Controls on alkalinity biogeochemistry in a coastal aquifer-aquitard system.

Inorganic carbon dynamics in coastal marine systems

High-nitrogen loadings of rivers and aquifers systems are a major concern because of potential effects on human health and water quality impacts such as eutrophication of lakes and coastal zones. This nitrogen enrichment is commonly attributed to anthropogenic sources such as sewage and agricultural and industrial wastes. The aims of this study were to delineate spatial distribution of groundwater ammonium in the coastal aquifer system in Pearl River Delta (PRD), China and to identify the origin of the abnormally high ammonium. A total of 40 boreholes were drilled to collect core samples of the aquitard and groundwater samples in the basal aquifer. The core samples were used for extraction of pore water for centrifugation and bulk chemical analyses in laboratory. Unlike previous studies which focused mainly on the aquifer, this study treated the aquifer-aquitard system as a hydrogeochemical continuum. The results show that the aquifer-aquitard system contains an exceptionally large total ammonium mass. Ammonium occurred at concentrations up to 390 mg/L in the basal sand Pleistocene aquifer 20-50 m deep, the largest concentration reported for groundwater globally. This ammonium was natural, areally extensive (1600 km(2)) and originated in the overlying Holocene-Pleistocene aquitard and entered the aquifer by groundwater transport and diffusion. Total ammonium in the aquifer (190 × 10(6) kg) was exceeded by total ammonium in the aquitard (8600 × 10(6) kg) by a factor of 45. Much organic nitrogen remained in the aquitard available for conversion to ammonium. This natural ammonium in the aquifer was slowly transported into the PRD river channels and the estuary of the South China Sea. The rate of this contribution will likely be greatly increased by sand dredging in the river channels and estuary. Although the ammonium in PRD groundwater occurred in the largest concentrations and mass reported globally, the literature shows no reports of other delta aquitards having been examined for ammonium occurrence and therefore abundant ammonium formed in aquitards rich in organic matter may not be uncommon and this "geologic" source of ammonium may present a large and hitherto unappreciated source of nitrogen discharging to surface waters.

An aquitard is not only a confining layer in the aquifer-aquitard system, but also a functional complex for nutrient reservation and microorganism inhabitation. Geochemical characteristics and geomicrobial processes in aquitards play an important role in groundwater quality, and should be well studied. The Pearl River Delta is dominated by clay-rich aquitards and was intensively influenced by Holocene transgression. In this thesis, the aquifer-aquitard system in the PRD was studied from various perspectives including geophysics, geochemistry and geomicrobiology. Sediment and groundwater samples were taken from representative locations in the PRD at different lithological units. Particle size distribution, computed axial tomography, optically stimulated luminescence dating and scanning electron microscopy were analyzed to understand the geophysical properties. Concentrations of major ions and abundances of environmental isotopes were measured for geochemical analysis. Bacterial 16S rRNA gene clone libraries were constructed to analyze microbial identifications and community structures in different strata. Gene abundances of anammox 16S and bacteria amoA in both sediment and groundwater samples were quantitatively analyzed with 15N isotope at the same depth.
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\nThe mixing process of seawater and river water was the dominant factor controlling the isotopic and chemical characterizes of groundwater in the system. Isotopic fractionation in the aquitards was majorly controlled by chemical and biological reactions other than diffusion. Chemical pattern in this system was mainly controlled by topography and sea level in the Holocene. Independent sedimentary centers of strong reducing environment were located in the depressions controlled by fault zones, and generated massive amount of pyrite minerals in the sediment and NH4+ in the groundwater. The sea level and depositional environment in the Holocene determined the physical structures of the sediment and seawater/river-water ratio in the estuary. Isotopic research also demonstrated that South Asia Monsoon was the major source of atmospheric precipitation in the PRD.
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\nBacterial species in the PRD sediment were identified. Canonical correspondence analysis between bacterial linages and environment factors showed that community structures were significantly modified by geological conditions. High bacterial diversity was observed in samples from the Holocene aquitard M1 and aquifer T1, while in the older aquitard M2 and basal aquifer T2, the bacterial diversity was much lower. Chloroflexi, γ-proteobacteria and δ-proteobacteria were the dominant phyla in the aquitard sediment. β-proteobacteria was the dominant phylum in sediment which was strongly influenced by fresh water. Anammox was the controlling biochemical process in microbial-derived ammonium loss, as demonstrated by gene abundance coupling with 15N isotope and ammonium concentration. The 16S gene abundance of anammox bacteria ranged from approximately 105 to 106 copies/g in the aquitard sediment, and ranged from 104 to 105 copies/g in aquifers. amoA gene abundance was 1-2 orders lower than anammox bacteria 16S in aquitard sediment, but in aquifers, the gene abundances of amoA and anammox 16S were similar. Interface between aquifer and aquitard was demonstrated as biochemically enhanced zone. The results of this study significantly benefited the understanding of geochemistry and microbiology in the aquifer-aquitard system, and showed directions for future work on geomicrobiology in aquitards.

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Carbon, nitrogen and sulfur isotopic features and the associated geochemical processes in a coastal aquifer system of the Pearl River Delta, China

Impact of salinity and carbonate saturation on stable Sr isotopes (δ88/86Sr) in a lagoon-estuarine system

A 1D numerical model is constructed to investigate the impact of sedimentation and sea level changes on transport of Cl− in the aquifer–aquitard system in the Pearl River Delta (PRD), China. The model simulates the evolution of the vertical Cl− concentration profiles during the Holocene. Sedimentation is modeled as a moving boundary problem. Chloride concentration profiles are reconstructed for nine boreholes, covering a wide area of the PRD, from northwest to southeast. Satisfactory agreement is obtained between simulated and measured Cl− concentration profiles. Diffusion solely is adequate to reproduce the vertical Cl− concentration profiles, which indicates that diffusion is the regionally dominant vertical transport mechanism across the aquitards in the PRD. The estimated effective diffusion coefficients of the aquitards range from 2.0 × 10–11 to 2.0 × 10–10 m2/s. The effective diffusion coefficients of the aquifers range from 3.0 × 10–11 to 4.0 × 10–10 m2/s. Advective transport tends to underestimate Cl− concentrations in the aquitard and overestimate Cl− concentrations in the basal aquifer. The results of this study will help understand the mechanisms of solute transport in the PRD and other deltas with similar geological and hydrogeological characteristics.

The stable isotopes of dissolved organic carbon (DOC) and inorganic carbon (DIC) provide insights into the carbon cycle, biogeochemical processes, and the fate of redox-sensitive elements in groundwater systems. The simultaneous determination of the stable isotope ratios (δ(13)C(DIC) and δ(13)C(DOC) values) in DIC and DOC in water samples would provide better understanding of those processes. The conditions for pretreating water samples prior to determining their DIC and DOC stable isotope ratios were optimized with a series of experiments on pre-purging (GasBench needle versus blowing concentrator) and reaction conditions. The carbon stable isotope ratios were determined by isotope ratio mass spectrometry. Sequential determination of the δ(13)C(DIC) and δ(13)C(DOC) values was also carried out using the optimized conditions. National Institute of Standards and Technology (NIST) δ(13)C standards were utilized to verify the precision of the proposed method. The optimized pretreatment conditions for DIC isotope analysis involved pre-purging the empty sample bottle for 60 min using a pressurized helium gas-blowing concentrator and then reacting the sample with 85% H3PO4 for 60 min in a water bath at 60°C. The optimized pretreatment conditions for DOC isotope determination involved pre-purging the sample for 60 min with the pressurized helium gas-blowing concentrator after adding 0.1 mol L(-1) AgNO3 and 85% H3PO4 and then reacting with Na2S2O8 as an oxidant and AgNO3 as a catalyst, in a 100°C water bath for 60 min. With the optimized methods, the analytical precision of the δ(13)C(DIC) and δ(13)C(DOC) values of the reference samples was 0.02‰. The precision of δ(13)C(DIC) and δ(13)C(DOC) values in groundwater samples was 0.03‰ and 0.17‰, respectively. For the sequential determination of the DIC and DOC carbon isotope ratios, the precision of the δ(13)C(DIC) and δ(13)C(DOC) values in reference samples was better than 0.2‰.

Impacts of an extreme Changjiang flood on variations in carbon cycle components in the Changjiang Estuary and adjacent East China sea

Abstract. Coastal and estuarine waters of the northern California Current system and southern Salish Sea host an observational network capable of characterizing biogeochemical dynamics related to ocean acidification, hypoxia, and marine heatwaves. Here, we compiled data sets from a set of cruises conducted in estuarine waters of Puget Sound (southern Salish Sea) and its boundary waters (Strait of Juan de Fuca and Washington coast). This data product provides data from a decade of cruises with consistent formatting, extended data quality control, and multiple units for parameters such as oxygen with different end use needs and conventions. All cruises obtained high-quality temperature, salinity, inorganic carbon, nutrient, and oxygen observations to provide insight into the dynamic distribution of physical and biogeochemical conditions in this large urban estuary complex on the west coast of North America. At all sampling stations, conductivity–temperature–depth (CTD) casts included sensors for measuring temperature, conductivity, pressure, and oxygen concentrations. Laboratory analyses of discrete water samples collected at all stations throughout the water column in Niskin bottles provided measurements of dissolved inorganic carbon (DIC), dissolved oxygen, nutrient (nitrate, nitrite, ammonium, phosphate, and silicate), and total alkalinity (TA) content. This data product includes observations from 35 research cruises, including 715 oceanographic profiles, with >7490 sensor measurements of temperature, salinity, and oxygen; ≥6070 measurements of discrete oxygen and nutrient samples; and ≥4462 measurements of inorganic carbon variables (i.e., DIC and TA). The observations comprising this cruise compilation collectively characterize the spatial and temporal variability in a region with large dynamic ranges of the physical (temperature = 6.0–21.8 ∘C, salinity = 15.6–34.0) and biogeochemical (oxygen = 12–481 µmol kg−1, dissolved inorganic carbon = 1074–2362 µmol kg−1, total alkalinity = 1274–2296 µmol kg−1) parameters central to understanding ocean acidification and hypoxia in this productive estuary system with numerous interacting human impacts on its ecosystems. All observations conform to the climate-quality observing guidelines of the Global Ocean Acidification Observing Network, the US National Oceanic and Atmospheric Administration's Ocean Acidification Program, and ocean carbon community best practices. This ongoing cruise time series supports the estuarine and coastal monitoring and research objectives of the Washington Ocean Acidification Center and US National Oceanic and Atmospheric Administration (NOAA) Ocean and Atmospheric Research programs, and it provides diverse end users with the information needed to frame biological impacts research, validate numerical models, inform state and tribal water quality and fisheries management, and support decision-makers. All 2008–2018 cruise time-series measurements used in this publication are available at https://doi.org/10.25921/zgk5-ep63 (Alin et al., 2022).

Vertical leaching of paleo-saltwater in a coastal aquifer–aquitard system of the Pearl River Delta

Human activities have disrupted the global carbon cycle, reducing carbon dioxide (CO₂) uptake by tidal wetlands and submerged vegetation. This exacerbates climate challenges, including rising temperatures and ocean acidification. Coastal systems such as mangroves and seagrasses serve as key carbon sinks, promising for CO₂ removal (CDR). Growing attention is being given to bivalves, whose calcification and reef-building activities shape coastal carbon dynamics. Most studies reduce bivalve impacts to a balance between individual CO₂ emissions and the carbon stored in their shells and tissues, often overlooking species interactions-such as symbioses-that may modulate carbon fluxes. Here, we examined the mussel-symbiont holobiont using Mytilus edulis under emersion in a controlled chamber to quantify CO₂ exchange. Mussels hosting cyanobacterial symbionts exhibited net atmospheric CO₂ uptake during daily air exposure, a critical phase of the tidal cycle. To evaluate the potential significance at larger ecological scales, we combined laboratory-derived CO₂ uptake data with field-based estimates of symbiont prevalence to model carbon fluxes at the mussel bed scale and compared them with values of established blue carbon systems. This research highlights the importance of species interactions in coastal carbon cycling and underscores the need to incorporate the mussel-symbiont holobiont into CDR models.

Dissolved organic matter in surface sediments along a river-to-ocean continuum: Molecular characteristics and sediment-water exchange dynamics.

<strong class="journal-contentHeaderColor">Abstract.</strong> Coastal and estuarine waters of the northern California Current System and southern Salish Sea host an observational network capable of characterizing biogeochemical dynamics related to ocean acidification, hypoxia, and marine heatwaves. Here we compiled data sets from a set of cruises conducted in estuarine waters of Puget Sound (southern Salish Sea) and its boundary waters (Strait of Juan de Fuca and Washington coast). This data product provides data from a decade of cruises with consistent formatting, extended data quality control, and multiple units for parameters such as oxygen with different end use needs and conventions. All cruises obtained high-quality temperature, salinity, inorganic carbon, nutrient, and oxygen observations to provide insight into the dynamic distribution of physical and biogeochemical conditions in this large urban estuary complex on the west coast of North America. At all sampling stations, CTD casts included sensors for measuring temperature, conductivity, pressure, and oxygen concentrations. Laboratory analyses of discrete water samples collected at all stations throughout the water column in Niskin bottles provided measurements of dissolved inorganic carbon (DIC), dissolved oxygen, nutrient (nitrate, nitrite, ammonium, phosphate, silicate), and total alkalinity (TA) content. This data product includes observations from 35 research cruises, including 715 oceanographic profiles, with &gt; 7490 sensor measurements of temperature, salinity, and oxygen; &ge; 6070 measurements of discrete oxygen and nutrient samples; and &ge; 4462 measurements of inorganic carbon variables (i.e., DIC and TA). The observations comprising this cruise compilation collectively characterize the spatial and temporal variability of a region with large dynamic ranges of the physical (temperature = 6.0&ndash;21.8 &deg;C, salinity = 15.6&ndash;34.0) and biogeochemical parameters (oxygen = 12&ndash;481 &micro;mol kg^&ndash;1, dissolved inorganic carbon = 1074&ndash;2362 &micro;mol kg^&ndash;1, total alkalinity = 1274&ndash;2296 &micro;mol kg^&ndash;1) central to understanding ocean acidification and hypoxia in this productive estuary system with numerous interacting human impacts on its ecosystems. All observations conform to the climate-quality observing guidelines of the Global Ocean Acidification Observing Network, the U.S. National Oceanic and Atmospheric Administration's Ocean Acidification Program, and ocean carbon community best practices. This on-going cruise time-series supports the estuarine and coastal monitoring and research objectives of the Washington Ocean Acidification Center and U.S. National Oceanic and Atmospheric Administration (NOAA) Ocean and Atmospheric Research programs, and provides diverse end users information needed to frame biological impacts research, validate numerical models, inform state and tribal water quality and fisheries management, and support decision makers. All 2008&ndash;2018 cruise time-series measurements used in this publication are available at <a href="https://doi.org/10.25921/zgk5-ep63" target="_blank" rel="noopener">https://doi.org/10.25921/zgk5-ep63</a> (Alin et al., 2022).

Accretionary prisms are thick layers of sedimentary material piled up at convergent plate boundaries. Large amounts of anaerobic groundwater and methane (CH4) are contained in the deep aquifers associated with accretionary prisms. In order to identify microbial activity and CH4 production processes in the deep aquifers associated with the Cretaceous accretionary prism in Okinawa Island, Japan, we performed geochemical and microbiological studies using anaerobic groundwater and natural gas (mainly CH4) samples collected through four deep wells. Chemical and stable hydrogen and oxygen isotope analyses of groundwater samples indicated that the groundwater samples obtained from each site originated from ancient seawater and a mixture of rainwater and seawater, respectively. Additionally, the chemical and stable carbon isotopic signatures of groundwater and natural gas samples suggested that CH4 in the natural gas samples was of a biogenic origin or a mixture of biogenic and thermogenic origins. Microscopic observations and a 16S rRNA gene analysis targeting microbial communities in groundwater samples revealed the predominance of dihydrogen (H2)-producing fermentative bacteria and H2-utilizing methanogenic archaea. Moreover, anaerobic cultures using groundwater samples suggested a high potential for CH4 production by a syntrophic consortium of H2-producing fermentative bacteria and H2-utilizing methanogenic archaea through the biodegradation of organic substrates. Collectively, our geochemical and microbiological data support the conclusion that the ongoing biodegradation of organic matter widely contributes to CH4 production in the deep aquifers associated with the Cretaceous accretionary prism.