Zone Of Water Table Fluctuation Research Articles

Effect of Pumping Speeds on the Fate of Aniline in Different Soil Layer

Helan Mountain is an important ecological safety barrier in northwest China. In this study, a heterogeneous site polluted by aniline on Helan Mountain was the research object, and the TMVOC (A Simulator For Multiple Volatile Organic Chemicals) model of aniline restoration by pumping was optimized by employing a column experiment. Four typical layers of the soil medium were selected to explore the influence of soil settlement caused by different pumping speeds on the fate of aniline in different zones. The results show that the optimal pumping speed at the site is 3.24 × 106 m3/month and the latest remediation time is the 10th month after the start of the remediation. The larger the pumping speed is, the more obvious the sedimentation effect is. When the remediation is carried out at 5.18 × 106 m3/month, the NAPL (Non-Aqueous-Phase Liquid) phase removal rate decreases by 33.75% and the distribution of aniline to the NAPL phase increases, compared to that without considering the soil settlement. The fate of aniline in the source zone is the least affected by sedimentation, while that in the vadose zone is the most affected. The phase redistribution phenomenon is the most obvious in the water table fluctuation zone, and the NAPL phase aniline changes into gas and liquid phases. In addition, the NAPL phase concentration in the water table fluctuation zone is two orders of magnitude higher than that at 0.2 m below the water table. NAPL is the most sensitive to the relative settlement in the aquifer. The simulation results can provide a technical reference for the future application of P&T (Pump-and-Treat) technology in the remediation of organically contaminated sites to facilitate the sustainable use of soil. It is suggested that more attention should be paid to the water table fluctuation zone during the remediation of contaminated sites.

Magnetic signatures of a creosote oil contaminated site: case study in São Paulo, Brazil

Soils and groundwater contamination modifies the physical–chemical conditions of the environment, altering natural biogeochemical processes of the ground. As a result, several mineral transformations occur, in which iron plays a decisive role. The presence of iron enables the study of magnetic properties, improving the understanding of the geophysical signatures of highly dynamic environments (e.g., biogeochemical hotspots and contamination plumes). In this work, we seek to identify creosote biodegradation related to the precipitation of magnetic minerals on sediments at a contaminated site in São Paulo, Brazil. Several rock magnetism analyses were carried out to provide the magnetic mineralogy of the samples in terms of their composition, size, and abundance. We conducted high-temperature thermomagnetic curves, frequency-dependent magnetic susceptibility, anesthetic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) data, superparamagnetic concentration and dipole moment (SPCDM), and scanning electron microscopy (SEM) analyses. The magnetic signatures of the contaminated samples suggest an increase of superparamagnetic grains in the water table fluctuation zone if compared to the magnetic signatures of the uncontaminated samples. Thermomagnetic curves of contaminated samples showed a lower heterogeneity of the magnetic mineral phases than the uncontaminated ones. This work contributes to the advancement of the understanding of how natural biogeochemical processes are impacted by human actions, such as soil contamination, and even by climate change, which should affect soil redox conditions in periods of drought and flooding.

Nitrate sink function of riparian zones induced by river stage fluctuations

River stage fluctuation (RSF) induced by tides, dam releases, or storms may lead to enhanced nitrogen cycling (N cycling) in riparian zones (RZ). We conducted a laboratory water table manipulation experiment and applied a multiphase flow and transport model (TOUGHREACT) to investigate the role of RSF in N cycling in the RZ. Coupled nitrification and denitrification occur in the water table fluctuation zone under alternating aerobic and anaerobic conditions. Nitrate removal was enhanced in the saturated and unsaturated zones of the RZ. The net nitrate reduction rate in groundwater increased with the increasing number of RSF, as a result, the cumulative water influx. The RZ nitrate sink function became stronger with increasing RSF amplitude and soil organic matter (SOM) content, and weaker with increasing [NH4+]/[NO3−] ratio and mineralization rate. RSF generally creates a hot moment for net nitrate removal in the RZ. However, a hot moment for the nitrate source function might occur if the [NH4+]/[NO3−] ratio of groundwater is much greater than 1 and/or if a large pool of nitrate accumulates in the topsoil over a prolonged dry period. The absence of oxygen diffusion in the model would overestimate the nitrate removal capacity of the RZ.

Base-level poljes in the Sivas gypsum karst, Türkiye

The Sivas Basin, Central Anatolia, includes one of the most outstanding gypsum karst terrains in the world, covering an area of 2140 km2. The karst landscapes are developed on late Eocene gypsum deposits. The fact that the polje shapes that are commonly seen in the limestone karst terrain also seen in the Sivas gypsum karst area further increases the importance of this area in terms of geomorphology. This study is focused on the explanation of the morphometric properties and formation mechanism of poljes around the Kızılırmak River in the Sivas gypsum karst area. The geomorphological evolution of the Sivas gypsum karst area was controlled by the Kızılırmak River drainage system formed in the Early Pliocene. Polygonal doline karst is common on the High Karst Plateau (or erosional-denudational surface) formed during this evolution process, while subsidence dolines, hanging valleys, and poljes are common on the Low Karst Plateau. A total of 14 poljes, most of which are drained underground, and a corrosion plain were identified around the Kızılırmak River in the Low Karst Plateau. The boundaries of some of the poljes, the area of the largest of which is 6.3 km2, and hence their strike is controlled by faults. These poljes are base-level poljes, which bases are approximately at the river level and developed in the water table fluctuation zone. In addition to the structural lines in the formation of the poljes, the beginning of a blind valley-like karstification process, especially in the lower parts of the hanging valleys, mostly as a result of the collapse doline formation (showing the estevalle feature), changes in the water table level in the epiphreatic zone, and in addition to the precipitation, the aggressive river floodwaters that invaded the polje floor were effective. Due to the rapid dissolution of gypsum, the development of polje must have been affected by the incision and deposition periods of the river. The Tödürge Lake depression most probably has been shaped by collapse dolines.

An Experimental Study on the Migration of Pb in the Groundwater Table Fluctuation Zone

As a result of fluctuations in the shallow groundwater table, hydrodynamic conditions change alongside environmental conditions and hydrogeochemical processes to affect pollutant migration. The study aimed to investigate the migration, adsorption, and desorption characteristics of Pb on fine, medium, and coarse sand in the water table fluctuation zone by using several laboratory methods, including the kinetic aspects of Pb2+ adsorption/desorption and water table fluctuation experiments. The results showed that the adsorption and desorption curves fit the Elovich equation well at a correlation coefficient above 0.9. In the adsorption and desorption kinetic experiments for fine, medium, and coarse sand collected and from the floodplain, the maximum adsorption capacity of Pb2+ was 2367 mg·kg−1, 1848 mg·kg−1, and 1544 mg·kg−1, respectively. The maximum desorption capacity of Pb2+ was 29.18 mg·kg−1, 62.38 mg·kg−1, and 81.60 mg·kg−1, respectively. In environments with pH greater than 4, the adsorption capacity was proportional to the pH, but the desorption capacity decreased as the pH increased in water. As the water table varied, the lowest pH occurred in the polluted medium we set initially. When the distance between the pollutants and sample solution grew further, pH increased, and the Pb2+ concentration decreased in the sample solution. In the column experiment of water table fluctuations on coarse sand, Pb2+ migrated nearly 5 cm upward from the original pollutant and migrated less than 10 cm downward from that. In our experiments on medium and fine sand, the upward and downward migration distances were <5 cm. The groundwater table fluctuations, pH variation, and Pb concentration currently influence the migration of Pb.

In-situ biofilm detection in field settings using multichannel seismic

Biofilm aids a variety of geoengineering applications but a general lack of in-situ detection and monitoring methods severely limits its field usage. In this pioneering study, we show that in-situ field detection of biofilm may be possible through multichannel surface seismic. Our study was conducted at a landfill site in Norman, Oklahoma, where an ongoing interaction of a leachate plume with a fluctuating water table has created optimal conditions for biofilm growth. We acquired multichannel seismic data along a 130 m long profile and inverted the ground roll and refractions respectively for S-wave (VS) and P-wave (VP) velocity structures of the shallow (0–5 m) subsurface. We find that within ~1.2 m–~3.0 m depth, which is the water table fluctuation zone (WTFZ), VS increases by ~50% over the background but without any appreciable changes in the VP. Environmental scanning electron microscopy of soil samples from various depths along a core shows the presence of biofilm exclusively within the WTFZ. A VS-only increase due to biofilm growth is consistent with laboratory studies of similar nature and can be realized through a mechanistic model where biofilm is simultaneously present in two morphologies: grain-cementing and pore-filling. Our findings open doors to detecting biofilm in a variety of settings such as hyporheic zones and contaminant plume fringe and for a range of purposes from soil remediation to carbon sequestration to detecting life in other planets.

Methanogens and Their Syntrophic Partners Dominate Zones of Enhanced Magnetic Susceptibility at a Petroleum Contaminated Site

Geophysical investigations documenting enhanced magnetic susceptibility (MS) within the water table fluctuation zone at hydrocarbon contaminated sites suggest that MS can be used as a proxy for investigating microbial mediated iron reduction during intrinsic bioremediation. Here, we investigated the microbial community composition over a 5-year period at a hydrocarbon-contaminated site that exhibited transient elevated MS responses. Our objective was to determine the key microbial populations in zones of elevated MS. We retrieved sediment cores from the petroleum-contaminated site near Bemidji, MN, United States, and performed MS measurements on these cores. We also characterized the microbial community composition by high-throughput 16S rRNA gene amplicon sequencing from samples collected along the complete core length. Our spatial and temporal analysis revealed that the microbial community composition was generally stable throughout the period of investigation. In addition, we observed distinct vertical redox zonations extending from the upper vadose zone into the saturated zone. These distinct redox zonations were concomitant with the dominant microbial metabolic processes as follows: (1) the upper vadose zone was dominated by aerobic microbial populations; (2) the lower vadose zone was dominated by methanotrophic populations, iron reducers and iron oxidizers; (3) the smear zone was dominated by iron reducers; and (4) the free product zone was dominated by syntrophic and methanogenic populations. Although the common notion is that high MS values are caused by high magnetite concentrations that can be biotically formed through the activities of iron-reducing bacteria, here we show that the highest magnetic susceptibilities were measured in the free-phase petroleum zone, where a methanogenic community was predominant. This field study may contribute to the emerging knowledge that methanogens can switch their metabolism from methanogenesis to iron reduction with associated magnetite precipitation in hydrocarbon contaminated sediments. Thus, geophysical methods such as MS may help to identify zones where iron cycling/reduction by methanogens is occurring.

Characterisation of fractured carbonate aquifers using ambient borehole dilution tests

Fractured carbonate aquifers derive their transmissivity essentially from a well-developed network of solutionally-enhanced fractures and conduits that can lead to high groundwater velocities and high vulnerability to contamination of water quality. Characterisation of the variation of hydraulic properties with depth is important for delineating source protection areas, characterising contaminant fate and transport, determination of the effectiveness of aquifer remediation, and parameter estimation for models. In this work, ambient open borehole uniform and point injection dilution tests were conducted on observation boreholes in the unconfined Cretaceous Chalk aquifer of East Yorkshire, UK, and interpreted in conjunction with other data via the implementation of a new work flow. This resulted in the characterisation of flow in these boreholes and the inference of properties such as groundwater flow patterns and velocities in the surrounding aquifer formation. Our workflow allowed sections of open boreholes showing horizontal versus vertical flow to be distinguished, and the magnitude of such flows and exchanges with the aquifer to be determined. Flow within boreholes were then used to characterise: i) presence and direction of vertical hydraulic gradients; ii) nature and depth distribution of flowing features; iii) depth interval porosity and permeability estimation of the flowing features from overall borehole transmissivity and geophysical image or caliper logs; iv) groundwater velocity estimation in the surrounding aquifer. Discrete flowing features were distributed across the range of depths sampled by the observation boreholes (typically up to 45–60 mbgl), but the majority were located in the zone of water table fluctuation marked by solutionally enlarged flow features. Quantitative interpretation of both uniform injection (tracer distributed throughout the open borehole section) and point injection (slug of tracer introduced at targeted depth) yielded vertical velocities within the borehole water column in broad agreement with those measured by flow logging. Depth specific fracture kinematic porosities inferred from the ambient dilution data combined with long-interval pump test and geophysical log data ranged between 3.7 × 10−4–4.1 × 10−3 with an average of 2.1 × 10−3; these values were in excellent agreement with those from other methods applied to the same aquifer such as larger scale pumping tests. A new approach to estimation of groundwater velocities from the dilution test data using externally measured hydraulic gradients gave inferred horizontal groundwater velocities ranging between 60 and 850 m/day, in full agreement with those from previously conducted borehole-to-borehole tracer tests. These results confirm that the studied aquifer is karstic, with rapid preferential pathways which have implication for flow and transport modelling, and pollution vulnerability. Our study results indicate that ambient single-borehole dilution approaches can provide an inexpensive and reliable approach for the characterisation of fractured and karstic aquifers.

Impacts of experimental alteration of water table regime and vascular plant community composition on peat mercury profiles and methylmercury production

Climate change is expected to alter the hydrology and vascular plant communities in peatland ecosystems. These changes may have as yet unexplored impacts on peat mercury (Hg) concentrations and net methylmercury (MeHg) production. In this study, peat was collected from PEATcosm, an outdoor, controlled mesocosm experiment where peatland water table regimes and vascular plant functional groups were manipulated over several years to simulate potential climate change effects. Potential Hg(II) methylation and MeHg demethylation rate constants were assessed using enriched stable isotope incubations at the end of the study in 2015, and ambient peat total Hg (THg) and MeHg concentration depth profiles were tracked annually from 2011 to 2014. Peat THg and MeHg concentrations and the proportion of THg methylated (%MeHg) increased significantly within the zone of water table fluctuation when water tables were lowered, but potential Hg(II) methylation rate constants were similar regardless of water table treatment. When sedges dominate over ericaceous shrubs, MeHg concentrations and %MeHg became significantly elevated within the sedge rooting zone. Increased desorption of Hg(II) and MeHg from the solid phase peat into pore water occurred with a lowered water table and predominant sedge cover, likely due to greater aerobic peat decomposition. Deeper, more variable water tables and a transition to sedge-dominated communities coincided with increased MeHg accumulation within the zone of water table fluctuation. Sustained high water tables promoted the net downward migration of Hg(II) and MeHg. The simultaneous decrease in Hg(II) and MeHg concentrations in the near-surface peat and accumulation deeper in the peat profile, combined with the trends in Hg(II) and MeHg partitioning to mobile pore waters, suggest that changes to peatland hydrology and vascular plant functional groups redistribute peat Hg(II) and MeHg via vertical hydrochemical transport mechanisms.

Redox Fluctuations and Organic Complexation Govern Uranium Redistribution from U(IV)-Phosphate Minerals in a Mining-Polluted Wetland Soil, Brittany, France

Wetlands have been proposed to naturally attenuate U transfers in the environment via U complexation by organic matter and potential U reduction. However, U mobility may depend on the identity of particulate/dissolved uranium source materials and their redox sensitivity. Here, we examined the fate of uranium in a highly contaminated wetland (up to 4500 mg·kg-1 U) impacted by former mine water discharges. Bulk U LIII-EXAFS and (micro-)XANES combined with SEM-EDXS analyses of undisturbed soil cores show a sharp U redox boundary at the water table, together with a major U redistribution from U(IV)-minerals to U(VI)-organic matter complexes. Above the water table, U is fully oxidized into mono- and bidentate U(VI)-carboxyl and monodentate U(VI)-phosphoryl complexes. Minute amounts of U(VI)-phosphate minerals are also observed. Below the water table, U is fully reduced andis partitioned between U(IV)-phosphate minerals (i.e., ningyoite and a lermontovite-like phase), and bidentate U(IV)-phosphoryl and monodentate U(IV)-carboxyl complexes. Such a U redistribution from U-minerals inherited from mine water discharge deposits could result from redox cycling nearby the water table fluctuation zone. Oxidative dissolution of U(IV)-phosphate minerals could have led to U(VI)-organic matter complexation, followed by subsequent reduction into U(IV)-organic complexes. However, uranium(IV) minerals could have been preserved in permanently waterlogged soil.

Well flow and dilution measurements for characterization of vertical hydraulic conductivity structure of a carbonate aquifer

The paper aims to characterize vertical variations in horizontal hydraulic properties in a fractured carbonate aquifer, the Cretaceous Chalk in East Yorkshire, UK. Two approaches are used: an inverse model of well flow applied to flow logs of pumped open wells, and open well dilution testing. In this case study, transmissivity in the unconfined part of the aquifer is dominated by the highly permeable zone of water table fluctuation, where carbonate dissolution has occurred enhancing fracture aperture; a similar enhanced permeability zone is present at the top of the aquifer where it is confined beneath glacial deposits, although periglacial physical weathering during Quaternary cold periods, rather than carbonate dissolution, is responsible. The aquifer is also shown to contain deeper permeable horizons of stratigraphic origin, which are better developed in the unconfined section.

THE ORETICAL FOUNDATION FOR CALCULATION OF FLUID SYSTEMS IN THE SUBSURFACE CONTAMINATED WITH LIGHT PETROLEUM PRODUCTS DURING GROUNDWATER FLUCTUATION<br/> Paper 4.Calculation and analysis of fluid systems in the subsurface contaminated with light petroleum products during groundwater table fluctuations

The article describes an algorithm for CFS (Calculation of Fluid Systems) computer program designed to calculate levels, state, distribution of content and volume of water and liquid light petroleum products in a lens formed on a water table during its fluctuations. The program consists of the blocks: data input, parameter transformation, value calculation and formation of input variable arrays on dates, identification and calculation of fluid system, analysis and correction of calculations, selection of zones with different states of liquid light petroleum products, formation of the array of water content and light petroleum products values, treatment of output data and calculation of changes in water and light petroleum products volumes vertically in a water table fluctuation zone, output of tables and results for chosen dates in a file.

Iron biomineralization controls on geophysical signatures of hydrocarbon contaminated sediments

The interpretation of geophysical data from mature hydrocarbon contaminated sites has relied on a conductive plume model where the conductivity of the subsurface contaminant volume is the result of microbial mediated changes in pore fluid chemistry. This conductive anomalous region is characterized by high total dissolved solids and occurs within the water table fluctuation zone where microbial activity is the maximum. Here we update this conductive plume model by providing new insights from recent laboratory investigations and geophysical data from hydrocarbon contaminated sites suggesting the unrecognized role of the impact that microbial-mediated metallic mineral precipitates have on geophysical signatures. We show that microbial redox processes (e.g., iron and sulfate reduction) during the biodegradation process involve mineralogical transformations and the precipitation of new minerals (e.g., magnetite, and pyrite) that can impact the electrical and magnetic properties of contaminated sediments. We provide examples from laboratory experiments and field studies and suggest that knowledge of the dominant redox processes occurring at hydrocarbon contaminated sites and the mineral phases formed is critical for a more robust interpretation of geophysical data associated with microbial-mediated changes at hydrocarbon contaminated sites. We also show that integration of both magnetic and electrical techniques may help reduce ambiguity in data interpretation.

High-resolution magnetic susceptibility measurements for investigating magnetic mineral formation during microbial mediated iron reduction

Disimilatory iron-reducing bacteria play an important role in the reduction of Fe(hydr)oxides and the production of secondary solid-iron mineral phases that can have magnetic properties. Magnetic susceptibility can therefore play an important role in identifying zones where microbial-mediated iron reduction is occurring. We investigated the magnetic susceptibility variations in a hydrocarbon-contaminated aquifer where methanogenesis and iron reduction are the main biogeochemical processes. Our objectives are to (1) determine the variability of magnetic susceptibility, (2) determine the hydrobiogeochemical controls on the magnetic susceptibility variability, and (3) evaluate the use of magnetic susceptibility as a viable technique for identifying zones where the coupling of iron and organic carbon cycling is occurring. Magnetic susceptibility data were acquired down 11 boreholes within contaminated and uncontaminated locations. We show that magnetic susceptibility values for boreholes within the free phase plume are higher than values for boreholes within the dissolved phase plume and background. Magnetic susceptibility values are highest within the zone of water table fluctuation with peaks predominantly occurring at the highest water table marks and are also coincident with high concentrations of dissolved Fe(II) and organic carbon content, suggesting that the zone of water table fluctuation is most biologically active. High magnetic susceptibility values within the vadose zone above the free phase plume are coincident with a zone of methane depletion suggesting aerobic or anaerobic oxidation of methane coupled to iron reduction. Our results suggest that magnetic susceptibility can be used as a viable tool in iron and carbon cycling studies.

Seasonal and spatial variation in groundwater quality along the hillslope of an agricultural research catchment (Western France)

AbstractShallow groundwater that develops on hillslopes is the main compartment in headwater catchments for flow and solute transport to rivers. Although spatial and temporal variations in its chemical composition are reported in the literature, there is no coherent description of the way these variations are organized, nor is there an accepted conceptual model for the mechanism involved. On the basis of the results from an experimental hillslope, we propose a description of the variation in spatial and temporal patterns of groundwater chemical composition and discuss the mechanisms involved. We instrumented a headwater catchment with a network of 42 nested piezometers along a 200 m hillslope and monitored major anion composition frequently over a 2‐year period. The results demonstrated that (i) the anionic composition in water table fluctuation zone varied significantly compared to deeper portions of the aquifer on the hillslope, confirming that this layer constitutes a main compartment for the mixing of new recharge water and old groundwater, (ii) deeper parts of the aquifer exhibited seasonal variations with structured hysteretic patterns, suggesting that mixing processes also occurred at greater depths and (iii) these hysteretic patterns were dampered from upslope to downslope, indicating an increased influence of lateral flow downslope. We recommend that the generality of these observations be assessed in different environments, as they can constitute a new set of processes to include in hydrological models. Copyright © 2010 John Wiley & Sons, Ltd.

Solute transfer in the unsaturated zone-groundwater continuum of a headwater catchment

This study deals with solute transfer in the vertical continuum between the unsaturated zone and shallow groundwater of a weathered granite aquifer in the Kerbernez headwater catchment of western France. The objectives are (i) to determine the mechanisms responsible for solute transfer in the unsaturated and water-table fluctuation zones of the aquifer, and (ii) to analyse the implications of these results on solute transfer times at the catchment scale. An experimental site located in the plateau area of the catchment was equipped with 6 tensiometers, 18 ceramic cups at depths from 0.25 to 2.5 m and 7 piezometers from 3 to 20 m. Measurements of hydraulic head and water sampling were carried out over a period of 2.5 years in the unsaturated zone (0–2 m), the water table fluctuation zone (2–9 m) and the permanently saturated zone (>9 m). Two tracer experiments were carried out by applying two pulses of water, one enriched with deuterium and the other with bromide. Natural chloride concentrations, as well as deuterium and bromide concentrations, were analysed from solution samples. From the artificial tracer concentrations, two porosity compartments can be identified and partly quantified: (1) the slow-mobile porosity (36% of the bulk volume), accounting for the slow piston-flow transfer (2–3 m per year), and (2) the rapid-mobile porosity, which transfers small quantities of bromide at a rate of 19 cm h1 down to the water table. Natural chloride concentrations are characterised by a high temporal variability in the water-table fluctuation zone, whereas the concentrations remain steady in time in the permanently saturated zone (42 mg l1 at 20 m depth). The temporal variability is related to the water-table fluctuations and follows the same pattern each hydrological year, i.e. low concentrations during rising water-table followed by a progressive increase in concentrations during the periods of high piezometric level and water-table recession. This pattern is explained in terms of the two mobile porosity compartments and groundwater hydraulics. Based on these findings, we propose a conceptual model of solute transfer along the hillslope of a headwater catchment. We conclude that mixing in the water-table fluctuation zone could occur at two spatial scales. Firstly, at the pore scale, with mixing of waters in slow mobile and rapid mobile porosity, and secondly, at the scale of the hillslope. The mixing at this latter scale would appear to result from differences of flow path geometry and velocity between the unsaturated zone and the groundwater.

Natural immobilization of uranium by phosphate mineralization in an oxidizing saprolite–soil profile: chemical weathering of the Coles Hill uranium deposit, Virginia

Geochemical and mineralogical studies of the 15- to 20-m-thick soil and saprolite profile developed over the Coles Hill uranium deposit demonstrate that uranium transport may be inhibited or naturally attenuated by the precipitation of U(VI) phosphate minerals in oxidizing and saturated soil environments. This study examines geochemical conditions in which these secondary uranium phosphates are formed and remain stable for hundreds of thousands of years. We also describe geochemical conditions that may lead to their dissolution. The lower part of the weathering profile, which consists of groundwater saturated saprolite, contains up to 1300 mg/kg uranium (within the solid). This concentration is approximately 1.5 times greater than the average ore grade of the deposit, indicating that the saturated saprolites are enriched relative to the underlying primary ore. Uranium within this zone is dominantly associated with U(VI) phosphates of the meta-autunite mineral group. Groundwaters from this zone contain less than 14 μg/l uranium suggesting that the U(VI) phosphate minerals present within the Coles Hill saprolites are capable of buffering dissolved uranium concentrations to values significantly lower (by a factor of 2 or more) than the US-EPA maximum contaminant level of 30 μg/l. Above the water table, U(VI) phosphates of the type found in the saturated saprolite zone are not present, and the soil uranium concentration drops to an average value of 250 mg/kg. Thus, the zone of water table fluctuation represents a major transition zone in uranium mineralogy. Mineralogical characterization of the upper part of the profile suggests that geochemical conditions in the unsaturated zone are not conducive to uranium stabilization by meta-autunite group minerals. Uranium within the vadose zone, however, is associated with phosphate (e.g. Al phosphate minerals and sorption with phosphate to ferric oxyhydroxides). As a whole, the Coles Hill soil profile is interpreted to have reached a steady state with respect to uranium transport and immobilization. Results of this study suggest that uranium is leached from the unsaturated part of the profile (meta-autunite unstable) and reprecipitated below the water table where the measured activity ratio of dissolved phosphate to carbonate is higher. Based on estimates of weathering rates for this region, the processes responsible for the dispersal of uranium within the Coles Hill system are estimated to have been active on a time scale of hundreds of thousands of years. This system thus represents an excellent natural laboratory for understanding the long-term mineralogical and geochemical processes involved in the coupling of uranium and phosphorus in natural, fluid-rich systems.

ALTERAÇÃO SUPERGÊNICA DO DEPÓSITO DE COBRE-OURO DO SALOBO, SERRA DOS CARJÁS-PA - ÊNFASE NO COMPORTAMENTO DO COBRE

The weathering mantle of the Salobo deposit was developed upon metagraywackes with interbedded lens of iron formations. The profile is of the truncated type and constituted, from the base to the top, of a transition horizon, a saprolitic horizon and colluvial deposits. The two former horizons show lateral, textural and mineralogical variations which reflect the heterogeneous nature of the parent rock. Thus, alteration of the barren graywackes resulted essentially in the formation of a saprolite rich in clay minerais (hydrobiotite, smectite, kaolinite). On the other hand, weathering of the iron formations, host-rocks of copper sulfides (bornite, calcocite, chalcopyrite), originated a gossan type profile composed from the base to the top of a supergene sulfide zone, an oxidized minerais zone and a ferruginous saprolite. The supergene sulfides are digenite, covellite and bornite (?) while cuprite, malachite, azurite and native copper constitute the oxidized copper assemblage. Within the saprolite, smectite and goethite are the main copper enriched minerais. The evolution of such supergene setting involved oxidization and dissolution of primary sulfides with mobilization of Cu and Fe as soluble sulfatos. The downward and lateral transfer of these metais caused the formation of new mineral associations according to the Cu/S ratio in the solutions and the ambient pH and Eh conditions.In the reducing zone the ionic exchange between copper sulfate and primary sulfides yielded mainly digenite and covellite, while in the water-table fluctuation zone and immediately above, in more oxidizing conditions, copper precipitated as cuprite and native metal. Besides, reaction of copper sulfate with calcite from veins that cuts iron formations resulted in the fixation of Cu as malachite and azurite in neutral to alkaline conditions. In the Carajas region, the lateritic mantle was formed during the Lower Tertiary, but, during the Upper Tertiary and Quaternary, underwent dissection due to regional uplift. Thus, in the Salobo area, the duricrust and the upper saprolite were removed. Each phase of lowering of the hydrostatic levei resulted in the deepenning of the weathering profile and alteration, in oxidizing conditions, of previously formed supergene oxides and sulfides. One parte of the released copper then underwent downward leaching and reprecipitation in deeper reconstituted cementation and oxidation zones, while another part was fixed by neoformed goethite and clay-minerals.

Rapid development of ferricretes on a subtropical valley side slope

Ferricretes can be formed along some valley‐side slopes in the southeastern United States coastal plain as a consequence of erosional exposure of zones of iron precipitation in areas of groundwater discharge. This mode of ferricrete formation was demonstrated due to the recession of estuarine shoreline bluffs after hurricanes in 1996. Iron‐precipitation zones exposed by bluff retreat at Flanner Beach, North Carolina in 1996 had formed indurated ferricretes by 1998. This confirms the valley‐side groundwater discharge model of ferricrete formation, and shows that, once the zone of iron precipitation is exposed, ferricrete can form in less than two years. The newly formed ferricretes also allow the identification of five distinct stages in their formation: (1) iron precipitation in the zone of water table fluctuation; (2) the formation of brittle iron‐cemented layers; (3) exposure by erosion or mass wasting and the first stage of hardening; (4) further hardening into indurated ferricrete; and (5) formation of limonite ferricretes, and impregnation with manganese oxides. The results from Flanner Beach show that the process may proceed from stages two and three to four and five in less than two years, suggesting that only short periods of stability following erosion or mass wasting episodes are necessary to allow ferricrete formation.

Mineralization of Sparsely Water-Soluble Polycyclic Aromatic Hydrocarbons in a Water Table Fluctuation Zone

The mineralization potential of sparsely water-soluble polycyclic aromatic hydrocarbons (PAHs) within a highly diesel-contaminated water table fluctuation zone (WTFZ) was investigated using core-scale column microcosms. Experimental conditions mimicked overall seasonal changes in water and oxygen content at the site. During the first aerobic winter, PAH mineralization rates in the freshly contaminated soil were fastest for contaminant [14C]naphthalene which was the least hydrophobic and most water-soluble. Lowering the water table nearly doubled the mineralization rates of all [14C]PAHs studied. During the oxygen-poor summer, all mineralization rates were insignificant and failed to respond to water table changes. Neither a return to water-saturated aerobic (winter) conditions nor lowering the water table under aerobic conditions induced detectable mineralization of [14C]naphthalene, but lowering the water table did markedly hasten the still slow mineralization of [14C]phenanthrene and [14C]anthracene. The time-dependent mineralization behavior and its response to water table fluctuations were explicable in terms of microbial responses to the changing oxygen content and depleting mineral nutrients.