Influence of Waves on Groundwater Flows and Geochemistry in a Sandy Nearshore Aquifer: A Combined Field and Modelling Study

Waves are known to influence the flux of pollutants to coastal waters via groundwater discharge. This study combines field measurements with numerical groundwater modelling to evaluate the influence of a period of intensified wave conditions (wave event) on nearshore groundwater flows and geochemistry in a sandy freshwater beach. Comprehensive vertical nested pressure transducer data obtained over a 2.5 day isolated wave event reveal the development of transient groundwater flow recirculations through the nearshore aquifer combined with enhanced water exchange across the sediment-water interface (i.e., beach face). The wave-induced groundwater flows were simulated in FEFLOW using a phase-averaged wave setup approach to represent waves acting on the sediment-water interface. The time-varying measured and simulated hydraulic gradients match well indicating that consideration of wave setup alone, rather than instantaneous (phase-resolved) wave effects, is able to adequately capture wave-induced perturbations in nearshore groundwater flows. Additionally, the impact of the wave-induced groundwater flows on geochemical conditions near the sediment-water interface is illustrated by redox and pH fluctuations over the wave event. The observed fluctuations may considerably impact the fate of reactive pollutants discharging and also recirculating through a nearshore aquifer. In addition to the importance of the phase-averaged effect of waves, the phase-resolved effects of waves also need to be understood to better predict the fate of chemical and microbial constituents close to the sediment-water interface. Additional high frequency field measurements indicate that the phase-resolved effects of waves account for considerably larger fluxes of water and associated constituents exchanged across the sediment-water interface than the phase-averaged flux. Despite the large magnitude, the flux generated by phase-resolved wave effects is rapidly reversing in direction and thus is not expected to considerably influence dissolved constituents due to its short residence time, estimated to be between \u3c 1 and 70 seconds. However, the exchange of particulates may be important as their transport is strongly governed by attachment and detachment processes. It is proposed that the rapid water exchange processes near the sediment-water interface control the fate and transport of particulate organic matter and fecal bacteria and thus are important for regulating nutrient and microbial coastal water quality

Influence of beach hydrodynamics on saltwater transport and oil persistence in beaches

The exchange of water and solutes across the aquifer-ocean interface plays an important ecological role in both aquifer and the open water. Saltwater intrusion could prevent utilization of groundwater for either drinking water or irrigation. In addition, the flux of solutes, such as nutrients, from inland groundwater sources plays an important role in shoreline ecology, especially in embayments. This research relies on a three-pronged approach, involving a laboratory beach experiment, field scale experiments and numerical analyses. The laboratory beach experiment is designed to investigate the fundamental processes occurring in beach aquifers. It explores the effects of buoyancy (or lack thereof) on the flushing of freshwater to sea. The results indicate that the increase in water pressure when freshwater inundates saltwater systems exceeds the pressure obtained assuming the system is filled with freshwater. This has implications for the evaluation of stress on aquifers and the management of coastal aquifers. The field study is conducted along two transects of a tidally influenced gravel beach in Prince William Sound (PWS), Alaska, which was heavily polluted by the Exxon Valdez oil. The observed data of water table, pore water salinity and tracer (lithium) concentration indicates high freshwater recharge to the clean transect and low freshwater recharge to the oiled transect. The numerical model MARLIN is used to reproduce the observations of water pressure and pore water salinity at both transects. Based on the field experiments and numerical simulations, the beach can be viewed as a two-layer system from a hydraulic point of view with a high permeable upper layer underlain by a layer with low permeability. Five hypotheses are designed and tested to explore factors affecting the beach hydrodynamics. One of the hypotheses establishes that the density gradient between saltwater and freshwater does not play a role in the intertidal zone of beaches. This hypothesis is tested through numerical modeling and laboratory experiments and is rejected. The results show that the density gradient has significant effect on solute transport in the intertidal zone. Another hypothesis states that depth (and slope) of bedrock greatly affects solute transport in beaches. Numerical investigations indicate that the depth of bedrock greatly affects solute transport in homogenous beaches while it has minor impact on that in the upper layer of two-layer beaches. A third hypothesis states that in locations of a large freshwater recharge, it is less likely to find oil. Numerical simulations reveal that freshwater recharge promotes the removal of oil in two-layer beaches by maintaining the water table at or above the interface of the two layers. Findings from this work in relation to oiled beaches include: 1) oil tends to persist at locations of small freshwater recharge, 2) Prior to oil arriving to the shoreline, one could minimize oil penetration into the beach by releasing water onto the beach at the high tide line during low tides, and 3) bioremediation of oil polluted beaches should be conducted using deep injection as amendments applied on the beach surface would washout rapidly to sea

Fate and Transport of Nutrients and Contaminants Under the Impact of Surface Water and Groundwater Interactions

Periodic river fluctuations are common in nature. River fluctuations propagate into the riparian aquifer meters to hundreds of meters inland. They greatly enhance the mixing of geochemically distinct river water with groundwater and lead to intensive biogeochemical transformation of solutes in the hyporheic zone (HZ). Here, we use a combination of field methods and numerical simulations to investigate the effects of BS induced by river fluctuations due to both natural (e.g. tides and floods) and anthropogenic (e.g. hydropeaking) events on nutrients, (i.e. nitrogen (N)) and contaminants, (i.e. arsenic (As)) fate and transport. We carried out our study in two study sites: a dam regulated river in Austin, Texas and a tidally fluctuating river in Bangladesh. In the first study site, we developed a two-dimensional (2-D) coupled flow, reactive transport model to study the influence of dam release induced river fluctuations on N cycling within the HZ. Sensitivity analyses were conducted to quantify the effects of river amplitude, sediment hydraulic conductivity (K) and dispersivity, and ambient groundwater flow on nitrate removal efficiency. Our results demonstrated that daily river fluctuations created denitrification hot spots within the HZ that would not otherwise exist under naturally neutral or gaining conditions. In the second study site, we investigated the effects of tidal fluctuations on the formation of a permeable natural reactive barrier (PNRB) consisting of iron oxide precipitates and the implications of this for As trapping and mobilization in an aquifer high in dissolved As concentrations adjacent to the Meghna River. We first characterized the hydraulic properties of riverbank aquifer by using slug tests, pumping test and tidal methods. The characterized aquifer properties were used in a 2-D, flow and reactive transport model to simulate the spatial and temporal distributions of an PNRB in response to tidal and seasonal river stage fluctuations. Our study found that tidal and seasonal river stage fluctuations accelerate the formation of PNRB and broadened their spatial extent. This work, therefore, contributes to the understanding of the fate of several very important biogeochemical cycles (i.e. N, Fe and As) in a dynamically fluctuating river-aquifer system

Saltwater-freshwater mixing fluctuation in shallow beach aquifers

YesField measurements and numerical simulations demonstrate the existence of an upper saline plume in tidally dominated beaches. The effect of tides on the saltwater-freshwater mixing occurring at both the upper saline plume and lower salt wedge is well understood. However, it is poorly understood whether the tidal driven force acts equally on the mixing behaviours of above two regions and what factors control the mixing fluctuation features. In this study, variable-density, saturated-unsaturated, transient groundwater flow and solute transport numerical models are proposed and performed for saltwater-freshwater mixing subject to tidal forcing on a sloping beach. A range of tidal amplitude, fresh groundwater flux, hydraulic conductivity, beach slope and dispersivity anisotropy are simulated. Based on time sequential salinity data, the gross mixing features are quantified by computing the spatial moments in three different aspects, namely, the centre point, length and width, and the volume (or area in a two-dimensional case). Simulated salinity distribution varies significantly at saltwater-freshwater interfaces. Mixing characteristics of the upper saline plume greatly differ from those in the salt wedge for both the transient and quasi-steady state. The mixing of the upper saline plume largely inherits the fluctuation characteristics of the sea tide in both the transverse and longitudinal directions when the quasi-steady state is reached. On the other hand, the mixing in the salt wedge is relatively steady and shows little fluctuation. The normalized mixing width and length, mixing volume and the fluctuation amplitude of the mass centre in the upper saline plume are, in general, one-magnitude-order larger than those in the salt wedge region. In the longitudinal direction, tidal amplitude, fresh groundwater flux, hydraulic conductivity and beach slope are significant control factors of fluctuation amplitude. In the transverse direction, tidal amplitude and beach slope are the main control parameters. Very small dispersivity anisotropy (e.g., α_L⁄α_T <5) could greatly suppress mixing fluctuation in the longitudinal direction. This work underlines the close connection between the sea tides and the upper saline plume in the aspect of mixing, thereby enhancing understanding of the interplay between tidal oscillations and mixing mechanisms in tidally dominated sloping beach systems.Shenzhen Key Laboratory for Coastal Ocean Dynamics and Environment (No. ZDSY20130402163735964), National High Technology Research & Development Program of China (No. 2012AA09A409)

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

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 the STE, 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