Impact of Submarine Groundwater Discharge on Marine Water Quality and Reef Biota of Maui

Unravelling the signatures of submarine groundwater discharge and seawater intrusion along the coastal plains of Odisha, India: a multi-proxy approach

Submarine Groundwater Discharge (SGD) and Seawater Intrusion (SWI) are two contrary hydrological processes that occur across the land-sea continuum and understanding their nature is essential for management and development of coastal groundwater resource. Present study has attempted to demarcate probable zones of SGD and SWI along highly populated Odisha coastal plains which is water stressed due to indiscriminate-exploitation of groundwater leading to salinization and fresh groundwater loss from the alluvial aquifers. A multi-proxy investigation approach including decadal groundwater level dynamics, LANDSAT derived sea surface temperature (SST) anomalies and in-situ physicochemical analysis (pH, EC, TDS, salinity and temperature) of porewater, groundwater and seawater were used to locate the SGD and SWI sites. A total of 340 samples for four seasons (85 samples i.e., 30 porewater, 30 seawater and 25 groundwater in each season) were collected and their in-situ parameters were measured at every 1-2 km gap along ~ 145 km coastline of central Odisha (excluding the estuarine region). Considering high groundwater EC values (> 3000 μS/cm), three probable SWI and low porewater salinities (< 32 ppt in pre- and < 25 ppt in post-monsoons), four probable SGD zones were identified. The identified zones were validated with observed high positive hydraulic gradient (> 10 m) at SGD and negative hydraulic gradient (< 0 m) at SWI sites along with anomalous SST (colder in pre- and warmer in post-monsoon) near probable SGD locations. This study is first of its kind along the Odisha coast and may act as initial basis for subsequent investigations on fresh-saline interaction along the coastal plains where environmental integrity supports the livelihood of coastal communities and the ecosystem.

The connection between Submarine Groundwater Discharge and seawater quality: The threat of treated wastewater injected into coastal aquifers

Submarine Groundwater Discharge (SGD) delivers nutrients to the coastal sea triggering phytoplankton blooms, eutrophication, and can also serve as a pathway for contaminants. Wastewater treatment plants (WWTP) including injection wells in coastal areas influence coastal aquifers and might impact the composition and magnitude of SGD fluxes. In tourist areas, wastewater treatment may be less efficient and larger in volume during high seasons, potentially impacting nutrient fluxes from SGD and exacerbating environmental impacts. This study analyzes the nutrient transfer from treated wastewater injection in karstic aquifers to the coastal sea via SGD, considering the impacts of tourism seasonality. This study is conducted in Cala Deià, a small cove in the Balearic Islands, a Mediterranean tourist destination. The findings suggest that the seasonality of tourism, leading to variations in the volume of wastewater treated in the WWTP, influences the dynamics of the coastal aquifer. This leads to increased SGD water and nutrient fluxes to the sea in summer, i.e. the peak tourist season. The measured DIN, DIP, and DSi inventories in the cove are much larger in August than in April (3, 10, and 1.5 times higher, respectively) due to higher input of nutrients in summer due to SGD impacted by the WWTP. These elevated nutrient flows can support algal blooms in the cove, compromising water quality for local swimmers and tourists. Indeed, in August, shoreline stations exhibited eutrophic Chl-a concentrations, with peaks reaching approximately 4 mg Chl-a L-1. These elevated levels suggest the presence of an algal bloom during the survey. The anthropogenic origin of SGD-driven nutrients is traced in seawater and seagrass meadows, as evidenced by high ∂15N signatures indicative of polluted areas. Thus, the high pressure exerted on coastal areas by tourism activities increased the magnitude of SGD nutrient fluxes, thereby threatening coastal ecosystems and the services they provide.

Review of Endocrine Disrupting Compounds (EDCs) in China's water environments: Implications for environmental fate, transport and health risks

Endocrine Disrupting Compounds (EDCs) are ubiquitous in soil and water system and have become a great issue of environmental and public health concern since the 1990s. However, the occurrence and mechanism(s) of EDCs' migration and transformation at the watershed scale are poorly understood. A review of EDCs pollution in China's major watersheds (and comparison to other countries) has been carried out to better assess these issues and associated ecological risks, compiling a large amount of data. Comparing the distribution characteristics of EDCs in water environments around the world and analyzing various measures and systems for managing EDCs internationally, the significant insights of the review are: 1) There are significant spatial differences and concentration variations of EDCs in surface water and groundwater in China, yet all regions present non-negligible ecological risks. 2) The hyporheic zone, as a transitional zone of surface water and groundwater interaction, can effectively adsorb and degrade EDCs and prevent the migration of high concentrations of EDCs from surface water to groundwater. This suggests that more attention needs to be paid to the role played by critical zones in water environments, when considering the removal of EDCs in water environments. 3) In China, there is a lack of comprehensive and effective regulations to limit and reduce EDCs generated during human activities and their discharge into the water environment. 4) To prevent the deterioration of surface water and groundwater quality, the monitoring and management of EDCs in water environments should be strengthened in China. This review provides a thorough survey of scientifically valid data and recommendations for the development of policies for the management of EDCs in China's water environment.

Measuring tissue water potential in marine macroalgae via an updated Chardakov method

Regulation of tissue water potential is a key mechanism in macroalgal osmotic responses to changing external osmotic conditions, which are common in tidally influenced estuarine and intertidal systems. Nevertheless, significant knowledge gaps exist in our understanding of osmotic responses in macroalgae because few methods measure osmotic potential within macroalgal tissues. Leaf psychrometers have furthered understanding of osmotic potentials in terrestrial plant water relations, yet these have not been developed to measure the range of highly negative potential values found in marine macroalgae. To address these gaps, we present an effective, updated version of the Chardakov method to measure tissue water potential in macroalgae. Here, we present a case study examining macroalgal response in tissue water potential by two morphologically and evolutionarily distinct species, Ulva lactuca (Chlorophyta) and Hypnea musciformis (Rhodophyta) to four paired salinity and nutrient treatments at two temperatures. These treatments simulate a gradient from full coastal ocean conditions to brackish submarine groundwater discharge, an ecosystem type found on basaltic shorelines. Both algae demonstrated plasticity in osmotic response to submarine groundwater discharge with significant positive correlations between tissue water potential and proportion of submarine groundwater discharge in the treatment. These results are the first to describe macroalgal response in tissue water potential, a first step to understanding algal physiological ecology in such complex coastal environments. This revised Chardakov method is a valuable tool to better understand species-specific osmotic responses to ecologically relevant conditions, and can augment the study of other tidal systems and ontogenetic stages.

Anchialine pool shrimp (Halocaridina rubra) as an indicator of sewage in coastal groundwater ecosystems on the island of Hawai'i

Groundwater is a primary pathway for wastewater and other pollutants to enter coastal ecosystems worldwide. Sewage associated pathogens, pharmaceuticals, and other emerging contaminants pose potential risks to marine life and human health. Anchialine pool ecosystems and the endemic species they support are at risk and provide an opportunity to sample for presence of contaminants prior to diffusion in the marine environment. In this study, we tested the potential use of nitrogen isotopes in the tissues of a dominant anchialine pool grazing shrimp (Halocaridina rubra), as a bioindicator for sewage in groundwater flowing through their habitats. Water quality parameters and shrimp tissue isotopes (N and C) were collected from pools exposed to a range of sewage contamination along the West Hawai'i coastal corridor from 2015 to 2017. Data were used to test for spatial and temporal variability both within and among pools and to examine the relationship between stable isotopes and water quality parameters. Within 22 pools, mean δ15N from whole tissue samples ranged between 2.74‰ and 22.46‰. Variability of isotope values was low within individual pools and within pool clusters. However, δ15N differed significantly between areas and indicated that sewage is entering groundwater in some of the sampled locations. The significant positive relationship between δ15N and dissolved nitrogen (p<0.001, R2 = 0.84) and δ15N and phosphorus (p<0.001, R2 = 0.9) support this conclusion. In a mesocosm experiment, the nitrogen half-life for H. rubra tissue was estimated to be 20.4 days, demonstrating that the grazer provides a time-integrative sample compared to grab-sample measurements of dissolved nutrients. Ubiquitous grazers such as H. rubra may prove a useful and cost-effective method for δ15N detection of sewage in conjunction with standard monitoring methods, enabling sampling of a large number of pools to establish and refine monitoring programs, especially because anchialine habitats typically support no macroalgae.

Divergent responses of native and invasive macroalgae to submarine groundwater discharge

Marine macroalgae are important indicators of healthy nearshore groundwater dependent ecosystems (GDEs), which are emergent global conservation priorities. Submarine groundwater discharge (SGD) supports abundant native algal communities in GDEs via elevated but naturally derived nutrients. GDEs are threatened by anthropogenic nutrient inputs that pollute SGD above ambient levels, favoring invasive algae. Accordingly, this case study draws on the GDE conditions of Kona, Hawai'i where we evaluated daily photosynthetic production and growth for two macroalgae; a culturally valued native (Ulva lactuca) and an invasive (Hypnea musciformis). Manipulative experiments-devised to address future land-use, climate change, and water-use scenarios for Kona-tested algal responses under a natural range of SGD nutrient and salinity levels. Our analyses demonstrate that photosynthesis and growth in U. lactuca are optimal in low-salinity, high-nutrient waters, whereas productivity for H. musciformis appears limited to higher salinities despite elevated nutrient subsidies. These findings suggest that reductions in SGD via climate change decreases in rainfall or increased water-use from the aquifer may relax physiological constraints on H. musciformis. Collectively, this study reveals divergent physiologies of a native and an invasive macroalga to SGD and highlights the importance of maintaining SGD quantity and quality to protect nearshore GDEs.

Geology and land use shape nitrogen and sulfur cycling groundwater microbial communities in Pacific Island aquifers

Resource-constrained island populations have thrived in Hawai'i for over a millennium, but now face aggressive new challenges to fundamental resources, including the security and sustainability of water resources. Characterizing the microbial community in groundwater ecosystems is a powerful approach to infer changes from human impacts due to land management in hydrogeological complex aquifers. In this study, we investigate how geology and land management influence geochemistry, microbial diversity and metabolic functions. We sampled a total of 19 wells over 2-years across the Hualālai watershed of Kona, Hawai'i analyzing geochemistry, and microbial communities by 16S rRNA amplicon sequencing. Geochemical analysis revealed significantly higher sulfate along the northwest volcanic rift zone, and high nitrogen (N) correlated with high on-site sewage disposal systems (OSDS) density. A total of 12,973 Amplicon Sequence Variants (ASV) were identified in 220 samples, including 865 ASVs classified as putative N and sulfur (S) cyclers. The N and S cyclers were dominated by a putative S-oxidizer coupled to complete denitrification (Acinetobacter), significantly enriched up to 4-times comparatively amongst samples grouped by geochemistry. The significant presence of Acinetobacter infers the bioremediation potential of volcanic groundwater for microbial-driven coupled S-oxidation and denitrification providing an ecosystem service for island populations dependent upon groundwater aquifers.

Trade-offs across values in cesspool management highlight challenges to policy making

On-site Sewage Disposal Systems (OSDS) are globally common, and in Hawai'i they present a risk of contamination to drinking water sources and nearshore waters. State legislation has commanded that all cesspools are to be banned by 2050, thus requiring tens of thousands of systems to be converted in the coming decades. This project followed a participatory structured decision-making (SDM) approach to collaboratively design cost-effective and equitable solutions for thousands of cesspools in the high elevation areas of north Maui, Hawai'i. Participatory workshops with a diverse group of stakeholders set ten objectives and brainstormed 33 alternatives, for which the technical team then modeled groundwater nutrients, costs, and equity. All alternatives posed trade-offs, though composting toilets performed best across most objectives, albeit with high maintenance burden. Discounting innovative toilets, the multi-objective analysis suggests that the state should invest in cluster sewering of high-density communities, followed by incentivizing septic tank solutions in properties with the highest effluent flow first, then expanding across the area. The total project cost (installation and operation/maintenance) would be $183-258 million, depending upon the sewer-septic combination. An efficiency frontier reveals sub-par combinations, including aerobic treatment units and passive absorption systems, which cost much more and deliver lower mass flux reduction than more cost-effective alternatives. This study contributes a novel case of rural sanitation to the literature in which decision support tools are used to facilitate evidence-based, collaborative decision-making for sanitation planning. The state could use a similar participatory SDM process when approaching other communities to discuss their cesspool upgrade strategies. Broadening the use of decision analytic techniques can have wider ecological, economic, and social benefits for the state and contexts beyond Hawai'i, as SDM provides a transparent and rigorous, evidence-based decision-theoretic framework to explore multiple values and strategies to address difficult resource management problems.

Detection and impact of sewage pollution on South Kohala's coral reefs, Hawai'i

Sewage pollution from on-site sewage disposal systems and injection wells is impacting coral reefs worldwide. Our study documented the presence and impact of sewage on South Kohala's coral reefs, on Hawai'i Island, through benthic water quality and macroalgal sampling (fecal indicator bacteria, nutrients, δ¹⁵N macroalgal tissue), NO₃^- stable isotope mixing models, water motion measurements, and coral reef surveys. Sewage pollution was moderate on the offshore reef from benthic seeps, and water motion mixed and diluted it across the benthos. These conditions likely contribute to the dominance of turf algae cover, and the severity and prevalence of growth anomalies and algal overgrowth on corals. Use of multiple indicators and studying water motion was necessary to assess sewage pollution and identify environmental drivers associated with impaired coral health conditions. Methods used in this study can be utilized by natural resource managers to identify and reduce anthropogenic stressors to coral reefs.

Groundwater discharge locally shapes the rocky shore macroinvertebrate community in South-Southwest Portugal

Groundwater discharge is an essential process in the functioning of coastal aquatic ecosystems due to its significant role in nutrient cycling, geochemical mass balances and primary productivity. However, the occurrence patterns, importance, and effects of this discharge on rocky shores communities remain largely unknown. We assessed the importance of groundwater discharge into the highly ecologically important intertidal ecosystems. We compared the benthic macroinvertebrate composition and abundance between discharge and no-discharge sites, replicated for five shores in South and Southwest Portugal. This robust replicated feature across shores and regions is a particularly novel contribution to the field. Groundwater discharge significantly affected the biological communities' abundance across all shores, but not biodiversity patterns. The algae Enteromorpha sp., snail Melaraphe neritoides and lichen Verrucaria maura can potentially be used as bioindication tools for shifts in groundwater discharge quantity and qualitative patterns. Our study validates the importance of this commonly overlooked local disturbance factor in regulating intertidal communities.

Thermal-Based Remote Sensing Solution for Identifying Coastal Zones with Potential Groundwater Discharge

Submarine Groundwater Discharge (SGD) is an essential process of the hydrological cycle by hydraulically connecting the land and sea. However, the occurrence, importance and effects of SGD remain largely underexplored. Here, we developed and validated a straightforward tool for mapping potential SGD areas in coastal ecosystems of Portugal. Our approach was based on the premise that relatively cooler groundwater discharging to warmer coastal waters manifests in the thermal band of satellite imagery acquired during the summer months. We then used Landsat 8 thermal infrared imagery (TIR) to derive sea surface temperature and standardized temperature anomalies maps. The results confirmed the capacity of TIR remote sensing for identifying SGD areas. The thermal analysis enabled us to acquire a useful visual-spatial correlation between the location of thermal anomalies and potentiometric surfaces of coastal aquifers. This way, over 20 potential SGD areas were identified. Our study makes an important contribute to our current SGD research status by developing a cost-efficient tool which can be used as a first level approach for large areas. Further investigation is needed to quantify the SGD and its potential effect in the receiving ecosystems, especially those located within environmentally protected areas.

Stable isotopes reveal overlooked incorporation of diffuse land-based sources of nutrients and organic matter by intertidal communities at Rapa Nui (Easter Island)

Rapa Nui is an important hotspot of endemic marine biodiversity, where diffuse land-based sources (e.g., nutrients and organic matter) entering into coastal waters could develop eutrophication in coastal environments, with deleterious impacts on the marine ecosystem. Stable isotopes (δ¹³C and δ¹⁵N) of intertidal communities (macroalgae and invertebrates) were studied from sites with contrasting human influence (populated and unpopulated), to evaluate the incorporation and transfer of diffuse land-based sources through food webs. Macroalgae showed differences between some sites, and invertebrates showed a ¹⁵N-enrichment pattern at populated areas relative to unpopulated, being these differences significant in gastropods, barnacles and sea urchins. Moreover, trophic structure metrics suggest a higher trophic diversity in populated areas relative to unpopulated and support the isotopic partitioning between sites, associated with the incorporation of sources with ¹⁵N-enriched values. The above suggests that diffuse land-based sources could be incorporated by macroalgae, transferred into benthic consumers, and altering the trophic structure.

Review of drivers and threats to coastal groundwater quality in China

With rapid socio-economic development, China's coastal areas are among the fastest growing and most economically dynamic regions in the world. Under the influence of climate change and human activities, protecting the quality of coastal groundwater has emerged as one of the key environmental and resource management issues for these areas. This paper reviews (for the first time) groundwater quality data for the coastal basins of China, where over 600 million people live, focussing on key inorganic indicators/pollutants; groundwater salinity, nitrate, fluoride, and arsenic. These pollutants present major water quality issues and are also valuable as indicators of wider processes and influences impacting coastal groundwater quality - e.g. saltwater intrusion, agricultural pollution and release of geo-genic contaminants. We discuss the major drivers causing water quality problems in different regions and assess future trajectories and challenges for controlling changes in coastal groundwater quality in China. Multiple processes, including modern and palaeo seawater/brine migration, groundwater pumping for agricultural irrigation, pollution from agrochemical application, rapid development of aquaculture, urban growth, and water transfer projects, may all be responsible (to different degrees) for changes observed in coastal groundwater quality, and associated long-term health and ecological effects. We discuss implications for sustainable coastal aquifer management in China, arguing that groundwater monitoring and contamination control measures require urgent improvement. The evolution and treatment of coastal groundwater quality problems in China will serve as an important warning and example for other countries facing similar pressures, due to climate change, coastal development, and intensification of anthropogenic activity in coming decades.

Spatial distribution and sources of nutrients at two coastal developments in South Kohala, Hawai'i

Nutrient sources to coastal waters with coral reefs are not well-characterized. This study documented spatial distributions of nutrients within coastal waters along two developments with coral reefs, and identified nutrient sources through nutrient mixing plots, δ¹⁵N measurements in macroalgal tissue, and NO₃^- stable isotope mixing models. Nutrients decreased from fresh groundwaters to offshore waters, with some surface waters higher in concentrations than benthic ones. Conservative and non-conservative mixing between fresh and ocean waters occurred, the latter suggestive of local nutrient sources and biological removal. δ¹⁵N in macroalgal tissue and NO₃^- concurred that fresh groundwater, ocean water, and fertilizers were dominant nutrient sources. Benthic salinity and NO₃^- + NO₂^- concentrations illustrated that submarine groundwater discharge delivered nutrients to reefs in pulses ranging from minutes to days. Information generated from this study is imperative for developing management actions to improve water quality and make coral reefs more resilient to stressors.

Identifying wastewater management tradeoffs: Costs, nearshore water quality, and implications for marine coastal ecosystems in Kona, Hawai'i

Untreated and minimally treated wastewater discharged into the environment have the potential to adversely affect groundwater dependent ecosystems and nearshore marine health. Addressing this issue requires a systems approach that links land use and wastewater management decisions to potential impacts on the nearshore marine environment via changes in water quality and quantity. To that end, a framework was developed to assess decisions that have cascading effects across multiple elements of the ridge-to-reef system. In an application to Kona (Hawai'i, USA), eight land use and wastewater management scenarios were evaluated in terms of wastewater system upgrade costs and wastewater related nutrient loads in groundwater, which eventually discharge to nearshore waters, resulting in potential impacts to marine habitat quality. Without any upgrades of cesspools or the existing wastewater treatment plant (WWTP), discharges of nutrients are expected to increase substantially with permitted development, with potential detrimental impacts to the marine environment. Results suggest that converting all of the existing cesspools to aerobic treatment units (ATU) and upgrading the existing WWTP to R-1 quality provide the highest protection to nearshore marine habitat at a cost of $569 million in present value terms. Other wastewater management options were less effective but also less costly. For example, targeted cesspool conversion (a combination of septic and ATU installation) in conjunction with the WWTP upgrade still provided a substantial reduction in nutrients and potential impacts to marine habitat quality relative to the present situation at a price point roughly $100 million less than the entirely ATU option. Of note, results were more sensitive to the inclusion of the WWTP upgrade option than they were to assumptions regarding the efficiency of the cesspool conversion technologies. The model outputs also suggest that the spatial distribution of potential impacts should be carefully considered when comparing different wastewater management scenarios. When evaluated separately, the WWTP option reduced total nutrients by more than the targeted cesspool conversion option at a fraction of the cost. However, potential improvements in marine habitat quality only occurred in the immediate vicinity of the WWTP, whereas the benefits under targeted cesspool conversion were more evenly distributed along the coast.

Submarine groundwater discharge as a potential driver of eastern oyster, Crassostrea virginica, populations in Georgia

Reef-building eastern oysters, Crassostrea virginica, provide many ecosystem services, including production of valuable commercial products, formation of complex habitats, improved water quality and shoreline protection. Despite this, oyster populations have experienced dramatic declines throughout their range, spawning massive investment in management and restoration. Restoration efforts typically consider several well-studied metrics that normally influence oyster success; however, one potential factor that has not received much prior attention is submarine groundwater discharge (SGD). We conducted a series of field surveys and field experiments to explore the relationship between SGD and oysters in a marsh-lined tidal creek in Georgia, USA. SGD was mapped across multiple time points using a natural radon tracer (radon-222), and fluxes were paired with discrete measurements of oyster density, condition, size, recruitment and growth at multiple locations along the creek. Variation in oyster metrics was best explained by a combination of SGD, pH, and DO, which displayed a high degree of multicollinearity. We found an overall negative, nonlinear relationship between oyster density and groundwater flux. Interestingly, juvenile and adult condition and growth were not negatively impacted by groundwater. Rather, our results suggest that the likely mechanism for the density-flux relationship was interruption of the larval recruitment, which was also negatively related to flux. We hypothesize that larval interruption is due to the low dissolved oxygen and pH conditions of the groundwater at high flux sites. Overall, the interaction between SGD and oysters appears complex, and may be affected by other variables. This study provides evidence of a potential negative effect of a previously understudied natural phenomena on oyster demographics, and we suggest that SGD be considered in future management efforts.

Evaluating the feasibility and advantage of a multi-purpose submerged breakwater for harbor protection and benthic habitat enhancement at Kahului Commercial Harbor, Hawai‘i: case study

Construction of breakwaters provides an engineering solution for coastal protection. However, little effort has been made toward understanding the ecological impact on local coral reef ecosystems and developing engineering structures that would enhance the coral reef environment. A submerged breakwater proposed for Kahului Commercial Harbor, Hawai‘i, provided an opportunity to design a multi-purpose ‘reef structure’ to mitigate wave impacts while providing new coral reef habitat. This design involved ecological and environmental considerations alongside engineering principles, serving as a model for environmentally sound harbor development. This field study evaluated environmental conditions and reef community composition at the proposed site in a gradient extending outward from the harbor, using in situ data with multivariate analyses. Benthic and topographic features in the area were assessed using a towed drop camera system to relate to biological factors. Results that support breakwater topography should follow the natural spur and groove and depth of the adjacent reef and orient with wave direction. A deep area characterized by unconsolidated substrata and low coral cover would be replaced with the shallow, sloping hard bottom of the breakwater, and provide an exemplary area for corals to flourish while protecting the harbor from large ocean swells. Surfaces on shallow sloping hard bottoms receive higher levels of irradiance that benefits coral growth. Optimal levels of water motion facilitate sediment removal and promote coral recruitment and growth. The design of the Kahului Harbor submerged multi-purpose structure serves as a model for design of shoreline modification that enhances, rather than degrades, the local coral reef environment.

Evaluation, effect and utilization of submarine groundwater discharge for coastal population and ecosystem: A special emphasis on Indian coastline

Submarine groundwater discharge (SGD) is an important process driven by marine and terrestrial forces. Low tide affects SGD the most, therefore the ideal time to detect SGD is the low tide, especially during spring tide. Techniques to detect and quantify SGD along with the understanding of the related aquifer characteristics is discussed in this study. Scientific community across the world is realizing the importance of studying and mapping SGD because in the scenario of climate change, this part of the global hydrological cycle is an important process and is known to have a significant effect on the marine ecosystem due to nutrient and metal inputs around the region of discharge. Therefore, understanding the processes governing SGD becomes very important. In this review, various components and processes related to SGD (e.g. Submarine Groundwater Recharge, Deep Porewater Upwelling, Recirculated Saline Groundwater Discharge), along with detailed discussion on impacts of SGD for marine ecosystem is presented. Also, it highlights the future research direction and emphasis is put on more research to be done keeping in mind the changing climate and its impacts on SGD.

Ridge to Reef Management Implications for the Development of an Open-Source Dissolved Inorganic Nitrogen-Loading Model in American Samoa

Excessive nutrient discharge to tropical island coastlines drives eutrophication and algal blooms with significant implications for reef ecosystem condition and provision of ecosystem services. Management actions to address nutrient pollution in coastal ecosystems include setting water-quality standards for surface waters discharging to the coast. However, these standards do not account for the effects of groundwater discharge, variability in flow, or dilution, all of which may influence the assessment of true nutrient impacts on nearshore reef habitats. We developed a method to estimate dissolved inorganic nitrogen (DIN) loads to coastal zones by integrating commonly available datasets within a geospatial modeling framework for Tutuila, American Samoa. The DIN-loading model integrated an open-source water budget model, water-sampling results, and publicly available streamflow data to predict watershed-scale DIN loading to the island's entire coastline. Submarine groundwater discharge (SGD) was found to deliver more terrigenous DIN to the coastal zone than surface water pathways, supporting findings from other tropical islands. On-site wastewater disposal systems were also found to be the primary anthropogenic sources of DIN to coastal waters. Our island-wide DIN-loading model provides a simple and robust metric to define spatially explicit sources and delivery mechanisms of nutrient pollution to nearshore reef habitats. Understanding the sources and primary transport modes of inorganic nitrogen to nearshore reef ecosystems can help coastal resource managers target the most impactful human activities in the most vulnerable locations, thereby increasing the adaptive capacity of unique island ecosystems to environmental variation and disturbances.

Whale burial and organic matter impacts on biogeochemical cycling in beach aquifers and leachate fluxes to the nearshore zone

Coastal managers are increasingly faced with the challenge of disposing of stranded whale carcasses on beaches. Direct burial in the beach is often used as a cost effective method of disposal. However, whale burial management plans are often met with public resistance owing to the perceived risk of shark attraction to burial leachate that may discharge from the seabed. A reactive transport model was combined with a numerical variable-density groundwater flow model to assess buried whale leachate plume formation, transport, influence on beach aquifer reactivity, and discharge to coastal surface water for a range of burial setback distances, depths, and whale sizes. A second set of simulations was performed to evaluate aquifer nitrate removal efficiencies for a range of buried wrack scenarios and to evaluate the role of organic carbon source on beach reactivity. A sensitivity analysis was performed for both sets of models across ten physical and reaction parameters. Simulations using the best estimate parameter set showed that whale burials can produce DOC and ammonium leachate plumes in the beach aquifer that are transported to and discharge near the low tide line in water depths of 0.4-2.4 m. DOC and ammonium concentrations in discharging whale leachate were1.6 and 26 times higher than typical surf zone concentrations, respectively. Of the factors tested, the burial distance inland from the high tide line was the most important factor affecting leachate fluxes to surface water. Burials placed farther inland led to smaller DOC fluxes to surface water, but increased ammonium fluxes. Burial depth also affected whale leachate to the subtidal zone, with deeper burials resulting in smaller fluxes of DOC. Leached DOC from whale decomposition and from buried wrack can fuel denitrification hotspots within beach sediments. The sensitivity analysis showed that nitrate removal supported by buried wrack and whale leachate fluxes are highly dependent on beach properties, hydrologic forcing, and reaction parameters. The wrack model results have implications for beach scraping and the whale burial models show that whale leachate can be delivered to the shallow subtidal zone via groundwater discharge pathways, with potential implications for shark attraction and whale burial management practices.

Microbial community composition across a coastal hydrological system affected by submarine groundwater discharge (SGD)

Mobile Bay, the fourth largest estuary in the USA located in the northern Gulf of Mexico, is known for extreme hypoxia in the water column during dry season caused by NH4+-rich and anoxic submarine groundwater discharge (SGD). Nutrient dynamics in the coastal ecosystem point to potentially elevated microbial activities; however, little is known about microbial community composition and their functional roles in this area. In this study, we investigated microbial community composition, distribution, and metabolic prediction along the coastal hydrological compartment of Mobile Bay using 16S rRNA gene sequencing. We collected microbial samples from surface (river and bay water) and subsurface water (groundwater and coastal pore water from two SGD sites with peat and sandy lithology, respectively). Salinity was identified as the primary factor affecting the distribution of microbial communities across surface water samples, while DON and PO43- were the major predictor of community shift within subsurface water samples. Higher microbial diversity was found in coastal pore water in comparison to surface water samples. Gammaproteobacteria, Bacteroidia, and Oxyphotobacteria dominated the bacterial community. Among the archaea, methanogens were prevalent in the peat-dominated SGD site, while the sandy SGD site was characterized by a higher proportion of ammonia-oxidizing archaea. Cyanobium PCC-6307 and unclassified Thermodesulfovibrionia were identified as dominant taxa strongly associated with trends in environmental parameters in surface and subsurface samples, respectively. Microbial communities found in the groundwater and peat layer consisted of taxa known for denitrification and dissimilatory nitrate reduction to ammonium (DNRA). This finding suggested that microbial communities might also play a significant role in mediating nitrogen transformation in the SGD flow path and in affecting the chemical composition of SGD discharging to the water column. Given the ecological importance of microorganisms, further studies at higher taxonomic and functional resolution are needed to accurately predict chemical biotransformation processes along the coastal hydrological continuum, which influence water quality and environmental condition in Mobile Bay.

Algal bioassays detect modeled loading of wastewater-derived nitrogen in coastal waters of O'AHU, HAWAI'I

Previous studies indicate coastlines are at risk of wastewater contamination from injection wells, cesspools, and septic systems. In this study, common marine algae were used to ground-truth modeled loading of wastewater-derived N to coastlines of O'ahu, Hawai'i. Macroalgae were collected and/or deployed at 118 sites and analyzed for tissue δ¹⁵N and N %. Wastewater source locations were used to estimate wastewater-derived N in groundwater with the modeling software MT3DMS/MODFLOW. Algal bioassays identified six coastal regions subjected to elevated wastewater-derived N loading. In a case study, submarine groundwater discharge (estimated by ²²²Rn mass balance) was related to wastewater loading from onsite sewage disposal systems (OSDS) and municipal wastewater injection wells in Waimānalo. The highest ²²²Rn-derived SGD rate and N flux were 21.4 m³/m/d and 62.6 g/m/d, respectively. The results of this study suggest that OSDS and injection wells discharge substantial volumes of wastewater and N across broad regions of coastal O'ahu.

Effects of tourism-derived sewage on coral reefs: Isotopic assessments identify effective bioindicators

Pulau Redang and Pulau Tioman have experienced huge tourism growth over the last two decades, but minimal sewage treatment may threaten the resilience of their coral reefs. This study uses stable isotope techniques to identify suitable bioindicators of sewage nutrients (δ¹⁵N) at these islands by measuring macroalgae (Lobophora spp.), gastropods (Drupella spp.), scleractinian coral (Acropora spp.), and leather coral (Sinularia spp.). At tourist hubs using seepage septic tank systems, enrichment of Acropora δ¹⁵N (Redang, +0.7‰) and Sinularia δ¹⁵N (Tioman, +0.4‰) compared to pristine background levels indicate enhanced sewage nutrient discharge. Carbon isotopes and survey data suggest that sedimentation did not confound these δ¹⁵N trends. Potential damaging effects of sewage discharge on the coral reef communities at both islands are highlighted by strong correlations between Acropora δ¹⁵N and regional variation in coral reef community structure, and exclusive occurrence of degraded reefs at regions of high sewage influence.

Remobilization of dissolved metals from a coastal mine tailing deposit driven by groundwater discharge and porewater exchange

Mining impacts on coastal environments have been extensively studied around the world. However, the role of Submarine Groundwater Discharge (SGD) and Porewater Exchange (PEX) as pathways for pollutants from mining waste deposits into seawater has been largely overlooked. Portmán Bay is located in the Cartagena-La Unión Pb-Zn sulphur mining district in Murcia, SE of Spain. The disposal of about 60 million tons of metal-rich mine tailings from 1957 to 1990 led to the infill of most of the bay. Although the effects of metals on indicator organisms have been shown previously, there is a major lack of knowledge on the release of dissolved metals from the emerged tailing deposit into the sea, more than 25 years after the closure of the mining activities. Samples for Ra isotopes (²²³Ra, ²²⁴Ra, ²²⁶Ra and ²²⁸Ra) and dissolved metals (Ag, Cd, Co, Pb, Zn) were analyzed in porewaters and seawater in order to separately estimate SGD and PEX driven dissolved metal fluxes. Our results show a continuous release of dissolved metals into the sea driven by both PEX and SGD. Most of dissolved metals are remobilized and released into the water column by PEX, which is a ubiquitous mechanism acting along the shoreline. Although SGD only represents 13% of the water flow, it drives large fluxes of dissolved Fe into the sea, mainly restricted to the west side of the bay. Large inputs of dissolved Fe²⁺ from the anoxic tailings deposit trigger a massive precipitation of iron hydroxides that enables the removal of most dissolved metals from the water column. This study highlights the role of PEX and SGD as significant mechanisms for the land to ocean transfer of dissolved metals from coastal mine tailings deposits.

Contributions of native forest protection to local water supplies in East Maui

Tropical forests provide a suite of benefits including biodiversity, cultural value, and a range of ecosystem services. Globally, there is increasing interest in incentivizing native forest protection as a multi-benefit natural infrastructure strategy to secure clean and ample water supplies. In addition to conversion to agriculture and other non-forest land uses, non-native species invasion represents a major threat to these systems, particularly on islands. Whereas several recent efforts have quantified the benefits of reforestation or avoided agricultural expansion in tropical forest areas, the hydrologic and associated economic benefits of avoided invasion have received less attention. To address this gap, we quantified the benefits of protecting native forest from conversion to non-native forest in East Maui, Hawai'i in terms of groundwater recharge, a highly valued hydrologic ecosystem service that water utilities increasingly seek to co-finance. Compared with two counterfactual invasion scenarios, the groundwater recharge benefits of planned conservation activities reached 40.9 to 146.3 million cubic meters over 100 years depending on invasion rate assumptions. This translated to 2.70 to 137.6 million dollars of cost savings to the water utility in present value terms (achieved through reducing reliance on more expensive water alternatives) under a range of discount rates and water scarcity assumptions. Our results suggest that investing in native forest conservation provides an important hydrologic ecosystem service benefit that complements the range of benefits provided by these ecosystems. These findings demonstrate that co-financing native forest conservation represents an important supply side option in water resources planning.

Clostridium perfringens testing improves the reliability of detecting non-point source sewage contamination in Hawaiian coastal waters compared to using Enterococci alone

Non-point sources of sewage-related pollution in tropical marine waters are difficult to ascertain. Enterococci (ENT) are widely used as indicators of human waste but their efficacy in tropical waters is highly debated due to natural presence in tropical soils. Clostridium perfringens (CP) is often used as a secondary indicator of fecal contamination because its presence indicates sewage, and in tropical waters environmental sources are unlikely. We analyzed a 27-year dataset containing over 29,000 samples collected by the State of Hawaii, to determine a proposed CP standard for detecting human sewage, which has applicability throughout tropical marine waters globally. Measured ENT concentrations were highly correlated with turbidity. In three instances, sewage contamination was not detected by ENT samples alone, and impairments from non-point pollution may be highly misinformed in Hawaii. The EPA should examine relationships between CP and human health and implement CP as the primary FIB in tropical marine waters.

Assessment of Terrigenous Nutrient Loading to Coastal Ecosystems along a Human Land-Use Gradient, Tutuila, American Samoa

Anthropogenic nutrient loading is well recognized as a stressor to coastal ecosystem health. However, resource managers are often focused on addressing point source or surface water discharge, whereas the impact of submarine groundwater discharge (SGD) as a nutrient vector is often unappreciated. This study examines connections between land use and nutrient loading through comparison of four watersheds and embayments spanning a gradient of human use impact on Tutuila, a high tropical oceanic island in American Samoa. In each study location, coastal radon-222 measurements, dissolved nutrient concentrations, and nitrogen isotope values (δ15N) in water and in situ macroalgal tissue were used to explore SGD and baseflow derived nutrient impacts, and to determine probable nutrient sources. In addition to sampling in situ macroalgae, pre-treated macroalgal specimens were deployed throughout each embayment to uptake ambient nutrients and provide a standardized assessment of differences between locations. Results show SGD-derived nutrient flux was more significant than baseflow nutrient flux in all watersheds, and δ15N values in water and algae suggested wastewater or manure are likely sources of elevated nutrient levels. While nutrient loading correlated well with expected anthropogenic impact, other factors such as differences in hydrogeology, distribution of development, and wastewater infrastructure also likely play a role in the visibility of impacts in each watershed.

Numerical assessments of recharge-dominated groundwater flow and transport in the nearshore reclamation area in western Taiwan

This study employed experimental and numerical methods to assess the behavior of conservative solute transport for a selected temporary solid waste site in a reclamation area in western Taiwan. Calibrating a site-specific numerical model, finite element model of water flow through saturated-unsaturated media (FEMWATER), relies on observations from field- and laboratory-scale hydraulic tests and spatial-temporal monitoring. The field-scale experiment used a modified hydraulic tomography survey (MHTS) to identify near surface aquifer stratifications and estimate the distribution of saturated hydraulic conductivity. The pressure plate experiments provided parameters for the van Genuchten soil characteristic model. Sensitivity analyses were then conducted based on varied recharge rates and dispersivities applied to the calibrated model. Observations of groundwater levels and salinity in the wells indicated that the regional groundwater flow was from southeast to northwest. In addition, a shallow freshwater layer was noted in the study area. The tidal-induced amplitudes for water level fluctuation in the wells ranged from 2 to 20 cm, depending on their distance from the seawater body. MHTS showed clear stratification, similar to that of well loggings at the storage site. The hydraulic conductivity at the test site ranged from 8 to 10 m/day, which is close to that obtained from the laboratory falling head test. The results of particle-tracking modeling showed that the critical recharge rate for the site needed to enhance plume traveling is 1000 mm/year. The increase in dispersivity values induced a decrease in plume travel time of up to 1000 days from the site to the coastal line. A special case for pulse releasing solute at the site shows that the key factor in controlling plume migration is the recharge rate. This is due to the low natural head gradient in the reclamation area. The results therefore suggest that a land drainage system near the site can play an important role in contaminant transport in the reclamation area.

Quantification of Submarine Groundwater Discharge in the Gaza Strip

Gaza Strip has suffered from seawater intrusion during the past three decades due to low rainfall and high abstraction from the groundwater resource. On a yearly basis, more than 170 million m3 of groundwater is abstracted, while the long-term average recharge from rainfall is 24.4 million m3/year. Submarine groundwater discharge (SGD) has never been studied in the Gaza Strip, due to lack of experience in this field, next to the ignorance of this subject due to the seawater intrusion process taking place. Continuous radon measurements were carried out in six sites along the Gaza Strip to quantify the SGD rate. The final result shows SGD to occur in all sampled sites. The range of SGD rates varies from 0.9 to 5.9 cm·day−1. High values of SGD are found in the south (Rafah and Khan Younis governorates). The high values are probably related to the shallow unconfined aquifer, while the lowest values of SGD are found in the middle of Gaza Strip, and they are probably related to the Sabkha formation. In the north of Gaza Strip, SGD values are in the range of 1.0 to 2.0 cm·day−1. Considering that SGD would occur with the measured rates in a strip of 100 m wide along the whole coast line, the results in a quantity of 38 million m3 of groundwater being discharged yearly to the Mediterranean Sea along Gaza coast. Nutrient samples were taken along Gaza Strip coastline, and they were compared to the onshore wells, 600 m away from the Mediterranean Sea. The results show that SGD has higher NO3− + NO2− than nutrient-poor seawater, and that it is close to the onshore results from the wells. This confirms that the source of SGD is groundwater, and not shallow seawater circulation. In a coastal strip of 100 m wide along the Gaza coast, a yearly discharge of over 400 tons of nitrate and 250 tons of ammonium occurs from groundwater to the Mediterranean Sea.

Submarine Groundwater Discharge Differentially Modifies Photosynthesis, Growth, and Morphology for Two Contrasting Species of Gracilaria (Rhodophyta)

Gracilaria coronopifolia and an invasive congener, Gracilaria salicornia, were examined across an SGD gradient in the field and laboratory. Tissue samples of both species were cultured for 16 days along an onshore-offshore SGD gradient at Wailupe, Oahu. G. salicornia tolerated the extremely variable salinity, temperature, and nutrient levels associated with SGD. In marked contrast, half of G. coronopifolia plants suffered tissue loss and even death at SGD-rich locations in the field and in laboratory treatments simulating high SGD flux. Measurements of growth, photosynthesis, and branch development via two novel metrics indicated that the 27‰ simulated-SGD treatment provided optimal conditions for the apparently less tolerant G. coronopifolia in the laboratory. Benthic community analyses revealed that G. salicornia dominated the nearshore reef exposed to SGD compared with the offshore reef, which had a greater diversity of native algae. Ultimately, SGD inputs to coastal environments likely influence benthic community structure and zonation on otherwise oligotrophic reefs.

Synthesizing the Effects of Submarine Groundwater Discharge on Marine Biota

Submarine groundwater discharge (SGD) is a global and well-studied geological process by which groundwater of varying salinities enters coastal waters. SGD is known to transport bioactive solutes, including but not limited to nutrients (nitrogen, phosphorous, silica), gases (methane, carbon dioxide), and trace metals (iron, nickel, zinc). In addition, physical changes to the water column, such as changes in temperature and mixing can be caused by SGD. Therefore SGD influences both autotrophic and heterotrophic marine biota across all kingdoms of life. This paper synthesizes the current literature in which the impacts of SGD on marine biota were measured and observed by field, modeling, or laboratory studies. The review is grouped by organismal complexity: bacteria and phytoplankton, macrophytes (macroalgae and marine plants), animals, and ecosystem studies. Directions for future research about the impacts of SGD on marine life, including increasing the number of ecosystem assessment studies and including biological parameters in SGD flux studies, are also discussed.

Are submarine groundwater discharges affecting the structure and physiological status of rocky intertidal communities?

This study evaluated the impacts of submarine groundwater discharges (SGD) on a rocky intertidal community of South Portugal, during April-November 2011. Chlorophyll-a concentration was higher at the SGD site in respect to the Reference site. Epibenthic community structure differed between sites, with an increase in Chthamalus spp. and a decrease in macroalgae coverage at the SGD site. The abundance and body size of Mytilus galloprovincialis were consistently higher at the SGD site. During mid-spring, under potentially higher SGD and less favorable conditions for coastal phytoplankton, the ecophysiological condition of M. galloprovincialis and G. umbilicalis was also higher at the SGD site. These beneficial effects on filter-feeders and herbivores probably resulted from local increases in prey availability, supported by SGD-driven nutrient inputs. Conversely, P. depressa was not favoured by SGD, probably due to a lower dependency on algae as food. The analysis of epibenthic community structure and ecophysiological condition represents a promising approach to disentangle the ecological impacts of SGD on intertidal ecosystems.

Spatial distribution of sewage pollution on a Hawaiian coral reef

While sewage pollution is contributing to the global decline of coral reefs, its offshore extent and direct reef impacts from water column mixing and benthic seeps are poorly documented. We addressed this knowledge gap on a Hawaiian coral reef using sewage indicator and benthic cover measurements, macroalgal bioassays, and a pollution scoring tool. Fecal indicator bacteria (FIB) and nutrient concentrations were spatially variable in surface and benthic waters, with shoreline values being highest. Shoreline macroalgae δ¹⁵N and %N indicated high nitrogen loads containing sewage, while offshore surface and benthic values suggested lower nitrogen loads from environmental sources. Coral cover was negatively correlated with FIB, macroalgal δ¹⁵N, and nutrient concentrations. Benthic salinity and temperature measurements detected daily tidal groundwater pulses which may explain these associations. While pollution scores revealed that sewage was largely concentrated along the shoreline, results showed some reached the reef and may be contributing to its declining condition.

A linked land-sea modeling framework to inform ridge-to-reef management in high oceanic islands

Declining natural resources have led to a cultural renaissance across the Pacific that seeks to revive customary ridge-to-reef management approaches to protect freshwater and restore abundant coral reef fisheries. Effective ridge-to-reef management requires improved understanding of land-sea linkages and decision-support tools to simultaneously evaluate the effects of terrestrial and marine drivers on coral reefs, mediated by anthropogenic activities. Although a few applications have linked the effects of land cover to coral reefs, these are too coarse in resolution to inform watershed-scale management for Pacific Islands. To address this gap, we developed a novel linked land-sea modeling framework based on local data, which coupled groundwater and coral reef models at fine spatial resolution, to determine the effects of terrestrial drivers (groundwater and nutrients), mediated by human activities (land cover/use), and marine drivers (waves, geography, and habitat) on coral reefs. We applied this framework in two 'ridge-to-reef' systems (Hā'ena and Ka'ūpūlehu) subject to different natural disturbance regimes, located in the Hawaiian Archipelago. Our results indicated that coral reefs in Ka'ūpūlehu are coral-dominated with many grazers and scrapers due to low rainfall and wave power. While coral reefs in Hā'ena are dominated by crustose coralline algae with many grazers and less scrapers due to high rainfall and wave power. In general, Ka'ūpūlehu is more vulnerable to land-based nutrients and coral bleaching than Hā'ena due to high coral cover and limited dilution and mixing from low rainfall and wave power. However, the shallow and wave sheltered back-reef areas of Hā'ena, which support high coral cover and act as nursery habitat for fishes, are also vulnerable to land-based nutrients and coral bleaching. Anthropogenic sources of nutrients located upstream from these vulnerable areas are relevant locations for nutrient mitigation, such as cesspool upgrades. In this study, we located coral reefs vulnerable to land-based nutrients and linked them to priority areas to manage sources of human-derived nutrients, thereby demonstrating how this framework can inform place-based ridge-to-reef management.