Submarine Groundwater Discharge Variations Research Articles

Seasonal dynamics of submarine groundwater discharge in Zhanjiang Bay: an investigative study utilizing 222Rn as a tracer

The spatial and temporal variation of submarine groundwater discharge (SGD) has been a topic of interest for researchers in recent years. However, the study of SGD is limited within the scope of Zhanjiang Bay. The Zhanjiang Bay is located in western Guangdong Province, known for its high temperatures and typhoon vulnerability. The present study examined the fluctuation patterns of seawater 222Rn in Zhanjiang Bay during the period from June 2021 to March 2022. This investigation employs the 222Rn tracing technique to establish a mass balance model to compute the SGD rate, and assess the significance of SGD in translocating nutrients into the bay. We conducted a four-month sampling, observing the differences between the rainy and dry seasons. Precipitation exerts a significant influence on the seasonal variations of SGD. The SGD rates of Zhanjiang Bay ranged from 7.14 to 10.75 cm·d-1 in the dry season and from 13.38 to 14.61 cm·d-1 in the rainy season. SGD-derived nutrients also exhibited seasonal fluctuations, the annual input of nutrients it supplies may be comparable to the quantity delivered by rivers, which significantly impacted the trophic status of the bay.

Temporal Variations of Submarine Groundwater Discharge into a Tide-Dominated Coastal Wetland (Gaomei Wetland, Western Taiwan) Indicated by Radon and Radium Isotopes

Submarine groundwater discharge (SGD) is evidenced around Taiwan, but the seasonal/temporal changes of SGD have not been fully examined. Here, we report a time-series investigation of SGD into a tide-dominated coastal wetland, the Gaomei Wetland, located to the south of the Da-Chia River’s mouth, western Taiwan, by using environmental tracers (222Rn, 224Raex, 228Ra, δD, and δ18O). Our results showed that regardless of dry and wet seasons, the 222Rn activities in coastal waters were high at low tide but low at high tide. It represents the continuous input of 222Rn-enriched groundwater. However, the 224Raex and 228Ra activities showed seasonal changes with tide conditions. In the dry season, the 224Raex and 228Ra activities in coastal waters were low at low tide but high at high tide; whereas in the wet season, an opposite relation was observed with quite high 224Raex and 228Ra activities in the low-tide waters. Coupled with the lower δD and δ18O values of coastal and pore waters in the dry season, in comparison to those in the wet season, it is suggested that these phenomena probably reflected a seasonal difference in the main SGD component with fresh SGD in the dry season, but saline ones in the wet season. Based on a 222Rn mass balance model, the estimated SGD fluxes into the Gaomei Wetland varied with tidal fluctuations and ranged from 0.2 to 25 cm d−1 and from 0.1 to 47 cm d−1 for the dry and wet seasons, respectively. A slightly high SGD flux occurring during the wet season at spring tide, implied a stronger tidal pumping coupled with a larger hydraulic gradient between land and sea. In this study, we demonstrated that the variation of SGD into the Gaomei Wetland is not only controlled by the seasonal changes of groundwater recharge, but also by the tidal pumping process.

Is Submarine Groundwater Discharge (SGD) Important for the Historical Fish Kills and Harmful Algal Bloom Events of Mobile Bay?

Large-scale fish and crustacean kills, locally known as Jubilees, and harmful algal blooms (HABs) have been occurring in Mobile Bay (Alabama) for more than a century. In fact, the first record describing a Jubilee event in Mobile Bay during 1867 was the first ever-documented case of mass mortalities of marine animals caused by hypoxia. To evaluate the importance of submarine groundwater discharge (SGD) in the occurrence of Jubilees and HABs in Mobile Bay, a 3-year study was conducted using a multi-method approach. Significant spatial and temporal variations of SGD were revealed in the bay only by applying a combination of geochemical and shallow geophysical techniques. The development of seasonal hypoxia observed in bay waters in areas impacted by Jubilees was the result of anoxic SGD inputs, which magnitude and spatial distribution were controlled by shallow lithological heterogeneities created during the modern development of the bay. Although when compared to the river discharge SGD contributed between 0.2 (wet season) and 5% (dry season) of the total freshwater inputs to Mobile Bay, 80% of the total SGD in the bay occurred in areas ecologically impacted by hypoxia and Jubilees. In these areas, SGD comprised up to 37% of the total water inputs during the dry season, coinciding with the time of the year when Jubilees and HABs occur. In conclusion, while SGD might not be a significant source of fresh water to Mobile Bay or other estuaries worldwide, enhanced SGD caused by site-specific lithological heterogeneities can have a critical role in the development of hypoxia and ecological issues in nearshore waters.

Unmanned aerial vehicles (UAVs)‐based thermal infrared (TIR) mapping, a novel approach to assess groundwater discharge into the coastal zone

Submarine groundwater discharge (SGD) is a widely recognized process that carries considerable amounts of groundwater and dissolved chemicals to the coastal ocean. Despite its importance, a lack of suitable tools to assess SGD's spatial and temporal variability has hampered a complete understanding of the process. Here we report, for the first time, use of an unmanned aerial vehicle (UAV or “drone”) to assess SGD variations. An octocopter UAV platform equipped with a thermal infrared (TIR) system was flown along a coastline on Jeju Island, Korea. The UAV clearly captured thermal signatures of SGD plumes and their dynamic temporal fluctuations modulated by tidal variations. Based on a plume area-SGD flux relation we developed by combining aerial and field data, we estimated that the SGD flux of the study site ranged from 33,000 to 54,000 m3 d−1. The drone approach enabled acquisition of time series plume imagery with easy control of spatial resolution, flexible field operations, and remote sensing of SGD at low cost compared to conventional aerial surveys. Combining the UAV-TIR images with on-site sampling enables one to determine fluxes of nutrients and other dissolved species. UAV-TIR mapping can thus serve as a powerful tool for study of SGD and other coastal processes.

Terrestrial-originated submarine groundwater discharge through deep multilayered aquifer systems beneath the seafloor

A large quantity of submarine groundwater discharge (SGD) of about 1000 m3 day−1 m−1 of the 600-km-long shoreline of South Atlantic Bight has been estimated by Moore (Global Biogeochemical Cycles, 2010b, 24, GB4005, doi:10.1029/2009GB003747). However, there is great uncertainty in estimating the percentage of net, land-originated groundwater recharge of SGD. Moreover, most previous studies considered the homogeneous case for the coastal superficial aquifers. Here, we investigated the terrestrial-originated SGD through a multilayered submarine aquifer system, which comprises two confined aquifers and two semi-permeable layers. The inland recharge includes a constant part representing the annual average and a periodical part representing its seasonal variation. An analytical solution was derived and used to analyse the distributions of the terrestrial-originated SGD from the multilayered aquifers along the Winyah Bay transect, South Atlantic Bight. It is found that the width of the zone of SGD from the upper aquifer ranges from ~0.8 to ~8.0 km depending on the leakance of the seabed semi-permeable layer. A head of the upper aquifer at a coastline 1.0 m higher than the mean sea level will cause a SGD of 1.82– 18.3 m3 day−1 m−1 from that aquifer as the seabed semi-permeable layer's leakance varies from 0.001 to 0.1 day−1, providing considerable possibility for considerable land-originated SGD. Seasonal terrestrial-originated SGD variations predicted by the analytical model provide consistent explanation of the seasonal variation of 226Ra observed by Moore (Journal of Geophysics, 2007, 112, C10013, doi:10.1029/2007JC004199). The contribution of the lower aquifer to SGD is only 1.2–12% of that of the upper aquifer. Copyright © 2014 John Wiley & Sons, Ltd.

제주도 연안에서 해저 지하수 및 지하수 기원 영양염류 유입량의 시간적 변화

To determine the temporal variations of submarine groundwater discharge (SGD) and SGD-driven nutrients inputs, we measured the seepage rate and the nutrient concentrations of pore water/groundwater in Bangdu Bay of Jeju Island at two and three month intervals from September 2009 to September 2010. The seepage rate of groundwater ranged from 0 to 330 cm/day (average ~170 cm/day) during the five sampling periods, which increased sharply from high tide to low tide due to changes in hydraulic pressure gradient between water table in land and water sea level in the coastal ocean by the tidal cycles. The submarine inputs of groundwater were also relatively higher in summer than in winter. The nutrient fluxes from SGD were about 90~100%, 70~95%, and 65~100% of the total input (except from open ocean waters) for dissolved inorganic nitrogen (DIN), phosphorus (DIP), and silicate (DSi), respectively, potentially supporting about 0.9~33 g of new primary production in Baugdu Bay. Thus, our study suggests that SGD-driven nutrients may play an important role in the eutrophication and biological production in the coastal ocean of Jeju Island.

Measurement of temporal and horizontal variations in 222Rn activity in estuarine waters for tracing groundwater inputs

We measured the time-series variations in 222 Rn activity at a fixed station in the Yeongsan River Estuary (YRE), Korea, where the upstream water is obstructed by an artificial dam, during November 18-24, 2008, and conducted a horizontal 222 Rn survey in the Seomjin River Estuary (SRE), Korea, during December 16-18, 2008. In the YRE, we observed constantly low levels of 222 Rn activity over the sampling period, relative to the summer in 2008. The 222 Rn variations and 222 Rn mass-balance modeling results indicate that the submarine groundwater discharge (SGD) in the YRE was relatively low and constant over the sampling period and that the temporal SGD variations were not related to tides or to the water elevation in the dam upstream. This low and constant SGD appears to be due to the lower water level in the dam relative to the local sea level during the study period, resulting in the SGD mostly from the surrounding land areas rather than from the dam. In the SRE, higher 222 Rn activities were observed at upstream stations of the river (salinity: <5) and decreased as the salinity increased, but there were marked increases of 222 Rn, together with dissolved inorganic silicate (DSi), at stations close to the river mouth (salinity: ~33). This suggests that the river has major groundwater inputs from the two extreme upstream and downstream areas. Because most chemical flux estimations via river discharge do not account for groundwater inputs from the downstream high- salinity zone, our results suggest that there is an important unaccounted source of river-driven chemical fluxes. Overall, our study shows that the continuous 222 Rn monitoring system provides high-resolution information on SGD over different locations and times.

Underwater in situ measurements of radionuclides in selected submarine groundwater springs, Mediterranean Sea

The application of the in situ measurement system 'KATERINA' for monitoring of radon progenies in submarine groundwater discharge (SGD) was investigated at different locations in the Mediterranean Sea (Chalkida, Stoupa, Korfos and Cabbé). At Chalkida and Stoupa radon progenies concentration exhibited almost constant values of 1.2 ± 0.1 and 2.5 ± 0.2 Bq l(-1), respectively. At Korfos these activities ranged between 1.4 ± 0.1 and 2.3 ± 0.2 Bq l(-1) exhibiting inverse relationship with salinity. At Cabbé the in situ measured data were compared with radon measurements obtained by liquid scintillation counter. The system also resolved radon progeny variations of SGD on time scales above 1 h. The radioactivity levels of radon progenies from all sites were found considerably lower (approximately 2 orders of magnitude) than the commonly accepted limits for radon in drinking water.

Submarine groundwater discharge from the Yellow River Delta to the Bohai Sea, China

Spatial and temporal variations of submarine groundwater discharge (SGD) have been evaluated by automated seepage meters from the Yellow River delta to 7 km offshore in the Bohai Sea, China. We identified three zones from the coast to offshore based on different relationships between tidal and SGD changes. Our results indicate that the point of maximum SGD shifted 2 km offshore from September 2004 to September 2006. This spatial change is thought to be caused by sediment deposition near the coast. Integrating submarine fresh groundwater discharge (SFGD) along the coastline of the Yellow River delta using measured values of SFGD per unit length of shoreline shows that discharge of fresh groundwater along the entire delta would be equivalent to 4.5 to 7.0 % of the river discharge.

Monitoring of submarine groundwater discharge along the Donnalucata coast in the south-eastern Sicily using underwater gamma-ray spectrometry

A novel technique for monitoring of submarine groundwater discharge (SGD) in coastal zones based on an in situ underwater gamma-ray spectrometry of radon-decay products is described. Several sites were visited during the IAEA’2002 expedition offshore Donnalucata in the south-eastern Sicily. Continuous monitoring of 222Rn in the beach spring at Donnalucata has shown variable 222Rn activity concentrations in groundwater (from 12 to 19 kBq m - 3 ), depending inversely on water levels during tide. Spatial variations of SGD have been observed in the Donnalucata boat basin. Average 222Rn activity concentrations in seawater varied from ∼ 0.1 to 3.7 kBq m - 3 , showing an inverse relationship with salinity. A continuous monitoring carried out at the site closest to the coast has also revealed an inverse relationship of 222Rn activity concentration on tide and salinity. The 222Rn concentrations in seawater varied from 2.3 kBq m - 3 during high tides to 4.8 kBq m - 3 during low tides, confirming the tidal influence on SGD.

Submarine groundwater discharge measured by seepage meters in sicilian coastal waters

Seepage measurements of submarine groundwater discharge (SGD) were carried out in March 2002 in the Donnalucata boat basin in the southeastern Sicily. SGD rates measured by manual seepage meters ranged from 5.5 to 19.3 L/min/m (with an average of 12.1 L/min/m), which was 0.6 and 5.9 times the SGD determined in Florida and Perth, respectively. The spatial distribution of SGD was found to be highly variable along ∼300 m long coast area. Semi-diurnal SGD variations which anti-correlated with tide were found using newly developed automated seepage meters that can provide continuous SGD data with high resolution. These variations may be attributed to the tidal effect, when a greater hydraulic gradient, observed at low tide, moves groundwater from land to the ocean, and an opposite effect is observed at high tide.

Characterisation of submarine groundwater discharge offshore south-eastern Sicily

A complex approach in characterisation of submarine groundwater discharge (SGD) off south-eastern Sicily comprising applications of radioactive and non-radioactive tracers, direct seepage measurements, geophysical surveys and a numerical modelling is presented. SGD fluxes in the Donnalucata boat basin were estimated by direct seepage measurements to be from 4 to 12 L s −1, which are comparable with the total SGD flux in the basin of 17 L s −1 obtained from radon measurements. The integrated SGD flux over the Donnalucata coast estimated on the basis of Ra isotopes was around 60 m 3 s −1 per km of the coast. Spatial variations of SGD were observed in the Donnalucata boat basin, the average 222Rn activity concentration in seawater varied from ∼0.1 kBq m −3 to 3.7 kBq m −3 showing an inverse relationship with salinity. The continuous monitoring carried out at the site closest to the coast has revealed an inverse relationship of 222Rn activity concentration on the tide. The 222Rn concentrations in seawater varied from 2.3 kBq m −3 during high tides to 4.8 kBq m −3 during low tides, thus confirming an influence of the tide on submarine groundwater discharge. Stable isotopes (δ 2H and δ 18O) showed that SGD samples consist up to 50% of groundwater. Geo-electrical measurements showed a spatial variability of the salt/fresh water interface and its complex transformation in the coastal zone. The presented results imply that in the studied Donnalucata site there are at least two different sources of SGD, one superficial, represented by mixed fresh water and seawater, and the second one which originates in a deeper limestone aquifer.

Dynamics of submarine groundwater discharge and freshwater‐seawater interface

Relationships between submarine groundwater discharge (SGD) and the freshwater‐saltwater interface are evaluated by continuous measurements of SGD rates, conductivity and temperature of SGD, and resistivity measurements across the coastal aquifer. Our measurements show that the processes of SGD differ between the offshore and nearshore environments. SGD and submarine fresh groundwater discharge (SFGD) rates were largest just landward of the saltwater‐freshwater interface. SGD variations landward the saltwater‐freshwater interface had negative correlations with tidal variations, because of the connections of terrestrial groundwater in the land and the ocean. SGD in the nearshore can be explained mainly by connections of terrestrial groundwater, while offshore SGD rate is controlled mostly by oceanic process such as recirculated saline groundwater discharge.

Tidal Load and Salt Water Influences on Submarine Ground Water Discharge

AbstractField measurements were conducted to characterize the influences of salt water and tidal loading on submarine ground water discharge (SGD) into West Neck Bay on Shelter Island, New York. SGD was continuously recorded at three sites using an ultrasonic ground water seepage meter, and the spatial extents of the terrestrially derived SGD and the salt water/fresh water mixing zone were inferred from electrical conductivity profiling. While SGD increases with increasing hydraulic gradient in the far field of the coastal aquifer, shorter temporal variations of SGD were inversely correlated with tidal stage. SGD and hydraulic heads in the onshore unconfined aquifer near the coast responded asymmetrically to tidal loading, with discharge and head increasing faster than decreasing. A time lag of ∼1 h was observed between the tidal load and responses in onshore head and SGD. This lag represents a manifestation of the travel time required for tidal influences to propagate landward before the hydraulic gradient adjusts and influences seepage. The spatial extent of the fresh water component of SGD determined by electrical conductivity profiles, seepage velocity, and the time lag between tide and SGD amplitudes are in reasonable agreement with analytic estimates.

Submarine groundwater discharge in Osaka Bay, Japan

Submarine groundwater discharge (SGD) rates in Osaka Bay were continuously measured and analyzed to evaluate seawater–groundwater interactions. Fast Fourier transfer and power spectrum density methods were applied to analyze the dominant periods of the SGD variations. Diurnal and semidiurnal periods of SGD variation were found, and they were caused by tidal effects. According to the separation of SGD into fresh and recirculated water components using automated seepage meter measurements and terrestrial groundwater flow analyses, the fresh groundwater component in SGD was evaluated to be in the range 4%–29% at Tannowa, Osaka. Therefore, SGD rates depend mainly on the volume of recirculated seawater. Correlation analyses between SGD and sea level show that SGD is delayed by 4 h after sea level changes.

Tidal effects on submarine groundwater discharge into the ocean

Semi‐monthly, diurnal and semi‐diurnal variations of Submarine Groundwater Discharge (SGD) were found in Osaka bay, Japan, by continuous measurements of SGD using newly developed automated seepage meters. SGD increased sharply from neap to spring tide with semi‐monthly period. SGD also changes with diurnal and semi‐diurnal periods due to changes in hydraulic gradient between the groundwater and seawater in the coastal zone. SGD variations with semi‐monthly period by neap‐spring tidal pumping is much larger than those with diurnal and semi‐diurnal variations by tidal height changes.