Subsurface Pore Water Research Articles

Spatio-temporal and depth-oriented evaluation of hyporheic zone processes in headwater catchments

The hyporheic zone (HZ) is vital for nutrient cycling and overall stream ecosystem health across groundwater-surface water interfaces, with biogeochemical and physical variations influencing exchange processes. To gain a depth-oriented insight into the various hyporheic functioning, isotopic (18O and 2H) and chemical analyses, along with physical parameters, were performed in two streams named Ahna and Losse in North Hesse, Germany. Multi-level interstitial probes were installed to extract subsurface pore water samples up to 0.45 m depth. We identified three distinct HZ in Ahna upstream, downstream and Losse. The Ahna upstream HZ was a less permeable barrier with low water fluxes but reactive, while the downstream HZ was less reactive with higher permeability and vertical fluxes. The Losse HZ, remained non-reactive but well-mixed with good hydraulic circulation due to fast downward flows. Isotopic composition revealed distinctive features, indicating increased evaporation and intricate mixing with other water sources and intensified subsurface flow in Ahna downstream compared to upstream. Hydro-chemical dynamics in Ahna exposed increased ion concentrations downstream, driven by anthropogenic factors along the course. Upstream denitrification was dominant under low oxygen conditions due to the slow infiltration and low mixing rates. The primary water source for both streams and HZs was groundwater, with the potential for rainwater to serve as a secondary source. This study shows the spatial heterogeneity of HZ processes and highlights that the specific HZ conditions need to be examined locally.

Sedimentary porewaters record regional tectonic and climate events that perturbed a deep-sea brine pool in the Gulf of Aqaba, Red Sea

Brine pools in deep-sea environments provide unique perspectives into planetary and geological processes, extremophile microbial communities, and sedimentary records. The NEOM Brine Pool Complex was the first deep-sea brine pool system found in the Gulf of Aqaba, representing a significant extension of the geographical range and depositional setting of Red Sea brine pools. Here, we use a combination of brine pool samples collected via cast using a conductivity, temperature, depth instrument (CTD), as well as interstitial porewaters extracted from a sediment core collected in the NEOM Brine Pool to characterize the chemical composition and subsurface evolution of the brine. New results indicate that the NEOM brines and the subsurface porewaters may originate from different sources. Elemental concentrations suggest the brines in the NEOM pool are likely derived from dissolution of sub-seabed evaporites. In contrast, the sedimentary porewaters appear to have been influenced by periodic turbidite flows, generated either by earthquakes, submarine landslides, or flash floods, in which normal marine waters from the overlying Red Sea became entrained, periodically disturbing the chemistry of the brine pool. Thus, sediment porewaters beneath brine pools may record transient and dynamic changes in these deep marine depositional environments, reflecting the interplay between climate, tectonics, and sedimentation patterns along a rapidly urbanizing coastline. In concert, new results from NEOM extend the range and chemical constraints on Red Sea Brine Pools and highlight the dynamic interplay between Red Sea Deep water, dissolving evaporites, turbidites, and subsurface fluids that produce these unique depositional environments which host microbial life at the edge of habitability. In concert with sedimentological indicators, the chemistry of porewaters beneath deep-sea brine pools may present detailed records of natural hazards arising from interactions between the atmosphere, lithosphere, hydrosphere, and anthroposphere.

Subsurface Porewater Flow Accelerates Talik Development Under the Alaska Highway, Yukon: A Prelude to Road Collapse and Final Permafrost Thaw?

AbstractThe presence of taliks (perennially unfrozen zones in permafrost areas) adversely affects the thermal stability of infrastructure in cold regions, including roads. The role of heat advection on talik development and feedback on permafrost degradation has not been quantified methodically in this context. We incorporate a surface energy balance model into a coupled groundwater flow and energy transport numerical model (SUTRA‐ice). The model, calibrated with long‐term observations (1997–2018 on the Alaska Highway), is used to investigate and quantify the role of heat advection on talik initiation and development under a road embankment. Over the 25‐year simulation period, the new model is driven by reconstructed meteorological data and has a good agreement with near surface soil temperatures. The model successfully reproduces the increasing depth to the permafrost table (mean absolute error <0.2 m), and talik development. The results demonstrate that heat advection provides an additional energy source that expedites the rate of permafrost thaw and roughly doubles the rate of permafrost table deepening, compared to purely conductive thawing. Talik initially formed and grew over time under the combined effect of water flow, snow insulation, road construction and climate warming. Talik formation creates a new thermal state under the road embankment, resulting in acceleration of underlying permafrost degradation, due to the positive feedback of heat accumulation created by trapped unfrozen water. In a changing climate, mobile water flow will play a more important role in permafrost thaw and talik development under road embankments, and is likely to significantly increase maintenance costs and reduce the long‐term stability of the infrastructure.

The extent to which soil hydraulics can explain ecohydrological separation

Field measurements of hydrologic tracers indicate varying magnitudes of geochemical separation between subsurface pore waters. The potential for conventional soil physics alone to explain isotopic differences between preferential flow and tightly-bound water remains unclear. Here, we explore physical drivers of isotopic separations using 650 different model configurations of soil, climate, and mobile/immobile soil-water domain characteristics, without confounding fractionation or plant uptake effects. We find simulations with coarser soils and less precipitation led to reduced separation between pore spaces and drainage. Amplified separations are found with larger immobile domains and, to a lesser extent, higher mobile-immobile transfer rates. Nonetheless, isotopic separations remained small (<4‰ for δ2H) across simulations, indicating that contrasting transport dynamics generate limited geochemical differences. Therefore, conventional soil physics alone are unlikely to explain large ecohydrological separations observed elsewhere, and further efforts aimed at reducing methodological artifacts, refining understanding of fractionation processes, and investigating new physiochemical mechanisms are needed.

Age and chemistry of dissolved organic carbon reveal enhanced leaching of ancient labile carbon at the permafrost thaw zone

Abstract. Climate change will alter the balance between frozen and thawed conditions in Arctic systems. Increased temperatures will make the extensive northern permafrost carbon stock vulnerable to decomposition and translocation. Production, cycling, and transport of dissolved organic carbon (DOC) are crucial processes for high-latitude ecosystem carbon loss that result in considerable export off the Arctic landscape. To identify where and under what conditions permafrost DOC is mobilized in an Arctic headwater catchment, we measured radiocarbon (14C) of DOC and assessed DOC composition with ultraviolet–visible spectroscopy (UV–Vis) of surface waters and shallow and deep subsurface porewaters from 17 drainages in the Barrow Environmental Observatory in Alaska. Samples were collected in July and September 2013 to assess changes in age and chemistry of DOC over time. DOC age was highly variable ranging from modern (19 ‰ Δ14C) to approximately 7000 BP (−583 ‰ Δ14C). DOC age increased with depth, over the summer as the active layer deepened, and with increasing drainage size. DOC quality indicators reflected a DOC source rich in high molecular-weight and aromatic compounds, characteristics consistent with vegetation-derived organic matter that had undergone little microbial processing, throughout the summer and a weak relationship with DOC age. In deep porewaters, DOC age was also correlated with several biogeochemical indicators (including dissolved methane concentration, δ13C, and the apparent fractionation factor), suggesting a coupling between carbon and redox biogeochemistry influencing methane production. In the drained thawed lake basins included in this study, DOC concentrations and contributions of vegetation-derived organic matter declined with increasing basin age. The weak relationship between DOC age and chemistry and consistency in DOC chemical indicators over the summer suggest a high lability of old DOC released by thawing permafrost.

Polychaete Invasion May Lead to Biogeochemical Change in Host Marine Environment

Marine invasive species may modify their host environment by altering ecosystem biogeochemistry. We hypothesized that the invasive polychaete Marenzelleria viridis in Baltic Sea areas increases sulfate reduction (SR) in sediment micro-zones surrounding its burrow. Consequently, higher free porewater sulfide (H2S) is expected in sediments dominated by M. viridis than in corresponding sediments inhabited by the native polychaete Hediste diversicolor. In a thin-aquaria experiment, we found high SR rates (220 to 539 nmol cm−3 d−1) around the burrow walls of M. viridis as well as in surface and subsurface sediments with overall rates 2-fold higher than in defaunated control sediment. Similarly, an in situ survey revealed subsurface porewater H2S peaks moving upward towards the sediment surface in M. viridis inhabited areas. Accordingly, 50–85% higher porewater H2S was found almost year-round in these areas compared with H. diversicolor inhabited areas, suggesting that the invasion of M. viridis probably led to a substantial change in sediment biogeochemistry. In conclusion, M. viridis stimulates SR in sediment micro-zones and increases H2S in coastal sediments. Such change to more reducing conditions after the invasion may have critical environmental implications on, e.g., the distribution of H2S intolerant flora and fauna species.

Creek Dynamics Determine Pond Subsurface Geochemical Heterogeneity in East Anglian (UK) Salt Marshes

Salt marshes are complex systems comprising of ephemerally flooded, vegetated platforms hydraulically fed by tidal creeks. Where drainage is poor, formation of saline-water ponds can occur. Within East Anglian (UK) salt marshes, two types of sediment chemistries can be found beneath these ponds; iron-rich sediment, which is characterized by high ferrous iron concentration in subsurface porewaters (up to 2 mM in the upper 30 cm); and sulfide-rich sediment, which is characterized by high porewater sulfide concentrations (up to 8 mM). We present five years of push-core sampling to explore the geochemistry of the porewater in these two types of sediment. We suggest that when carbon is present in quantities sufficient to exhaust the oxygen and iron content within pond sediments, conditions favour the presence of sulfide-rich sediments. In contrast, in pond sediments where oxygen is present—primarily through bioirrigation—reduced iron can be reoxidised and thus recycled for further reduction, favouring the perpetuation of iron-rich sedimentary conditions. We find these pond sediments can alter significantly over an annual timescale. We carried out a drone survey, with ground-truthed measurements, to explore the spatial distribution of geochemistry in these ponds. Our results suggest a pond’s proximity to the creek partially determines the pond subsurface geochemistry with iron-rich ponds tending to be closer to large creeks relative to sulfide-rich ponds. We suggest differences in surface delivery of organic carbon, due to differences in the energy of the ebb flow, or the surface/ subsurface delivery of iron may control the distribution. This could be amplified if tidal inundations flush ponds closer to creeks more frequently, removing carbon and flushing with oxygen. These results suggest that anthropogenic creation of drainage ditches could shift the distribution of iron- and sulfide-rich ponds and thus have an impact on nutrient, trace metal and carbon cycling in salt marsh ecosystems.

The drivers of biogeochemistry in beach ecosystems: A cross-shore transect from the dunes to the low-water line

This study addresses key processes in high-energy beach systems using an interdisciplinary approach. We assess spatial variations in subsurface pore water residence times, salinity, organic matter (OM) availability, and redox conditions and their effects on nutrient cycles as well as on microbial community patterns and microphytobenthos growth. At the study site on Spiekeroog Island, southern North Sea, beach hydrology is characterized by the classical zonation with an upper saline plume (USP), a saltwater wedge, and a freshwater discharge tube in between. Sediment and pore water samples were taken along a cross-shore transect from the dunes to the low water line reaching sediment depths down to 5 m below sediment surface. Spatial variations in pore water residence time, salinity, and organic matter availability lead to steep redox and nutrient gradients. Vertical and horizontal differences in the microbial community indicate the influence of these gradients and salinity on the community structure. Modeled seawater flux through the USP and freshwater flux through the tube are on average 2.8 and 0.75 m3 per day and meter of shoreline, respectively. Furthermore, ridge sediments at the lower beach discharge seawater at rates of 0.5 and 1.0 m3 per day and meter of shoreline towards the runnel and seaside, respectively. Applying seawater and freshwater fluxes and representative nutrient concentrations for the discharge zones, nutrient fluxes to adjacent nearshore waters are 117 mmol NH4+, 55 mmol PO43 − and 575 mmol Si(OH)4 per day and meter of shoreline. We propose that this nutrient efflux triggers growth of microphytobenthos on sediment surfaces of the discharge zone. A first comparison of nutrient discharge rates of the beach site with a nearby sandy backbarrier tidal flat margin indicates that the beach system might be of less importance in supplying recycled nutrients to nearshore waters than the backbarrier tidal flat area.

Pathways and transformations of dissolved methane and dissolved inorganic carbon in Arctic tundra watersheds: Evidence from analysis of stable isotopes

AbstractArctic soils contain a large pool of terrestrial C and are of interest due to their potential for releasing significant carbon dioxide (CO2) and methane (CH4) to the atmosphere. Due to substantial landscape heterogeneity, predicting ecosystem‐scale CH4 and CO2 production is challenging. This study assessed dissolved inorganic carbon (DIC = Σ (total) dissolved CO2) and CH4 in watershed drainages in Barrow, Alaska as critical convergent zones of regional geochemistry, substrates, and nutrients. In July and September of 2013, surface waters and saturated subsurface pore waters were collected from 17 drainages. Based on simultaneous DIC and CH4 cycling, we synthesized isotopic and geochemical methods to develop a subsurface CH4 and DIC balance by estimating mechanisms of CH4 and DIC production and transport pathways and oxidation of subsurface CH4. We observed a shift from acetoclastic (July) toward hydrogenotropic (September) methanogenesis at sites located toward the end of major freshwater drainages, adjacent to salty estuarine waters, suggesting an interesting landscape‐scale effect on CH4 production mechanism. The majority of subsurface CH4 was transported upward by plant‐mediated transport and ebullition, predominantly bypassing the potential for CH4 oxidation. Thus, surprisingly, CH4 oxidation only consumed approximately 2.51 ± 0.82% (July) and 0.79 ± 0.79% (September) of CH4 produced at the frost table, contributing to &lt;0.1% of DIC production. DIC was primarily produced from respiration, with iron and organic matter serving as likely e‐ acceptors. This work highlights the importance of spatial and temporal variability of CH4 production at the watershed scale and suggests broad scale investigations are required to build better regional or pan‐Arctic representations of CH4 and CO2 production.

In situ measurement and simulation of nano-magnetite mobility in porous media subject to transient salinity.

Nanotechnologies have been proposed for a variety of environmental applications, including subsurface characterization, enhanced oil recovery, and in situ contaminant remediation. For such applications, quantitative predictive models will be of great utility for system design and implementation. Electrolyte chemistry, which can vary substantially within subsurface pore waters, has been shown to strongly influence nanoparticle aggregation and deposition in porous media. Thus, it is essential that mathematical models be capable of tracking changes in electrolyte chemistry and predicting its influence on nanoparticle mobility. In this work, a modified version of a multi-dimensional multispecies transport simulator (SEAWAT) was employed to model nanoparticle transport under transient electrolyte conditions. The modeling effort was supported by experimental measurements of paramagnetic magnetite (Fe3O4) nanoparticle, coated with polyacrylamide-methylpropane sulfonic acid - lauryl acrylate (nMag-PAMPS), mobility in columns packed with 40-50 mesh Ottawa sand. Column effluent analyses and magnetic resonance imaging (MRI) were used to quantify nanoparticle breakthrough and in situ aqueous phase concentrations, respectively. Experimental observations revealed that introduction of de-ionized water into the brine saturated column (80 g L(-1) NaCl + 20 g L(-1) CaCl2) promoted release and remobilization of deposited nanoparticles along a diagonal front, coincident with the variable density flow field. This behavior was accurately captured by the simulation results, which indicated that a two-site deposition-release model provided the best fit to experimental observations, suggesting that heterogeneous nanoparticle-surface interactions governed nanoparticle attachment. These findings illustrate the importance of accounting for both physical and chemical processes associated with changes in electrolyte chemistry when predicting nanoparticle transport behavior in subsurface formations.

Production, degradation, and flux of dissolved organic matter in the subterranean estuary of a large tidal flat

The distributions of dissolved organic carbon (DOC), total dissolved hydrolysable amino acids (THAA), and colored dissolved organic matter (CDOM) were studied in pore-water/groundwater samples (including seeping water) from a subterranean estuary (STE) of a large tidal flat in Hampyeong Bay, Korea, in July 2011. The relatively low alanine D/L ratios and high THAA concentrations in the pore-water closest to the sediment surface (0–10cm) indicate the active production of dissolved organic matter (DOM) from benthic algae, and the relatively low THAA concentrations and high D/L ratios in the subsurface pore-water (10–35cm) indicate a relatively large presence of bio-degraded DOM. In the deep pore-water (35–75cm), relatively low D/L ratios, high DOC concentrations, and intense humic-like fluorescence were observed, suggesting a net accumulation of less-reactive DOC in this layer. Overall, this STE appears to have net DOM sources because the concentrations of DOC (60–1700μM) in the pore-water decreased toward the land, the surface, and the low-salinity waters. The concentrations of DOC in the seeping water (185±52μM) were higher than those in the overlying seawater (144±9μM), resulting in net DOC fluxes of 2–5×109g·C·yr−1 through submarine groundwater discharge (SGD) into Hampyeong Bay. The organic matter compositions in the seeping water indicated that SGD introduced DOM from both the surface and subsurface layers. Our results highlight that tidal flats are important sources for DOM, implying that SGD-driven DOM plays an important role in coastal carbon cycles and biogeochemistry.

Seasonal Changes In Mineralogy, Geochemistry and Microbial Community of Bacteriogenic Iron Oxides (BIOS) Deposited in a Circumneutral Wetland

Changes in the mineralogy, microbial community structure and aquatic geochemistry of bacteriogenic iron oxides (BIOS) deposited at a circumneutral groundwater seep were recorded seasonally. X-ray diffraction and X-ray absorption spectroscopy demonstrated that 2-line ferrihydrite dominated the iron-bearing mineralogy of the BIOS (>60%). Minor proportions of the more crystalline lepidocrocite were also detected in BIOS collected throughout the year (<30%), but goethite (<15%) was only identified in sediments collected during the spring, summer and fall months. Subsurface porewater profiles generally showed an increase in dissolved equilibrium concentrations of Fe(II), indicative of microbial Fe(III) reduction, and the depth at which appreciable levels of dissolved Fe(II) (and to a lesser extent manganese) arose, and sulfate concentrations declined, become shallower as the seasons progressed from spring to fall. Clone libraries constructed from 16S rRNA gene sequence analysis of surficial BIOS showed the presence of sequences closely affiliated to known Fe(II)-oxidizing and Fe(III)-reducing prokaryotes, pointing towards a tightly coupled microbial iron cycle. Clones that showed closest identity to Fe(II)-oxidizing bacteria in culture, including Gallionella spp. and Sideroxydans spp., made up a significant portion of the clone library, but their relative proportion declined through the winter (73%), spring (26%) and summer (9%). The dominance of the putative microaerophilic Fe(II)-oxidizing chemolithoautotrophs in the winter clone library may be partly due to the lower temperature and dissolved organic carbon concentrations observed during this season, conditions that could have limited mixotrophic or organotrophic growth by mesophilic bacteria. Supplemental materials are available for this article. Go to the publisher's online edition of Geomicrobiology Journal to view the free supplemental file.

Development of the DET technique for high-resolution determination of soluble reactive phosphate profiles in sediment pore waters

The DET (diffusive equilibrium in thin films) technique is developed to measure soluble reactive phosphate (SRP) profiles in sediment pore waters at a millimetre resolution. The analytical procedure includes equilibration of the gels in sediments, section of the gels after retrieval from the sediments, back elution of phosphorus in the gels, and analysis of SRP in the eluents. Recovery of phosphorus is improved from back elution with 0.25 M nitric acid relative to deionised water. SRP concentrations in pore waters of different sediments measured by DET probes agree well with those directly measured by the colorimetric method. Pore water profiles obtained simultaneously using gel probes and other techniques (including Rhizon and dialysis peeper) also show comparability at similar resolutions. The DET probes were used to investigate pore water SRP profiles in the sediments of two contrasting regions (algal-dominated and macrophyte-dominated) in Lake Taihu. An increasingly upward movement of SRP was observed in subsurface pore waters of the algal-dominated region coupled with an increase in water temperature from March to May. Peak-shape distribution of SRP and horizontal heterogeneity was observed in pore waters of the macrophyte-dominated region, which is most likely caused by the activity of submerged macrophyte roots.

Microbial and geochemical features suggest iron redox cycling within bacteriogenic iron oxide-rich sediments

The geochemistry, mineralogy and microbiology of bacteriogenic iron oxides (BIOS) deposited at a circumneutral groundwater seep were examined to evaluate the biogeochemical controls on the redox cycle of iron in such environments. X-ray diffraction and extended X-ray absorption fine structure (EXAFS) spectroscopy showed that 2-line ferrihydrite dominated (60–70%) the BIOS mineralogy, with more crystalline lepidocrocite and goethite phases comprising the remainder, likely the product of Fe(II)-induced dissolution–reprecipitation pathways. Subsurface porewater profiles displayed an increase in dissolved (<0.2μm) equilibrium concentrations of Fe(II), also reflected in peaks in HCl-extractable sedimentary Fe(II) at depth, indicative of biological iron reduction. Interestingly EXAFS analysis of sediments collected from such depths revealed little mineralogical change from the surficial BIOS. Dissolved concentrations of Fe(III), likely present as nanoparticulates or organic complexes, also rose with depth. Scanning electron microscopy images of the surface BIOS samples showed a proliferation of sheath structures characteristic of Fe(II)-oxidizing Leptothrix spp. A 16S rRNA clone library obtained from the surficial BIOS included microorganisms closely associated to known Fe(II)-oxidizing (Gallionella spp., Sideroxydans spp.) and Fe(III)-reducing (Rhodoferax ferrireducens and Geothrix fermentans) bacteria. Analysis of subsurface sediment collected from an Fe(III)-reducing horizon revealed a drop in the proportion of β-Proteobacteria, which hosts numerous Fe(II)-oxidizing genera, and a rise in the δ-Proteobacteria, home to anaerobic metal-reducing prokaryotes. The recognition of clones with close identity to bacteria intimately involved in complementary iron redox transformations in the same surficial BIOS sediment suggests that microbial oxidation and reduction of iron may be tightly coupled in this, and other, shallow sedimentary environments.

Liquefaction potential of coastal slopes induced by solitary waves

Tsunami runup and drawdown can cause liquefaction failure of coastal fine sand slopes due to the generation of high excess pore pressure and the reduction of the effective over burden pressure during the drawdown. The region immediately seaward of the initial shoreline is the most susceptible to tsunami-induced liquefaction failure because the water level drops significantly below the still water level during the set down phase of the drawdown. The objective of this work is to develop and validate a numerical model to assess the potential for tsunami-induced liquefaction failure of coastal sandy slopes. The transient pressure distribution acting on the slope due to wave runup and drawdown is computed by solving for the hybrid Boussinesq—nonlinear shallow water equations using a finite volume method. The subsurface pore water pressure and deformation fields are solved simultaneously using a finite element method. Two different soil constitutive models have been examined: a linear elastic model and a non-associative Mohr–Coulomb model. The numerical methods are validated by comparing the results with analytical models, and with experimental measurements from a large-scale laboratory study of breaking solitary waves over a planar fine sand beach. Good comparisons were observed from both the analytical and experimental validation studies. Numerical case studies are shown for a full-scale simulation of a 10-m solitary wave over a 1:15 and 1:5 sloped fine sand beach. The results show that the soil near the bed surface, particularly along the seepage face, is at risk to liquefaction failure. The depth of the seepage face increases and the width of the seepage face decreases with increasing bed slope. The rate of bed surface loading and unloading due to wave runup and drawdown, respectively, also increases with increasing bed slope. Consequently, the case with the steeper slope is more susceptible to liquefaction failure due to the higher hydraulic gradient. The analysis also suggests that the results are strongly influenced by the soil permeability and relative compressibility between the pore fluid and solid skeleton, and that a coupled solid/fluid formulation is needed for the soil solver. Finally, the results show the drawdown pore pressure response is strongly influenced by nonlinear material behavior for the full-scale simulation.

Measurement of soluble reactive phosphorus concentration profiles and fluxes in river-bed sediments using DET gel probes

DET (diffusive equilibrium in thin films) gel probes were used for sampling river-bed sediment porewaters, to characterise in situ soluble reactive phosphorus (SRP) concentration profiles and fluxes. DET probes were deployed in three contrasting rural streams: (1) a headwater ‘pristine’ stream, with minimal P inputs from low intensity grassland and no point sources, (2) an intensively cultivated arable catchment, and (3) a stream subject to high P loadings from sewage effluent and intensive arable farming. The DET results showed highly enriched porewater SRP concentrations of between ca. 400 and 5000 ug-P/1 in the sewage-impacted stream. In contrast, the arable and pristine streams had porewater SRP concentrations <70 ug-P/1 and <20 ug-P/1, respectively. Porewater SRP concentration profiles in both the sewage-impacted and arable-impacted streams showed well-defined vertical structure, indicating internal sources and sinks of SRP within the sediment. However, there was little variability in porewater SRP concentrations in the pristine stream. The DET porewater profiles indicated net diffusion of SRP (a) from the overlying river water into the surface sediment and (b) from subsurface sediment upwards towards the sediment–water interface. A mass balance for the sewage-impacted site showed that the influx of SRP into the surface sediments from the overlying river water was small (ca. 1% of the daily river SRP load). The DET results indicated that, in the arable and sewage-impacted streams, the surface ‘cap’ of fine sediment may play an important role in inhibiting upward movement of SRP from subsurface porewaters into the overlying river water, under steady-state, low-flow conditions.

Evaluating Ground Water–Sea Water Interactions via Resistivity and Seepage Meters

We investigated submarine ground water discharge and salt water-fresh water interactions at two locations along the shoreline of the Upper Gulf of Thailand to evaluate mechanisms of water and material transport into the coastal zone. Our data set illustrates the value of using a combined approach consisting of automatic seepage meters to monitor flow rates while assessing the conductivity (salinity) of the subterranean fluids via remote resistivity measurements. Negative correlations between electric conductivities of fluids measured directly inside seepage meter chambers and the remotely assessed resistivities of subsurface pore water show that such measurements may evaluate the spatial distribution of flow rates as well as the subterranean water quality in the coastal zone. Combined seepage and resistivity measurements may thus provide a more complete understanding of coastal ground water dynamics.

Measurement and Modelling of Gravel Beach Groundwater Response to Wave Run-up: Effects on Beach Profile Changes

Abstract Beach profile change in the swash zone on gravel beaches is characterised by enhanced onshore sediment transport and berm formation, and infiltration loss in the swash zone is often given as the reason why gravel beaches are steeper than sand beaches. In this paper, we report on field measurements conducted on a gravel beach, Slapton Sands, Devon, UK, in April 2001. Collected data included surface pressures under uprush and backwash, subsurface pore water pressures, uprush and backwash velocities, and bed elevations. The field data were compared with predictions of the BeachWin model, which simulates interacting wave run-up/run-down, beach groundwater flow, swash sediment transport, and resulting beach profile changes. With a relatively large value of hydraulic conductivity, the model was able to predict the observed berm formation at the upper part of the beach. In contrast, the berm feature was absent in the simulation without swash infiltration effects included. The predicted erosion at the mi...

Variations of absolute gravity accompanying earthquake‐induced changes in subsurface pore water pressure at the Mizunami Underground Research Institute construction site, central Japan

The Tono Research Institute of Earthquake Science has been measuring gravity using an FG5 absolute gravimeter located at the Mizunami Geoscience Academy (MGA) in central Japan since January 2004. Measured gravity decreased immediately following the 2004 earthquake off the Kii peninsula (MJMA 7.4) by about 6 μGal. Here, we investigate the empirical relationship between pore water pressure change in a borehole near the MGA and gravity change measured at the MGA. We reveal that (1) gravity change correlates inversely with pore water pressure change at 81 m below the surface at a particular borehole and (2) several different sets of conversion coefficients from pressure head to gravity can be used to explain 60–70% of gravity variations with less than 2 μGal uncertainty. These newly identified relationships may suggest that an absolute gravimeter alone could be used to observe the change of groundwater quantity.

Effects of pH, temperature, and aqueous organic material on the dissolution kinetics of meta-autunite minerals, (Na, Ca)2−1[(UO2)(PO4)]2· 3H2O

Autunite-group minerals have been frequently identified in contaminated sediments as the long-term controlling phase of U. Under these conditions, the mobility of U in subsurface pore waters is limited by the rate of dissolution of autunite and meta-autunite group minerals, X 3-n (n)+ [(UO 2 )(PO 4 )] 2 ⋅xH 2 O, where X = Ca or Na. Single-pass ß ow-through (SPFT) tests were conducted to quantify the dissolution kinetics of natural Ca meta-autunite, Ca[(UO 2 )(PO 4 )] 2 ⋅3H 2 O, and synthetic Na meta-autunite, Na 2 [(UO 2 )(PO 4 )] 2 ⋅3H 2 O, as a function of pH (7.10) and temperature (5.70 °C) in the presence and absence of aqueous organic material. The data indicate that release of U and P are non-stoichiometric over the range of experimental conditions investigated. In a 0.1 M NH 4 OH buffer solution, acquisition of valid dissolution rate data was limited by uramphite solubility, NH 4 [(UO 2 )(PO 4 )]2·xH 2 O. Dissolution rates obtained in a 0.01 M TRIS [tris (hydroxymethyl) aminomethane] buffered solution increased by a factor of ~100× over the pH interval of 7 to 10 (η = 0.90 ± 0.08), irrespective of temperature. At constant pH the rate data showed a minor increase with temperature. Data from experiments using a more concentrated 0.05 M TRIS buffer exhibited a ~35-fold increase in rates compared to those in a 0.01 M TRIS buffer at constant temperature and pH. The difference in release rate between interlayer cation (Na + or Ca 2+ ) and U is ~10 000 in neutral solutions; however, the difference diminishes to ~10 at higher pH values. The combination of structural dissolution and ion exchange explain these trends in interlayer cation behavior. Data presented here illustrate the significance of pH and dissolved organic material on the dissolution of autunite minerals.