Mixing Of Surface Water Research Articles

Groundwater-surface water mixing shifts ecological assembly processes and stimulates organic carbon turnover.

Environmental transitions often result in resource mixtures that overcome limitations to microbial metabolism, resulting in biogeochemical hotspots and moments. Riverine systems, where groundwater mixes with surface water (the hyporheic zone), are spatially complex and temporally dynamic, making development of predictive models challenging. Spatial and temporal variations in hyporheic zone microbial communities are a key, but understudied, component of riverine biogeochemical function. Here, to investigate the coupling among groundwater–surface water mixing, microbial communities and biogeochemistry, we apply ecological theory, aqueous biogeochemistry, DNA sequencing and ultra-high-resolution organic carbon profiling to field samples collected across times and locations representing a broad range of mixing conditions. Our results indicate that groundwater–surface water mixing in the hyporheic zone stimulates heterotrophic respiration, alters organic carbon composition, causes ecological processes to shift from stochastic to deterministic and is associated with elevated abundances of microbial taxa that may degrade a broad suite of organic compounds.

Bacterial growth rate and the relative abundance of bacteria to heterotrophic nanoflagellates in the euphotic and disphotic layers in temperate coastal waters of Sagami Bay, Japan

This study aimed to clarify the vertical differences in bacterial growth and grazing pressure on bacteria by heterotrophic nanoflagellates (HNF) and to identify the controlling factors of bacterial growth in temperate coastal waters of Sagami Bay, Japan. In addition to environmental factors, the annual monthly variations in bacterial growth rate (BGR) and the relative abundance of bacteria to HNF (BA/HNFA) were investigated in the euphotic and disphotic layers between May 2012 and May 2013. Significant vertical differences in BGR and BA/HNFA were evident between the two layers during the thermal stratification times of May to October 2012 and April to May 2013. BGR indicated significantly stronger limitation of bacterial growth in the euphotic layer compared to the disphotic layer. In contrast, significantly lower BA/HNFA was observed in the euphotic layer, suggesting significantly higher grazing pressure on bacteria by HNF. However, significant differences in BGR and BA/HNFA were not observed between the two layers from November 2012 to Match 2013, when the water column was well-mixed vertically due to the cooling and wind-induced mixing of surface water. This study indicates that bacteria in the euphotic layer grow less actively and are more vulnerable to predatory grazing by HNF relative to the disphotic layer during the stratification period. Further, multiple regression analyses indicate that bacterial growth was most controlled by the concentrations of chlorophyll a and dissolved organic carbon in the euphotic and disphotic layers, respectively.

Contrasting distributions of dissolved gaseous mercury concentration and evasion in the North Pacific Subarctic Gyre and the Subarctic Front

The distribution of dissolved gaseous mercury (DGM) and the oxidation–reduction processes of mercury (Hg) in the surface and subsurface ocean are currently understudied despite their importance in ocean–atmosphere interactions. We investigated the Hg(0) evasion and the DGM distribution at water depths of 2–500m in the Subarctic Front, Western Subarctic Gyre, and Bering Sea of the Northwestern Pacific. The mean DGM concentration in the surface mixed water (<10m) and the mean Hg(0) evasion flux were significantly higher in the Subarctic Front (125±5.0fM and 15pmolm−2h−1, respectively), which typically has lower nutrient levels and higher primary production, than in the Western Subarctic Gyre and the Bering Sea (74±18fM and 3.2±1.2pmolm−2h−1, respectively). The variation in the chlorophyll-a concentration and extracellular protease activity predicted 54% and 48% of the DGM variation, respectively, in the euphotic zone (2–50m). The DGM concentration in aphotic intermediate water (415±286fM) was positively correlated to the apparent oxygen utilization (AOU; r2=0.94 and p<0.001 for the Western Subarctic Gyre and the Bering Sea; r2=0.61 and p=0.01 for the Subarctic Front), emphasizing the importance of microbial oxidation of organic matter. The DGM-to-AOU ratio in aphotic water was significantly (p<0.05, ANCOVA) higher at the Western Subarctic Gyre and Bering Sea sites (2.5±0.14) than the ratio at the Subarctic Front sites (0.89±0.27) that mainly consisted of newly formed North Pacific Intermediate Water. The overall results imply that variation of DGM and Hg(0) evasion is closely linked to primary production in euphotic water and organic remineralization in aphotic intermediate water. The oceanic alterations in these factors may induce significant modification in Hg redox speciation in the Northwestern Pacific.

A Pliocene to recent history of the Bering Sea at Site U1340A, IODP Expedition 323

Fossil diatoms are the principal component of Bering Sea sediments and reflect the paleoceanographic history of the region. Diatom accumulation rates and relative abundances at International Ocean Discovery Program (IODP) Site U1340A are presented. Overall, the total diatom productivity record from 4.9 Ma to the present day reveals a fourfold reduction at circa 4.2 Ma from ~45 × 107 down to 11 × 107 valves/g (wet sediment), signifying a major shift in the upwelling and/or nutrient regime, coinciding with the end of the late Miocene-early Pliocene bloom identified in the eastern equatorial Pacific and California margin. Further abrupt shifts in the diatom assemblage occur at (1) 2.78–2.55 Ma, (2) 2.0–1.8 Ma, and (3) 1.0–0.88 Ma. (1) At 2.78–2.55 Ma, the appearance of sea ice-related species marks the regional cooling associated with the expansion of Northern Hemisphere ice sheets, subsequent development of stratified, nutrient-depleted waters, and increased influence of Western Basin Water masses (most likely due to the suppressed inflow of the Alaskan Stream). (2) Rapid cooling between 2.0 and 1.8 Ma indicates increased sea ice duration and/or frequency. This, coupled with low sea level stands caused prolonged closure of the Aleutian Passes, coupled with further increased Western Basin Water inflow. (3) The shift to 100 ka glacial/interglacial cycles at the middle-Pleistocene transition (1.0–0.88 Ma) marked an increase in upwelling-related species, indicating enhanced surface water mixing. These records confirm that the development and changing dynamics of sea ice in the Bering Sea played a major role in sub-Arctic Ocean circulation and is an integral component of global climate change.

Reductive weathering of black shale and release of barium during hydraulic fracturing

Hydraulic fracturing is an important technological advance in the extraction of natural gas and petroleum from black shales, but water injected into shale formations in the fracturing process returns with extraordinarily high total-dissolved-solids (TDS) and high concentrations of barium, Ba. It is generally assumed that high TDS comes from the mixing of surface water (injected fluid) with Na–Ca–Cl formation brines containing elevated Ba, but the mechanisms by which such mixing might occur are disputed. Here we show that Ba in water co-produced with gas could originate from water-rock reactions, with Ba levels observed in produced waters reached on a time scale relevant to hydraulic fracturing operations. We examined samples from three drill cores from the Marcellus Shale in Pennsylvania and New York to determine the possible water-rock reactions that release barium during hydraulic fracturing. Two samples, one containing microcrystalline barite (BaSO4) and one without barite, contain elevated concentrations of Ba relative to the crustal average for shale rocks. A third sample is slightly depleted in Ba relative to the crustal average. Micro-XRF measurements and SEM/EDS analysis combined with chemical sequential extraction methods reveal that a majority of the Ba in all samples (55–77 wt.%) is present in clays and can only be leached from the rock by dissolution in hydrofluoric acid. Thus, a majority of barium in our samples is relatively inaccessible to leaching under hydraulic fracturing conditions. However, the balance of Ba in the rocks is contained in phases that are potentially leachable during hydraulic fracturing (e.g., soluble salts, exchangeable sites on clays, carbonates, barite, organics).We next studied how shale reacts with water at elevated temperatures (80 °C), low Eh (−100 to −200 mV), and a range of ionic strengths (IS = 0.85–6.4) that emulate conditions prevalent at depth during hydraulic fracturing. Our experimental results indicate that the amount of Ba released from the bulk rock has a positive correlation with the ionic strength of the reacting fluid. Between 5 and 25% of the total Ba in the rock can be leached from shale under ionic strength conditions and leachate compositions typical of produced waters over a contact time of just 7 days. We suggest that reductive weathering of black shale occurs during hydraulic fracturing due to: 1) Ba2+ in clays exchanging with Na+ and Ca2+ ions that are present in high concentrations in produced water, and 2) increased solubility and dissolution kinetics of barite under high ionic strength conditions. At the low Eh conditions prevalent during hydraulic fracturing the sulfate deficient water allows Ba to be dissolved into the produced water. Based on Ba yields determined from laboratory leaching experiments of Marcellus Shale and a reasonable estimate of the water/rock mass ratio during hydraulic fracturing, we suggest that all of the Ba in produced water can be reconciled with leaching directly from the fractured rock.

Rare earth elements (REE) in deep groundwater from granite and fracture-filling calcite in the Tono area, central Japan: Prediction of REE fractionation in paleo- to present-day groundwater

Rare earth elements (REEs) combined with yttrium (YREE) in deep groundwater from granite and fracture-filling calcite are being studied at the Mizunami Underground Research Laboratory (MIU, Tono area, central Japan). The groundwater was sampled from cored boreholes drilled from the MIU galleries at 200, 300, 400 and 500m depths. The fracture-filling calcite was collected from the same boreholes and dissolved in 1M NH4 acetate buffer solution (pH4.8). Major chemical composition of groundwater changed with depth; and the concentrations of some major elements (Na+, K+, Ca2+ Mg2+, Cl−, Br− and B) increased with depth, while alkalinity, SO42−, and F− decreased. Isotopic compositions of δ18O and δD indicated that the groundwater originated from meteoric water. YREE patterns of groundwater also changed with depth; with heavy REE (HREE) enrichment in “shallow” groundwater (−200 to −400m) and light REE (LREE) enrichment in “deep” groundwater (−500m). The predominant YREE species are YREECO3+ and YREE(CO3)2− (LaCO3+: 65–70%, La(CO3)2−: 8–27%, LuCO3+: 20–38% and Lu(CO3)2−: 53–77%). The LREE enrichment in the “shallow” groundwater resulted in preferential scavenging of positively charged YREECO3+ by negatively charged bedrock surfaces in slightly alkaline groundwater pH (7.4–8.6). Because of the construction and operation of the underground facility, the “shallow” groundwater has been influenced by infiltration of surface water undersaturated in calcite. In contrast, the “deep” groundwater was barely affected by the mixing of surface water, indicating that the YREE originated from YREE fractionation by bedrock granite. Irrespective of depth, a negative Ce anomaly preserved in the groundwater, is probably indicative of paleo-oxic conditions during paleo-recharge. The negative Ce anomaly in the groundwater was possibly fractionated from sedimentary calcite with positive Ce anomalies during the paleo-recharge. The YREE patterns of the fracture-filling calcite were enriched in LREE. The isotopic variations of δ18O and δ13C in the fracture-filling calcite indicated that it originated from a mixture of sea water and fresh water at a temperature <100°C. The YREE patterns of paleo-groundwater from which the fracture calcite precipitated were calculated using YREE abundances in the fracture calcite and distribution coefficient reported in previous laboratory experiments. The paleo-groundwater was found to be enriched in LREE and HREE depending on the aqueous YREE species. Our results suggest that changes in the paleo- to present-day YREE fractionation range roughly from that of present-day shallow to deep groundwater.

Oxygenation of an anoxic fjord basin strongly stimulates benthic denitrification and DNRA

Hypoxia hampers eutrophication reduction efforts by enabling high nutrient fluxes from sediment to bottom waters. Oxygenation of hypoxic water bodies is often proposed to reduce benthic ammonium and phosphate release. This study investigates the functional response of benthic nitrate-reducing processes to a long-term engineered oxygenation effort in a density-stratified fjord with euxinic bottom waters. Oxygenation was achieved by mixing surface water with deep, euxinic water, which increased oxygen and nitrate concentrations in the deep water column. The presence of nitrate instigated benthic nitrate reduction in the newly oxidized sediments by equally stimulating denitrification and dissimilatory nitrate reduction to ammonium (DNRA). DNRA and total nitrate reduction rates, as well as the contribution of DNRA to total nitrate reduction, decreased with increasing exposure time of the sediments to oxygen. The relative importance of DNRA as a nitrate sink was correlated to nitrate concentrations, with more nitrate being reduced to ammonium at higher bottom water nitrate concentrations. Overall, engineered oxygenation decreased the net efflux of dissolved inorganic nitrogen from the sediments by stimulating net nitrate removal through denitrification.

Effect of water de-stratification on dissolved oxygen and ammonia in tilapia ponds in Northern Thailand

Episodes of low concentrations of dissolved oxygen and high concentrations of ammonia are major causes of fish stress, which in turn, reduces growth and increases mortality rates in aquaculture ponds. This study measured the effects of water de-stratification on dissolved oxygen and ammonia concentrations in tilapia ponds in Northern Thailand. Fifteen ponds in five provinces in Northern Thailand were sampled on multiple dates in the hot, wet, and dry seasons. Thermal water stratification peaked around 14:00-16:00 h each day; wherein the differences between surface and lower water temperatures in the 0.8-2.0 m deep ponds reached 1.3-4.0 C. Thermal de-stratification in the hot season and dry season usually occurred late at night; in the wet season however, it occurred early in the evening due to the cooling effects of rain. The mixing of surface and bottom waters decreased dissolved oxygen levels in water near the surface, and, increased concentration levels in the bottom water layer. Mean DO concentrations of integrated and commercial ponds were lower than 1 mg/L from 02:00 to 06:00 h, in 68 % of the observations. Repeated measures ANOVA analysis showed that water depth and fish culture systems have significant effects on total ammonia nitrogen (TAN). TAN concentrations near the surface were lower than at the bottom, and increased after water de- stratification. TAN in the integrated culture system was significantly higher than in commercial and subsis- tence systems. Toxic, un-ionized ammonia (NH3) concentrations in all culture systems, however, were still lower than standard toxicity level for tilapia. The improved understanding of the seasonal and culture-system specific effects of de-stratification on pond water quality provided by this study, are useful for improving pond management practices to reduce the risks of exceeding DO thresholds. In particular, fish farmers should: (1) take care to avoid over-feeding and manage water and sediments to prevent excessive accumulation of organic matter and waste at the bottom of ponds, which can influence other water quality parameters; and, (2) use aeration and mechanical mixing interventions at critical times to reduce stress on fish from low DO con- centrations, and thus avoid risks of mass mortality events.

Mixing effects on nitrogen and oxygen concentrations and the relationship to mean residence time in a hyporheic zone of a riffle-pool sequence

Flow paths and residence times in the hyporheic zone are known to influence biogeochemical processes such as nitrification and denitrification. The exchange across the sediment-water interface may involve mixing of surface water and groundwater through complex hyporheic flow paths that contribute to highly variable biogeochemically active zones. Despite the recognition of these patterns in the literature, conceptualization and analysis of flow paths and nitrogen transformations beneath riffle-pool sequences often neglect to consider bed form driven exchange along the entire reach. In this study, the spatial and temporal distribution of dissolved oxygen (DO), nitrate ( NO3−) and ammonium ( NH4+) were monitored in the hyporheic zone beneath a riffle-pool sequence on a losing section of the Truckee River, NV. Spatially varying hyporheic exchange and the occurrence of multi-scale hyporheic mixing cells are shown to influence concentrations of DO and NO3− and the mean residence time (MRT) of riffle and pool areas. Distinct patterns observed in piezometers are shown to be influenced by the first large flow event following a steady 8 month period of low flow conditions. Increases in surface water discharge resulted in reversed hydraulic gradients and production of nitrate through nitrification at small vertical spatial scales (0.10–0.25 m) beneath the sediment-water interface. In areas with high downward flow rates and low MRT, denitrification may be limited. The use of a longitudinal two-dimensional flow model helped identify important mechanisms such as multi-scale hyporheic mixing cells and spatially varying MRT, an important driver for nitrogen transformation in the riverbed. Our observations of DO and NO3− concentrations and model simulations highlight the role of multi-scale hyporheic mixing cells on MRT and nitrogen transformations in the hyporheic zone of riffle-pool sequences.

Zanclean/Piacenzian transition on Cyprus (SE Mediterranean): calcareous nannofossil evidence of sapropel formation

Quantitative analyses of calcareous nannofossils in the sediments of Pissouri South section on the island of Cyprus have produced a paleoceanographic record reflecting the paleoclimatic conditions during the Zanclean/Piacenzian transition. Pissouri South cyclical lithological alternations between organic-rich laminated layers and grey marls reflect the Earth’s orbital precession. According to the calcareous nannofossil biostratigraphy which has been performed, the studied section is correlated with MNN14/15 and MNN16 calcareous nannofossil biozones and is astronomically dated between 4.065 and 3.217 Ma. Intervals of increased organic carbon content, along with the positive values of Florisphaera profunda, Helicosphaera sellii, Discoaster spp. and the subsequent increase of stratification S-index, correspond to the sapropel deposition during periods of wetter climate and intense continental runoff, especially from the river Nile. These layers alternate with grey marly intervals, featured by the increased values of small placoliths of Reticulofenestra and Gephyrocapsa species, which are indicative of eutrophic conditions during intense surface-water mixing. Our data support the prevalence of a generally warm phase characterized by the absence of high-frequency climate variations in the southeastern Mediterranean during the Zanclean/Piacenzian (Early/Late Pliocene) transition.

Coccolithophore variability from the Shackleton Site (IODP Site U1385) through MIS 16-10

Coccolithophore assemblages have been investigated at Integrated Ocean Drilling Program Site U1385, on the western Iberian margin, through Marine Isotope Stage (MIS) 16 to 10, between the end of the Mid-Pleistocene Transition and the Mid-Brunhes interval, with the aim to reconstruct orbital and millennial-scale surface water modifications. Assemblage variations are interpreted in terms of paleoclimate and paleoproductivity proxies. The pattern of C37 alkenones is also presented as an additional indicator of primary paleoproductivity. The overall proxies are compared with the available benthic and planktonic δ18O records and Ca/Ti profile. A new benthic and planktonic δ13C dataset is also shown. The coccolithophore abundance mirrors the Ca/Ti pattern indicating that coccolith-derived carbonate is the dominant contributor to carbonate production in the studied interval. The distinct increase in the coccolithophore abundance, as well as in the accumulation rate, occurring at the MIS 14/13 transition, reflects the beginning of the worldwide-scale mid-Brunhes blooming of Gephyrocapsa caribbeanica and triggers the increase in carbonate production imprinted on the Ca/Ti profile. Interglacials are marked by enhanced abundances of the coccolithophore warm water group (wwt group) that also displays high frequency variability related to precessional/insolation forcing. Warmest surface water conditions are recorded during MIS 15, suggesting an intensified contribution of the subtropical AzC, essentially during MIS 15.5 and 15.1. Reduced productivity in these intervals is in agreement with a major influence of nutrient-poor and less ventilated subtropical waters. On the other hand, productive and mixed surface water conditions can be inferred during MIS 13 in agreement with other North Atlantic records. A long lasting period of warm, stratified and oligotrophic waters is inferred during MIS 11.3, indicating a continuous and more persistent influence of subtropical waters at the site location. Glacial phases are marked by increases of Coccolithus pelagicus ssp. pelagicus and of Gephyrocapsa margereli–Gephyrocapsa muellerae. The pattern of C. pelagicus ssp. pelagicus during MIS 16 is in agreement with a southern position of the Polar Front at the end of the Mid-Pleistocene Transition with respect to younger counterparts, whereas the pattern of the wwt group during MIS 14 attests the influence of subtropical water during this weak glacial. Throughout the interval, short-lived increases of C. pelagicus ssp. pelagicus, G. margereli–G. muellerae>4μm and of reworked taxa are concomitant to decreases of coccolithophore productivity and heavier values of planktonic δ18O, testifying the occurrence of abrupt cold episodes related to North Hemisphere millennial-scale climate oscillations.

Glacial-interglacial productivity contrasts along the eastern Arabian Sea: Dominance of convective mixing over upwelling

The western continental margin of India is one of the highly productive regions in the global ocean. Primary productivity is induced by upwelling and convective mixing during the southwest and northeast monsoons respectively. Realizing the importance of high primary productivity, a sediment core was collected below the current oxygen minimum zone (OMZ) from the southwestern continental margin of India. This was dated by AMS radiocarbon and as many as 60 paleoclimate/paceoceanographic proxies, such as particle size, biogenic components, major, trace and rare earth elements (REEs) which were measured for the first time to determine sources of sediment, biogeochemical processes operating in the water column and their variations since the last glacial cycle. R-mode factor analysis of comprehensive data indicates that the dominant regulator of paleoproductivity is the southwest monsoon wind induced upwelling. Other paleoproductivity related factors identified are the marine biogenic component and biogenic detritus (as an exported component from the water column added to the bottom sediment). All paleoproductivity components increased significantly during the marine isotope stage-1 (MIS-1) compared to those accumulated from MIS-4 to MIS-2. The second group of factors identified are the terrigenous sediments with heavy minerals like zircon and ilmenite. The terrigenous sediment, in particular, increased during MIS-2 when the sea-level was lower; however, the heavy mineral component fluctuated over time implying pulsed inputs of sediment. The diagenetic fraction and reducing component are the third group of factors identified which varied with time with increased accumulation during the MIS transitions.The primary productivity along the southwestern continental margin of India seems to have been controlled principally by the upwelling during the southwest monsoon season that was weaker from MIS-4 to MIS-2, as relative to that during the MIS-1. In contrast, increased glacial productivity noticed in sediments deposited below the current oxygen minimum zone (OMZ) along the north of the study area that can be linked to entrainment of nutrients through the intensified convective mixing of surface water during the northeast monsoon. The sequestration of greenhouse gases by the western continental margin of India was higher during glacial than interglacial cycles.

Microbial community of the bottom sediments of the brackish Lake Beloe (Transbaikal region)

Investigation of microbial taxonomic diversity and of the rates of microbial processes of production and decomposition of organic matter made it possible to establish considerable diversity and activity of the sulfur cycle microorganisms in the microbial community of Lake Beloe (pH 9.4, salinity 3.1 g/L) upper sediment layer. According to the results of pyrosequencing, of the 16S rRNA gene, bacteria involved in H2 formation and oxidation were numerically predominant and highly diverse. The Hydrogenophaga spp. dominating in the community are aerobic or facultatively anaerobic chemoorgano- and chemolithoautotrophs using hydrogen oxidation as the source of energy. They play an important role in the transitory zones of mixing of subterranean and surface water.

Palaeoenvironmental turnover across the Cenomanian-Turonian transition in Oued Bahloul, Tunisia: foraminifera and geochemical proxies

The integrated analysis of foraminiferal assemblages, geochemical proxies, and stable isotopes in the Oued Bahloul section (Tunisia) allowed us to reconstruct the environmental turnover across the Cenomanian–Turonian boundary. An increase in palaeoproductivity proxies (P/Ti, U/Al, Sr/Al) and in δ13C values, and a decrease in foraminiferal diversity and δ18O values mark the beginning of the Oceanic Anoxic Event 2 (OAE2) at the Rotalipora cushmani and Whiteinella archaeocretacea biozones boundary. Eutrophic conditions at the seafloor and in the water column are evidenced by high proportions of buliminids and the replacement of planktic oligotrophic specialist Rotalipora by eutrophic opportunist Hedbergella. The enrichment in organic matter and redox sensitive elements, together with the abundance of low-oxygen tolerant benthic foraminifera, indicate dysoxic conditions in the deep-water column and at the seafloor (higher Uaut than Moaut). Among planktic foraminifera, deep- and intermediate-dwellers disappear (Rotalipora and Globigerinelloides), and surface-dwellers proliferate (Hedbergella). The persistency of the poorly oxygenated conditions during the W. archaeocretacea Biozone locally produced euxinic conditions where MoEF and Moaut reach high values, diversity presents minimum values, and benthic foraminifera temporarily disappear. The maximum percentage of heterohelicids indicates a stratified water column with a well-developed oxygen minimum zone. Improved oxygen conditions returned in the upper part of the W. archaeocretacea Biozone and Helvetoglobotruncana helvetica Biozone, with a slow recovery of foraminiferal assemblages, decrease in eutrophic genera (Heterohelix) and increase in mesotrophic genera (Whiteinella). A gradual increase in δ18O values suggests decreased temperatures in surface waters. The OAE2 has been attributed to global temperature changes and palaeoceanographic reorganization. The poor mixing of surface and deep waters and enhanced primary productivity related to global warming – associated with increasing continental weathering and nutrient runoff – may have favored the eutrophication of the ocean and the expansion of the oxygen minimum zone.

Plankton blooms, ocean circulation and the European slope current: Response to weather and climate in the Bay of Biscay and W English Channel (NE Atlantic)

The flow of upper-layer surface water and circulation for the Bay of Biscay, continental slope and in the wider region of the NE Atlantic is presented, as well as the seasonality of flow and internal tides. The marine plankton environments of Biscay Ocean, Biscay Eddies, Biscay Slope and Biscay Shelf are defined. The Shelf region (Armorican and Celtic) is further divided into Stratified Shelf, Frontal and Tidally Mixed. Seasonal distributions of chlorophyll a are given for all environment from in situ measurements and remote sensing data. Mixing and stabilisation of surface water in the euphotic layer for the start of the spring bloom using in situ profiling measurements is examined. Some regional responses for the slope current, dinoflagellate blooms and interannual variations in spring diatom numbers with respect to weather and climate in the Bay of Biscay and around the British Isles are suggested and discussed. An example of the Eastern European Ocean Margin continental slope response to winter weather (sea level atmospheric pressure forcing) resulting in warm winter water in the southern Bay of Biscay (Navidad, with eddy production) and off the Shetland continental slopes (the warm-water supply route to the Arctic) is given from the slope climate observation series.

Numerical modeling of bed form induced hyporheic exchange

The hyporheic zone is a region beneath and alongside a stream, river, or lake bed where shallow groundwater and surface water mix. Field and experimental observations, along with modeling studies, indicate that hyporheic exchange occurs mainly in response to pressure gradients driven by the geomorphological features of stream beds. Flow over a pool-riffle sequence creates an irregular pressure gradient that drives hyporheic exchange. Currently to analyze the overall flow pattern in different types of pool-riffle structures, hyporheic exchange flow was analyzed using a fully coupled hydro-dynamic model. Simulation results showed that recirculation zones and stagnation points in the pool-riffle structures dominantly controlled the upwelling and downwelling patterns. Numerical simulations were analyzed for the velocity distribution, velocity vectors, and the streamline and flux of groundwater and surface water. Upwelling flow was dominated by a pressure gradient generated by the apex of riffles. Downwelling flow patterns were affected by the flow pattern formed in pools, which was related to the geometric shapes of the pools. The mixing pattern in the groundwater was also affected by the pool shape. The results will be applicable for river restoration projects and stream ecology related to hyporheic exchange, in the prediction and management of upwelling and downwelling flow induced by bed forms.

Sources of major ions and processes affecting the geochemical and isotopic signatures of subsurface waters along a tropical river, Southwestern India

Systematic monitoring of subsurface hydrogeochemistry has been carried out for a period of one year in a humid tropical region along the Nethravati-Gurupur River. The major ion and stable isotope (delta O-18 and delta H-2) compositions are used to understand the hydrogeochemistry of groundwater and its interaction with surface water. In the study, it is observed that intense weathering of source rocks is the major source of chemical elements to the surface and subsurface waters. In addition, agricultural activities and atmospheric contributions also control the major ion chemistry of water in the study area. There is a clear seasonality in the groundwater chemistry, which is related to the recharge and discharge of the hydrological system. On a temporal scale, there is a decrease in major cation concentrations during the monsoon which is a result of dilution of sources from the weathering of rock minerals, and an increase in anion concentrations which is contributed by the atmosphere, accompanied by an increase in water level during the monsoon. The stable isotope composition indicates that groundwater in the basin is of meteoric origin and recharged directly from the local precipitation during the monsoonal season. Soon after the monsoon, groundwater and surface water mix in the subsurface region. The groundwater feeds the surface water during the lean river flow season.

Characterization of groundwater and surface water mixing in a semiconfined karst aquifer using time‐lapse electrical resistivity tomography

AbstractGroundwater flow in karst includes exchange of water between large fractures, conduits, and the surrounding porous matrix, which impacts both water quality and quantity. Electrical resistivity tomography combined with end‐member mixing analysis (EMMA) and numerical flow and transport modeling was used to study mixing of karst conduit and matrix waters to understand spatial and temporal patterns of mixing during high flow and base flow conditions. To our knowledge, this is the first time EMMA and synthetic geophysical simulations have been combined. Here we interpret an 8 week time‐lapse electrical resistivity data set to assess groundwater‐surface mixing. We simulate flow between the karst conduits and the porous matrix to determine fractions of water recharged to conduits that has mixed with groundwater stored in the pore space of the matrix using a flow and transport model in a synthetic time‐lapse resistivity inversion. Comparing the field and synthetic inversions, our results enable us to estimate exchange dynamics, spatial mixing, and flow conditions. Results showed that mixing occurred at a volumetric flux of 56 m3/d with a dispersivity around 1.69 m during the geophysical experiment. For these conditions, it was determined that conduit water composition ranged from 75% groundwater during base flow conditions to less than 50% groundwater in high flow conditions. Though subject to some uncertainties, the time‐lapse inversion process provides a means to predict changing hydrologic conditions, leading to mixing of surface water and ground water and thus changes to water quantity and quality, as well as potential for water‐rock reactions, in a semiconfined, sink‐rise system.

Numerical verification of hyporheic zone depth estimation using streambed temperature

Summary The hyporheic zone is the active region of groundwater and surface water mixing, and consequently, through which the contaminants released to surface water are transferred to groundwater. Because of the high energy demand and expense associated with the remediation of contaminated groundwater, it has become a matter of primary concern to find an efficient and cost-effective way to restore the contaminated groundwater. Therefore delineating the hyporheic zone is of vital importance for the prevention and management of groundwater contamination. This paper proposes a method to estimate the depth of the hyporheic zone using heat transfer analysis for a streambed. In order to assess the adequacy of the proposed method, the sensitivity of the results of heat transfer analyses to the hyporheic flux was evaluated. Due to the high sensitivity to the hyporheic zone depth, the heat transfer analysis was determined to be appropriate to delineate the hyporheic zone depth. The depth estimated from the heat transfer analysis was comparable with that from a conventional tracer test. The proposed method has an advantage over existing methods of determining the hyporheic zone depth due to the fact that it only requires field temperature measurements.

Surface water renewal and mixing mechanisms in a semi-enclosed microtidal domain. The Barcelona harbour case

Water renewal and mixing are highly related to the evolution of the ecological status in harbour areas and are crucial for environmental harbour management. For the first time, these processes have been studied in the Barcelona harbour with a high resolution 3D hydrodynamic model. This harbour is representative of a semi-enclosed domain with a complex coastline in a micro-tidal environment (like most Mediterranean harbours). The tracking of numerical Lagrangian particles deployed in the flow was used to parameterize the surface water renewal mechanisms. The use of Finite Size Lyapunov Exponents has proven to be a relatively easy and efficient way to characterize the mixing patterns. The complex geometry of the harbour and the meteo-oceanographic forcings lead to intricate hydrodynamics that define spatial heterogeneity of water renewal and mixing. The most favourable conditions for enhancing surface water renewal and mixing have been identified in idealized scenarios and realistic simulations. In general, renewal is enhanced near the harbour mouths and strongly limited in the inner areas. However, under particular forcing conditions, the residence time can decrease even in the most sheltered areas. The presence of two mouths in the harbour seems to favour water renewal in comparison to harbours with only one mouth. Mixing is mainly induced by the action of wind forcing, while external shelf currents are much less efficient. The study of two realistic simulations suggests that harbour response to real forcing cannot be deduced from the combination of simple idealized scenarios. The time evolution of the forcings and the transient response of the system play a key role in defining the residence time patterns.