Estimation of submarine groundwater discharge and associated nutrient fluxes in Tolo Harbour, Hong Kong

Submarine groundwater discharge and ocean acidification: Implications from China's coastal waters

Ocean acidification (OA) is a global environmental concern, and submarine groundwater discharge (SGD) is a potentially process that enhances OA. This review summarizes the relationship between two types of constituents carried by SGD into China's seawater and OA. 1) Current research predominantly concentrates on constituent fluxes from SGD, neglecting its ecological impacts on carbon and nutrients budgets, as well as the mechanisms between carbon and nutrients. 2) Uncertainties persist in SGD research methods and acidification characterization. 3) There's a need to enhance quantitative research methods of SGD-OA, particularly in areas with intricate biogeochemical processes. Effective identification methods are crucial to quantify SGD's contribution to OA. Investigating core scientific questions, including SGD's impact on OA rates and scales, is paramount. While the primary focus is on SGD-OA research in China, insights gained from novel perspectives could have broader value for coastal management globally.

Comparing nearshore and embayment scale assessments of submarine groundwater discharge: Significance of offshore groundwater discharge as a nutrient pathway

Submarine groundwater discharge (SGD) can influence biogeochemical cycles in coastal seas by delivering nutrients from the seafloor. Comparison between the nearshore and embayment scale assessments of SGD against river water discharge would be crucial for understanding biogeochemical impacts on the coastal seas because the discharge pattern (non-point or point pathway) is different. Here, we quantified SGD contribution to rivers in nutrient budgets at two scales within a coastal embayment (Obama Bay, Japan) by mass balance models of radon and radium isotopes. We then compared the SGD contribution between the two scales by the meta-analysis for regional data sets conducted in nearshore and embayment scales. The estimated SGD rates in the nearshore and embayment scales in the bay were 7.8 cm d-1 and 20.0 cm d-1, indicating that offshore SGD was more significant than nearshore. The ratios of nutrient fluxes derived from SGD to rivers (SGD:River) in the nearshore scale were 1.7 for dissolved inorganic nitrogen (DIN), 3.0 for phosphorus (DIP), and 0.5 for silica (DSi), while those in the embayment scale increased to 10.4 for DIN, 18.5 for DIP, and 3.9 for DSi. This result indicates that SGD-derived nutrients become more important at larger spatial scales. Meta-analysis revealed that the difference in the contribution of SGD to rivers was affected by the seafloor size and there was no significant difference in SGD rates between nearshore and embayment scale studies. However, our regional study shows the site-specific pattern that SGD rates in the embayment scale were higher than those in the nearshore scale. Overall, we clarified that SGD can be a crucial nutrient pathway for coastal embayments regardless of the spatial scales and contribute to coastal nutrient biogeochemistry in more offshore areas.

Using 222Rn temporal and spatial distributions to estimate the groundwater discharge rate and associated nutrient fluxes into high salinity lakes in Badain Jaran Desert, Northwest China

Groundwater is the main source of water and salt recharge for to lakes in arid regions. Quantifying the groundwater discharge and its nutrient input is important in the evolution of lake environments in the Badain Jaran Desert (BJD), Northwest China. In this study, ten BJD lakes were sampled for ²²²Rn in April and September 2021, and the ²²²Rn mass balance model was used to quantify the groundwater discharge rates and derived nutrient fluxes to these lakes. The results showed that the ²²²Rn activity and the groundwater recharge rate of lake water both present a positively correlated with lake water depth. The hot points of high ²²²Rn activity in the lake water were consistent with the locations of groundwater discharge areas. According to the ²²²Rn temporal and spatial distributions, the mean groundwater recharge rates for the ten lakes in April and September were 5.4 ± 0.6 and 7.7 ± 1 mm/d, respectively, and the annual mean groundwater discharge rates varied between 1.1 ± 0.2 and 14.6 ± 1.6 mm/d, with a mean of 7 ± 0.9 mm/d. Given that all the perennial lakes in the BJD have the same groundwater recharge rate as the mean recharge rate of the ten studied lakes, the annual mean groundwater recharge amount received by the lakes in the entire BJD is (5.6 ± 0.7) × 10⁷ m³/a. According to the groundwater recharge amount, the inputs of dissolved inorganic nitrogen, dissolved inorganic phosphorus, dissolved inorganic silicon, total nitrogen, and total phosphorus to the BJD lakes from groundwater were (4.7 ± 0.6) × 10⁵, (3.8 ± 0.5) × 10⁴, (7.9 ± 1) × 10⁵, (7.2 ± 0.9) × 10⁵, (2.5 ± 0.3) × 10⁴ kg/a, respectively. This study provides a reference for quantifying of groundwater discharge rates into salt lakes in other arid regions.

Storm accelerated subsurface Escherichia coli growth and exports to coastal waters

Storm significantly deteriorates coastal water fecal pollution now and beyond. Questions relating to storm exerting on coastal water safety are often intertwined with both surface water and subsurface processes. Stormwater runoff is a vital metric for coastal water fecal pollution under current cognition, while the controls of subsurface system remain unclear. Here, this study leveraged two time-series field data collected in a sandy beach during storm and non-storm periods to probe subsurface Escherichia coli (E. coli) growth and exports to coastal waters under storm events. Results demonstrated that storm events can not only stimulate subsurface E. coli growth, but also accelerate subsurface E. coli exports into the receiving water. Storm-intensified rainfall injected more oxygenous rainwater in the shallow groundwater, subsequently stimulating subsurface E. coli growth. Storm-strengthened wave energy was responsible for accelerating subsurface E. coli exports through enhanced wave-induced recirculated seawater. This study proposes a new insight for the stress of storm events on microbial pollution in coastal waters. The findings are constructive to the prevention of beach ecosystem pollution and can pave the way for coastal safety management to future extreme weather.

Assessment of red tide risk by integrating CRITIC weight method, TOPSIS-ASSETS method, and Monte Carlo simulation

This study proposes a red tide risk assessment method based on intercriteria correlation (CRITIC), technique for order preference by similarity to an ideal solution (TOPSIS), assessment of estuarine trophic status (ASSETS) methods and Monte Carlo simulation (MCS) to calculate the probability of each risk level. The integrated TOPSIS-ASSETS method is used to calculate the risk levels of each year, where index weight is determined by CRITIC method. MCS method is employed to calculate the probability of each risk level. The results showed that level III to level V indicates high possibility of red tides in the case study area (Tolo Harbor). The highest risk rating was level V in 1988. The change of the risk level of red tide is consistent with the real situation of the occurrence of red tide. Another case of the east part of Skagerrak Strait shows that the results of this method are consistent with field situation. When there is an error between the evaluation results and the real situation, MCS can further suggest the probability of error in the evaluation results. Meanwhile, sensitivity analysis was used to test the performance of the evaluation model and two comparative methods. The results show that the proposed risk assessment method has better performance than other methods and can provide an effective risk evaluation for red tide management.

Elucidating sources of atmospheric NOX pollution in a heavily urbanized environment using multiple stable isotopes

Atmospheric deposition of nitrogen (N) from rain and aerosols can be a significant non-point source - particularly in urbanized coastal areas and contribute to coastal eutrophication and hypoxia. Here, we present geochemical and isotopic data from surface waters coupled with an 18-month time series of geochemical and isotopic data measured on wet and dry deposition over Hong Kong from June 2018. Dual stable isotopes of nitrate (δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^-) of rain and total suspended particulates (TSP) were analyzed to trace the sources and understand seasonal pattern of atmospheric nitrate. The δ¹⁵N of TSP, δ¹⁵N-NO₃ in rain and TSP ranged from +0.94 to +17.6‰, -4.1 to +3.0‰ and -1.3 to +9.0‰ respectively. δ¹⁵N varied seasonally with higher values in winter and lower values in summer. This variation can be explained by a change in the sources of atmospheric NO_x driven by the East Asian Monsoon. It was found that most NO_x comes from coal burning in winter and a mix of vehicle emissions, fossil fuel combustion and lightning in summer. Moreover, the estimated dry and wet deposition of nitrate and ammonium in Hong Kong is around 18 kg N ha^-1 annually, which is of the same order of magnitude as N released by sewage effluents and groundwater. This implies that atmospheric N deposition over the N-limited waters of the eastern side of Hong Kong could contribute significantly to the N budget. Therefore, atmospheric N deposition may alter the local N marine cycling, thus monitoring its impact is crucial for water quality in Southern China.

Dominance of evaporation on lacustrine groundwater discharge to regulate lake nutrient state and algal blooms

As global threats to freshwater lakes, eutrophication and harmful algal blooms (HABs) are governed by various biogeochemical, climatological and anthropogenic processes. Groundwater is key to join these processes in regulating HABs, but the underlying mechanisms remain unclear. Here, we leveraged basin-wide field data of Lake Taihu (China's largest eutrophic lake) and global archives, and demonstrate the dominance of evaporation on lacustrine groundwater discharge (LGD) in shallow lakes. We extrapolated decadal LGD and the derived nutrient loadings and found that HABs promptly consume ubiquitous groundwater borne nutrients, leading lake water N: P ratios 2-3 months time lagged behind LGD N: P ratios. We conclude that evaporation dominated LGD is an unraveled but crucial regulator of nutrient states and HABs in shallow lakes, which advocates synergistical studies from both climatological and hydrogeological perspective when restoring lake ecosystems.

Salinity and temperature profiling for the submarine groundwater discharge simulations: Quantification through heat and solute transport model

Developed coastal regions are the hotspots for contaminated groundwater discharge, affecting sensitive marine ecosystems. The present study aims to identify submarine groundwater discharge (SGD) locations and quantify the contaminant load reaching to the western coast of India (Gujarat coast) using stable isotopes, seepage meter, heat and solute transport model. The coastal aquifers are highly enriched in trace metals due to various active natural processes and anthropogenic activities across the coast. Terrestrial and recirculated SGD was a significant contributor to flow and metal load, which ranged from 1.04 to 181.1 m³.year^-1 and 0-77.41 kg.year^-1, respectively. The highest estimated SGD in the Gujarat coast was relatively less than the SGD reported in the Bay of Bengal and comparable to the South Chennai coast. The order of metal flux found in the study was Zn > Fe > Cr > Pb > Ni > Cu > Mn, whereas the highest flux of Zn (77.41 kg. year^-1) was reported at Fansa beach, which was 7x Fe-flux and 45 x Cr-flux, respectively. Higher micronutrients (Fe and Zn) load in the southern coast leads to increased vulnerability of eutrophication, algal blooms and biotic ligand formation in aquatic species. This enrichment of micronutrients in the coastal ecosystem was evident by the growth of seaweeds on the seabed at SGD identified locations.

Effects of salinity and particle size on radium desorption from river sediments in the Qinghai-Tibet Plateau

Natural radium isotopes have been widely used to study groundwater discharge in different systems. Therefore, it is of great significance to understand the desorption behavior of radium isotopes on sediments to trace water-land exchange processes. However, there is very limited studies observing the desorption Ra isotopes to lake water of the brine lake. ²²⁴Ra desorption experiments with different salinities and particle sizes were carried out by collecting samples of brackish water from Qinghai Lake, brine from Dabuxun Lake and river sediments entering the lakes. The results show that the desorption activity of ²²⁴Ra from the river sediments to lake water of Qinghai Lake is 0.2 dpm/g when the salinity is 10.07‰. The maximum desorption activity of ²²⁴Ra from river sediments to lake water of Dabuxun Lake is 0.195 dpm/g at a salinity of 40.81‰. A salinity of 41.81‰ and particle size of 16.28 μm are the threshold points affecting the desorption behavior of Ra. When the salinity is less than 40.81‰, the desorption activity of Ra increases linearly with increasing salinity. When the salinity is greater than 40.81‰, the desorption activity of Ra decreases nonlinearly with increasing salinity and tends toward a stable low value. When the particle size is larger than 16.28 μm, the small particle size promotes desorption. The smaller the particle size is, the greater the desorption activity is. When the particle size is less than 16.28 μm, the small particle size inhibits desorption. The smaller the particle size is, the smaller the desorption activity. The co-precipitation of Ra²⁺ with supersaturated Ca²⁺, SO₄^2- and other ions may be the main reason for the threshold point of salinity and particle size in Ra desorption process in salt lake system.

Modification of fatty acid profile and biosynthetic pathway in symbiotic corals under eutrophication

Symbiotic corals receive energy not only by ingesting food (e.g. plankton, inorganic/organic matter, i.e. heterotrophy), but also by endosymbiosis, which supplies photosynthates (dissolved inorganic carbon, i.e. autotrophy). These two sources of energy have distinct fatty acid (FA) profiles, which can be used to differentiate corals by their primary feeding mode. FA profiles have been applied as biomarkers to evaluate the quality of nutrition in the midst of environmental change. However, species-specific responses of coral FA profiles and biosynthetic pathway under cultural eutrophication are still unknown. We collected two coral species (Acropora samoensis, Platygyra carnosa) from sites with different levels of eutrophication to test for variations in FA profiles. Gas Chromatography-Mass Spectrometry (GC-MS) was performed to identify FA profiles and quantify their concentration. Our main findings are threefold: 1) chronic eutrophication inhibits corals' ability to synthesize essential FA; 2) PUFA:SFA ratio and certain FA biomarkers or their pathway can be successfully utilized to determine the relative degree of autotrophy and heterotrophy in corals; 3) under eutrophication, different FA profiles of coral host tissue are attributed to different feeding strategies. Thus, our research provides significant new insights into the roles of FA as a risk assessment tool in coral reef ecosystems under the pressure of eutrophication.

Hydroclimatic variability drives submarine groundwater discharge and nutrient fluxes in an anthropogenically disturbed, semi-arid estuary

Nutrient budgets in semi-arid estuaries, with ephemeral freshwater inflows and limited nutrient sources, are likely incomplete if contributions from submarine groundwater discharge (SGD) are not included. Here, the relative importance of saline/recirculated SGD-derived nutrient fluxes spatiotemporal variability to the overall nutrient budget is quantified for Nueces Bay, Texas, U.S.A., across hydroclimatic conditions ranging from drought to normal, to flood. On average, 67% of the variance in water quality is due to temporal differences while 16% is explained by spatial differences. Principal component analysis (PCA) reveals three principal components: freshwater inflow (PC1 28.8%), saline/recirculated SGD and recycled nitrogen (PC2 15.6%), and total SGD and "new" nitrogen (PC3 11.2%). Total SGD porewater fluxes ranged from 29.9-690.3 mmol∙m^-2d^-1 for ammonium, 0.21-18.7 mmol∙m^-2d^-1 for nitrite+nitrate, 3.1-51.3 mmol∙m^-2d^-1 for phosphate, 57.1-719.7 mmol∙m^-2d^-1 for silicate, and 95.9-36,838.5 mmol∙m^-2d^-1 for dissolved organic carbon. Total and saline/recirculated SGD fluxes were on average 150-26,000 and 5.8-466 times, respectively, greater than surface runoff fluxes across all seasons. Nitrogen (N) enrichment in porewater occurs near the agricultural fields because of soil N flushing and percolation to groundwater, which facilitates N-rich groundwater fluxes. There were substantial "new" N inputs from terrestrial groundwater following precipitation while saline/recirculated SGD of recycled N accounts for only <4% of total SGD inputs. The "new" N inputs occur in the river and river mouth during flooding, and near the north shore where topography and hydraulic gradients are steeper during drought. Thus, while significant inputs of N may be associated with atmospheric deposition, or remineralization in the porewater, groundwater is the highest contributor to the nutrient budget in Nueces Bay. This result implies that nutrient management strategies should focus on land-use practices to reduce N contamination of shallow groundwater and subsequent contamination of estuaries.

Two-decade variations of fresh submarine groundwater discharge to Tolo Harbour and their ecological significance by coupled remote sensing and radon-222 model

Although submarine groundwater discharge (SGD) comprises an insignificant proportion of the global hydrologic cycle, it contributes significantly to chemical fluxes into the coastal waters due to concentrated constituents in coastal groundwater. Large nutrient loadings derived from SGD can lead to a series of environmental and ecological problems such as algal blooms, resulting in water discoloration, severe dissolved oxygen depletion, and eventually beach closures and massive fish kills. Previous studies have demonstrated the relationship between algal blooms and SGD obtained from direct measurement with seepage meters or from geo-tracer (i.e., radon and radium) based models; these traditional methods are time-consuming, laborious and point monitoring, and can hardly achieve a high spatiotemporal resolution SGD estimation, which is vital in revealing the effects of SGD to algal blooms over a long period. Alternatively, remote sensing methods for high spatiotemporal resolution SGD localization and quantification are applicable and effective. The temperature difference or anomaly between groundwater and coastal water extracted from satellite thermal images can be used as the indicator to localize and detect SGD especially its fresh component (or fresh SGD). In this study, multi-year (2005, 2011 and 2018) radon samples in Tolo Harbour were used to train regression models between in-situ radon (Rn) activity and the temperature anomaly by Landsat satellite thermal images. The models were used to estimate two-decade variations of fresh SGD in Tolo Harbour. The synergistic analysis between the time series of fresh SGD derived from regression models and high spatiotemporal resolution ecological metrics (chlorophyll-a, algal cell counts, and E.coli) leads to the findings that the increase of the fresh SGD associated with high nutrient concentrations is witnessed 10-20 days before the observations of algal bloom events. This study makes the first attempt to demonstrate the strong relation between the SGD and algal blooms over a vicennial span, and also provides a cost effective and robust technique to estimate SGD on a bay scale.

The dynamics of dissolved inorganic nitrogen species mediated by fresh submarine groundwater discharge and their impact on phytoplankton community structure

Submarine groundwater discharge (SGD)-driven nutrient inputs have long been speculated to sustain the high frequency of red tide occurrence in Tolo Harbour, Hong Kong, for its larger flux and higher nutrient loadings than river discharge. Based on analysis of high resolution time series biogeochemical and climatological data from 2000 to 2015, fresh SGD-derived dissolved inorganic nitrogen (DIN) is found to be a significant regulator of the annual cycle of phytoplankton community structure in the harbour. In the wet season, fresh SGD supplies nutrients with NH₄⁺:NO₃^- ratio < 1 to the seawater, meanwhile creates an intensive vertical stratification environment. As a result, diatom which is a NO₃^- specialist, is prone to be the major group in the harbour. Fresh SGD delivers a same orders of magnitude of DIN as river and precipitation, but it is more important to phytoplankton community structure dynamics because fresh groundwater has smaller NH₄⁺:NO₃^- ratio that significantly changes the ratio in the harbour. In the dry season, with the decline of fresh SGD and the ease of stratification, vertical mixing uplifts the nutrient (NH₄⁺:NO₃^- ratio > 1) released from the bottom sediment leading to a NH₄⁺ dominant environment in water column. Dinoflagellate and other groups then become dominant species of phytoplankton in the harbour. Fresh SGD has a major influence on the NH₄⁺:NO₃^- ratio in the seawater compared to tide-driven SGD, even though the latter contributes a larger proportion SGD. Tide-driven SGD also produces NH₄⁺ and NO₃^-, but NH₄⁺:NO₃^- ratio are mainly subject to the beach environment (bare/mangrove beach), which does not change much seasonally, thus dominant DIN species do not change significantly throughout a year. In a conclusion, fresh SGD plays the most important role among all the endmembers in regulating the DIN composition in Tolo Harbour and its fluctuation mediates the phytoplankton community structure.

Combining hydrological investigations and radium isotopes to understand the environmental effect of groundwater discharge to a typical urbanized estuary in China

Pollution of urbanized rivers with excess nutrients due to groundwater discharge is an increasing environmental concern worldwide. Dan'ao river, a typical urbanized river in the Guangdong-Hong Kong-Macao Greater Bay Area, is experiencing heavy water pollution. However, the groundwater-derived nutrient loads had not yet been thoroughly quantified. In order to quantify the contribution of groundwater-derived nutrient inputs, we combined the methods of hydrological investigations and radium isotopes. Groundwater and river water samples were collected from the river upstream to the estuary for the analyses of radium quartets and nutrients including DIN, DIP and DSi. The results showed that the radium activities in both surface water and groundwater decreased from the estuary to the upstream. The groundwater discharge rate was estimated by the radium mass balance model using short-lived radium isotopes (²²³Ra and ²²⁴Ra). The estimated groundwater discharge rate ranged from 1.99 × 10⁵ to 6.67 × 10⁵ m³ d^-1, comparable to the upstream river discharge rate of 4.23 × 10⁵ m³ d^-1. The groundwater-derived nutrient fluxes were 165.66-554.98 mmol m^-2 d^-1 for DIN, 2.47-8.26 mmol m^-2 d^-1 for DIP and 63.73-213.49 mmol m^-2 d^-1 for DSi, respectively. They contributed 19%~44% DIN, 16%~39% DIP, and 31%~60% DSi of all the nutrient inputs into the Dan'ao River, respectively. In addition, the nutrient inputs by groundwater discharge has an average DIN:DIP ratio of as high as 190, which is able to potentially affect the riverine and marine nutrient structures. These findings may provide useful information for designing control strategies for reducing massive nutrient inputs to Dan'ao River in the future.

A combined-method approach to trace submarine groundwater discharge from a coastal karst aquifer in Ireland

Knowledge about the hydraulic connections between submarine groundwater discharge (SGD) and its terrestrial coastal catchment is relevant with regard to the management of marine and coastal waters in karst areas. This study applies different methods and monitoring approaches to trace SGD between the Burren Limestone Plateau and Galway Bay in western Ireland, via an excavated sinkhole shaft and deep conduit. Areas of potential SGD were first delineated based on sea surface temperature anomalies using Landsat satellite images. Two fluorescent dyes and solid wood chips were then used as tracers. Solid wood chips were tested as potential means to circumvent the problem of high dispersion in the sea, impacting on the fluorescent dyes to yield readings below the detection limits. Sampling was conducted at 10 different terrestrial locations and in the sea at Galway Bay. Offshore sampling was conducted in transects over a period of four successive days onboard of a vessel using an automated field fluorometer and a conductivity-temperature-depth sensor. No wood chips were recovered in the sea but both fluorescent dyes were successfully sampled. The estimated travel times are in the order of 100 to 354 m/h, and localised tracer readings correlate well in space and time with low conductivity readings. By confirming hydraulic connections between the two karst features and Galway Bay, the study substantiates the hypothesised importance of Variscan veins with regard to regional groundwater flow in the region.

Tidal induced dynamics and geochemical reactions of trace metals (Fe, Mn, and Sr) in the salinity transition zone of an intertidal aquifer

Biogeochemical reactions in an intertidal aquifer influences the submarine groundwater discharge (SGD) associated trace metal flux to the ocean. Tidal fluctuation greatly affects the physical mixing, and biogeochemical transformation of trace metals in the intertidal aquifer. This study presents the dynamics of trace metals (Fe, Mn, and Sr) and the production of Fe²⁺ in the salinity transition zone is discovered. The variations of Fe²⁺ are led by the shifts of both physical mixing and biogeochemical reaction during tidal fluctuation. The transformation from amorphous Fe(OH)₃ to FeS is the main reason for the enrichment of Fe²⁺ in the zone with a salinity of 0.5-10. Mn behaves much less active than Fe in the intertidal aquifer due to the very limited Mn in the solid phase and the major driving force of Mn²⁺ variation is the physical mixing rather than geochemical reaction. Sr²⁺ behaves conservatively and shows a synchronous with salinity in the salinity transition zone. This study found that Fe²⁺ precipitates in a form not limited to Fe (hydro)oxides and the FeS minerals is the most possible form of precipitation in reduced aquifers. In that case, only a small part of Fe²⁺ discharges to the sea associated with SGD, but Mn²⁺ has a comparatively conservative property during the transport in the intertidal aquifer and majority of the Mn²⁺ originated from fresh groundwater will discharge with SGD in this study. The biogeochemical transformation pathways of Fe and Mn observed in this study provides insights into the cycles of Fe and Mn in an intertidal aquifer, which is of significance to accurately estimate the SGD derived Fe and Mn fluxes to the ocean.

Significant chemical fluxes from natural terrestrial groundwater rival anthropogenic and fluvial input in a large-river deltaic estuary

The shores of the Pearl River estuary are home to 35 million people. Their wastes are discharged into the large river delta-front estuary (LDE), one of the most highly polluted systems in the world. Here we construct a radium reactive transport model to estimate the terrestrial groundwater discharge (TGD) into the highly urbanized Pearl River LDE. We find the TGD comprises only approximately 0.9% in term of water discharge compared to the river discharge. The TGD in the Pearl River LDE delivers significant chemical fluxes to the coast, which are comparable to the fluvial loadings from Pearl River and other world major rivers. Of particular importance is the flux of ammonium because of its considerable role in Pearl River estuary eutrophication and hypoxia. Unlike the ammonium in many other aquifers, the ammonium in the Pearl River aquifer system is natural and originated from organic matter remineralization by sulfate reduction in the extremely reducing environment. The TGD derived NH₄⁺ is as much as 5% of the upstream Pearl River fluvial loading and 42% of the anthropogenic inputs. This high groundwater NH₄⁺ flux may greatly intensify the eutrophication, shift the trophic states, and lead to alarming hypoxia within the affected ecosystems in the Pearl River LDE. The large TGD derived chemical fluxes will lead to deterioration of water and will potentially affect human health.

Submarine groundwater discharge and chemical behavior of tracers in Laizhou Bay, China

Naturally occurring radon (²²²Rn) and radium isotopes are widely used to trace water mixing and submarine groundwater discharge (SGD) in the coastal zones. However, their activities in groundwater are variable both spatially and temporally. Here, time series sampling of ²²²Rn and radium was conducted to investigate their behavior in intertidal groundwater of Laizhou Bay, China. The result shows that groundwater redox conditions have an important impact on the behavior of tracers. The activities of tracers will decrease under oxidizing conditions and increase under reducing conditions. Radon and radium mass balance models were used to evaluate the flushing time and SGD based on spatial surveys in Laizhou Bay. The flushing time is estimated to be 32.9-55.3 d with coupled models, which agrees well with the result of tidal prism model. The trace-derived SGD in the whole bay ranges from 6.1 × 10⁸ to 9.0 × 10⁸ m³/d and the re-circulated seawater (RSGD) ranges from 5.5 × 10⁸ to 8.5 × 10⁸ m³/d. The average SGD and RSGD fluxes are 22.8 and 21.1 times greater than the Yellow River discharge in April 2014, respectively. The study provides a better understanding of the dynamics of coastal groundwater and behavior of tracers in a well-studied bay system.

Influence of Coastal Submarine Groundwater Discharges on Seagrass Communities in a Subtropical Karstic Environment

The influence of coastal submarine groundwater discharges (SGD) on the distribution and abundance of seagrass meadows was investigated. In 2012, hydrological variability, nutrient variability in sediments and the biotic characteristics of two seagrass beds, one with SGD present and one without, were studied. Findings showed that SGD inputs were related with one dominant seagrass species. To further understand this, a generalized additive model (GAM) was used to explore the relationship between seagrass biomass and environment conditions (water and sediment variables). Salinity range (21-35.5 PSU) was the most influential variable (85%), explaining why H. wrightii was the sole plant species present at the SGD site. At the site without SGD, GAM could not be performed since environmental variables could not explain a total variance of > 60%. This research shows the relevance of monitoring SGD inputs in coastal karstic areas since they significantly affect biotic characteristics of seagrass beds.

Constraining the temporal variations of Ra isotopes and Rn in the groundwater end-member: Implications for derived SGD estimates

Submarine groundwater discharge (SGD) has been recognized as an important supplier of chemical compounds to the ocean that may influence coastal geochemical cycles. Radium isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra_,²²⁸Ra) and radon (²²²Rn) have been widely applied as tracers of SGD. Their application requires the appropriate characterization of both the concentrations of tracers in the discharging groundwater and their distribution in the coastal water column. This study evaluates the temporal evolution of Ra isotopes and ²²²Rn concentrations in a dynamic subterranean estuary of a microtidal Mediterranean coastal aquifer that experiences large displacements of the fresh-saltwater interface as a necessary initial step in evaluating the influence of SGD in coastal waters. We show that changes in groundwater salinities due to the seaward displacement of the fresh-saltwater interface produced large variations in Ra activities in groundwater (by a factor of ~19, ~14, ~6, and ~11 for ²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra, respectively), most importantly during rainfall events. In contrast, the ²²²Rn activities in groundwater oscillated only by a factor of 3 during these rainy periods. The large temporal variability in Ra activities hampers the characterization of the SGD end-member when using Ra isotopes as tracers, and thus presents a challenge for obtaining accurate SGD estimates. This study emphasizes the need to understand the hydrodynamics of coastal aquifers to appropriately constrain the Ra isotopes and ²²²Rn concentrations in groundwater and when applying both tracers in dynamic microtidal coastal systems.

A Permanent Multilevel Monitoring and Sampling System in the Coastal Groundwater Mixing Zones

To study the spatial and temporal variability of water dynamics and chemical reactions within the coastal groundwater mixing zones (CGMZs), high-resolution periodical and spatial groundwater sampling within CGMZs is needed. However, current samplers and sampling systems may require heavy driving machines to install. There is also possible contamination from the metal materials for current samplers and sampling systems. Here, a permanent multilevel sampling system is designed to sample coastal groundwater within CGMZs. This cost-effective system consists of metal-free materials and can be installed easily. The system is tested in Po Sam Pai and Tingkok, Tolo Harbor and Hong Kong. Major ions, nutrients, stable isotopes and radium and radon isotopes were analyzed and the data provided scientific information to study the fresh-saltwater interface fluctuations, and temporal variations and spatial heterogeneity of geochemical processes occurred within CGMZs. The reliable spatial and temporal data from the sampling system demonstrate that the system functions well and can provide scientific data for coastal aquifer studies.

Submarine groundwater discharge and nutrient loadings in Tolo Harbor, Hong Kong using multiple geotracer-based models, and their implications of red tide outbreaks

Multiple tracers, including radium quartet, (222)Rn and silica are used to quantify submarine groundwater discharge (SGD) into Tolo Harbor, Hong Kong in 2005 and 2011. Five geotracer models based on the end member model of (228)Ra and salinity and mass balance models of (226)Ra, (228)Ra, (222)Rn, and silica were established and all the models lead to an estimate of the SGD rate of the same order of magnitude. In 2005 and 2011, respectively, the averaged SGD based on these models is estimated to be ≈ 5.42 cm d(-1) and ≈2.66 cm d(-1), the SGD derived DIN loadings to be 3.5 × 10(5) mol d(-1) and 1.5 × 10(5) mol d(-1), and DIP loadings to be 6.2 × 10(3) mol d(-1) and 1.1 × 10(3) mol d(-1). Groundwater borne nutrients are 1-2 orders of magnitude larger than other nutrient sources and the interannual variation of nutrient concentration in the embayment is more influenced by the SGD derived loadings. Annual DIP concentrations in the harbor water is positively correlated with the precipitation and annual mean tidal range, and negatively correlated with evapotranspiration from 2000 to 2013. Climatologically driven SGD variability alters the SGD derived DIP loadings in this phosphate limited environment and may be the causative factor of interannual variability of red tide outbreaks from 2000 to 2013. Finally, a conceptual model is proposed to characterize the response of red tide outbreaks to climatological factors linked by SGD. The findings from this study shed light on the prediction of red tide outbreaks and coastal management of Tolo Harbor and similar coastal embayments elsewhere.

Estimating the input of submarine groundwater discharge (SGD) and SGD-derived nutrients in Geoje Bay, Korea using (222)Rn-Si mass balance model

In order to evaluate the main source of nutrients for maintaining the high production in shellfish farming bay, we have measured (222)Rn activities and the concentrations of nutrients in stream water, seawater, and coastal groundwater around Geoje Bay, one of the largest cultivation areas of oyster in the southern sea of Korea in April 2013. Using the (222)Rn and Si mass balance model, the residence time of bay seawater was about 5days and the submarine groundwater discharge (SGD) into the bay was estimated to be approximately 1.8×10(6)m(3) d(-1). The SGD-derived nutrient fluxes contributed approximately 54% for DIN, 5% for DIP, and 50% for DSi of total nutrient input entering into the bay. Thus, our results suggest that SGD is the major source of nutrients in Geoje Bay, and SGD-derived nutrients are very important to support the biological production of this shellfish farming bay.

Submarine fresh groundwater discharge into Laizhou Bay comparable to the Yellow River flux

Near- and off-shore fresh groundwater resources become increasingly important with the social and economic development in coastal areas. Although large scale (hundreds of km) submarine groundwater discharge (SGD) to the ocean has been shown to be of the same magnitude order as river discharge, submarine fresh groundwater discharge (SFGD) with magnitude comparable to large river discharge is never reported. Here, we proposed a method coupling mass-balance models of water, salt and radium isotopes based on field data of (223)Ra, (226)Ra and salinity to estimate the SFGD, SGD. By applying the method in Laizhou Bay (a water area of ~6000 km(2)), we showed that the SFGD and SGD are 0.57 ~ 0.88 times and 7.35 ~ 8.57 times the annual Yellow River flux in August 2012, respectively. The estimate of SFGD ranges from 4.12 × 10(7) m(3)/d to 6.36 × 10(7) m(3)/d, while SGD ranges from 5.32 × 10(8) m(3)/d to 6.20 × 10(8) m(3)/d. The proportion of the Yellow River input into Laizhou Bay was less than 14% of the total in August 2012. Our method can be used to estimate SFGD in various coastal waters.

Chemical inputs from a karstic submarine groundwater discharge (SGD) into an oligotrophic Mediterranean coastal area

The impacts of nutrient and other chemical inputs released by a submarine groundwater discharge (SGD) on the marine environment of an oligotrophic Mediterranean coastal area (Messiniakos Gulf, SE Ionian Sea) are investigated through a multidisciplinary approach. Nutrients and organic pollutants associated with the SGD are presented to study the chemical characteristics of the SGD and to investigate its effect on the marine ecosystem in comparison to freshwater discharges of the water bodies of Messinia Prefecture. Nutrient and organic pollutant fluxes were calculated from (214)Bi-based SGD estimates. An average of 22×10(3) mol of silicate per month and 8×10(3) mol of nitrate per month were released via the SGD. Nutrient concentrations at the mouth of the SGD were three times higher than in Messiniakos Gulf, and NO3(-) was the primary Dissolved Inorganic Nitrogen form discharged by SGD. Organic pollutant concentrations associated with agricultural activities were low at the SGD. The implementation of a Eutrophication Index (E.I.) showed that the water column at the SGD site corresponds to Moderate/Bad ecological quality, whereas the status switches rapidly to Good at a small distance from the SGD. Coastal areas influenced by river or sewage discharge correspond to a Moderate/Good ecological status. The BENTIX index used for the classification of the ecological quality status of the benthic macroinvertebrate communities showed that the SGD has a minor influence compared to the other freshwater discharges in Messiniakos Gulf. Though the SGD has a considerable outflow, morphology and hydrodynamics of the area favor the rapid dispersion of the upwelling water and degrades the SGD's effect even on a regional scale.

Submarine groundwater discharge estimation in an urbanized embayment in Hong Kong via short-lived radium isotopes and its implication of nutrient loadings and primary production

(224)Ra and (223)Ra are adopted as tracers to qualify submarine groundwater discharge (SGD) in Tolo Harbor, a highly urbanized embayment in Hong Kong. Based on the sampling data, a two-layered radium mass balance model is used to estimate lateral SGD and bottom SGD. Total SGD is estimated to be 1.2-3.0 cm d(-1), including lateral SGD of 5.7-7.9 cm d(-1) and bottom SGD of 0.3-2.0 cm d(-1). Fresh SGD is estimated to be (2.1-5.5) × 10(5)m(3)d(-1). Nutrient fluxes (mold(-1)) from SGD are estimated to be (3-7.4) × 10(4) (dissolved inorganic nitrogen), (2.4-6.2) × 10(2) (dissolved inorganic phosphate) and (6.5-16) × 10(4) (dissolved silicate). Primary productivity is estimated to be (1.5-15) × 10(6)gCd(-1), 2-53% of which is supported by SGD-induced phosphate fluxes. The study indicates that SGD is a significant source of nutrients to coastal waters and may cause an obvious increase of primary production. These findings must be considered in future coastal ecological management.