Submarine Groundwater Discharge Research Articles

Morphological and chemical features of submarine groundwater discharge zones in the south-eastern part of the Baltic Sea

The complex study of the bottom sediments and near-bottom water layer of the Gdansk Deep revealed the Ca maximum anomaly, marking the discharge of the Oxford-Titonian aquifer. The discharge zone is associated with gas-saturated sediments, which is caused by a common pathway of aqueous and gaseous fluids upraise along tectonic faults. The detailed survey of a seabed topography and acoustic survey of the sediments upper layer helped to specify the area of gas-saturated sediments and the size of the pockmarks.

Radium isotope assessment of submarine groundwater discharge and associated nutrient inputs in Eastern Liaodong Bay, China

The accurate assessment of submarine groundwater discharge (SGD) and associated nutrient fluxes plays a significant role in water resources management and ecological environment protection in the coastal area. Currently, Liaodong Bay has become one of the most polluted marine areas in China. However, the nutrient fluxes carried by the SGD and its environmental effects have not yet been thoroughly reported in eastern Liaodong Bay, China. In this study, the distribution of 224Ra/223Ra activity ratio and its influencing factors under the geochemical conditions were explored. The activity of radium was affected by the water type and the geochemical factors. The radium isotopes in seawater were positively correlated with the ratios of Mg2+/Na+ and Ca2+/Na+ but negatively correlated with salinity and SO42−. The average apparent water age was evaluated to be 13.0 days. Furthermore, based on the radium mass balance model, the total average SGD flux was calculated to be 1.31 × 108 m3 day–1. The nutrient fluxes carried by the SGD were 7.16 × 107, 1.01 × 106, 1.61 × 107, 0.92 × 106, and 5.41 × 107 mol day−1 for DIN, DIP, NH4+, NO2−, and NO3−, respectively. The nutrient inputs through the SGD have an average DIN : DIP ratio of 70, which was able to seriously affect the ecological environment. The SGD-derived nutrient fluxes were higher than the local river input, which demonstrated that the SGD was a major source of nutrients affecting regional marine ecosystems.

Submarine Groundwater Discharge in the Northern Bohai Sea, China: Implications for Coastal Carbon Budgets and Buffering Capacity

AbstractSubmarine groundwater discharge (SGD) has been widely recognized as an important source of dissolved nutrients in coastal waters and affects nutrient biogeochemistry. In contrast, little information is available on SGD impacts on coastal carbon budgets. Here, we assessed the SGD and associated carbon (dissolved inorganic carbon [DIC] and total alkalinity [TA]) fluxes in Liaodong Bay (the largest bay of the Bohai Sea, China) and discussed their border implications for coastal DIC budget and buffering capacity. Based on 223Ra and 228Ra mass balance models, the SGD flux was estimated to be (0.92–1.43) × 109 m3 d−1. SGD was the largest contributor of DIC, accounting for 55%–77% of the total DIC sources. The low ratio (<1) of SGD‐derived TA to DIC fluxes and negative correlation between radium isotopes and pH in seawater implied that SGD would potentially reduce seawater pH in Liaodong Bay. Combining the groundwater carbon data in Liaodong Bay with literature data, we found that the SGD‐derived DIC flux off China was 4–9 times greater than those from rivers. By analyzing the TA/DIC ratios in groundwater along the Chinese coast and related carbon fluxes, SGD was thought to partially reduce the CO2 buffer capacity in receiving seawater. These results obtained at the bay scale and national scale suggest that SGD is a significant component of carbon budget and may play a critical role in modulating coastal buffering capacity and atmospheric CO2 sequestration.

Analytical study for wave-induced submarine groundwater discharge in subtidal zone

Both tide and wave are significant coastal driving forces of submarine groundwater discharge (SGD). The hydrodynamic process of tidal SGD is well studied while wave induced part is still challenging to quantify. In this paper, an analytical study was performed to investigate wave-induced SGD in a subtidal aquifer extending horizontally infinitely with impermeable horizontal bottom. The loading effect of seawater weight overlying the seabed was taken into account. The analytical solution was validated by two-dimensional numerical simulations with modeling domains of finite horizontal extension length. Sensitivity analyses of the analytical solution showed that the amplitude of groundwater head fluctuations generally decayed exponentially with the aquifer thickness, and the decay rate decreased with increasing loading efficiency. Loading effect can decrease the head gradients between seawater and porewater. Wave-induced SGD generally increased with the aquifer’s thickness, permeability, wavenumber, wave period, and decreased with the loading efficiency and specific storage of the aquifer. The analytical and simulated results may provide guidance for assessing the wave-induced SGD and shed light on modeling the biogeochemistry process in wave dominated porous seabed environment.

Occurrence of enteric viruses and faecal indicators in submarine groundwater discharges in the coastal environment of the Mexican Caribbean

AbstractThe occurrence of faecal microorganisms such as total coliforms (TCs), Escherichia coli (E. coli), coliphages (somatic and F+ specific) and the genomes of faecal viruses: human adenovirus (HAdV), norovirus (NoV) genogroup I (GI) and II (GII), enterovirus (EV), aichi virus (AiV) and Pepper mild mottle virus (PMMoV), was explored in submarine groundwater discharges (SGDs) in the Mexican Caribbean. Ultra‐filtrated water samples were subjected to nucleic acid extraction and real‐time quantitative polymerase chain reaction (PCR). Results showed the presence of TC bacteria at all sites, whereas the detection of E. coli occurred only during the dry season. Coliphages occurred during the rainy and dry seasons, with concentrations ranging from 10 to 2560 plaque forming units (PFU/100 ml). HAdV and AiV were detected in 50% and 41% of the samples, at concentrations ranging from 102 to 104 genome copies per litre (GC/L) and from 102 to 106 GC/L, respectively. NoV GI and II were detected in 25% and 16% of the samples, at concentrations of 101 GC/L and 102–103 GC/L, respectively. PMMoV was detected in 50% of the samples at concentrations ranging from 101 to 103 GC/L. The EV genomes were not detected. These findings demonstrate that faecal microorganisms can be transported through SGDs in the Mexican Caribbean, potentially contributing to human health risks for recreation.

The effects of subterranean estuary dynamics on nutrient resource ratio availability to microphytobenthos in a coastal lagoon

Causal links between subterranean estuary (STE) dynamics, their climatological drivers, and the ecology of coastal ecosystems have remained elusive. Yet, establishing these connections is essential for fully integrated management of coastal ecosystems. We test, in a semi-arid climate, whether the composition of submarine groundwater discharge (SGD) to a lagoon can be regulated by the annual oscillation of the local STE, itself driven by groundwater recharge variability. We study STE outflow samples gathered monthly for a year in the Ria Formosa lagoon, examining the temporal dynamics of salinity, EH, pH (Total scale), dissolved oxygen and nutrient (PO43−, NO2−, NO3−, NH4+, and Si(OH)4) concentrations under the local hydrological regime. The objectives were threefold: (1) to determine the annual variability of nutrient content and N:P:Si stoichiometry in SGD into the lagoon; (2) to identify the main drivers of variability in SGD composition and stoichiometry and their interactive effects; (3) to discuss links to, and implications for, ecosystem function that could help define expectations of cause-effect relationships and be useful for environmental management of the lagoon and similar systems elsewhere.We find that the terrestrial groundwater recharge cycle drives the expansion and contraction of the subterranean estuary on annual timescales, causing the pH of SGD to fluctuate in opposition to continental groundwater level. The annual dynamics of the STE and the resulting pH oscillation determine the annual variability of nutrient composition ratio in SGD and shape benthic primary production dynamics. When saltwater intrusion occurs, the pH within SGD increases, enhancing nitrification and desorption of exchangeable phosphorus, while silicate fluxes increase with seawater retreat. The result is that nutrient resource ratio availability for coastal primary production depends on the fresh groundwater level. This implies that ecosystem function in such systems is more tightly related to the dynamics of linked groundwater reservoirs than previously thought.

Impact of submarine groundwater discharge on biogeochemistry and microbial communities in pockmarks

The impact of submarine groundwater discharge (SGD) on coastal sea biogeochemistry has been demonstrated in many recent studies. However, only a few studies have integrated biogeochemical and microbiological analyses, especially at sites with pockmarks of different degrees of groundwater influence. This study investigated biogeochemical processes and microbial community structure in sediment cores from three pockmarks in Hanko, Finland, in the northern Baltic Sea. Pockmark data were supplemented by groundwater and seawater measurements. Two active pockmarks showed SGD rates of 0.02 cm d−1 and 0.31 cm d−1, respectively, based on porewater Cl− profiles, while a third pockmark had no SGD influence. Reactive transport modelling (RTM) established that the porewater systems of these active pockmarks are dominated by advection, resulting in the focusing of biogeochemical reactions and the microbial community into a thin zone at the sediment surface. The advection further reduces the accumulation of organic matter in the surface sediments, resulting in the absence of a sulfate-methane transition zone (SMTZ) at these pockmarks. Furthermore, the RTM estimated low rates of consumption of SO42−, and low rates of production of CH4, NH4+, DIC at the active pockmarks. Archaeal communities in the active pockmarks were dominated by ammonia-oxidizing archaea of predominantly groundwater origin. In contrast, at the inactive pockmark, the lack of SGD has permitted rapid deposition of organic-rich mud. The porewater system in the inactive pockmark is dominated by diffusion, leading to orders of magnitude higher metabolite concentrations at depth compared to the active pockmarks. The biogeochemical environment in the inactive pockmark resembles typical organic-rich mud seafloor in the area, with sulphate reduction and methanogenesis dominating organic matter remineralization. Accordingly, methanogens dominate the archaeal community, whereas sulfate reducers dominate the bacterial community. RTM results suggest that sulfate-mediated anaerobic oxidation of methane (S-AOM) also occurs at this site. Although depth-integrated fluxes of SO42−, CH4, NH4, DIC at the inactive pockmark are orders of magnitude higher compared to the active pockmarks, processes at the inactive pockmark represent internal recycling in the coastal sea. Fluxes observed at the active pockmarks, although comparatively small in magnitude, are partly influenced by external inputs to the sea through SGD. Hence, effluxes across the sediment–water interface at these sites partly represent direct external fluxes to the marine environment, in addition to diagenetic recycling at the benthic interface. The study highlights that SGD can result in significant spatial heterogeneity of biogeochemical processes and microbial community structure in the coastal zone, and that the overall effects of SGD and associated solute fluxes at an SGD site are a function of the number of pockmarks, the rate of SGD, and the ratio of active to inactive pockmarks.

Impacts of Coastal Shrimp Ponds on Saltwater Intrusion and Submarine Groundwater Discharge

AbstractShrimp aquaculture has expanded rapidly in coastal zones worldwide over the past few decades. Saline water stored in shrimp farm ponds can infiltrate into the underlying aquifer causing groundwater salinization and increased submarine groundwater discharge (SGD) to coastal water. However, little research has assessed salinization resulting from these shrimp ponds. To understand the impacts of shrimp farm irrigation on groundwater salinization and SGD, we numerically simulated a series of aquaculture management scenarios in a two‐dimensional conceptual coastal aquifer using a coupled surface‐subsurface approach. We characterized sensitivities to pond water salinity, pond water depth, and farm width. Salinization was assessed by three indicators (salinized area, infiltrated salt mass, and recovery rate), and three SGD indicators were evaluated (fresh SGD, saline SGD, and saltwater circulation rate). Our results show that pond water depth is the primary control on the mass of saltwater infiltration while farm width is the primary control for recovery rate. Pond water salinity and depth affect both fresh and saline SGD. We show that aquaculture is a previously unrecognized mechanism of salinization affecting coastal aquifer vulnerability and SGD. A regional graphical information system analysis shows transformation into aquacultural ponds could introduce considerable SGD variability spatially and temporally. These findings will enable coastal managers to better evaluate groundwater vulnerability in regions with expanding onshore aquaculture and demonstrates the impact of aquaculture on coastal groundwater resources and the need for further study to understand the impact of aquaculture across Asia and the globe.

Physical and biogeochemical controls of the carbonate system of the Yucatan Shelf

The dynamics and distribution of the carbonate system and the processes that regulate it over the Yucatan Shelf (YS), a region of karst geology and high productivity that is influenced by submarine groundwater discharges (SGDs) and upwelling, were explored with data from two oceanographic cruises. The first oceanographic cruise was conducted in November 2015 during the Nortes season, a period of intense northerly wind activity, and a second cruise was conducted during the rainy season in August/September 2016. Notable biogeochemical differences were present between them. At the surface, Caribbean Surface Water (CSW) predominated over the shelf in both periods. During the Nortes cruise, a surface nearshore-offshore gradient showed high dissolved inorganic carbon (DIC; ∼2470 μmol kg−1) and total alkalinity (TA; ∼2460 μmol kg−1) values near the coast and average pHTotal and pCO2 values of 7.42 ± 0.10 and 2206 ± 546 μatm, respectively. These geochemical characteristics were attributed to the influence of SGDs, punctuated by relatively low δ13CDIC values between −4.18‰ and −2.49‰, which reflects an important oxidation of organic carbon and the dissolution of carbonate minerals. The presence of upwelled water on the eastern side of the YS showed average DIC and pHTotal values of 2260 ± 15 μmol kg−1 and 7.69 ± 0.08, respectively, which were lower than coastal values. During the rainy cruise, the advection of CSW by the Yucatan Current was traced by its thermohaline properties. However, the surface water carbonate system was relatively homogeneous, with average DIC, TA, pHTotal, and pCO2 values of 2047 ± 16 μmol kg−1, 2388 ± 11 μmol kg−1, 8.02 ± 0.02, and 440 ± 27 μatm, respectively. Lastly, the δ13CDIC values during this cruise ranged from −1.21‰ to 1.25‰, which suggests that the carbonate system is mainly regulated by organic matter production and respiration.

Tracing impact of El Niño Southern Oscillation on coastal hydrology using coral 87Sr/86Sr record from Lakshadweep, South-Eastern Arabian Sea

El Niño Southern Oscillation (ENSO) is one of the dominant climate modes influencing global precipitation and temperature. ENSO has a large impact on the monsoonal precipitations over the Indian subcontinent and thereby influences hydrological conditions. Due to dearth of long-term instrumental records of the hydrological parameters on sufficient spatial resolution, it is difficult to assess the impact of ENSO on regional hydrology. Though several geochemical proxies have been used to reconstruct past ENSO events through tracing the changes in past hydrological and climatic parameters, however, such reconstructions are often complicated by the influence of multiple processes and/or factors and their nonlinear relation with the proxy records. In this study, Sr isotope composition (87Sr/86Sr) was measured in Porites coral from the Lakshadweep, south-eastern Arabian Sea to reconstruct past ENSO events and to trace its regional hydrological impacts. The high precision measurements of 87Sr/86Sr in Lakshadweep coral show resolvable variations ranging from 0.709080 to 0.709210. The 87Sr/86Sr record shows an inverse relation with Niño 3.4 record; lower values matched with El Niño years and higher values with La Niña years. Our investigation reveals that ENSO driven precipitation changes impacted submarine groundwater discharge (SGD) to the Minicoy Atoll and resulted in 87Sr/86Sr variations of the Minicoy Atoll water. Therefore, deviation from the average seawater 87Sr/86Sr value can be quantified in terms of SGD contribution to the Minicoy Atoll. Our estimates based on binary mixing between seawater and SGD 87Sr/86Sr suggest a significant supply of SGD, maximum up to ~20 % of the total volume of the Minicoy Atoll during La Niña years due to higher rainfall compared to El Niño years. This finding highlights potential application of coral 87Sr/86Sr record as an alternate proxy to reconstruct past ENSO events and to trace its quantitative impact on regional hydrology, chemical and nutrient fluxes to coastal oceans via SGD.

Submarine groundwater discharge detection through remote sensing: An application of Landsat 7 and 8 in Hawaiʻi and Ireland

Submarine groundwater discharge (SGD) represents an important component of the global water cycle. Improved assessment of SGD occurrences provides information to constrain current estimates of total freshwater discharges, which are crucial for evaluation of changes in the global water cycle.This study leverages SGD linkages to patterns in sea surface temperature (SST) and color. Typically, groundwater temperatures exhibit less variability than SST and tend to have temperature distinct from sea water. Consequently, thermally anomalous plumes often occur in coastal areas with SGD. Nutrient-enriched groundwater flows have been linked to changes in ocean color as a consequence of algae growth and of the presence of colored dissolved organic matter. Under such conditions, SGD presence may also manifest as plumes of different colors. Changes in SST and color can be captured through satellite imagery for the thermal infrared and visible bands, which are used here to detect potential SGD (PSGD) occurrences. A method based on clustering and spectroscopy techniques is applied to automatically detect PSGD plumes. SST data are clustered to map SST anomalies. Derivative analysis and angular distance are applied to identify a color signature linked to SGD. In a novel approach, SST and color are combined to identify PSGD through remote sensing.The methods presented in this study were applied to Landsat 7 and 8 imagery captured over southern Ireland and the west coast of the Island of Hawaiʻi, where past field surveys of radon isotopes activities and salinity have documented SGD occurrences. These selected sites are used as validation points. Derivative analysis and angular distance resulted in PSGD plumes in the vicinity of sampled locations, mainly locations with samples that are strong indicators of SGD. In general, the PSGD plumes derived from each method are compared as independent, parallel lines of evidence of PSGD occurrences. Results also show that PSGD pixels from derivative analysis and angular distance alone, without considering SST, can be obfuscated by confounding factors, such as low tides and unrelated chlorophyll enrichment, and that combining SST and color data reduces those effects. These results provide further evidence for PSGD occurrences along the south coast of Ireland and the west coast of the Island of Hawaiʻi, and showcase that satellite data can be used to better identify the location of PSGD occurrences over large areas.

Geochemical ratios mediated understanding of estuarine dynamics in submarine groundwater discharge prevalent basaltic aquifer

Submarine groundwater discharge (SGD) has been recognized as an integral pathway of hydrological cycle. The role of SGD as a mechanism for material transport from terrestrial to marine ecosystems has also been investigated, considering the large hydrological flux and a massive load of solutes is found to be carried as SGD to the coastal oceans. However, there are challenges in recognizing the process and delineating specific areas which are susceptible to SGD. Considering the unique geochemical signature of groundwaters found in varied lithology, this paper investigates the possibility of using geochemical ratios and their variability in coastal areas as a tracer for the identification of SGD. The Br−/Cl− ratio, prevalently used for identification of seawater-groundwater mixing is investigated. The study raises questions over the viability of Br−/Cl− as a tracer as Br− rich groundwater in the SGD prevalent area may be a cause of elevated Br−/Cl− ratio in the coastal oceans. Furthermore, the peculiar observation of Fe+2 enrichment in coastal seawater also indicated the influence of differential weathering on the subterranean estuarine (STE) sediments releasing Fe+2 as the groundwater in the area is not enriched in groundwater. This observation is very important in the context of rising sea levels as a larger STE due to rising sea levels can cause higher mobilization of Fe+2 in coastal oceans causing a direct influence on the coastal ecosystem.

Distribution of 222Rn in Seawater Intrusion Area and Its Implications on Tracing Submarine Groundwater Discharge on the Upper Gulf of Thailand

Abstract Radon (222Rn) has been widely employed as a tracer for estimating submarine groundwater discharge (SGD). However, the uncertainty of the SGD estimation remains significant, due to the spatial variability of radon in groundwater. In this study, we analyzed the hydrochemical proprieties of seawater and coastal groundwater in the Upper Gulf of Thailand and discussed the distribution characteristics of 222Rn in aquifers in terms of aquifer lithology, groundwater system recharge conditions, and water retention time. The results suggested that the residence time of groundwater and the process of groundwater salinization have the greatest impact on the distribution of 222Rn activity. A 222Rn mass balance model, synthesizing the distribution characteristics of 222Rn in groundwater and tidal influences on SGD, was built to estimate the submarine groundwater discharge in the Upper Gulf of Thailand. The result showed that the SGD flux of the Upper Gulf of Thailand was 0.0203 m/d. Moreover, there is a positive correlation between tidal height and the activity of 222Rn in groundwater. The SGD observed during the low tide was about 1.25 times higher than that observed during the high tide. This may influence the marine geochemical cycles of elements and their impact on marine ecosystems.

Closing the Global Marine 226Ra Budget Reveals the Biological Pump as a Dominant Removal Flux in the Upper Ocean

AbstractRadium isotopes are powerful proxies in oceanography and hydrology. Radium mass balance models, including assessments of submarine groundwater discharge (SGD), often overlook particle scavenging (PS) as a pathway for dissolved radium removal from the world ocean. Here, we build a global ocean 226Ra mass balance model and reevaluate the potential importance of PS. We find that PS is the major 226Ra sink for the upper ocean, removing about 96% of the total input from various sources. Aside from vertical exchange with the lower ocean, SGD is the largest 226Ra source into the upper ocean. The biological pump transfers particles to the deep ocean, resulting in a major but often overlooked impact on the global 226Ra marine budget. Our findings suggest that radium mass balance models should consider PS in systems with high siliceous algae production and export fluxes and long water residence times to prevent underestimation of large‐scale SGD fluxes.

Open-Source Code for Radium-Derived Ocean-Groundwater Modeling: Project Open RaDOM

Radium has been commonly used as a tracer of submarine groundwater discharge to the ocean and embankments, as radium activities are commonly input into box models to calculate a groundwater flux. Similarly, isotopes of radium (Ra224, Ra223, Ra226, Ra228) have been used to calculate water mass ages, which have been used as a proxy for residence times. Less commonly, radium and other tracers have been utilized in mixing models to determine the relative contribution of groundwater to a marine system. In the literature, all of these methods have almost exclusively been solved using analytical methods prone to large errors and other issues. Project Open RaDOM, introduced here, is a collection of open-source R scripts that numerically solve for groundwater flux, residence time, and relative contribution of groundwater to coastal systems. Solving these models numerically allows for over-constrained systems to increase their accuracy and force real solutions. The scripts are written in a way to make them user-friendly, even to scientists unfamiliar with R. This communication includes a description of the scripts in Project Open RaDOM, a discussion of examples in the literature, and case studies of the scripts using previously published data.

Calibration and Verification of a Hydrodynamic Model for a Narrow Estuary Receiving Submarine Groundwater Discharges

This paper presents calibration and verification of the laterally averaged model for estuaries (LAMFE) for the Chassahowitzka River, a spring-fed estuary located on the Gulf coast of central Florida, USA. The river is a narrow and complex riverine system that receives submarine groundwater discharges (SGDs), which are difficult to measure. This study tried to make a reasonable estimate of some unquantified or missing SGD data with a trial-and-error approach in the model calibration process. The model was successfully calibrated against measured real-time data of water level, salinity, and temperature at three locations within the simulation domain from November 2012 to December 2015, before it was verified for the period from January 2016 to March 2017. Skill assessment of the model performance included visual comparisons and statistical quantifications of the difference between measured real-time data and model results. Both have shown that the LAMFE model performs well in simulating hydrodynamics, salinity transport, and thermal dynamics in the estuary. The good agreement of model results with field data suggests that the estimation of unquantified or missing SGDs in the spring-fed estuary is reasonable.

Submarine groundwater discharge and associated nutrient influx in surroundings of the estuary region at Gulf of Mannar coast, Indian Ocean

The influx of fresh groundwater and re-circulated sea water into coastal ecosystem occurs through the submarine groundwater discharge (SGD). Measurement of salinity, radium tracers (224Ra, and 226Ra isotopes) and nutrients in estuarine water, coastal surface water and groundwater during December 2019 estimated the SGD and associated nutrient fluxes near the Karameniyar estuary (Gulf of Mannar) and surroundings of the Manapad region at southern part of Tamil Nadu state in India. The presence of excessive radium tracers revealed that the SGD was contributing to Ra desorption from the sediments and enrichment in the coastal waters. We estimated SGD of approximately 0.03–0.59 m3 m−2 d−1 for the Manapad region and relatively more homogeneous but comparatively less values in the Karameniyar estuary (0.03–0.34 m3 m−2 d−1). Higher average values of dissolved inorganic nitrogen (DIN; 43.62 μmol L−1) and soluble reactive phosphate (SRP; 1.848 μmol L−1) suggested greater influence of SGD on the overall coastal water nutrient budget. This study also indicated simultaneous occurrence of fresh and saline SGD in this region.

Geophysical and geochemical prospecting for submarine groundwater discharge in a karst leaky aquifer from Valliyur Hills to Uvari Beach

Geophysical and geochemical prospecting for submarine groundwater discharge in a karst leaky aquifer from Valliyur Hills to Uvari Beach

CrAssphage as an indicator of groundwater-borne pollution in coastal ecosystems

Novel approaches for monitoring coastal water quality changes and identifying associated contaminant source(s) are of growing importance as climate change and population redistribution to coastal zones continue to impact coastal systems. CrAssphage, a virus found in the human gut and shed with fecal matter, is currently gaining popularity as an indicator of human fecal contamination in surface water and groundwater. Here we demonstrate that DNA assays targeting crAssphage genetic fragments can be used to detect pollution from nearshore onsite wastewater treatment systems discharging to the ocean via submarine groundwater discharge. We integrated this novel viral monitoring tool into a field study that characterized the physical hydrogeology (hydraulic gradients, hydraulic conductivity, and seepage fluxes) and surface water and groundwater quality at a study site on the north shore of Nova Scotia, Canada. Increased use of onsite wastewater treatment systems during the summer cottage season coincided with widespread detections of crAssphage in submarine groundwater discharge (4/4 samples) and coastal surface waters (3/8 samples). Conversely, classical fecal pollution indicators based on bacterial targets (Escherichia coli and human-specific Bacteroidales genetic marker (HF183)) were sparsely detected in the samples in the coastal environment (2/12 E. coli samples, 0/12 HF183 samples), likely due to greater attenuation of bacterial contaminants within the subsurface environments. Results from this first application of crAssphage in coastal groundwater contribute to a growing body of research reporting the application of this emerging tracer in various environments impacted by sewage pollution sources.

Radium-Derived Water Mixing and Associated Nutrient in the Northern South China Sea

Nutrients play an important role as biogenic elements in modulating marine productivity, and water mixing usually facilitates the transportation of nutrients in the coastal ocean. In this study, the distributions of naturally occurring radioisotopes 226Ra and 228Ra in the surface and water column of the northern South China Sea (NSCS) have been investigated to estimate oceanic mixing and nutrient supplies. We identified three masses of the South China Sea Warm Current (SCSWC), the South China Sea Branch of the Kuroshio (SCSBK), and shelf water in the summer of June 2015, but only SCSWC and SCSBK were observed in the spring of March 2017. The fraction of the SCSBK in summer was estimated to be an average of 0.25 ± 0.16, which was lower than that in the spring of 0.57 ± 0.32 in our study area. The horizontal mixing from the Pearl River plume revealed eddy diffusion of (1.2 ± 0.79) × 105 cm2/s and advection velocity ω of 0.25 ± 0.16 cm/s in the slope region. In the water column, the best-fit exponential curve gradient of 228Ra led to a vertical diffusion coefficient of 0.43 ± 0.33 cm2/s that went down to the subsurface of the upper 1,000 m, and an upward vertical diffusion coefficient was revealed as 18 ± 9.9 cm2/s from the near-bottom. Combining the nutrient distributions, horizontal mixing from the Pearl River plume carried (5.6 ± 4.9) × 102 mmol N/m2/d, 2.2 ± 2.0 mmol P/m2/d, and (4.1 ± 3.9) × 102 mmol Si/m2/d in the very surface layer, suggesting that shelf water plays a significant role in the nutrient sources of the slope of the NSCS during June 2015. The upward vertical mixing supplied 2.7 ± 1.6 mmol N/m2/d, 0.18 ± 0.11 mmol P/m2/d, and 15 ± 8.4 mmol Si/m2/d to the upper layer, which appeared more important than atmospheric deposition and rivaled submarine groundwater discharge.