Intertidal Groundwater Research Articles

Tidal pumping and intertidal groundwater springs create pronounced spatiotemporal thermal variability in a coastal lagoon

AbstractIntertidal groundwater springs provide thermally stable, nutrient‐rich freshwater that causes fine‐scale variability in water temperature and salinity and promotes biodiversity in coastal estuaries and lagoons. In this study, we combined three thermal sensing techniques to elucidate summer water temperature patterns at the mouths of intertidal groundwater springs and the ambient coastal lagoon in Prince Edward Island, Canada. Thermal infrared imagery captured the spatiotemporal variability in relative temperatures of the lagoon channel and cold‐water plumes from the springs. Thermal anomalies due to the springs were detected at the surface throughout a tidal cycle, suggesting that the thermal plumes were buoyant. Fiber‐optic distributed temperature sensing paired with temperature loggers near the spring outlets recorded groundwater temperatures at low tide (~ 7°C) but ambient lagoon temperatures (up to 26°C) at high tide due to the vertical thermal gradient in the stratified water column. Calculated estuarine Richardson numbers throughout the study confirm intertidal spring buoyancy due to salinity differences. The fiber‐optic distributed temperature sensing results revealed pronounced variations in temperature as evidenced by spatial (3.7°C) and temporal (5.5°C) standard deviations over the cable length during the study and lower mean lagoon bed water temperatures at high tide (22.5°C) than low tide (24.3°C) due to heat advection from tidal pumping. Lagoon temperatures fluctuate at diurnal (solar) and semi‐diurnal (tidal) frequencies. The findings emphasize the efficacy of a multi‐technology approach to reveal fine‐scale and dynamic thermal processes that drive temperature patterns and habitat distribution in coastal lagoon ecosystems and highlight the thermal influence of intertidal springs.

Impacts of climate change and best management practices on nitrate loading to a eutrophic coastal lagoon

Anthropogenic climate change and associated increasing nutrient loading to coasts will worsen coastal eutrophication on a global scale. Basin Head is a coastal lagoon located in northeastern Prince Edward Island, Canada, with a federally protected ecosystem. Nitrate-nitrogen (NO3-N) is conveyed from agricultural fields in the watershed to the eutrophic lagoon via intertidal groundwater springs and groundwater-dominated tributaries. A field program focused on four main tributaries that discharge into the lagoon was conducted to measure year-round NO3-N loading. These measurements were used to calibrate a SWAT+ hydrologic model capable of simulating hydrologic and NO3-N loads to the lagoon. Several climate change scenarios incorporating different agricultural best management practices (BMPs) were simulated to better understand potential future NO3-N loading dynamics. Results indicate that all climate change scenarios produced increased annual NO3-N loading to the lagoon when comparing historical (1990–2020) to end of century time periods (2070–2100); however, only one climate scenario (MRI-ESM2-0 SSP5-8.5) resulted in a statistically significant (p-value <0.05) increase. Enlarged buffer strips and delayed tillage BMP simulations produced small (0%–8%) effects on loading, while changing the crop rotation from potato-barley-clover to potato-soybean-barley yielded a small reduction in NO3-N loading between the historical period and the end of the century (26%–33%). Modeling revealed changes in seasonal loading dynamics under climate change where NO3-N loads remained more consistent throughout the year as opposed to current conditions where the dominant load is in the spring. An increase in baseflow contributions to streamflow was also noted under climate change, with the largest change occurring in the winter (e.g., up to a five-fold increase in February). These findings have direct implications for coastal management in groundwater-dominated agricultural watersheds in a changing climate.

굴 껍데기 피복에 따른 조간대 점토질 퇴적물의 투수성 변화에 관한 연구

굴 껍데기 피복에 따른 조간대 점토질 퇴적물의 투수성 변화에 관한 연구

Intertidal spring dynamics and coastal nutrient loading revealed through geophysics, drone surveys, and in‐situ monitoring

AbstractCoastal eutrophication poses an increasing risk to ecosystem health due to enhanced nutrient loading to the global coastline. Submarine groundwater discharge (SGD) represents a significant pathway for nitrate‐nitrogen (NO3‐N) transport to the coast, but diffusive SGD transport is difficult to monitor directly, given the low flux rates and expansive discharge areas. In contrast, focused SGD from intertidal springs can potentially be sampled and directly gauged, providing unique insight into SGD and associated contaminant transport. Basin Head is a coastal lagoon in Prince Edward Island, Canada that is a federally protected ecosystem. Nitrate‐nitrogen is conveyed from agricultural fields in the contributing watershed to the eutrophic lagoon via intertidal groundwater springs and groundwater‐dominated tributaries. We used several field methods to characterize groundwater discharge, nutrient loading, and in‐channel mixing associated with intertidal springs. The tributaries and intertidal springs were gauged and sampled to estimate a representative summer nitrate load to the lagoon. Our analysis revealed that NO3‐N export to the lagoon through tributaries and springs throughout summer 2023 was on average 401 kg N/month, with the combined spring loading comparable in magnitude to the combined tributary loading. We collected thermal infrared and visual imagery using drone surveys and found spatial overlap between cold‐water plumes from the spring discharge and macroalgae blooms, indicating the local thermal and ecosystem impacts of the focused SGD. We also mapped the electrical resistivity (salinity) distribution in the water column around one large spring with electromagnetic geophysics at different tidal stages to reveal the three‐dimensional spring plume dynamics. Results showed that the fresher spring water floated above the saline lagoon water with the brackish plume oriented in the direction of the tidal current. Collectively, our multi‐pronged field investigations help elucidate the hydrologic, thermal, and nutrient dynamics of intertidal springs and the cascading ecosystem impacts.

Hydrodynamic and anthropogenic disturbances co-shape microbiota rhythmicity and community assembly within intertidal groundwater-surface water continuum

Tidal hydrodynamics drive the groundwater-seawater exchange and shifts in microbiota structure in the coastal zone. However, how the coastal water microbiota structure and assembly patterns respond to periodic tidal fluctuations and anthropogenic disturbance remains unexplored in the intertidal groundwater-surface water (GW-SW) continuum, although it affects biogeochemical cycles and coastal water quality therein. Here, through hourly time-series sampling in the saltmarsh tidal creek, rhythmic patterns of microbiota structure in response to daily and monthly tidal fluctuations in intertidal surface water are disentangled for the first time. The similarity in archaeal community structures between groundwater and ebb-tide surface water (R2=0.06, p = 0.2) demonstrated archaeal transport through groundwater discharge, whereas multi-source transport mechanisms led to unique bacterial biota in ebb-tide water. Homogeneous selection (58.6%-69.3%) dominated microbiota assembly in the natural intertidal GW-SW continuum and the presence of 157 rhythmic ASVs identified at ebb tide and 141 at flood tide could be attributed to the difference in environmental selection between groundwater and seawater. For intertidal groundwater in the tidal creek affected by anthropogenically contaminated riverine inputs, higher microbial diversity and shift in community structure were primarily controlled by increased co-contribution of dispersal limitation and drift (jointly 57.8%) and enhanced microbial interactions. Overall, this study fills the knowledge gaps in the tide-driven water microbial dynamics in coastal transition zone and the response of intertidal groundwater microbiota to anthropogenic pollution of overlying waters. It also highlights the potential of microbiome analysis in enhancing coastal water quality monitoring and identifying anthropogenic pollution sources (e.g., pathogenic Vibrio in aquaculture) through the detection of rhythmic microbial variances associated with intertidal groundwater discharge and seawater intrusion.

Present and future thermal regimes of intertidal groundwater springs in a threatened coastal ecosystem

Abstract. In inland settings, groundwater discharge thermally modulates receiving surface water bodies and provides localized thermal refuges; however, the thermal influence of intertidal springs on coastal waters and their thermal sensitivity to climate change are not well studied. We addressed this knowledge gap with a field- and model-based study of a threatened coastal lagoon ecosystem in southeastern Canada. We paired analyses of drone-based thermal imagery with in situ thermal and hydrologic monitoring to estimate discharge to the lagoon from intertidal springs and groundwater-dominated streams in summer 2020. Results, which were generally supported by independent radon-based groundwater discharge estimates, revealed that combined summertime spring inflows (0.047 m3 s−1) were comparable to combined stream inflows (0.050 m3 s−1). Net advection values for the streams and springs were also comparable to each other but were 2 orders of magnitude less than the downwelling shortwave radiation across the lagoon. Although lagoon-scale thermal effects of groundwater inflows were small compared to atmospheric forcing, spring discharge dominated heat transfer at a local scale, creating pronounced cold-water plumes along the shoreline. A numerical model was used to interpret measured groundwater temperature data and investigate seasonal and multi-decadal groundwater temperature patterns. Modelled seasonal temperatures were used to relate measured spring temperatures to their respective aquifer source depths, while multi-decadal simulations forced by historic and projected climate data were used to assess long-term groundwater warming. Based on the 2020–2100 climate scenarios (for which 5-year-averaged air temperature increased up to 4.32∘), modelled 5-year-averaged subsurface temperatures increased 0.08–2.23∘ in shallow groundwater (4.2 m depth) and 0.32–1.42∘ in the deeper portion of the aquifer (13.9 m), indicating the depth dependency of warming. This study presents the first analysis of the thermal sensitivity of groundwater-dependent coastal ecosystems to climate change and indicates that coastal ecosystem management should consider potential impacts of groundwater warming.

Characterisation of intertidal springs in a faulted multi-aquifer setting

In intertidal zones, groundwater is often present as seepage that provides freshwater and nutrients to marine ecosystems. Point discharge or springs in intertidal zones have been observed in many locations, often in the form of sand boils. The spatial extent, temporal variability and source of intertidal springs are rarely documented and typically, not well understood. This study examined four intertidal groundwater springs at Sellicks Beach, South Australia, during May 2017, November 2019 and September 2020 using a combination of hydrogeophysical methods. A thermal infrared survey undertaken in 2017 showed springs as groupings of closely spaced sand boils that were warmer (15 °C) than the surrounding saturated beach sediments (7 °C). The four springs ranged in diameter from 0.20 to 0.45 m. Electromagnetic geophysical surveys identified a resistive anomaly (3.5 to 5.0 ohm.m), assumed to represent freshwater upwelling at the location of a spring, that extended 10 m horizontally and at least 6.7 m vertically. The average electrical conductivity of water discharging from the springs was 18.4 mS/cm, while seawater was 54.8 mS/cm. δ18O and δ2H data from the springs showed a variation between winter and spring, likely caused by variations in mixing ratios between seawater and groundwater. The springs are proximal to major regional fault systems that likely create preferential flow paths that control spring location and flow rates. The observations of spring characteristics highlight the critical role of seawater-groundwater mixing ratios, preferential flow paths and tidal variations in creating temporal variability in spring discharge and salinity.

Heterogeneity Affects Intertidal Flow Topology in Coastal Beach Aquifers

AbstractIntertidal aquifers are hotspots of biogeochemical cycling where nutrients and contaminants are processed prior to discharge to the ocean. The nature of the dynamic subsurface mixing zone is a critical control on mitigating reactions. Simulation of density‐dependent, variably saturated flow and salt transport incorporating realistic representations of aquifer heterogeneity was conducted within a Monte Carlo framework to investigate influence of nonuniform permeability on intertidal groundwater flow and salt transport dynamics. Results show that heterogeneity coupled with tides creates transient preferential flow paths within the intertidal zone, evolving multiple circulation cells and fingering‐type salinity distributions. Due to heterogeneity, strain‐dominated (intense mixing) and vorticity‐dominated (low mixing) flow regions coexist at small spatial scales, and their spatial extent reaches peaks at high tide and low tide. Such topological characteristics reveal complex tempo‐spatial mixing patterns for intertidal flow with localized areas of high and low mixing intensities, which have implications for intertidal biogeochemical processing.

Oxygenation of aquifers with fluctuating water table: A laboratory and modeling study

Intertidal sediments host many biogeochemical processes which drastically affect coastal ecosystems. Dissolved Oxygen (DO) plays a major role in many of these biogeochemical reactions, and is of primary importance for intertidal groundwater quality. Groundwater level of intertidal zone fluctuates regularly corresponding to the tidal cycle. During the fluctuation of water table, the oxygen-rich air could be entrapped in the pore space of sediments, and hence serve as an important oxygen source to replenish oxygen-depleted groundwater. Although oxygenation enhanced by fluctuated water table has been studied in river banks, no study has been conducted in ocean intertidal sediments under tide-induced groundwater table fluctuations, and quantification of interphase oxygen mass transfer is lacking. In this study, combined laboratory experimental and numerical modeling approach is developed to study the mechanism of oxygenation of aquifers with regularly fluctuated water table. Two column experiments are conducted with regular multiple drainage-imbibition periods to mimic the Oman sea tidal cycles controlled by 3.5 and 5.75 mL/min of pumping rate respectively. The numerical model is developed and calibrated by the column experiment data, and used to simulate two additional columns by increasing the entrapped air saturation to 0.25 from 0.15 and using 30 ppt of saline water respectively. The results suggest that the larger water table fluctuation leads to 35% more dissolved oxygen, and higher entrapped air saturation enables 22% more oxygen transferred to the anoxic aqueous phase and significantly enhanced the oxygenation of the aquifer, especially the deep zone below the water table. Dissolved oxygen is 4.81% less in the column with 30 ppt of saline water than with fresh water.

Distributions, quality assessments and fluxes of heavy metals carried by submarine groundwater discharge in different types of wetlands in Jiaozhou Bay, China

Intertidal groundwater and seawater were sampled to analyze the distribution characteristics, the contamination status and the submarine groundwater discharge (SGD)-associated fluxes of heavy metals Cu, Pb, Zn, Cd, Cr, and Hg as well as the metalloid As at four typical intertidal wetlands (including a sandy beach, a mud flat, a tidal marsh and an estuarine intertidal zone) of Jiaozhou Bay, China. Results show that the surface water near the Dagu River estuary suffers from a severe Cu pollution. The groundwater in the sandy beach and mud flat has stronger enrichment abilities of heavy metals than those at the other two sites. The contents of Pb and Zn in groundwater are mainly controlled by the sulfate reduction. At the mud flat, human activities may cause potential Pb contamination to groundwater. The heavy metal effluxes in the sandy beach are the largest of all the four wetlands.

Coastal water quality assessment and groundwater transport in a subtropical mangrove swamp in Daya Bay, China

Coastal water quality assessment is challenging due to the complex hydrological environment in mangrove swamps. Such assessment requires a good understanding of swamp hydrology and potential solute reactions. In this study, we investigated the concentration variations of a suite of major elements (Na+, K+, Ca2+, Mg2+, SO42−, HCO3– and Cl−), nutrients such as dissolved inorganic phosphorous (DIP) and silicate (DSi), and heavy metals (Cu, Zn, As, Hg, Cd, Pb and Cr) along a typical mangrove transect in Daya Bay, China. The transect comprises a tidal creek, a mudflat and two mangrove zones. The major elements exhibited various degrees of dilution and loss, such as losses of K+ due to uptake by mangrove trees and of SO42− due to microbial sulfate reduction. Numerical simulations of groundwater flow showed that the inland fresh groundwater can continuously discharge through the landward mangrove zone, where high concentrations of DIP and DSi occurred. However, the middle mangrove zone served as a sink of DIP and DSi due to the weak hydrodynamic environment. The spatial distribution of heavy metal concentrations showed that only Cu pollution occurred in the creek zone. The Metal Pollution Index (MPI) was developed to compare the enrichment capability of heavy metals at the different sampling sites. Results showed that the intertidal groundwater has the strongest heavy metal enrichment capability, followed by sea water, inland groundwater and river water. Within the intertidal zone, the creek zone has the highest MPI, followed by the mangrove zone and the mudflat zone. The local water circulation occurred around the creek should contribute the high MPI in the creek zone.

Submarine and intertidal groundwater discharge through a complex multi-level karst conduit aquifer

The quantification of submarine and intertidal groundwater discharge (SiGD) or purely submarine groundwater discharge (SGD) from coastal karst aquifers presents a major challenge, as neither is directly measurable. In addition, the expected heterogeneity and intrinsic structure of such karst aquifers must be considered when quantifying SGD or SiGD. This study applies a set of methods for the coastal karst aquifer of Bell Harbour in western Ireland, using long-term onshore and offshore time series from a high-resolution monitoring network, to link catchment groundwater flow dynamics to groundwater discharge as SiGD. The SiGD is estimated using the “pollution flushing model”, i.e. a mass-balance approach, while catchment dynamics are quantified using borehole hydrograph analysis, single-borehole dilution tests, a water balance calculation, and cross-correlation analysis. The results of these analyses are then synthesised, describing a multi-level conduit-dominated coastal aquifer with a highly fluctuating overflow regime draining as SiGD, which is in part highly correlated with the overall piezometric level in the aquifer. This concept was simulated using a hydraulic pipe network model built in InfoWorks ICM [Integrated Catchment Modeling]® version 7.0 software (Innovyze). The model is capable of representing the overall highly variable discharge dynamics, predicting SiGD from the catchment to range from almost 0 to 4.3 m3/s. The study emphasises the need for long-term monitoring as the basis for any discharge studies of coastal karst aquifers. It further highlights the fact that multiple discharge locations may drain the aquifer, and therefore must be taken into consideration in the assessment of coastal karst aquifers.

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

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

Combining Remote Temperature Sensing with in-Situ Sensing to Track Marine/Freshwater Mixing Dynamics.

The ability to track the dynamics of processes in natural water bodies on a global scale, and at a resolution that enables highly localised behaviour to be visualized, is an ideal scenario for understanding how local events can influence the global environment. While advances in in-situ chem/bio-sensing continue to be reported, costs and reliability issues still inhibit the implementation of large-scale deployments. In contrast, physical parameters like surface temperature can be tracked on a global scale using satellite remote sensing, and locally at high resolution via flyovers and drones using multi-spectral imaging. In this study, we show how a much more complete picture of submarine and intertidal groundwater discharge patterns in Kinvara Bay, Galway can be achieved using a fusion of data collected from the Earth Observation satellite (Landsat 8), small aircraft and in-situ sensors. Over the course of the four-day field campaign, over 65,000 in-situ temperatures, salinity and nutrient measurements were collected in parallel with high-resolution thermal imaging from aircraft flyovers. The processed in-situ data show highly correlated patterns between temperature and salinity at the southern end of the bay where freshwater springs can be identified at low tide. Salinity values range from 1 to 2 ppt at the southern end of the bay to 30 ppt at the mouth of the bay, indicating the presence of a freshwater wedge. The data clearly show that temperature differences can be used to track the dynamics of freshwater and seawater mixing in the inner bay region. This outcome suggests that combining the tremendous spatial density and wide geographical reach of remote temperature sensing (using drones, flyovers and satellites) with ground-truthing via appropriately located in-situ sensors (temperature, salinity, chemical, and biological) can produce a much more complete and accurate picture of the water dynamics than each modality used in isolation.

Nitrogen and oxygen isotopes in nitrate in the groundwater and surface water discharge from two rural catchments: implications for nitrogen loading to coastal waters

In Prince Edward Island, Canada, widespread intensive potato production has contributed to elevated nitrate concentrations in groundwater and streams, and eutrophic or anoxic conditions occur regularly in several estuarine systems. In this research, the stable isotopes of nitrogen and oxygen in nitrate in intertidal groundwater discharge and stream water were used, in conjunction with water quality and quantity data and land use information, to better understand the characteristics of nitrate delivered to two small estuaries with contrasting land use in their contributory catchments. Most of the water samples collected during the two-year study had isotopic signatures that fell in the range expected for nitrate derived from ammonium-based fertilizers (26.5 % of the samples) or in the overlapping range formed between ammonium-based fertilizers and nitrate derived from soil (64 % of the samples). Overall, isotopic signatures spanned over relatively narrow ranges, and correlations with other water quality parameters, or catchment characteristics, were weak. Nitrate in groundwater discharge and surface water in the Trout River catchment exhibited significantly different isotopic signatures only for the nitrogen isotope, while in the McIntyre Creek catchment groundwater discharge and surface water had similar isotopic signatures. When the isotopic results for the waters from the two catchments were compared, the surface waters were found to be similar, while the isotopic signatures of nitrate in groundwater were distinct only for the nitrogen isotope. Denitrification in the two study catchments was not evident based on the isotopic results for nitrate; however, in the case of the Trout River catchment, where a small freshwater pond exists, an average nitrate load reduction of 14 % was inferred based on a comparison of nitrate loads entering and leaving the pond. Overall, it appears that natural attenuation processes, occurring either in the streams or groundwater flow systems, do not significantly reduce nitrate loading to these estuaries.

Intertidal and submarine groundwater discharge on the west coast of Ireland

Submarine Groundwater Discharge is now a phenomenon of global interest, as studies show that it represents both a significant proportion of the fresh water input to the ocean, and a significant contribution to the loads of many substances. At present, little monitoring of groundwater in Ireland is carried out at its point of entry to seawater, and consequently the volumes of fresh water, and the loads of nutrients and contaminants being carried into Irish coastal waters by submarine and intertidal groundwater discharge (SiGD), are unknown. SiGD is the principal source of fresh water entering Irish coastal waters between the major west coast estuaries of the Corrib and the Shannon. Calculations of the volume of submarine SiGD delivered to southern Galway bay in winter indicate it equals 10–25% of the discharge of the R. Corrib, and that its nutrient load may be of the same order of magnitude as that from the R. Corrib. This coastal karst area includes important commercial shellfish waters, which may be strongly impacted by SiGD.

Differences in microphytobenthos and macrofaunal abundances associated with groundwater discharge in the intertidal zone

We investigated the impact of intertidal groundwater seepage on benthic microalgae and macrofauna in 4 study sites located in 2 large tidal flat ecosystems along the western coast of Korea by comparing the chemical, physical, and biological characteristics of 'glossy' seepage sites with those of nearby areas without visually distinct groundwater discharge (dry sediment surface). At 3 of the 4 sites, sediment properties as well as pore water chemistry were similar in groundwater seepage and dry areas. At the 4th study site, the groundwater seepage areas were more coarse- grained compared to the dry areas. Here, the groundwater seepage also had lower salinity and higher nutrient concentrations than the pore water of the dry area and the seawater in a nearby tide pool. Although diatoms were the dominant algal class in seepage and dry areas alike, the seepage areas in 3 of the sites had elevated contributions of other marker pigments such as chlorophyll (chl) b compared to the dry areas. Chl a concentrations were higher in all seepage areas compared to dry areas, and all dry areas had high pheophytin a:chla ratios, indicating a substantial amount of degraded algal material. In the seepage areas of 3 of the sites, we found large numbers of the snail Batillaria cumingi, while crab burrows of Scopimera sp. were only present in the neighboring dry areas. Correlations of sediment chl a concentrations with physicochemical properties of the ambient pore water indicated that microphytobenthos responded specifically to groundwater seepage, which may provide shelter from desiccation and salt stress during emersion of the tidal flat. Our results sug- gest that globally common groundwater seepage significantly impacts the ecosystem structures and microphytobenthos production of the tidal flats.

Porewater nitrate profiles in sandy sediments hosting submarine groundwater discharge described by an advection–dispersion-reaction model

In order to separate the effects of reaction from those of transport on vertical porewater concentration profiles of nitrate at an intertidal groundwater seepage site (Ria Formosa, Portugal), a free-boundary solution of an Advection–Dispersion-Reaction (ADR) model was used to describe the shape of NO3− concentration profiles collected in situ. The model includes three sequential reaction layers, postulated with basis on the local distribution of the benthic organic C:N ratio and major identifiable changes in concentration gradients with depth. The advective nature of the system was used to propose a mass balance simplification to the constitutive equations permitting a free-boundary solution, which in turn allowed prediction of sediment–water fluxes of NO3−. Sensitivity analysis confirmed that in similarly advective-dominated environments, both the porewater concentration distribution and the interfacial fluxes are strongly dependant on seepage rate and benthic reactivity. The model fitted the measured profiles with high correlation (usually higher than 90%), and model-derived sediment–water NO3− fluxes were in good agreement to fluxes measured in situ with Lee-type seepage meters (0.9948 slope, R2 = 0.8672, n = 8). Nitrate oxidation and reduction rates extracted from model fits to the data (10−2–100 mmol m−2 h−1) agreed with literature values. Because dispersive effects are not included in direct mass balances based on the porewater concentrations, the model presented here increases the accuracy of apparent reaction rate estimates and geochemical zonation at Submarine Groundwater Discharge (SGD) sites. The results establish the importance of sandy sediments as reactive interfaces, able to modulate mass transfer of continental-derived contaminants into coastal ecosystems. We suggest that tools such as the one described here might be used to advantage in preparing further experimental studies to elucidate how benthic reactivity affects nitrate distribution and fluxes in sediments affected by SGD.

Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 363:205-215 (2008) - DOI: https://doi.org/10.3354/meps07455 Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge Ryan K. Dale, Douglas C. Miller* University of Delaware, College of Marine and Earth Studies, 700 Pilottown Rd Lewes, Delaware 19958, USA *Corresponding author. Email: dmiller@udel.edu ABSTRACT: Groundwater discharge is a common phenomenon along sandy coasts. At Cape Henlopen, Delaware, USA, it creates low porewater salinity in spatially heterogeneous patterns over 1 to 10 m horizontally. In the present study, porewater salinity and macrofaunal communities were sampled along intertidal transects in summer and spring. We consistently observed high spatial variability in porewater salinity at 10 cm sediment depth, with changes up to 15.8 m–1. Community composition and species abundance differed between the seasons, but, in each season, communities associated with low porewater salinity (<15) differed from those associated with higher porewater salinity (15 to 24 or >24). In general, species from a diverse pool of sand flat species gradually became absent as salinity decreased. These local community changes correlated with porewater salinity could compromise the interpretation of various benthic indices that are based on water-column salinity, which, at this site, remains around 28. In most samples, the polychaete Marenzelleria viridis was numerically dominant (up to 10000 m–2) in burrows up to 50 cm deep. Both in the field and in cores constructed in the laboratory, we found that the number of worm burrows was highly correlated with hydraulic conductivity of the sediment. We propose that burrowing worms (especially M. viridis) at this groundwater discharge site act as hydraulic ecosystem engineers by means of their burrowing and tube building. In general, there appear to be strong physical–biological interactions among the distribution of organisms, their infaunal behavior, the hydrological properties of the sediment and the magnitude of groundwater discharge that may contribute to infaunal zonation on intertidal sand flats. KEY WORDS: Benthic communities · Groundwater · Salinity · Species distribution · Intertidal · Hydraulic conductivity Full text in pdf format PreviousNextCite this article as: Dale RK, Miller DC (2008) Hydrologic interactions of infaunal polychaetes and intertidal groundwater discharge. Mar Ecol Prog Ser 363:205-215. https://doi.org/10.3354/meps07455 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 363. Online publication date: July 15, 2008 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2008 Inter-Research.

Radium and radon radioisotopes in regional groundwater, intertidal groundwater, and seawater in the Adelaide Coastal Waters Study area: Implications for the evaluation of submarine groundwater discharge

The input of groundwater-borne nutrients to Adelaide's (South Australia) coastal zone is not well known but could contribute to the ongoing decline of seagrass in the area. As a component of the Adelaide Coastal Waters Study (ACWS), the potential for using the radium quartet ( 223Ra, 224Ra, 226Ra and 228Ra) and 222Rn to evaluate submarine groundwater discharge (SGD) was evaluated. Potential isotopic signatures for SGD were assessed by sampling groundwater from three regional aquifers potentially contributing SGD to the ACWS area. In addition, intertidal groundwater was sampled at two sand beach sites. In general, the regional groundwaters were enriched in long-lived Ra isotopes ( 226Ra and 228Ra) and in 222Rn relative to intertidal groundwater. Radium activity (but not 222Rn activity) was positively correlated to salinity in groundwater from one of the regional aquifers and in intertidal groundwater. Radium isotope ratios ( 223Ra/ 226Ra, 224Ra/ 226Ra and 228Ra/ 226Ra) were less variable than individual Ra isotope activities within potential SGD sources. Recirculated seawater (estimated from the intertidal groundwater samples with seawater-like salinities) also had distinctly higher Ra isotope ratios than the regional groundwaters. The activities for all radioisotopes were relatively low in seawater. The activity of the short-lived 223Ra and 224Ra were highest at the shoreline and declined exponentially with distance offshore. In contrast, 228Ra and 226Ra activities had a weak linear declining trend with distance offshore. Rn-222 activity was at or near background in all seawater samples. The pattern of enrichment in short-lived Ra isotopes and the lack of 222Rn in seawater suggest that seawater recirculation is the main contributor to SGD in the ACWS area. Preliminary modeling of the offshore flux of 228Ra and 226Ra suggest that the SGD flux to the ACWS area ranges between 0.2 and 3 · 10 − 3 m 3 (m of shoreline) − 1 s − 1 .