Effects of temperature on tidally influenced coastal unconfined aquifers

Aquifer-ocean temperature contrasts are common worldwide. Their effects on flow and salinity distributions in unconfined coastal aquifers are, however, poorly understood. Based on laboratory experiments and numerical simulations, we examined the responses of flow processes in tidally influenced aquifers to aquifer-ocean temperature differences. The extent of seawater intrusion and seawater circulation were found to vary with the aquifer-ocean temperature contrast. Compared with the isothermal case, an increase of up to 40% of the tide-induced seawater circulation rate in the intertidal zone was observed when seawater is warmer than groundwater. In contrast, saltwater circulation in the lower saltwater wedge declines notably no matter whether the seawater is warmer or colder than groundwater. As the seawater temperature rises, the contribution of tide-induced circulation to the overall increase of submarine groundwater discharge becomes more important compared with that of density-driven seawater circulation. Both the upper saline plume and the freshwater discharge zone expand significantly with warmer seawater whereas the lower saltwater wedge contracts.

Thermohaline Flow in Seawater Intrusion in Shallow Coastal Aquifers

David Andrew Barry, Chenming Zhang

Temperature and salinity, with largely different diffusivities, make a perfect couple that drives thermohaline flow (double diffusive convection where heat and salt are the two main concerned properties) in porous media wherever their gradients co-exist. Studies of double diffusion convection have revealed various flow regimes generated by instable and even stable vertical stratification of heat and salt including diffusive, finger, and mixed flow regimes. In the context of seawater intrusion in shallow coastal aquifers, however, temperature and salinity gradients present more likely horizontal than vertical. Moreover, while salinity gradient is apparently seaward, the direction of temperature gradient may vary over the seasons. In such cases, understanding on how thermohaline flow exhibit and influence the overall state of seawater intrusion in near-shore aquifers remains largely deficient. In this study, we examine the occurrence and characteristics of thermohaline flow in seawater intrusion in coastal aquifers under various scenarios of salinity and temperature distribution by laboratory experiments and numerical simulation. Its effects on solute transport and saltwater circulation are also investigated.

2018

Effects of episodic rainfall on a subterranean estuary

David Andrew Barry, Clare Robinson

Numerical simulations were conducted to examine the effect of episodic rainfall on nearshore groundwater dynamics in a tidally influenced unconfined coastal aquifer, with a focus on both long- (yearly) and short-term (daily) behaviour of submarine groundwater discharge (SGD) and seawater intrusion (SWI). The results showed non-linear interactions among the processes driven by rainfall, tides and density gradients. Rainfall-induced infiltration increased the yearly averaged fresh groundwater discharge to the ocean but reduced the extents of the saltwater wedge and upper saline plume as well as the total rate of seawater circulation through both zones. Overall, the net effect of the interactions led to an increase of the SGD. The nearshore groundwater responded to individual rainfall events in a delayed and cumulative fashion, as evident in the variations of daily averaged SGD and salt stored in the saltwater wedge (quantifying the extent of SWI). A generalized linear model (GLM) along with a Gamma distribution function was developed to describe the delayed and prolonged effect of rainfall events on short-term groundwater behaviour. This model, validated with results of daily averaged SGD and SWI from the simulations of groundwater and solute transport using independent rainfall datasets, performed well in predicting the behaviour of the nearshore groundwater system under the combined influence of episodic rainfall, tides and density gradients. The findings and developed GLM form a basis for evaluating and predicting SGD, SWI and associated mass fluxes from unconfined coastal aquifers under natural conditions, including episodic rainfall.

American Geophysical Union2017

Deep-water coastal upwelling events during wintertime in a large lake (Lake Geneva)

David Andrew Barry, Andrea Cimatoribus, Ulrich Lemmin, Rafael Sebastian Reiss

Convective winter cooling is an important process in the annual cycle of a lake ecosystem, because it brings oxygen to the deeper layers. However, in deep lakes, convective processes are often not strong enough to extend the mixing down to the deepest layers. This may become even more problematic in the future considering climate change and potentially milder winters. In large deep lakes, two other processes originating in the coastal zone can significantly contribute to winter deep water renewal: cold water density currents on the lateral slopes caused by differential cooling between the shallow literal zone and the deep off-shore region, and upwelling of deep water masses over the lateral slopes which results from wind forcing and may be influenced by the Coriolis force. Previous field experiments in Lake Geneva, the largest freshwater lake in Western Europe (max. depth 310 m), have shown that cold-water density currents are a common phenomenon during wintertime, frequently reaching the thermocline at O(100m) depth. As part of a project investigating winter mixing dynamics in Lake Geneva, we examine wind-driven, Coriolis-influenced upwelling in order to better understand its role in the deep-water exchange during the weakly stratified period (December-March). The study is based on field observations taken on the northern shore of the lake, 20 km west of Lausanne in the winters of 2017-19. Near-bottom water temperatures are inferred from a fiber-optic based Distributed Temperature Sensing (DTS) system, laid down on the lateral slope starting at the shore, with a spatial and temporal resolution of O(1m) and O(0.1°C), respectively. Temperature and current velocities in the water column are recorded by several ADCP and thermistor chain moorings. The data set is completed by regular CTD campaigns following wind events favoring upwelling. These field measurements were combined with a validated 3D hydrodynamic model (MITgcm) in order to analyze and quantify the exchange between the littoral zones and deeper parts of the lake. It was observed that following medium to strong northeast-bound wind events (i.e. winds parallel to the main axis of the central lake) lasting for at least one day, near-shore temperatures at all instruments suddenly dropped to the same temperatures as those in the deep open waters and remained low for several days. This indicates that, triggered by persistent wind-stress and under the influence of the Coriolis force, hypolimnetic water reached the surface in the near shore zone, where it is subject to heat and gas exchange with the atmosphere, before returning to the deep layers. Numerical modeling of these events showed comparable results and allowed to trace the origin of this cold water to the deep layers. Our field observations and the numerical results suggest that in addition to the previously observed cold-water density currents, wind-driven upwelling is a common phenomenon in Lake Geneva during the weakly stratified winter period and potentially a significant mechanism for deep-water renewal.

2019

Deep-water coastal upwelling events during wintertime in a large lake (Lake Geneva)

David Andrew Barry, Andrea Cimatoribus, Ulrich Lemmin, Rafael Sebastian Reiss

2019