Advective Regime Research Articles

Hydrogeochemistry, groundwater ages and sources of salts in a granitic batholith on the Canadian Shield, southeastern Manitoba

The hydrogeochemistry of the Lac du Bonnet granitic batholith has been determined for the region of the Whiteshell Research Area (WRA) in southeastern Manitoba, Canada. This work forms part of the geosciences studies performed for the Canadian Nuclear Fuel Waste Management Program over the period 1980–1995 by Atomic Energy of Canada Limited (AECL). Knowledge of the variation of groundwater chemistry and its causes is useful in assessing the performance and safety of a nuclear fuel waste vault located at depths of up to 1000 m in a crystalline rock formation of the Canadian Shield. Groundwaters and matrix pore fluids have been obtained by standard sampling methods from shallow piezometers in clay-rich overburden, from packer-isolated borehole zones intersecting fractures or fault zones in the bedrock, and from boreholes in unfractured rock in AECL's Underground Research Laboratory (URL). Eighty-six individual fracture groundwaters have been sampled and analysed from permeable zones in 53 boreholes drilled to depths of up to 1000 m in the Lac du Bonnet batholith. In addition, 28 groundwaters from piezometers in a large wetland area near the URL have been sampled and analysed to determine the influence of clay-rich overburden on the bedrock hydrogeochemistry. Analyses have been made for major and minor ions, pH, Eh, trace metals, and stable and radioactive isotopes, to characterise these groundwaters and relate them to their hydrogeologic regimes. Shallow groundwaters in the fractured bedrock are generally dilute (TDS <0.3 g/l), Ca–Na–HCO 3 waters and show little indication of mixing with Ca–Mg–HCO 3–SO 4 groundwater from overburden sediments. The near-modern levels of 3H and 14C, and a warm-climate 2H/ 18O signature in these groundwaters, indicates that the upper ∼200 m of fractured bedrock contains an active groundwater circulation system with a residence time of tens to hundreds of years. Deeper fracture groundwaters (200–400 m depth) in recharge areas, are more alkaline, Na–Ca–HCO 3 waters and evolve to Na–Ca–HCO 3–Cl–SO 4 waters with increasing distance along the flow path. Isotopic data indicate the presence of a glacial melt-water component suggesting that the residence times of these waters are 10 3–10 5 a. These waters form a transition zone between the upper, advective flow regime and a deeper regime in sparsely fractured rock where groundwater in fractures and fracture zones is largely stagnant. At these depths (> 500 m), Na–Ca–Cl–SO 4 waters of increasing salinity (up to 50 g/l) with depth are found and in some fractures the waters have evolved to a Ca–Na–Cl composition. Isotopic data indicate that these waters are warm-climate and pre-glacial in origin, with residence times of over 1 Ma. Pore fluids observed to drain from the unfractured rock matrix in the URL facility are almost pure Ca–Cl in composition, ∼90 g/l salinity, and have a 2H/ 18O composition displaced well to the left of the global meteoric water line, about which all other WRA groundwaters lie. This information indicates that these pore fluids have undergone prolonged water-rock interaction and have residence times of 10 1–10 3 Ma. Most of the deeper fracture groundwaters and pore fluids have low Br/Cl ratios and moderate to high δ 34S values of dissolved SO 4 which indicates that their salinity could be derived from a marine source such as the basinal sedimentary brines and evaporites to the west of the batholith. These fluids may have entered the batholith during early Paleozoic times when sedimentary rocks were deposited over the granite and were driven by a hydraulic gradient resulting from higher ground in western Canada. The hydrogeochemical data and interpretations show that below ∼500 m in the WRA, fracture-hosted groundwaters are very saline, reducing and old, and are, therefore, indicative of stagnant conditions over the period of concern for nuclear waste disposal (1 Ma). The intact rock matrix at these depths is extremely impermeable as indicated by the presence of pore fluids with unusual geochemical and isotopic characteristics. The pore fluids may represent basinal brines that have evolved geochemically and isotopically to their current composition over periods as long as 10 3 Ma.

Flame enhancement and quenching in fluid flows

We perform direct numerical simulations of an advected scalar field which diffuses and reacts according to a nonlinear reaction law. The objective is to study how the bulk burning rate of the reaction is affected by an imposed flow. In particular, we are interested in comparing the numerical results with recently predicted analytical upper and lower bounds. We focus on the reaction enhancement and quenching phenomena for two classes of imposed model flows with different geometries: periodic shear flow and cellular flow. We are primarily interested in the fast advection regime. We find that the bulk burning rate v in a shear flow satisfies ν ∼ aU + b where U is the typical flow velocity and a is a constant depending on the relationship between the oscillation length scale of the flow and laminar front thickness. For cellular flow, we obtain ν ∼ U 1/4. We also study the flame extinction (quenching) for an ignition-type reaction law and compactly supported initial data for the scalar field. We find that in a shear flow the flame of size W can be typically quenched by a flow with amplitude U ∼ αW. The constant α depends on the geometry of the flow and tends to infinity if the flow profile has a plateau larger than a critical size. In a cellular flow, we find that the advection strength required for quenching is U ∼ W 4 if the cell size is smaller than a critical value.

Pore water anomalies of submarine gas-hydrate zones as tool to assess hydrate abundance and distribution in the subsurface: What have we learned in the past decade?

The most significant new contributions to the study of natural gas hydrates in the past decade have come from findings of the Ocean Drilling Program (ODP), notably legs 112, 141, 146 and 164, and to a lesser extent legs 131, 160, 170 and 186. Leg 164, in particular, was dedicated to gas-hydrate drilling in the Blake Ridge gas-hydrate field in the West Atlantic in an unprecedented multidisciplinary research effort [Paull et al., 1996. Proc. Ocean Drill. Program, Initial Rep. 164, 623 pp.; Paull C.K., Matsumoto, R., Wallace, P.J., Dillon, W.P. (Eds.), 2000a. Proc. Ocean Drill. Program Sci. Results 164, Ocean Drilling Program, College Station, TX, 459 pp.]. Most important for the progress of hydrochemical studies related to gas hydrates has been the growing awareness of the significance of diffusion-modulated advective processes shaping the chemical and isotopic pore water profiles in hydrate zones. This started with qualitative evidence for advective flow from drilling the (hydrate-bearing) Peru and (hydrate-free) Barbados active margins, continued with the Nankai Trough accretionary prism, the Japan Trench Slope and the Cascadia and Costa Rica active margins and culminated in the quantitative advection-diffusion model of Egeberg and Dickens [Chem. Geol. 153 (1999) 53.] for the passive margin setting of the Blake Ridge. Advective flow regimes are different at active and passive margins, as there is a tendency for the flow at active margins to be focussed along landward-dipping thrust planes and faults in the wedge of imbricated thrust sheets that finds expression in a step-pattern of the pore water profiles. For passive margins, but also for some active margin sites, advection is from sources below the drilled section, either through subvertical faults (passive margins) or from the décollement zone (active margins). We have learned that the well-known coupled pore water anomalies that are ascribed to hydrate dissociation—downward chlorinity decrease combined with δ 18O increase [Hesse and Harrison, 1981, Earth Planet. Sci. Lett. 55 (1981) 453.]—need not occur together in the presence of hydrates because the isotope effect may be overprinted by the effects of other reactions such as volcanic ash alteration or by the advection of low-δ 18O fluids. However, if the anomalies show up, hydrates are present almost invariably (with the exception of advected low-Cl −/high-δ 18O waters). Coming to terms with the effects of advection and diffusion has allowed successful modeling of the simpler hydrate-affected pore water profiles at passive margins. Instrumental for modeling are chlorine isotopes, which provide an effective tool to assess advection rates. The Egeberg and Dickens model allows estimation of hydrate concentration and distribution in the subsurface because it separates the effects of advection, diffusion and hydrate dissociation but critically depends on samples taken under in situ pressure and temperature conditions. Modeling the more complex pore water profiles of active margins is a challenge for the future. Compared to geophysical methods to estimate hydrate concentration, the geochemical method gives minimum amounts.

Design and optimization of on-chip capillary electrophoresis.

We present a systematic, experimentally validated method of designing electrokinetic injections for on-chip capillary electrophoresis applications. This method can be used to predict point-wise and charge-coupled device (CCD)-imaged electropherograms using estimates of species mobilities, diffusivities and initial sample plug parameters. A simple Taylor dispersion model is used to characterize electrophoretic separations in terms of resolution and signal-to-noise ratio (SNR). Detection convolutions using Gaussian and Boxcar detector response functions are used to relate optimal conditions for resolution and signal as a function of relevant system parameters including electroosmotic mobility, sample injection length, detector length scale, and the length-to-detector. Analytical solutions show a tradeoff between signal-to-noise ratio and resolution with respect to dimensionless injection width and length to the detector. In contrast, there is no tradeoff with respect to the Peclet number as increases in Peclet number favor both SNR and separation solution (R). We validate our model with quantitative epifluorescence visualizations of electrophoretic separation experiments in a simple cross channel microchip. For the pure advection regime of dispersion, we use numerical simulations of the transient convective diffusion processes associated with electrokinetics together with an optimization algorithm to design a voltage control scheme which produces an injection plug that has minimal advective dispersion. We also validate this optimal injection scheme using fluorescence visualizations. These validations show that optimized voltage scheme produces injections with a standard deviation less than one-fifth of the width of the microchannel.

Fractionation of sulfur isotopes during bacterial sulfate reduction in deep ocean sediments at elevated temperatures

A numerical model of sulfate reduction and isotopic fractionation has been applied to pore fluid SO 4 2− and δ 34S data from four sites drilled during Ocean Drilling Program (ODP) Leg 168 in the Cascadia Basin at 48°N, where basement temperatures reach up to 62°C. There is a source of sulfate both at the top and the bottom of the sediment column due to the presence of basement fluid flow, which promotes bacterial sulfate reduction below the sulfate minimum zone at elevated temperatures. Pore fluid δ 34S data show the highest values (135 ‰) yet found in the marine environment. The bacterial sulfur isotopic fractionation factor, α, is severely underestimated if the pore fluids of anoxic marine sediments are assumed to be closed systems and Rayleigh fractionation plots yield erroneous values for α by as much as 15‰ in diffusive and advective pore fluid regimes. Model results are consistent with α = 1.077 ± 0.007 with no temperature effect over the range 1.8 to 62°C and no effect of sulfate reduction rate over the range 2 to 10 pmol cm −3 d −1. The reason for this large isotopic fractionation is unknown, but one difference with previous studies is the very low sulfate reduction rates recorded, about two orders of magnitude lower than literature values that are in the range of μmol cm −3 d −1 to tens of nmol cm −3 d −1. In general, the greatest 34S depletions are associated with the lowest sulfate reduction rates and vice versa, and it is possible that such extreme fractionation is a characteristic of open systems with low sulfate reduction rates.

Experimental study of chaotic advection regime in a twisted duct flow

The generation of Lagrangian chaos has been studied experimentally in a twisted duct flow, a configuration representing a three-dimensional steady open flow in which various signatures of Lagrangian chaos are documented. The twisted duct consists of four 90 ∘ bends of square cross-section; the plane of curvature of each bend is at 90 ∘ to that of its neighbors. Dean roll-cells, generated by centrifugal forces and the geometrical perturbation due to the change in curvature plane, are the source of the irregular trajectories of the fluid particles. The Eulerian nature of the flow was investigated using a laser Doppler velocimeter (LDV). From the Eulerian point of view, the flow is completely regular. We show by laser-induced fluorescence (LIF) visualization that many characteristics of a Lagrangian chaotic system are present in this flow: − strong stretching and folding of material lines and surfaces; − sensitivity to initial conditions; − exponential growth of stretching in some flow regions. It is also shown that in some regions of the flow stretchings grow linearly with space, indicating non-chaotic behavior. Due to the chaotic nature of the flow, an equalization of the fly-time of fluid particles was observed during their passage through the twisted duct.

Mesoscale simulations of thermodynamic fluxes over complex terrain

Due to photochemical processes, most of the air quality problems arise during anticyclonic weather conditions in summer. Therefore, the problem is how thermodynamic fields and atmospheric processes (and especially wind fields) interact with the transport of pollutants in a valley. To answer such questions, knowledge about typical wind flow patterns occurring in the investigation area, leading to a classification of characteristic situations is useful. In particular, the most important of these flow fields and their generating processes can be analysed and understood by numerical simulations. Fourteen meteorological stations located at various topographic sites in the Fecht Valley (≃ 230 km 2 , Vosges Mountains, Alsace, France) provided the data necessary for processing an automatic wind-field classification. This classification identifies the characteristic summer flow patterns occurring in the valley. Two broad types of wind regimes can be schematically identified: during anticyclonic periods, thermally driven wind regimes are dominant; in contrast during cyclonic periods advective wind regimes are channelled by the orography. On the basis of this classification the authors were able to choose a typical summer day with anticyclonic conditions. A 3D simulation was performed for this day with the new Meso-Nh atmospheric mesoscale model to describe the thermodynamic fluxes and to understand the underlying physical processes. To obtain the most realistic results in this small area, high resolution surface information was introduced into the model. The data collected during a measurement campaign are used to validate the simulation results. The diurnal cycle of wind fields, the thermal stratification, and the heat fluxes are compared with observations.

Radiolarian fluxes, stocks, and population residence times in surface waters of the central equatorial Pacific

Radiolarian settling fluxes and standing stocks from the central equatorial Pacific are more indicative of physical process than the living environment of the organism. Polycystine radiolaria were collected from plankton and in moored sediment traps along longitude 140°W during both warm and cold phases of the 1992 ENSO as part of the US JGOFS Equatorial Pacific program (EqPac). Highest shell fluxes occurred at the 5°N and 5°S trapsites, roughly the boundary between equatorial and subtropical environments. The sediment trap samples collected at these sites were composites of different living assemblages that live equatorward, poleward and, in some cases, directly within the overlying water. Stock to flux ratios indicate an average radiolarian residence time of around one week within the 5°N to 5°S equatorial band. The population residence time for Didymocyrtis tetrathalamus, which dominated the plankton during El Niño, is 3 weeks. Key indicator species of the cold tongue period, Tetrapyle octacantha (subtropical) and Lophophaena hispida (equatorial), have residence times slightly less than one week. This suggests environmental forcing can fundamentally alter community composition by limiting which species populations can be maintained under certain advective and mixing regimes. In this way, seasonal to interannual oscillations in the meridional current can control the composition of radiolarian communities. T. octacantha exhibited the highest stocks and fluxes of any radiolaria, which implies high sediment abundance of this species reflects high input versus preferential preservation. Correlations between fluxes of radiolaria and organic carbon indicate the radiolaria play a larger role in the plankton community within the gyre margin environment than at the lower latitude sites.

Under-ice observations of water column temperature, salinity and spring phytoplankton dynamics: Eastern Bering Sea shelf

The inundation of two moored platforms by sea ice in late winter and early spring of 1995 provided unique time series of water column temperature, salinity, estimated chlorophyll-a, and phytoplankton fluorescence under advancing and retreating sea ice. One platform was located at 72 m in the weakly advective middle shelf regime. Here, chlorophyll-a concentrations began increasing shortly after the arrival of the ice (March) during the period of weak stratification and continued to increase while wind actively mixed the water column to >60 m. Changes in water column structure and properties resulted from an event of strong advection rather than vertical fluxes. During winter, such advective events can replenish the nutrients required to support the rich blooms that occur over the middle shelf during spring. The advancing ice was associated with the coldest waters and a deep (>50 m) mixed layer. The ice melt enhanced the two-layer system previously established by advection. A second mooring was located at the 120 m isobath on the more advective outer shelf. The ice reached this site on April 6, and chlorophyll-a concentrations increased as the sea ice melted. At the third mooring, located on the shelf farther south beyond the range of ice, the spring bloom began on ∼may 9.

Temporal evolution of CFC 11 and CFC 12 concentrations in the ocean interior

We study the temporal evolution of concentrations of the chlorofluorocarbons CFC 11 and CFC 12 in the ocean, under the assumption of circulation and mixing being invariant in time. This allows us to define a time‐invariant age distribution for a given point in the ocean, where the age is defined as time since the last contact with the atmosphere occurred. This concept is evaluated for a number of fundamental situations. We deduce a tendency for low CFC 11 and CFC 12 concentrations in advective regimes to increase exponentially in time and for concentrations near to a solubility equilibrium with atmospheric concentrations to increase rather more linearly. The apparent saturations, i.e., the ratios of interior to mixed‐layer CFC concentrations, increase monotonically in time, typical rates being 5–10% per decade. The theoretical results are compatible with time trends found in repeated CFC observations in the ocean. Diagrams on the temporal evolution for different age distributions are presented for the period 1970–2000, which can serve as a general orientation. The diagrams furthermore can provide time corrections for quasi‐synoptic evaluation of CFC observations taken over an extended period of time and assist in constructing time‐dependent CFC boundary conditions for numerical models of ocean circulation.

Possible links between ozone and temperature profiles

With a multiple linear regression analysis ozone and temperature trends are determined for a cold and warm half‐year cycle, based on concurrent ozone and temperature data recorded at various ozonesonde stations over the past decades. It is attempted to quantify the derived temperature trends, either in terms of radiative cooling caused by the trends in ozone and other greenhouse gases, or in terms of changes in the vertical advection regime. In the lower stratosphere, it is found that an increase in the ascent‐rate of air over the observed period often explains the observed cooling and ozone depletion, whereas higher up in the stratosphere the temperature trends are often in agreement with radiative transfer calculations. With the ozone and temperature residuals, correlation profiles are constructed, as well as sensitivity curves which give the local air temperature response to local changes in ozone. These sensitivity curves are compared with radiative transfer calculations for the stratosphere.

Fluid Flow in Fault Zones: Analysis of the Interplay of Convective Circulation and Topographically Driven Groundwater Flow

High‐permeability faults, acting as preferential pathways for fluid migration, are important geological structures for fluid, energy, and solute transport. This paper examines the interaction of thermally driven convective circulation in a steeply dipping fault zone and groundwater flow through the surrounding country rock that is driven by a regional topographic gradient. We consider a geometry where a fault zone with a homogeneous, isotropic permeability is located beneath a narrow valley in a region with substantial topographic relief. System behavior is best characterized in terms of the large‐scale permeabilities of the country rock and the fault zone. Using three‐dimensional numerical simulations, we map in permeability space four fluid flow and heat transfer regimes within a fault zone: conductive, advective, steady convective, and unsteady convective. The patterns of fluid flow and/or heat transfer are substantially different in each of these regimes. Maximum discharge temperatures can also be plotted in permeability space; the maximum discharge temperature in the advective regime is in general lower than that in the steady convective regime. A higher basal heat flux expands the convective regime in permeability space, as does a greater fault depth. Higher topographic relief on the regional water table compresses the convective regime, with the advective regime suppressing convective circulation at lower country rock permeabilities. If convective cells with aspect ratios close to 1 cannot form, the steady convective regime is smaller in permeability space, and the boundary between steady and unsteady convection occurs at lower values of fault zone permeability. At low country rock permeabilities a water table gradient along the surface trace of the fault of approximately 0.3% suppresses convective cells; at higher country rock permeabilities, convection can be suppressed by smaller gradients on the water table.

Circulation and representativeness of precipitation and air temperature in the southeast of the Quinghai-Xizang Plateau

Based on time series of 98 stations from the Qinghai-Xizang Plateau, the Indo-Gagnetic Plains, the River Gorge Country and Eastern China, the representativeness of the locally observed interannual variations of temperature and precipitation was analyzed by a statistical method. After an overview, which regards the main circulation patterns as well as the spatial distribution of temperature and precipitation in winter (January) and summer (July), the monthly distribution of the representativeness is discussed in a climato-genetic context. The monthly temperature representativeness is characterized by strong spatial and seasonal contrasts. The values range from less than 200 km in the basins and valley bottoms of NW Sichuan, caused by local summer circulation patterns, and more than 1600 km in the lower areas of E China, affected by the far reaching influence of the winter monsoonal surges of cold continental airmasses. With only a few exceptions mainly occurring in December, the precipitation representativeness does not exceed a 300 km limit. Due to the comparatively modest spatial disparities — except December when a steep rise, greater than 600 km occurs in the upper Tsangpo valley and adjacent areas — it can only be concluded, that a tendency of higher (lower) representativeness is predominantly related to advective (convective) regimes.

Formation of blooms by the symbiotic ciliate Mesodinium rubrum: the significance of nitrogen uptake

Mesodinium rubrum , a holotrichous ciliate that harbors endosymbiotic chloroplasts, was responsible for patches of red water in the coastal upweUing ecosystem at 15°S, Peru during March—May 1977. Stations within red patches showed elevated concentrations of paniculate nitrogen, chlorophyll a and dissolved organic nitrogen (DON), and lower concentrations of nitrate compared with areas where there were no ciliate blooms. These naturally occurring populations of M.rubrum took up nitrate, ammonium and DON. Light was required for nitrate uptake and to a lesser extent for ammonium uptake. Dark uptake of ammonium occurred in all experiments. There was no effect of light on DON uptake. Possession of endosymbiotic chloroplasts enables this protozoan to use upwelled nitrate efficiently for development of large blooms and concentrate into patches in the face of relatively strong advective regimes.

Radon 222 in the benthic boundary layer

A detailed survey of radon 222 and temperature profiles of the benthic boundary layer in the Hatteras Abyssal Plain shows a strong correlation between the structure of both. The apparent vertical diffusivities estimated from radon 222 are of the order of 50 cm2 s−1 in the mixed layer, and of the order of 1 cm−2 s−1 above it. One profile appears to have been taken in a frontal zone where isotherms dip sharply into the sediments. This is the only profile where there is significant penetration of radon above the mixed layer. Several other profiles suggest that the temperature may take longer than several radon half‐lives to adjust to new mixing and advection regimes. In such cases, one often sees considerable structure in the radon profile within the region where the potential temperature is well mixed.

Test of an equation for evaporation from bare soil

An empirical equation developed by Idso et al. (1979) at Phoenix, Arizona, to calculate daily average evaporation rates during all three drying stages of a bare soil was tested using measurements made at Agassiz, British Columbia, and is discussed on the basis of available evaporation theory. The results show that their expression for potential evaporation rate did not apply at Agassiz due to differences in the advection regimes at the two locations. The Agassiz potential evaporation rate data was well represented by the Priestley‐Taylor equation with aPT (‘alpha’) = 1.27 ± 0.1. It was concluded that the Idso et al. equation for potential evaporation rate has no greater generality than the Priestley‐Taylor or other such semiempirical approaches. The concept of expressing the stage III rate as proportional to the expression for potential evaporation rate worked marginally well at a culti‐packed site and quite well at a disc‐harrowed site. It was concluded that for soils with stage III rates much greater than 50% of potential evaporation rate, more complete procedures are necessary for calculating evaporation rates during extended drying periods.

The North Atlantic tritium distribution in 1972

The distribution of tritium in the North Atlantic in 1972 is compared with the distribution of salinity and the first‐order potential voracity as mapped on six constant potential density surfaces in the North Atlantic. The picture presented suggests an advective transport regime which is consistent with currently developing notions of the large‐scale gyre structure. Lateral mixing along isopycnals appears to be important in the northwestern region of the subtropical gyre, and vertical (cross‐isopycnal) mixing needs to be invoked only in near surface layers.

Entrainment and Diffusion in a Gulf Stream Cyclonic Ring

The mixing and entrainment processes present in a cyclonic ring are investigated by means of a parametric model which is fitted to serial temperature data for a North Atlantic ring. The physical model assumes an axially symmetric ring in which the temperature is presumed to be governed bywhere KA and KZ are the horizontal and vertical eddy diffusivity coefficients and J(ψT) is the (r,z) Jacobian of the transverse streamfunction ψ and the temperature T. Data from two cruises during the 1967 observation of a ring by the Woods Hole Oceanographic Institution provided estimates of the derivatives of the temperature. Regression analysis was used to determine the coefficients for polynomial representations of ψ(r,z) for selected combinations of Kh and Kz. The study indicates upper bounds on the order of magnitude for the diffusivities (Kh,Kz)=(105,10) cm2 s−1 based upon near-minimum least-squares error estimates from the regression analysis. An important result is that little differentiation exists between a purely advective entrainment regime and those regimes including both entrainment and diffusion; i.e., the entrainment circulation appears to be the dominant mechanism in the temporal changes of the ring for a time scale of at least two months. The results provide streamline patterns for the transverse flow from the surface to 1000 m depth consistent with isotherm movement and changes in the ring water masses.