groundwater flows and geochemistry in a sandy nearshore aquifer over a wave event

AbstractDynamic coastal forcing influences the transport of pollutants in nearshore aquifers and their ultimate flux to coastal waters. In this study, field data are presented that show, for the first time, the influence of a period of intensified wave conditions (wave event) on nearshore groundwater flows and geochemistry in a sandy beach. Field measurements at a freshwater beach allow wave effects to be quantified without other complex forcing that are present along marine shorelines (e.g., tides). Pressure transducer data obtained over an isolated wave event reveal the development of transient groundwater flow recirculations. The groundwater flows were simulated in FEFLOW using a phase‐averaged wave setup approach to represent waves acting on the sediment‐water interface. Comparison of measured and simulated data indicates that consideration of wave setup alone is able to adequately capture wave‐induced perturbations in groundwater flows. While prior studies have shown sharp pH and redox spatial zonations in nearshore aquifers, this study reveals rapid temporal variations in conductivity, pH, and redox (ORP) in shallow sediments (up to 0.5 m depth) in response to varying wave conditions. Comparison of head gradients with calculated conductivity and pH mixing ratios indicates the controlling effect of the wave‐induced water exchange and flows in driving the observed geochemical dynamics. While we are not able to conclusively determine the extent to which temporal variations are caused by conservative mixing versus reactive processes, the pH and ORP variations observed will have significant implications for the fate of reactive pollutants discharging through sandy nearshore aquifers.

Effects of alongshore morphology on groundwater flow and solute transport in a nearshore aquifer

AbstractVariations of beach morphology in both the cross‐shore and alongshore directions, associated with tidal creeks, are common at natural coasts, as observed at a field site on the east coast of Rarotonga, Cook Islands. Field investigations and three‐dimensional (3‐D) numerical simulations were conducted to study the nearshore groundwater flow and solute transport in such a system. The results show that the beach morphology, combined with tides, induced a significant alongshore flow and modified local pore water circulation and salt transport in the intertidal zone substantially. The bathymetry and hydraulic head of the creek enabled further and more rapid landward intrusion of seawater along the creek than in the aquifer, which created alongshore hydraulic gradient and solute concentration gradient to drive pore water flow and salt transport in the alongshore direction within the aquifer. The effects of the creek led to the formation of a saltwater plume in groundwater at an intermediate depth between fresher water zones on a cross‐shore transect. The 3‐D pore water flow in the nearshore zone was also complicated by the landward hydraulic head condition, resulting in freshwater drainage across the inland section of the creek while seawater infiltrating the seaward section. These results provided new insights into the complexity, intensity, and time scales of mixing among fresh groundwater, recirculating seawater and creek water in three dimensions. The 3‐D characteristics of nearshore pore water flow and solute transport have important implications for studies of submarine groundwater discharge and associated chemical input to the coastal sea, and for evaluation of the beach habitat conditions.