Seawater Mixing Zone Research Articles

Permeability–porosity model considering oxidative precipitation of Fe(II) in granular porous media

Despite decades of research into the interactions between groundwater and seawater, the evolution of permeability produced by iron precipitation has received little attention, resulting in uncertain predictions of permeability in porous media. This study establishes a novel permeability–porosity model for estimating permeability considering the oxidative precipitation of Fe(II) in granular porous media. The accuracy of this model is validated by a series of sand column tests investigating iron precipitation driven by subsurface freshwater–saltwater mixing. The experiments showed a localized pattern of iron precipitation concentrated near the freshwater–saltwater interface in those columns, where a low-permeability zone with intensive precipitation was generated, subsequently leading to a decrease in sand permeability of 45%. Extracted samples analyzed by scanning electron microscopy showed quartz sand particles coated by iron precipitates. With those successful experiments, the newly developed model is validated and fully capable of estimating the sand permeability underlying this geochemical process, providing predictions to within an order of magnitude. The present experimental results and model predictions bring valuable insights into how the pore matrix of a granular porous media evolves during iron precipitation in the subsurface groundwater–seawater mixing zone.

Response of the Coastal Phytoplankton Community to the Runoff from Small Rivers in the Northeastern Black Sea

River runoff is an important source of nutrients as well as suspended and dissolved organic matter that in coastal zones and on the shelf are transformed due to local production cycles. River runoff affects the hydrological regime, salinity, temperature, and irradiance in river–seawater mixing zone. Our study focuses on the response of phytoplankton to the impact of small Caucasian rivers in the Northeastern (NE) Black Sea, as one of the most sensitive components of marine ecosystems with respect to the changes in abiotic factors. The leading role of marine species of diatoms, dinoflagellates, and coccolithophores in the structure and functioning when impacted by runoff from small rivers is demonstrated in comparison to the freshwater community. Variability of the taxonomic composition and quantitative and productive characteristics of marine phytoplankton communities impacted by small rivers were comparable to or exceed the seasonal and interannual variability on the NE Black Sea shelf. This indicates the significant role of runoff from small Caucasian rivers in maintaining of a high production level of phytoplankton overall and of the coccolithophore Emiliania huxleyi in particular in the coastal zone.

Hydrogeochemical Characteristics and Processes of Shallow Groundwater in the Yellow River Delta, China

The Yellow River Delta is one of the biggest river deltas in China, and the shallow groundwater plays an important role in the development of the local agriculture and ecosystem. However, people are still unclear about the hydrochemical characteristics and mechanisms of the shallow groundwater. In this study, the authors collected and analyzed 81 groundwater samples from the delta plain and piedmont alluvial plain, and explored the hydrochemical features and causes through Piper diagrams, correlation analysis, ionic ratios, and speciation calculations. The results showed that anions were dominated by Cl and HCO3, the concentration of which was much more than that of SO4 and CO3. The groundwater can be divided into various types, including Na–Cl, Ca–Mg–HCO3, Na–HCO3 and Ca–Mg–Cl. This study tested an alternative method–ionic ratios based on the cumulative frequency distribution to characterizing the hydrochemical groups. According to different ion ratios and hydrogeological conditions, three hydrogeochemical zones with different dominant factors have been determined: Weathering—Fresh Water Zone (Zone I), Evaporation—Saline Water Zone (Zone II), and Seawater Mixing Zone (Zone III). As the calculated saturation index show, the calcite and dolomite are saturated, while the halite and gypsum from Zone I to Zone III tend to be saturated. In addition, cation exchange is an important hydrochemical process in the area, and Zone III experiences inverse ironic exchange. In conclusion, this hydrogeochemical zonation would be favorable for water resource management in the Yellow River Delta.

Numerical investigation on seawater intrusion and water leakage in different types of stratified coastal aquifers

Prediction of seawater intrusion (SWI) will increase the proper planning for withdrawal of groundwater, and it will help the freshwater management in the future. Since heterogeneous aquifers are more widespread than homogeneous one, this study aims to use the density-dependent SEAWAT code to investigate the influence of different combinations of hydraulic conductivity (K) on SWI. The effects of numbers and arrangement of layers with different K are examined on (a) toe position, (b) freshwater–seawater mixing zone thickness (Wt) and (c) leakage from adjacent layers in a steady-state condition. For this purpose, numerous scenarios with different K were simulated and analyzed. The results of horizontally layered aquifers showed that the streamlines are broken on boundary of layers. It was observed that when K is decreased, especially at the middle layer, the refraction angle is increased and consequently Wt is increased. The Wt in vertically layered cases was less than horizontally layered aquifers because of less streamline breakage. Also, random arrangement of K directs the saline flow to higher permeability zones. Thus, the SWI extent in this type of aquifer was more tangible than other types. Because of the difference in hydraulic head at layer borders, the seepage rate significantly increases nearby toe position and the dominant direction is upward. However, in the steady-state condition, the type of flow did not differ significantly for boundary cells of layers at the vicinity of toe position.

Quantification of non-ideal effects on diagenetic processes along extreme salinity gradients at the Mercator mud volcano in the Gulf of Cadiz

We present a transport-reaction model (TRACTION) specifically designed to account for non-ideal transport effects in the presence of thermodynamic (e.g. salinity or temperature) gradients. The model relies on the most fundamental concept of solute diffusion, which states that the chemical potential gradient (Maxwell’s model) rather than the concentration gradient (Fick’s law) is the driving force for diffusion. In turn, this requires accounting for species interactions by applying Pitzer’s method to derive species chemical potentials and Onsager coefficients instead of using the classical diffusion coefficients. Electrical imbalances arising from varying diffusive fluxes in multicomponent systems, like seawater, are avoided by applying an electrostatic gradient as an additional transport contribution.We apply the model to pore water data derived from the seawater mixing zone at the submarine Mercator mud volcano (MMV) in the Gulf of Cadiz. Two features are particularly striking at this site: (i) Ascending halite-saturated fluids create strong salinity (NaCl) gradients in the seawater mixing zone that result in marked chemical activity, and thus chemical potential gradients. The model predicts strong transport-driven deviations from the mixing profile derived from the commonly used Fick’s diffusion model, and is capable of matching well with the profile shapes observed in the pore water concentration data. Even better agreement to the observed data is achieved when ion pairs are transported separately. (ii) The formation of authigenic gypsum (several wt%) occurs in the surface sediments, which is typically restricted to evaporitic surface processes. Very little is known about the gypsum paragenesis in the subseafloor and we first present possible controls on gypsum solubility, such as pressure, temperature, and salinity (pTS), as well as the common ion and ion pairing effects. Due to leaching of deep diapiric salt, rising fluids of the MMV are saturated with respect to gypsum (as well as celestite and barite). Several processes that could drive these fluids towards gypsum supersaturation and hence precipitation were postulated and numerically quantified. In line with the varied morphology of the observed gypsum crystals, gypsum paragenesis at the MMV is likely a combination of two temperature-related processes. Gypsum solubility increases with increasing temperature, especially in strong electrolyte solutions and the first mechanism involves the cooling of saturated fluids along the geothermal gradient during their ascent. Secondly, local temperature changes, i.e. cooling during the transition from MMV activity towards dormancy results in the cyclic build-up of gypsum.The model showed that the interpretation of field data can be majorly misguided when ignoring non-ideal effects in extreme diagenetic settings. While at first glance the pore water profiles at the Mercator mud volcano would indicate strong reactive influences in the seawater mixing zone, our model shows that the observed species distributions are in fact primarily transport-controlled. The model results for SO4 are particularly intriguing, as SO4 is shown to diffuse into the sediment along its increasing (!) concentration gradient. Also, a pronounced gypsum saturation peak can be observed in the seawater mixing zone. This peak is not related to the dissolution of gypsum but is simply a result of the non-ideal transport forces acting on the activity profile of SO4 and Ca profiles.

Features of hydrocarbon distribution in the coastal zone of the northeastern Black Sea

Data on the content and composition of aliphatic and polycyclic aromatic hydrocarbons in surface water and bottom sediments are reported as compared to the distribution of total organic carbon, suspended particulate matter, lipids, and chlorophyll for the Greater Sochi area and the Gelendzhik and Blue bays. It is established that an influx of oil products leads to the increase of hydrocarbon concentrations in the water and bottom sediments, thus providing the present-day hydrocarbon background. Active transformation of organic matter in the water succession and on the water–bottom interface results in the prevalence of natural components in alkanes in spite of the high hydrocarbon concentrations (119–262 μg/g). The river–seawater mixing zone serves as a geochemical barrier preventing the influx of most pollutants delivered by rivers into open sea.

Steady-state freshwater–seawater mixing zone in stratified coastal aquifers

Summary The mixing zone between freshwater and intruding seawater, which controls regional groundwater flow dynamics and reactive transport processes, is the most important component in coastal aquifers. Laboratory experiments and numerical simulations were conducted to investigate the effects of aquifer stratification on the thickness of a steady-state freshwater–seawater mixing zone. A series of experiments were implemented in a laboratory tank packed with three layers of coastal sands and manipulated by two flow cells for maintaining constant-head boundary conditions. Packed aquifers included a homogenous aquifer, a high hydraulic conductivity (K)–low K–high K stratified aquifer, and a low K–high K–low K stratified aquifer. Both tank-scale and field-scale density-dependent models were developed to simulate the laboratory experiments and to systematically investigate aquifer stratification effects on the thickness of a steady mixing zone with varying layer configurations. Results showed that when an aquifer layer with a low K overlies a layer with a relatively high K, slanting upward flow of diluted saltwater and re-circulated seawater flow are refracted at the interface between the two layers, resulting in streamline separation and a broad mixing zone in the low-K layer. By contrast, the mixing zone in a high-K layer is slightly narrowed when it overlies a layer with a relatively low K because flow refraction squeezes the streamlines within the mixing zone. The mixing-zone profile in head-controlled systems is a function of K contrast, but independent of the magnitude of K. Our results demonstrated the significant impact of aquifer stratification on controlling groundwater flow and solute transport in coastal subsurface environments and would have significant implications for studying physical, chemical and biological processes in coastal subsurface environments, as aquifer stratification is naturally prevalent.

Identification of hydrogeochemical process linked to marine intrusion induced by pumping of a semiconfined mediterranean coastal aquifer

This article analyses the hydrogeochemical processes, linked to the freshwater–seawater mixing zone, which can be caused by continuous pumping from a detrital coastal aquifer. It was necessary to construct an experimental plot, drilling three boreholes along a line perpendicular to the coast. A complete physico-chemical analysis was done of all water samples taken. The percentage of seawater, calculated from Chloride and 18Oxygen concentrations, varied between 55 and 90 %. The ionic deltas (Δ) calculated, and the saturation indices (SI) of mineral phases susceptible to precipitation or dissolution, allowed a series of hydrogeochemical processes to be identified that occur as a consequence of the advance of marine intrusion into the coastal band, and of aquifer flushing. Based on the major elements, the fraction of exchange (βI) was calculated for samples ranging from seawater to freshwater, and this revealed that differences in βI could explain the hydrochemistry of the mixing zone. The main processes recognised include precipitation of dolomite, dissolution of gypsum, fixation of sulphur salts and cation exchange. Most of the ion exchange took place between Na and Calcium + Magnesium ions. The process of fixation or liberation of these ions is probably determined by the advance or recession of the saline wedge, and/or by recharge during rainy periods. The behaviour of Magnesium is more sensitive to small variations in salinity, whilst Calcium behaves more homogeneously. The high percentage of seawater in the samples studied favours the speed and magnitude of processes such as ion exchange, and the intervention of magnesium is also a key.

Calculation of river-seawater endmembers and differential trace metal scavenging in the Columbia River plume

A simple model is presented for calculating river and seawater mixing zone endmembers. This model allows construction of a conservative mixing line with respect to trace metals scavenged from surface waters, in the absence of direct endmember sampling. Results of the model suggest a transition from non-conservative to conservative behavior of the trace metals gallium (Ga) and zirconium (Zr) along a transect extending offshore of the Columbia River, indicating environmental controls on differential scavenging. Mechanistic explanation of observed differential scavenging is supported by wind data indicating northward transport of Columbia River plume waters, followed by southwest, offshore, transport. Columbia River plume waters transported to the southwest at the time of sample collection were therefore flanked offshore by aged plume waters returning from the north. While upwelling conditions provide nutrients during southerly transport of plume waters, northward plume transport is associated with downwelling, low nutrient conditions. Low phytoplankton abundance during periods of northward plume transport reduces trace metal scavenging, therefore dilution of river waters with ocean surface waters of comparatively low Ga and Zr concentrations results in near-conservative mixing of Ga and Zr in offshore, aged, plume waters. Results of this work suggest dynamic modification of trace metal residence time within close geographic proximity established by physical and biologic controls, dictated by local winds. Lower trace metal scavenging rates, resulting in a longer residence time, are suggested during northward transport of Columbia River plume waters, compared to higher scavenging rates, and thus shorter residence times, during southerly transport of plume waters. Gallium, demonstrating high analytical precision, reduced contamination risk, and decreased complications of upwelled sources relative to Zr, is proposed to provide identification of aged Columbia River plume waters.

Behaviour of colloidal trace metals (Cu, Pb and Cd) in estuarine waters: An approach using frontal ultrafiltration (UF) and stripping chronopotentiometric methods (SCP)

The frontal cascade ultrafiltration (UF) technique in conjunction with stripping chronopotentiometry (SCP) has been evaluated for determining the colloidal distribution of Cu, Pb and Cd in estuarine waters. Metallic concentrations in seven size fractions (0.45 μm–0.22 μm; 0.22 μm–300 kDa; 300–50 kDa; 50–30 kDa; 30–10 kDa; 10–5 kDa; <5 kDa) were determined with the aim to investigate their changes along the salinity gradient of the Penzé system (NW France). These data, completed by analysis of the total dissolved metals at 10 stations over the whole freshwater–seawater mixing zone, provided some insights in the removal and addition processes that affect Cu, Pb and Cd in estuaries.

Submarine groundwater discharge of nutrients to the ocean along a coastal lagoon barrier, Southern Brazil

The Patos–Mirim Lagoon system along the southern coast of Brazil is linked to the coastal ocean by a narrow mouth and by groundwater transport through a Holocene barrier. Although other groundwater systems are apparently active in this region, the hydraulic head of the lagoon, the largest in South America, drives groundwater transport to the coast. Water levels in wells placed in the barrier respond to changing water level in the lagoon. The wells also provide a measure of the nutrient concentrations of groundwater flowing toward the ocean. Additionally, temporary well points were used to obtain nutrient samples in groundwater on the beach face of the barrier. These samples revealed a subterranean freshwater–seawater mixing zone over a ca. 240 km shoreline. Previously published results of radium isotopic analyses of groundwater and of surface water from cross-shelf transects were used to estimate a water flux of submarine groundwater discharge (SGD) to nearshore surface waters of 8.5 × 10 7 m 3/day. Using this SGD and the nutrient concentrations in different compartments, nutrient fluxes between groundwater and surface water were estimated. Fluxes were computed using both average and median reservoir (i.e. groundwater and surface water) nutrient concentrations. The SGD total dissolved inorganic nitrogen, phosphate and silicate fluxes (2.42, 0.52, 5.92 × 10 6 mol day − 1, respectively) may represent as much as 55% (total N) to 10% (Si) of the nutrient fluxes to the adjacent shelf environment. Assuming nitrogen limitation, SGD may be capable of supporting a production rate of ca. 3000 g C m 2 year − 1in the nearshore surf zone in this region.

Experimental estimation of sorption removal influence on the transformation of dissolved uranium (VI) runoff in the river water–seawater mixing zone

Experimental estimation of sorption removal influence on the transformation of dissolved uranium (VI) runoff in the river water–seawater mixing zone

Exposure of inner-shelf reefs to nutrient enriched runoff entering the Great Barrier Reef Lagoon: Post-European changes and the design of water quality targets

We used historical flood plume extent data (modelled) to quantify the typical spatial extent of the summer runoff–seawater mixing zone of the Great Barrier Reef (GBR) lagoon. Spatially explicit analysis of the variability of in situ chlorophyll a concentrations (observed) across the runoff–seawater mixing zone, then allowed us to explore regional differences in the nutrient enrichment impact of runoff events from the various river systems that drain the GBR catchment. We demonstrate the existence of a discernable north–south gradient along the length of the GBR, such that for equivalent runoff:seawater dilutions ratios, lower levels of nutrient enrichment (as indicated by chlorophyll a observations) result from the river systems that drain the relatively undisturbed northern areas of the GBR catchment, compared to more human-impacted central and south areas. We identify a strong correlation between this north–south enrichment gradient and the flood concentration of dissolved inorganic nitrogen (DIN) entrained by the various river systems. By substituting the nutrient enrichment characteristics of the human-impacted river discharges with those of the undisturbed northern rivers, we provide a means to compare the short-term enriching ‘footprint’ for existing runoff intrusions with those that are likely to have occurred under pre-European catchment conditions. We demonstrate that under pre-European conditions, the nutrient enriching impact from river runoff was likely to have been largely constrained within 1–2 km of the coast, whereas existing conditions support the impact of reefs some 20–30 km off the coast. By using the developed spatial relations, we show that for the heavily human-impacted river systems, reductions in the end-of-river concentrations of DIN in the order 50–80% are needed in order to restore parity with pre-European conditions. We discuss these results in regard to developing end-of-catchment water quality targets for the region.

Trace element cycling in a subterranean estuary: Part 2. Geochemistry of the pore water

Submarine groundwater discharge (SGD) is an important source of dissolved elements to the ocean, yet little is known regarding the chemical reactions that control their flux from sandy coastal aquifers. The net flux of elements from SGD to the coastal ocean is dependent on biogeochemical reactions in the groundwater–seawater mixing zone, recently termed the “subterranean estuary.” This paper is the second in a two part series on the biogeochemistry of the Waquoit Bay coastal aquifer/subterranean estuary. The first paper addressed the biogeochemistry of Fe, Mn, P, Ba, U, and Th from the perspective of the sediment composition of cores Charette et al. [Charette, M.A., Sholkovitz, E.R., Hansell, C.M., 2005. Trace element cycling in a subterranean estuary: Part 1. Geochemistry of the permeable sediments. Geochim. Cosmochim. Acta, 69, 2095–2109]. This paper uses pore water data from the subterranean estuary, along with Bay surface water data, to establish a more detailed view into the estuarine chemistry and the chemical diagenesis of Fe, Mn, U, Ba and Sr in coastal aquifers. Nine high-resolution pore water (groundwater) profiles were collected from the head of the Bay during July 2002. There were non-conservative additions of both Ba and Sr in the salinity transition zone of the subterranean estuary. However, the extent of Sr release was significantly less than that of its alkaline earth neighbor Ba. Pore water Ba concentrations approached 3000 nM compared with 25–50 nM in the surface waters of the Bay; the pore water Sr-salinity distribution suggests a 26% elevation in the amount of Sr added to the subterranean estuary. The release of dissolved Ba to the mixing zone of surface estuaries is frequently attributed to an ion-exchange process whereby seawater cations react with Ba from river suspended clay mineral particles at low to intermediate salinity. Results presented here suggest that reductive dissolution of Mn oxides, in conjunction with changes in salinity, may also be an important process in maintaining high concentrations of Ba in the pore water of subterranean estuaries. In contrast, pore water U was significantly depleted in the subterranean estuary, a result of SGD-driven circulation of seawater through reducing permeable sediments. This finding is supported by surface water concentrations of U in the Bay, which were significantly depleted in U compared with adjacent coastal waters. Using a global estimate of SGD, we calculate U removal in subterranean estuaries at 20 × 10 6 mol U y −1, which is the same order of magnitude as the other major U sinks for the ocean. Our results suggest a need to revisit and reevaluate the oceanic budgets for elements that are likely influenced by SGD-associated processes.

Seasonal variations of cadmium speciation in the Penzé estuary, NW France

The study reported here was carried out in the macrotidal estuary of Penzé (Brittany, Western Channel, France). Ten field stations along the freshwater–seawater mixing zone were sampled each month in order to examine the speciation of dissolved and particulate cadmium. Different biogeochemical parameters (suspended particulate matter, Chlorophyll- a, pH and dissolved organic carbon) were also measured. Levels of total dissolved and total particulate metal ranged from 0.06 to 0.46 nM and from 0.4 to 2.4 μg g −1, respectively. Our data showed a non-conservative behaviour for cadmium as a result of desorption from particles. The rise in total dissolved cadmium in the salinity range 0–20 was about 0.2 nM. Incidentally, a decrease of total particulate concentrations of about 0.5 μg g −1, which corresponds approximately to 0.2 nM, was observed. The study of the various cadmium species led us to identify desorption of cadmium occurred from the iron/manganese oxide-associated fraction. This desorption process leads to the formation of labile complexes, i.e. chlorocomplexes in spring and summer. However, in winter and autumn, a large part of the dissolved fraction was constituted by organic complexes. These complexes can be formed from suspended particles desorption or could be a released from sediment. Analysis of flux indicated that most of the metal inputs to the estuary (usually more than 80%) corresponded to organic cadmium. Spring and summer outputs were mainly constituted of chlorocomplexes whereas cadmium organic complexes were predominant in winter and autumn output fluxes.

Seasonal variations of dissolved and particulate copper species in estuarine waters

This one-year survey conducted in the macrotidal estuary of Penzé (Brittany, Western part of the Channel, France) was aimed at examining the variations of the various dissolved and particulate copper species. Ten field stations along the whole freshwater–seawater mixing zone were sampled each month. Different biogeochemical parameters (SPM, chl- a, pH and DOC) were also measured. The levels in total dissolved and total particulate copper ranged from 1.8 to 9.5 nM and from 5 to 98 μg g −1, respectively; such amounts are indicative of a pollution-free system. Extractable C 18 copper (non-polar hydrophobic organic copper species), in winter and spring, accounted for 30–40% of the total dissolved copper. In summer, this contribution rapidly rose to 60% in the salinity range 20–30; over the same period of time, total particulate copper decreased. The change in dissolved copper speciation and the lowering of particulate copper concentrations were attributed to the release of strong organic ligands by phytoplankton. Our field data evidenced a highly variable behaviour for the various copper species over the seasonal cycle, and then led us to identify the following mechanisms: (i) metal desorption from organic river-flown particles (winter and spring), (ii) metal desorption from resuspended sediment in the upstream section (summer), (iii) competition between particles, non-extractable C 18 organic ligands and phytoplankton-released extractable C 18 organic ligands to complex copper in the downstream section (summer), and (iv) removal of non-extractable C 18 organic copper by adsorption (autumn). Dissolved copper species fluxes were assessed: most of metal inputs to the estuary (60–74%) corresponded to non-extractable C 18 organic copper. Winter and spring metal output fluxes were mainly constituted of non-extractable C 18 organic complexes; on the other hand, extractable C 18 organic complexes were predominant in summer and autumn output fluxes.

Geochemistry of Rare Earth Elements in the Ocean

This work briefly outlines modern ideas on geochemistry of rare earth elements (REE) in the ocean. Sources of REE and chemical properties of these elements, which govern their migration ability in natural processes, are considered. The REE behavior in the river water–seawater mixing zone is analyzed. The fractionation of dissolved and suspended REE in oceanic water in both aerobic and anaerobic conditions is also considered. It is shown that the variability of REE composition in pelagic sediments reflects the fractionation of these elements in the oceanic water as a consequence of material differentiation in the ocean. The REE distribution in terrigenous, authigenic, hydrothermal, and biogenic constituents of sediments, such as clay, bone debris, barite, phillipsite, Fe–Mn oxyhydroxides (ferromanganese nodules and micronodules), Fe–Ca hydroxo-phosphate, diatoms, and foraminifers, is considered.

Dynamics of particulate major and trace elements in the lower reaches of the Daugava River and adjacent area of the Gulf of Riga (Baltic Sea)

Suspended particulate matter (SPM) was collected in the freshwater–seawater mixing zone in the lower reaches of the Daugava River (Latvia) and adjacent marine area, during five cruises in 1998–2001. The study focused on biogeochemical phase exchange processes. SPM in the freshwater was found to be mainly allochthonous with a high content of organic matter, Mn and sorbed phosphate. Property-salinity plots suggested flocculation of humic-Fe complexes across the salinity gradient. The variability of sorbed phosphate was related to particulate Fe, although no dependence on pH and ionic strength was observed. The Mn contents of SPM mainly follow conservative mixing, but there are also indications of interface exchange of Mn in the mixing zone. The geochemical behaviour of particulate Al appears to differ from that of Fe. In early spring, trace element contents in SPM (Cr, Ni, Cu, Zn, Cd, Pb) correlate tightly with particulate organic matter, whose distribution is linked to phytoplankton distributions.

Use of tritium and helium to define groundwater flow conditions in Everglades National Park

The concentrations of tritium (3H) and helium isotopes (3He and 4He) were used as tracers of groundwater flow in the surficial aquifer system (SAS) beneath Everglades National Park (ENP), south Florida. From ages determined by 3H/3He dating techniques, groundwater within the upper 28 m originated within the last 30 years. Below 28 m, waters originated prior to 30 years before present with evidence of mixing at the interface. Interannual variation of the 3H/3He ages within the upper 28 m was significant throughout the 3 year investigation, corresponding with varying hydrologic conditions. In the region of Taylor Slough Bridge, younger groundwater was consistently detected below older groundwater in the Biscayne Aquifer, suggesting preferential flow to the lower part of the aquifer. An increase in 4He with depth in the SAS indicated that radiogenic 4He produced in the underlying Hawthorn Group migrates into the SAS by diffusion. Higher Δ4He values in brackish groundwaters compared to fresh waters from similar depths suggested a possible enhanced vertical transport of 4He in the seawater mixing zone. Groundwater salinity measurements indicated the presence of a wide (6–28 km) seawater mixing zone. Comparison of groundwater levels with surface water levels in this zone indicated the potential for brackish groundwater discharge to the overlying Everglades surface water.

Biogeochemical processes in a freshwater–seawater mixing zone in permeable sediments along the coast of Southern Brazil

The southern portion of the Brazilian coast is dominated by coastal lagoons formed by sandy barrier spits with small inlets. This coastal configuration is a barrier to the surface flow of freshwater to the sea; thus, we suspect that a significant amount of freshwater flows through the permeable sands, beneath the barrier spits, where it mixes with seawater. We excavated an 18-m-deep well into the barrier spit which separates the Patos Lagoon from the South Atlantic. Using this well, we were able to sample interstitial waters from discrete layers, at 1-m intervals, which were analyzed for salinity, temperature, pH, nutrients (ammonium, nitrate, phosphate, and silicate), uranium, molybdenum, and barium. Similar analyses were made on surface water samples from the Patos Lagoon estuarine mixing zone. Results of well samples show a continuous increase in salinity with depth reaching 18 at the bottom. Ammonium and silicate are high, generally around 100 and 100–150 μM, respectively, throughout the subterranean profile. Phosphate shows a distinct maximum at about 6 m (ca. 25 μM), and nitrate is generally low in all well samples. Uranium and molybdenum exhibit a minimum in the well profile at about the same location where barium exhibits a maximum (greater than 2 μM). When results are compared to the surface lagoon–seawater mixing data, ammonium, phosphate, silicate, and barium in well samples of similar salinity show considerable enrichment, while a comparison of uranium and molybdenum data indicates significant depletion of these metals in most well samples. Based on these and other data, we deduce that the following processes are active: products of remineralization of organic detritus accumulated in lagoon sediments are advected through permeable sediments to the oceans; dissolution of biogenic solids and/or solid silicates mobilizes silicate; phosphate, uranium, and molybdenum are mobilized from phosphate-rich sediment layers; sulfate reducers remove uranium and perhaps molybdenum from solution throughout most of the well profile; barium is desorbed from solids in the subterranean mixing zone. These results demonstrate that freshwater discharged to the ocean through permeable sediments may have a significantly different composition than that discharged at the surface.