Possible impact of shallow interfluve traps on groundwater remediation

Shallow stratigraphic traps exist in certain geologic environments, which might complicate groundwater remediation efforts. Too often, pump-and-treat remediation wells are installed in geometric patterns designed to maximize cleanup operations. If, however, shallow stratigraphic traps are present, certain contaminants may be locally trapped and thus missed during cleanup operations. Alternatively, if shallow stratigraphic traps are delineated prior to location of cleanup wells, fewer wells may be needed for remediation. For example, a single well located at the trap apex, coupled with a single injection well, might be sufficient if: (1) all contaminants were contained in the trap, and (2) the trap was not laterally extensive. Interfluve stratigraphic traps are formed when braided outwash sands are subsequently covered by backswamp or lacustrine clay deposits. Such traps are common near-surface features of the valley train deposits of the Lower Mississippi River Valley and have relief of a few tens of centimeters to perhaps in excess of 10 m. Interfluve traps are also expected to commonly occur in many glaciated regions. Due to channel migration during backswamp clay deposition, and differing thicknesses of clay cover, it is unlikely that all interfluve traps can be delineated by a single method. Therefore, several geological techniques that might be useful in delineating shallow interfluve traps are briefly discussed here, as well as potential complications in using the described methods.     

A comparison of estimated and calculated effective porosity

 Effective porosity in solute-transport analyses is usually estimated rather than calculated from tracer tests in the field or laboratory. Calculated values of effective porosity in the laboratory on three different textured samples were compared to estimates derived from particle-size distributions and soil–water characteristic curves. The agreement was poor and it seems that no clear relationships exist between effective porosity calculated from laboratory tracer tests and effective porosity estimated from particle-size distributions and soil–water characteristic curves. A field tracer test in a sand-and-gravel aquifer produced a calculated effective porosity of approximately 0.17. By comparison, estimates of effective porosity from textural data, moisture retention, and published values were approximately 50–90% greater than the field calibrated value. Thus, estimation of effective porosity for chemical transport is highly dependent on the chosen transport model and is best obtained by laboratory or field tracer tests. Received, March 1997 · Revised, August 1997 · Accepted, August 1997     

A geochemical approach to constraining the formation of glassy fallout debris from nuclear tests

Glassy nuclear fallout debris from near-surface nuclear tests is fundamentally reprocessed earth material. A geochemical approach to analysis of glassy fallout is uniquely suited to determine the means of reprocessing and shed light on the mechanisms of fallout formation. An improved understanding of fallout formation is of interest both for its potential to guide post-detonation nuclear forensic investigations and in the context of possible affinities between glassy debris and other glasses generated by high-energy natural events, such as meteorite impacts and lightning strikes. This study presents a large major-element compositional dataset for glasses within aerodynamic fallout from the Trinity nuclear test (“trinitite”) and a geochemically based analysis of the glass compositional trends. Silica-rich and alkali-rich trinitite glasses show compositions and textures consistent with formation through melting of individual mineral grains—quartz and alkali feldspar, respectively—from the test-site sediment. The volumetrically dominant glass phase—called the CaMgFe glass—shows extreme major-element compositional variability. Compositional trends in the CaMgFe glass are most consistent with formation through volatility-controlled condensation from compositionally heterogeneous plasma. Radioactivity occurs only in CaMgFe glass, indicating that co-condensation of evaporated bulk ground material and trace device material was the main mechanism of radioisotope incorporation into trinitite. CaMgFe trinitite glasses overlap compositionally with basalts, rhyolites, fulgurites, tektites, and microtektites but display greater compositional diversity than all of these naturally formed glasses. Indeed, the most refractory CaMgFe glasses compositionally resemble early solar system condensates—specifically, CAIs.     

Field investigation of deformation characteristics and stress mobilisation of a soil slope

Stress mobilisation and deformation of a slope are important for engineers to carry out reliable design of retaining systems. However, most case histories reported mainly on the response of pore water pressure (PWP), whereas knowledge about the stress deformation characteristics of slope is limited. In this study, a saprolitic soil slope was instrumented to monitor not only the responses of PWP but also horizontal stress and horizontal displacement. To assist in the interpretation of field data, a series of laboratory tests was conducted to characterise volume change behaviour of the soil taken from the site, under the effects of both net stress and suction. During a rainstorm event when positive PWP built up, a remarkably large displacement of 20 mm was recorded between 5.5- and 6-m depths, and the top 5 m of the slope exhibited translational downslope movement. This caused an increase in Bishop’s effective horizontal stress by 350 %, which reached a peak value close to 40 % of a Bishop’s effective passive stress. During the subsequent dry season when suction was recovered, an upslope rebound of 10 mm was recorded. Comparison of field and laboratory data reveals that the rebound was attributed to suction-induced soil shrinkage. This rebound led to a decrease in the Bishop’s effective horizontal stress previously built up during the storm event.     

Seismic motion and response prediction alternatives

Statistical methods are available which predict the maximum response of simple oscillators given the peak acceleration (A_p), peak velocity (V_p) or peak displacement (D_p) of seismic ground motions. An alternative parameter, namely an ordinate (or ordinates) of the Fourier amplitude spectrum of ground motion acceleration, FS(f), may in fact be a preferred predictor of peak response, especially in a frequency range close to f. Other statistical methods (attenuation laws) use distance R and other parameters such as magnitude (M), Modified Mercalli epicentral Intensity (I_o) and Modified Mercalli site Intensity (MMI or I_s) to predict spectral velocity (S_v(f)), etc. In using such approaches, it is most desirable to know the total uncertainty in the predicted peak response of the system given the starting parameter values. An extensive strong motion data set is used to study these questions, The most direct prediction models are found to be preferable (have lower prediction dispersion) but data may not be available in all regions to permit their use.     

Surficial geophysical deduction of the geomaterial and aquifer distributions at Ngor-Okpala local government area of Imo State, South Eastern Nigeria

Twenty seven vertical electrical sounding (VES) profiles surrounding four known traverses were obtained in Ngor-Okpala local government area of Imo state to examine the subsurface geomaterials and the associated groundwater potential. The VES data, constrained by borehole data, provided useful information about the subsurface hydrogeologic and lithologic conditions. From the validated interpretation, the area assessed has loamy soil, medium grained sands, well-sorted medium-grained/gravelly sands and river sand as the lithologic succession from top to the bottom of the depth penetrated. The aquifers in the area were found in the medium-grained sands and well-sorted medium-coarse-grained sands. The aquifer depth for all-season groundwater that would be devoid of draw-down can be found at a depth range of 42–50 m. The resistivity maps of selected depths exhibit sharp resistivity changes at depth due mainly to undulating subsurface topography. A map of the distribution of the kσ-values shows that good quality groundwater can be found in most parts of the area.     

Paleoenvironmental records of water level and climatic changes from the middle to late Holocene at a Lake Erie coastal wetland, Ontario, Canada

Pollen and diatom assemblages, and peat stratigraphies, from a coastal wetland on the northern shore of Lake Erie were used to analyze water level and climatic changes since the middle Holocene and their effects on wetland plant communities. Peat deposition began 4700 cal yr B.P. during the Nipissing II transgression, which was driven by isostatic rebound. At that time, a diatom-rich wild rice marsh existed at the site. Water level dropped at the end of the Nipissing rise at least 2 m within 200 yr, leading to the development of shallower-water plant communities and an environment too dry for most diatoms to persist. The sharp decline in water level was probably driven primarily by outlet incision, but climate likely played some role. The paleoecological records provide evidence for post-Nipissing century-scale transgressions occurring around 2300, 1160, 700 and 450 cal yr B.P. The chronology for these transgressions correlates with other studies from the region and implies climatic forcing. Peat inception in shallow sloughs across part of the study area around 700 cal yr B.P. coincides with the Little Ice Age. These records, considered alongside others from the region, suggest that the Little Ice Age may have resulted in a wetter climate across the eastern Great Lakes region.     

Ferrihydrite dissolution by pyridine-2,6-bis(monothiocarboxylic acid) and hydrolysis products

Pyridine-2,6-bis(monothiocarboxylate) (pdtc), a metabolic product of microorganisms, including Pseudomonas putida and Pseudomonas stutzeri was investigated for its ability of dissolve Fe(III)(hydr)oxides at pH 7.5. Concentration dependent dissolution of ferrihydrite under anaerobic environment showed saturation of the dissolution rate at the higher concentration of pdtc. The surface controlled ferrihydrite dissolution rate was determined to be 1.2 × 10⁻⁶ mol m⁻² h⁻¹. Anaerobic dissolution of ferrihydrite by pyridine-2,6-dicarboxylic acid or dipicolinic acid (dpa), a hydrolysis product of pdtc, was investigated to study the mechanism(s) involved in the pdtc facilitated ferrihydrite dissolution. These studies suggest that pdtc dissolved ferrihydrite using a reduction step, where dpa chelates the Fe reduced by a second hydrolysis product, H₂S. Dpa facilitated dissolution of ferrihydrite showed very small increase in the Fe dissolution when the concentration of external reductant, ascorbate, was doubled, suggesting the surface dynamics being dominated by the interactions between dpa and ferrihydrite. Greater than stoichiometric amounts of Fe were mobilized during dpa dissolution of ferrihydrite assisted by ascorbate and cysteine. This is attributed to the catalytic dissolution of Fe(III)(hydr)oxides by the in situ generated Fe(II) in the presence of a complex former, dpa.     

Elemental and mineralogical changes in soils due to bioturbation along an earthworm invasion chronosequence in Northern Minnesota

Minnesota forested soils have evolved without the presence of earthworms since the last glacial retreat. When exotic earthworms arrive, enhanced soil bioturbation often results in dramatic morphological and chemical changes in soils with negative implications for the forests’ sustainability. However, the impacts of earthworm invasion on geochemical processes in soils are not well understood. This study attempts to quantify the role of earthworm invasion in mineral chemical weathering and nutrient dynamics along an earthworm invasion chronosequence in a sugar maple forest in Northern Minnesota. Depth and rates of soil mixing can be tracked with atmospherically derived short lived radioisotopes ²¹⁰Pb and ¹³⁷Cs. Their radioactivities increase in the lower A horizon at the expense of the peak activities near the soil surface, which indicate that soil mixing rate and its depth reach have been enhanced by earthworms. Enhanced soil mixing by earthworms is consistent with the ways that the vertical profiles of elemental and mineralogical compositions were affected by earthworm invasion. Biologically cycled Ca and P have peak concentrations near the soil surface prior to earthworm invasion. However, these peak abundances significantly declined in the earthworm invaded soils presumably due to enhanced soil mixing. It is clear that enhanced soil mixing due to earthworms also profoundly altered the vertical distribution of most mineral species within A horizons. Though the mechanisms are not clear yet, earthworm invasion appears to have contributed to net losses of clay mineral species and opal from the A horizons. As much as earthworms vertically relocated minerals and elements, they also intensify the contacts between organic matter and cations as shown in the increased amount of Ca and Fe in organically complexed and in exchangeable pools. With future studies on soil mixing rates and elemental leaching, this study will quantitatively and mechanically address the role of earthworms in geochemical evolution of soils and forests’ nutrient dynamics.