Quantitative estimation of submarine groundwater discharge using airborne thermal infrared data acquired at two different tidal heights

Submarine groundwater discharge (SGD) plays an important role in coastal biogeochemical processes and hydrological cycles, particularly off volcanic islands in oligotrophic oceans. However, the spatial and temporal variations of SGD are still poorly understood owing to difficulty in taking rapid SGD measurements over a large scale. In this study, we used four airborne thermal infrared surveys (twice each during high and low tides) to quantify the spatiotemporal variations of SGD over the entire coast of Jeju Island, Korea. On the basis of an analytical model, we found a linear positive correlation between the thermal anomaly and squares of the groundwater discharge velocity and a negative exponential correlation between the anomaly and water depth (including tide height and bathymetry). We then derived a new equation for quantitatively estimating the SGD flow rates from thermal anomalies acquired at two different tide heights. The proposed method was validated with the measured SGD flow rates using a current meter at Gongcheonpo Beach. We believe that the method can be effectively applied for rapid estimation of SGD over coastal areas, where fresh groundwater discharge is significant, using airborne thermal infrared surveys.     

Submarine groundwater discharge revealed by aerial thermal infrared imagery: a case study on Jeju Island,Korea

Submarine groundwater discharge (SGD) is a global phenomenon that carries large volumes of groundwater and dissolved chemical species such as nutrient, metals, and organic compounds to coastal zones. We report the influence of SGD on the coastal waters of Jeju Island, Korea, using high‐resolution aerial thermal infrared (TIR) mapping techniques and field investigations. An aircraft‐based system was implemented using a cost‐effective TIR camera for aerial TIR mapping. Ground‐based calibrations and system integration with GPS/IMU (global positioning system/inertial measurement unit) were performed for the aerial systems. The aerial surveys showed distinct low‐temperature signatures of SGD along the coasts of Jeju Island, revealing large groundwater inputs from the coastal aquifers to the ocean. Multiple aerial surveys over a range of seasons and tidal stages revealed that SGD rates dynamically affect the sea surface temperature (SST) of the coastal zone. The in‐situ measurements supported that SGD has a substantial influence on the coastal water chemistry as well as SST. Our observations highlight the extent to which aerial‐based TIR mapping can serve as a powerful tool for studying SGD and other coastal processes. Copyright © 2016 John Wiley & Sons, Ltd.     

The enhancing effect on tidal signals of a submarine spring connected to a semi-infinite confined aquifer

Abstract

Submarine springs play an important role in submarine groundwater discharge (SGD). To investigate the effects of these springs on the propagation of tidal signals in coastal confined aquifers, this paper considers a general coastal aquifer system with a submarine spring on the seabed where the length of the aquifer's offshore extent is finite and its submarine outlet is covered by an impermeable outlet-capping. An approximate analytical solution is obtained for describing the tidal head fluctuations in the aquifer. Solution analyses indicate that the error of the approximate analytical solution is negligible when both distances from the spring hole to the coastline and to the submarine outlet-capping are much greater than the radius of the spring hole. Sensitivity tests are conducted to investigate the effects of hydraulic properties, tidal and spring geometric configuration parameters on the tidal signal propagation in the inland aquifer. For aquifers with infinite offshore length, or without submarine springs, existing solutions in the literature are obtained. The comparison of groundwater head fluctuations for the cases with and without a submarine spring demonstrate the enhancing effect of the submarine spring on tidal signal propagation in the inland aquifer. Three situations that fit our model assumptions are given for future potential applications. A hypothetical example is used to show the possibility of identifying a spring's location using the present analytical solution together with tidal signals observed from inland wells.

Editor D. Koutsoyiannis; Associate editor Y. Guttmann

Citation Xia, Y.Q., Li, H.L., Yang, Y., and Huang, W., 2012. Enhancing effect on tidal signals of a submarine spring related to a semi-infinite confined aquifer. Hydrological Sciences Journal, 57 (6), 1231–1248.     

Evaluating the source and seasonality of submarine groundwater discharge using a radon-222 pore water transport model

Pore water radon (²²²Rn) distributions from Indian River Lagoon, Florida, are characterized by three zones: a lower zone where pore water ²²²Rn and sediment-bound radium (²²⁶Ra) are in equilibrium and concentration gradients are vertical; a middle zone where ²²²Rn is in excess of sediment-bound ²²⁶Ra and concentration gradients are concave-downward; and an upper zone where ²²²Rn concentration gradients are nearly vertical. These ²²²Rn data are simulated in a one-dimensional numerical model including advection, diffusion, and non-local exchange to estimate magnitudes of submarine groundwater discharge components (fresh or marine). The numerical model estimates three parameters, fresh groundwater seepage velocity, irrigation intensity, and irrigation attenuation, using two Monte Carlo (MC) simulations that (1) ensure the minimization algorithm converges on a global minimum of the merit function and the parameter estimates are consistent within this global minimum, and (2) provide 90% confidence intervals on the parameter estimates using the measured ²²²Rn activity variance. Model estimates of seepage velocities and discharge agree with previous estimates obtained from numerical groundwater flow models and seepage meter measurements and show the fresh water component decreases offshore and varies seasonally by a factor of nine or less. Comparison between the discharge estimates and precipitation patterns suggests a mean residence time in unsaturated and saturated zones on the order of 5 to 7 months. Irrigation rates generally decrease offshore for all sampling periods. The mean irrigation rate is approximately three times greater than the mean seepage velocity although the ranges of irrigation rates and seepage velocities are the same. Possible mechanisms for irrigation include density-driven convection, wave pumping, and bio-irrigation. Simulation of both advection and irrigation allows the separation of submarine groundwater discharge into fresh groundwater and (re)circulated lagoon water.     

Using Rn to estimate submarine groundwater discharge (SGD) and the associated nutrient fluxes into Xiangshan Bay,East China Sea

Continuous radon (²²²Rn) monitoring was conducted at two stations (site A and site B) with different perpendicular distance from the shoreline in Xiangshan Bay, East China Sea. Based on a ²²²Rn balance model (various sources and sinks of ²²²Rn in coastal water), the average rate of SGD was estimated to be 0.69 cm/day and 0.23 cm/day for site A and site B, respectively. The results from a nutrient analysis of the groundwater indicate that the associated nutrients fluxes loading through the SGD pathway were 4.27 × 10⁶ mol/day for DIN, 2.24 × 10⁴ mol/day for DIP and 1.82 × 10⁶ mol/day for DSi, respectively, which were comparable to or even higher than the levels observed in the local streams. Therefore, adequate attention should be paid to the importance of SGD as one source of nutrients during the eutrophication control process in this area.     

Delineating coastal groundwater discharge processes in a wetland area by means of electrical resistivity imaging, 224Ra and 222Rn

Coastal groundwater discharge (CGD) plays an important role in coastal hydrogeological systems as they are a water resource that needs to be managed, particularly in wetland areas. Despite its importance, identifying and monitoring CGD often presents physical and logistical constraints, restraining the application of more traditional submarine groundwater discharge surveying techniques. Here we investigate the capability of electrical resistivity imaging (ERI) in the Peníscola wetland (Mediterranean coast, Spain). ERI surveying made it possible to identify and delineate an ascending regional groundwater flow of thermal and Ra‐enriched groundwater converging with local flows and seawater intrusion. The continuous inputs of Ra‐rich groundwater have induced high activities of Ra isotopes and ²²²Rn into the marsh area, becoming among the highest previously reported in wetlands and coastal lagoons. Geoelectrical imaging enabled inferring focused upward discharging areas, leaking from the aquifer roof through a confining unit and culminating as spring pools nourishing the wetland system. Forward modelling over idealized subsurface configurations, borehole datasets, potentiometric records from standpipe piezometers, petrophysical analysis, and four natural and independent tracers (²²⁴Ra, ²²²Rn, temperature and salinity) permitted assessing the geoelectrical model and a derived hydrogeological pattern. The research highlights the potential of ERI to improve hydrogeological characterization of subsurface processes in complex contexts, with different converging flows. Additionally, a hydrogeological conceptual model for a groundwater‐fed coastal wetland was proposed, based on the integration of surveying datasets. Copyright © 2013 John Wiley & Sons, Ltd.     

Combining isotopic tracers (222Rn and δ13C) for improved modelling of groundwater discharge to small rivers

In regions where aquifers sustain rivers, the location and quantification of groundwater discharge to surface water are important to prevent pollution hazards, to quantify and predict low flows and to manage water supplies. ²²²Rn is commonly used to determine groundwater discharge to rivers. However, using this isotopic tracer is challenging because of the high diffusion capacity of ²²²Rn in open water. This study illustrates how a combination of isotopic tracers can contribute to an enhanced understanding of groundwater discharge patterns in small rivers. The aim of this paper is to combine ²²²Rn and δ¹³C_DIC to better constrain the physical parameters related to the degassing process of these tracers in rivers. The Hallue River (northern France) was targeted for this study because it is sustained almost exclusively by a fractured chalk aquifer. The isotopes ²²²Rn, δ¹³C_DIC, δ²H and δ¹⁸O were analysed along with other natural geochemical tracers. A mass balance model was used to simulate ²²²Rn and δ¹³C_DIC. The results of δ²H and δ¹⁸O analyses prove that evaporation did not occur in the river. The calibration of a numerical model to reproduce ²²²Rn and δ¹³C_DIC provides a best‐fit diffusive layer thickness of 3.21 × 10^?5 m. This approach is particularly useful for small rivers flowing over carbonate aquifers with high groundwater DIC where the evolution of river DIC reflects the competing processes of groundwater inflow and CO₂ degassing. This approach provides a means to evaluate groundwater discharge in small ungauged rivers. Copyright © 2014 John Wiley & Sons, Ltd.