Upward Gradient Research Articles

Comments on “Evaluation and Application of Conditional Symmetric Instability, Equivalent Potential Vorticity, and Frontogenetic Forcing in an Operational Forecast Environment”

After reading the paper by Wiesmueller and Zubrick (1998) on conditional symmetric instability (CSI) and equivalent potential vorticity (EPV), I agree with them that there has been a problem in getting operational forecasters to understand and properly use CSI concepts in their forecast routines. However, the problem is not entirely for a lack of model gridded data and tools to slice and dice them. The basic concepts for CSI have been poorly presented to forecasters. There are widespread misconceptions about CSI that appear in the Wiesmueller–Zubrick paper. CSI is a relatively easy concept for forecasters to grasp, so the confusion appears to be unnecessary. First I will provide an explanation for CSI that I have not seen anywhere in the literature. It was explained to me in this way at a scientific conference, and I apologize for forgetting the person’s name who offered me the explanation. Then I will point out some specific problems with the Wiesmueller–Zubrick paper with hopes that I can correct some of the misconceptions. First I want to review some basic parcel dynamics. There are three major forces in the atmosphere that can act on a parcel. The forces in the vertical are the pressure gradient force and gravity. In the horizontal they are again the pressure gradient force and the Coriolis force. Meteorologists call the balance of these vertical forces ‘‘hydrostatic’’ balance and the balance of these horizontal forces ‘‘geostrophic’’ balance. Imbalances occur when the pressure gradient force changes for the parcel. For example, a parcel of air warmer than its environment will experience an imbalance of forces with the larger upward pressure gradient force accelerating the parcel upward. Under certain conditions, perturbed air parcels may be accelerated away from their previous positions in space. Meteorologists call those unbalanced force

Numerical Analysis of the Vertical Motion of a Single Micron-Sized Particle in a Thermal Gradient Diffusion Chamber with Electrostatic Levitation

A model to describe the vertical motion of a single micron-sized insoluble particle electrically suspended in an upward thermal gradient diffusion chamber is developed. Motion of the particle is determined by five forces: gravitational, electrostatic, aerodynamic drag, thermophoretic, and diffusiophoretic. Condensation/evaporation effects are taken into account. The model predicts the behavior of an aerosol particle with specified characteristics in a diffusion chamber. The use of an aqueous salt solution as the lower pool establishes two regions in the chamber: an undersaturated region with respect to water vapor in the lower portion of the chamber and a supersaturated region in the upper portion. This unique saturation profile combined with certain operating conditions of the chamber leads to an oscillatory motion of the particle.

On the long-term hydraulic gradient in the thick clayey aquitard in the Sarnia region, Ontario

Much of the southwestern part of Ontario between Lake Huron and Lake Erie has a thin freshwater aquifer overlain by an aquitard of Late Pleistocene clayey glaciolacustrine deposits and underlain by a thick Devonian shale aquitard. In a large area east of the St. Clair River, where the Quaternary aquitard is 30-50 m thick, groundwater of Pleistocene origin (identified by 18O and 2H signature) occurs in the aquifer and in the bottom part of the Quaternary aquitard. Numerous piezometer nests in the aquitard show a downward hydraulic gradient with depth. In some areas, the aquitard has downward gradients only in the upper part and upward gradient in the lower part, indicating a transient condition. The piezometer nests in the clayey aquitard also show an increase in Cl- concentration with depth. Long-term piezometer monitoring at two sites show a major shift in the aquitard hydraulic gradient since 1983 and a large rise in head in the underlying aquifer. Analyses of initial aquifer water levels, reported in well drilling records, indicate a large decline in the potentiometric surface of the aquifer between the 1940's and the 1970's followed by a recent rise in the surface in part of the region. This pattern is consistent with well drilling and water use records indicating that 7000 wells were installed in the aquifer in the three decades since 1940 and that groundwater use has greatly diminished in the past 10-15 years due to rural pipeline distribution of lake and river water. The hydraulic gradient in the aquitard is slowly adjusting to the rise in the aquifer potentiometric surface. One-dimensional solute transport modelling provides close matches to the vertical profiles of Cl- migrating upward from the aquifer since deglaciation, 15 000 - 18 000 years before present, by diffusion with little or no advection. The lack of advection indicates a near-neutral long-term hydraulic gradient. As the withdrawal rate of water from the aquifer continues to decline, it is expected that the hydraulic head in the aquitard in much of the area westward of the recharge area will continue to adjust for many decades.Key words: clay, aquitard, aquifer, water use, hydraulic gradient.

Locating groundwater discharge in large lakes using bottom sediment electrical conductivity mapping

Groundwater‐surface water studies that use conventional near‐shore piezometers and /or seepage meters are impractical in larger, areal extensive lakes, as they require exorbitant numbers of instruments to quantify groundwater discharge zones. In smaller lakes an electrical conductivity mapping method has proven useful in mapping groundwater discharge zones. The technique identifies groundwater discharge by measuring variations in sediment pore water electrical conductivity and reduces the number of instruments necessary to quantify inflow, thereby lowering instrumentation costs and increasing a study's efficiency. This study sought to determine the technique's applicability in larger lakes. Thus the method was tested within the Hamilton Harbour, at the western end of Lake Ontario. This study found systematic variations between nearshore and offshore sediments and identified three anomalous zones that were thought to represent groundwater inflow. Onshore and offshore piezometers were used to verify the presence of upward gradients and elevated electrical conductivities. The sediment probe survey provided qualitative maps of areas of elevated electrical conductivity indicative of groundwater discharge and allowed a fairly extensive shoreline to be mapped quickly and economically. Survey results guided the installation of nearshore piezometers to discharge zones, eliminating the inefficiency of more conventional “hit or miss” point source installation approaches. This research demonstrated that the sediment probe was a valuable tool for studying groundwater inputs into large lakes.

Removal of pooled dense, nonaqueous phase liquid from saturated porous media using upward gradient alcohol floods

Laboratory experiments employing 90% by volume alcohol solutions are used to compare the abilities of ethanol and 1‐propanol to remove pooled tetrachloroethene (PCE) from saturated porous media using low upward hydraulic gradients. Equilibrium ternary phase diagrams measured for the systems water/PCE/ethanol and water/PCE/1‐propanol indicate that for alcohol concentrations below the miscibility envelope, 1‐propanol will partition predominantly into the dense, nonaqueous phase liquid (DNAPL) phase while ethanol remains in the aqueous phase. Interfacial tension and phase density measurements show that while both systems demonstrate a reduction in interfacial tension with increasing alcohol content, the density difference between the aqueous and DNAPL phases is only reduced for the 1‐propanol system. Two‐dimensional experiments in saturated porous media using alcohol floods ranging in size from 0.125 pore volumes (PV) to 1.0 PV recovered between 5.7% and 98.7% of the PCE mass. The removal mechanisms for the ethanol floods included enhanced dissolution followed by miscible displacement, while the 1‐propanol floods removed PCE by DNAPL swelling and interfacial tension reduction leading to immiscible displacement followed by miscible displacement. Recovery results and effluent composition histories indicate that hydrodynamic instabilities and dispersion cause significant alcohol slug deterioration and confirm the necessity of using an appropriate size alcohol slug of sufficient concentration for efficient PCE mass recovery.

Declining effect of latitude on melanoma mortality rates in the United States. A preliminary study.

The gradient of mortality from melanoma of the skin with latitude among US whites was estimated from the slopes of semilogarithmic models fitted to the state-specific mortality rates and the latitudes of the states' capital cities. The upward gradient of mortality from north to south for malignant melanoma of the skin has been decreasing since 1950-1959, when data first became available, through 1960-1969, 1970-1979, and 1988-1992. By the early years of the 21st century, rates of melanoma mortality in the contiguous United States are expected to be unaffected by latitude. For the country as a whole, melanoma mortality rates have been rising for many years. This rise has become progressively slower, such that national rates have been projected to stabilize in the near future. While increasing geographic mobility has probably played a role in reducing the latitude effect, melanoma mortality rates may have reached levels at which increased exposure of US whites to sunlight has little incremental effect.

Measurement of hydraulic properties in deep lake sediments using a tethered pore pressure probe: Applications in the Hamilton Harbour, western Lake Ontario

Estimates of groundwater seepage flux in lake bottom sediments require knowledge of the hydraulic gradient at the sediment‐surface water interface and the hydraulic conductivity of the lake‐bottom materials. In deep waters, in situ measurement of these parameters can be accomplished through the use of piezometer probes lowered and monitored remotely from a surface vessel. In this research work a new tethered piezometer probe was developed and tested for use in collecting hydraulic property data in deep‐lake bottom sediments. The probe uses a variable‐reluctance transducer to measure the differential sediment pore pressure between two ports spaced 100 cm apart. The dissipation of pore pressure transients that develop during rapid emplacement of the probe were extrapolated in time to estimate equilibrium hydraulic gradients. In addition, various data analysis techniques were evaluated for determining sediment hydraulic conductivity and specific storage through interpretation of the pore‐pressure dissipation data. The probe was used to estimate groundwater seepage in the bottom sediments of the Hamilton Harbour, at the western end of Lake Ontario. Upward gradients were measured at nine locations within the harbor ranging from 0.010 to 0.425 and a downward gradient of −0.015 was recorded at one site along the harbor's eastern boundary. Hydraulic conductivities determined from pore‐pressure dissipation over time ranged from 6.9 × 10−9to 4.8 × 10−7 m/s. Specific storage values ranged from 0.08 to 0.19 m−1. Calculated average linear seepage velocities ranged from 4.3 × 10−8 to −8.5 × 10−9 m/s. The groundwater contribution to the harbor through the deeper, fine‐grained sediments was estimated to be 9.1 × 10−2 m3/s, or 2.9 × 106 m3/yr. This represents approximately 1.0% of the harbor basin's total volume, 15% of precipitation's contribution, 1.2% of the contribution of surface inflows (excluding the Burlington ship canal) and 0.22% of the total surface outflow passing through the Burlington shipping canal, which connects the harbor to Lake Ontario.

The Use of Upward Hydraulic Gradients to Arrest Downward DNAPL Migration in Rock Fractures

AbstractUpward water flow can arrest the downward migration of dense, non aqueous phase liquids (DNAPLS) through rough‐walled fractures provided that a sufficient hydraulic gradient exists. An exact analytical solution to predict the arresting gradient demonstrates that there is little difference between the gradient required to arrest DNAPL migration near the top of the fracture, and the gradient required to arrest migration once DNAPL has extended to the bottom of the fracture. Laboratory experiments involving the migration of tetrachloroethylene (PCE) through two samples of fractured limestone demonstrate the ability of upward water flow to arrest downward DNAPL migration under both wetting and drainage conditions. It is suggested that upward gradients can be generated beneath contaminated regions of the subsurface to provide a “hydraulic bottom.” A hydraulic bottom at a site would protect against potential downward mobilization of DNAPL in response to the application of aggressive remediation technologies such as surfactant flooding, alcohol flooding, and steam flooding. Upward gradients applied during drilling may also protect against downward mobilization of DNAPL in the formation while drilling through source zones.

HYDRODYNAMICS AND FAULT SEAL ASSESSMENT IN THE VULCAN SUB-BASIN, TIMOR SEA

The study of hydrodynamics sheds light on the current day fluid flow regime of traps of different integrity and on the mechanisms leading to seal breach. It also provides insights into potential migration pathways and zones of secondary hydrocarbon accumulation. This is a preliminary hydrodynamic investigation carried out on a few selected wells in the Vulcan Sub-basin from high, moderate and low integrity traps, referred to here as HIT, MIT and LIT respectively (after O'Brien et al, 1996). The fields sampled include East Swan (LIT), Skua and Jabiru (MIT) and Montara and Oliver (HIT). The findings of this study suggest a trend between trap type and current day fluid flow. The best result was obtained for Skua-2, where pressure testing was conducted over the bounding fault for the field, indicating high integrity. Some wells showed upward head gradients, such as East Swan-2; others showed a low or no vertical gradient, such as Jabiru-1 A,−2, −3, −10 and Oliver-1. There was general agreement between the flow direction and the conductivity of the faults, as indicated by mud loss data. Preliminary potentiometric surface mapping of Jabiru indicated lateral flow, controlled by production, and hydraulic communication within the faulted reservoir. Wellbore hydrodynamics and potentiometric surface mapping of the reservoir heads on a regional scale promise to give indications of major structural controls on flow as well as secondary migration pathways.

Numerical Simulation of Surfactant-Enhanced Solubilization of Pooled DNAPL

A one-dimensional numerical model that simulates the surfactant-enhanced solubilization of pooled DNAPL is presented. Two non-equilibrium expressions describing mass transfer are examined. The mass transfer coefficient is based on the wetting phase Darcy flux, with the mass transfer rate coefficient accounting for the variation of interfacial area available for mass transfer between the DNAPL and the surfactant solution through the pool and over time. The non-wetting phase saturation distribution is calculated as a function of the hydraulic gradient, allowing for exact calculation of the local velocity. The model is calibrated using data from laboratory column experi ments where pooled tetrachloroethylene was solubilized under upward gradient conditions. Upward gradients were utilized to prevent downward pool mobilization in response to a lowering of interfacial tension upon exposure to solubilizing surfactants. Effluent concentrations are generally well predicted by the model, although simulated effluent...

The use of upward gradients to arrest downward dense, nonaqueous phase liquid (DNAPL) migration in the presence of solubilizing surfactants

This paper presents a mathematical analysis and the results of a series of laboratory experiments investigating in situ dissolution of dense, nonaqueous phase liquid (DNAPL) pools from porous media using solubilizing surfactants under upward-flowing conditions. Surfactant flood experiments were conducted in a one-dimensional glass column packed with natural sand. Perchloroethylene (PCE) was emplaced in the column as a pool perched above a coarse–fine sand interface to reproduce conditions occurring when DNAPL pools on a low permeability layer in the subsurface. Aqueous surfactant solution was directed vertically upward to counteract downward pool mobilization resulting from reduction of the PCE-water interfacial tension in the presence of surfactant. Experiments were conducted at overall hydraulic gradients across the sand pack of 0.0, 0.1, 0.2, 0.3, and 0.5. The surfactant flood solution consisted of a 1:1 blend of two commercial surfactants, Witconol NP-100 and Rexophos 25/97, in a 2% aqueous solution. Surfactant flooding was conducted until all visible traces of PCE were removed from the column. Subsequent water flooding was continued until effluent PCE concentrations exiting the column dropped below 5 ppm. In the zero gradient experiment, the PCE pool invaded the fine sand layer immediately upon contact with the surfactant because of a lowering of interfacial tension. PCE entry into the fine sand did not occur in the 0.3 and 0.5 gradient runs. Minor amounts of PCE mobilization occurred in the late stages of the 0.1 and 0.2 gradient runs and is attributed to a lowering of interfacial tension with time upon exposure to surfactant. Key words : solubilizing surfactant, groundwater remediation, DNAPL, tetrachloroethylene.

Formation of regolith-collapse sinkholes in southern Illinois: Interpretation and identification of associated buried cavities

Three regolith-collapse sinkholes formed near the Dongola Unit School and the Pentecostal Church in the southern Illinois village of Dongola (Union County) during the spring of 1993. The sinkholes appeared over a three-month period that coincided with development of a new municipal well. The new well was drilled through clay-rich, valley-fill sediment into karstified limestone bedrock. The piezometric surface of the limestone aquifer is above land surface, indicating the presence of an upward hydraulic gradient in the valley and that the valley fill is acting as a confining unit. Pumping during development of the well lowered the piezometric surface of the limestone aquifer to an elevation below the base of the valley fill. It is hypothesized that drainage of water from the sediments, the resulting loss of hydrostatic pressure and buoyant force in overlying sediments, increased intergranular pressure, and the initiation of groundwater flow toward the well resulted in rapid sediment transport, subsurface erosion, and collapse of the valley-fill sediment. The sinkholes follow an approximately east-west alignment, which is consistent with one of the two dominant alignments of passages of nearby joint-controlled caves. A constant electrode-separation resistivity survey of the school playground was conducted to locate areas that might contain incipient sinkholes. The survey revealed a positive resistivity anomaly trending N75E in the southern part of the study area. The anomaly is linear, between 5 and 10 m wide, and its trend either intersects or is immediately adjacent to the three sinkholes. The anomaly is interpreted to be a series of pumping-induced cavities in the valley-fill sediments that formed over a preexisting crevice in the karstified bedrock limestone.

Evaluation of a compacted till liner test pad constructed over a granular subliner contingency layer

The construction and evaluation of a compacted clayey till test pad constructed over a stone layer are described. The evaluation of the clayey liner involved (i) excavation of six test pits through the liner, followed by careful visual inspection for defects in the liner; (ii) sampling of the liner using standard 75 mm diameter Shelby tubes, a 150 mm diameter piston sampler, and block sampling; (iii) triaxial hydraulic conductivity tests on samples of liner material consolidated to a number of stress levels relevant to the proposed design; and (iv) diffusion tests on samples of liner material. Based on the results it is concluded that it was possible to construct a low-permeability liner (hydraulic conductivity less than 1.4 × 10−8 cm/s under expected field stress conditions). Geotextiles from above and below the compacted liner were carefully exhumed and subjected to a series of laboratory tests to examine (i) the effect of construction damage on the geotextile's strength; (ii) the effectiveness of the geotextile to minimize intrusion of the clay liner through the geotextile and into the stone layer(s) under expected field stress conditions; (iii) the effectiveness of the geotextile as a filter for the compacted liner material under high upward gradient conditions; and (iv) the friction angle between the geotextile and clay, and geotextile and stone. The geotextiles exhumed from the test liner showed some evidence of construction damage; however, based on the field observations and subsequent laboratory tests, it is concluded that they performed adequately. Key words : waste disposal, clay liners, geotextiles, field performance, hydraulic conductivity, landfills.

Use of a groundwater flow model to explain strong upward gradients in rock formation underneath the Mercier site. Note : Isabel, D; Gelinas, P; Locat, J Can Geotech JV29, N4, Aug 1992, P698–701

Use of a groundwater flow model to explain strong upward gradients in rock formation underneath the Mercier site. Note : Isabel, D; Gelinas, P; Locat, J Can Geotech JV29, N4, Aug 1992, P698–701

Use of a groundwater flow model to explain strong upward gradients in rock formation underneath the Mercier site

The groundwater pollution case at Mercier is a very complex one. Groundwater flow modeling has been a valuable tool in the assessment of this large environmental problem. However, due to the complexity of the hydrogeological setting, the modeling has been performed with various simple case models in lieu of a large complex model. Here we report the results of one of these piecewise modeling tasks that proved very useful in the explanation of the strong upward gradients observed in the bedrock aquifer. These results and their interpretation prove the usefulness of the piecewise modeling strategy in this case. Key words : ground water modeling, finite elements.

Study of the Performance of Sealing Systems for Access Shafts in a High-Level Waste Repository

ABSTRACTShaft sealing in a high-level waste(HLW) disposal system functions to minimize the water flow passage, and retard the radionuclide transport from the repository to the accessible environment. It is important to estimate the radionuclide migration along the sealed shaft from the viewpoint of the design and the performance assessment of the sealing system.This study presents the results of sensitivity analyses on the radionuclide migration in the vicinity of the access shaft of a repository in order to evaluate the effects of the length of a plug, as well as the number of plugs, and curtain grouts.In this study, the upward hydraulic gradient of the groundwater flow along shafts was used, based on transient coupled thermo-hydraulic analyses around a repository. Hydraulic conductivities of the backfill material and the disturbed zones around the shaft tunnels were also assumed to be one order and two orders of magnitude higher than that of the host rock, respectively.The results show that the velocity of the groundwater within the shaft and the disturbed zone is reduced by a factor of one third by installing a few plugs into the shaft filled with backfill material. The curtain grouts have the effect of retarding the radionuclide migration from the repository to the ground surface at a factor of approximately five. A few plug installations have the same effect. The sealing system properly constituted with backfill, plugs, and grouts can provide the same performance as the original host rock.

Penetration of a quasi‐statically slipping crack into a seismogenic zone of heterogeneous fracture resistance

This paper is concerned with some aspects of nonuniform stressing above a deep creeping portion of a fault zone prior to a large crust‐breaking earthquake. The model that we use involves a slipping crack, representing the deeper, more stably sliding portions of the fault zone, which penetrates upward from depth and is blocked in the lower region of the seismogenic zone. When conditions are uniform along strike, the upward penetration at the crack front is by mode III in strike‐slip fault zones but by mode II in thrust or normal fault zones. Two major results are reported. First, we analyze approximately, via a linear perturbation formulation, how a tectonic crack front encounters and ultimately shears through arrays of localized “asperities” that are distributed parallel to the crack front and have a toughness which is greater than that of adjoining segments of the fault zone. Using a fast Fourier transform based numerical procedure to simulate crack penetration into asperities, we find a notable difference between mode II and mode III crack fronts in that the former penetrates approximately twice as far between the asperities as the latter under the same loading level. This is interesting because observations of slip distribution in large earthquakes suggest that there is a significant aseismic component to the total slip budget in subduction zone earthquakes, which in contrast does not seem to be present in strike‐slip zone earthquakes, and also that the surface slip distribution in continental dip‐slip faulting is typically much more irregular than for strike‐slip faulting. In a simulation involving multiple rows of periodic asperities we note that the more deeply penetrating mode II crack front contacts more asperities simultaneously while breaking them at different load levels compared to the less flexible mode III crack front, which simply breaks one row of asperities and jumps (unstably) to the next. The second major result concerns whether a straight crack front in the lithosphere along a strike‐slip fault zone is configurationally stable, that is, whether the crack front will tend to remain straight as the crack penetrates upward from depth. It is found that for infinitesimal perturbations of the straight front beyond a critical wavelength, of the order of the crustal lithosphere thickness, the stress intensity factor is higher at the most advanced portions of the crack front rather than at the least advanced; the opposite is true at shorter wavelengths. When resistance to crack growth is essentially uniform over the fault plane, this means that the straight crack front is configurationally unstable at long wavelengths. The issue of configurational stability is related to the concept of fault segmentation, which is based on the observation that fault zones, particularly long ones, do not rupture along their entire length during a single earthquake. Effect of a vertical gradient of fracture resistance is discussed in the appendix, where it is shown that a significant upward gradient of resistance to crack growth may completely stabilize the straight crack configuration.

Modelling of the Uptake, Flow and Utilization of C, N and H 2O within Whole Plants of Ricinus communis L. Based On Empirical Data

Summary An empirically based modelling technique was used to quantitatively depict uptake, flow and utilization of C, N and H2O for a 9 day period in mid vegetative growth of NO3-fed castor bean (Ricinus communis L.) exposed to a mean salinity stress of 128 mol m-3 NaCl. The models incorporated data on C : N weight ratios of solutes of phloem sap and pressure-induced xylem exudates of leaves, stem internodes and petioles, net increments or losses of C, N and H2O in plant parts, transpirational losses of shoot organs and respiratory losses of C from shoot parts and root. A computational technique was developed to assess the extent of xylem to phloem transfer of C and N within internally sited organs (stem segments and petioles) in addition to traffic of C and N through these organs to terminal organs (root, shoot apex and leaves). Molar ratios of inputs of H2O: C : N were 4803: 22: 1. Half of the net daytime photosynthetic gain of C by the shoot was translocated initially to the root, 33 % was lost in root respiration, 17% in shoot night respiration, 8% cycled through the root system and 40% was finally incorporated into shoot dry matter, 10% into root dry matter. The corresponding N budget showed 93% initial transfer in xylem from root to leaf laminae, 32% backflow in phloem from shoot to root and 15 % cycled through the root. The water budget involved a 98% loss in transpiration, 2% incorporation into tissues and 2% commitment to phloem transport. Xylem to phloem exchanges of C and N in shoot mostly involved transfer from xylem to phloem. The models predicted a steep upward gradient in decreasing water use efficiency with leaf age, due principally to poor CO2 fixation in relation to water loss by youngest leaves. Comparison of fluxes of N and H2O between stem internode segments with those passing out to leaves indicated progressive lateral abstraction of N from leaf traces serving lower leaves and subsequent passage of this N to the xylem of cauline traces serving upper regions of the shoot. Data were compared with earlier obtained information on C, N and H2O partitioning in nodulated white lupin (Lupinus albus L.).

Absolute electron density measurements in the equatorial ionosphere

Accurate measurement of the electron density profile and its variations is crucial to further progress in understanding the physics of the disturbed equatorial ionosphere. To accomplish this, a plasma frequency probe was included in the payload complement of two rockets flown during the CONDOR rocket campaign conducted from Peru in March 1983. In this paper we present density profiles of the disturbed equatorial ionosphere from a night-time flight in which spread- F conditions were present and from a day-time flight during strong electrojet conditions. Results from both flights are in excellent agreement with simultaneous radar data in that the regions of highly disturbed plasma coincide with the radar signatures. The spread- F rocket penetrated a topside depletion during both the upleg and downleg. The electrojet measurements showed a profile peaking at 1.3 × 10 5cm −3 at 106 km, with large scale fluctuations having amplitudes of roughly 10 % seen only on the upward gradient in electron density. This is in agreement with plasma instability theory. We further show that simultaneous measurements by fixed-bias Langmuir probes, when normalized at a single point to the altitude profile of electron density, are inadequate to correctly parameterize the observed enhancements and depletions.

Hydraulic potential in Lake Michigan bottom sediments

The magnitude and direction of groundwater flux in the bottom sediments of Lake Michigan were deduced from measurements made during three shipboard cruises between 1973 and 1975. These factors affect the geochemical environment of the sediments and therefore the distribution of trace elements reported to be present. The near-shore, sandy-bottom and fine-grained, soft, deep-lake sediments were investigated; areas of hard till or bedrock were not included in the study. Thirty-three piezometers were placed in near-shore sands in waters 5–15 m deep. The piezometers were placed an average of 3 m into the bottom sediment. Water levels from the piezometers averaged 0.6 cm above the lake level, equivalent to an upward hydraulic gradient of about 0.002 cm/cm. Water samples taken from the piezometers have a distinctly different chemical composition from that of the lake water. The total dissolved mineral content and hardness of the groundwater are about twice those of the lake water. Twenty-two hydraulic gradient measurements were made in the fine-grained soft deep-lake sediments in waters 48–140 m deep by using a differential-pressure transducer dropped into the sediments. These measurements show an upward gradient averaging 0.2 cm/cm. No chemical data were obtained for the groundwater in the deep-lake sediments. The results of this study indicate that the groundwater flux is upward through the bottom sediments into Lake Michigan and that there is a chemical change in the water near the water-sediment contact.