Investigation of the different binding edge sites for Zn on montmorillonite using P-EXAFS - The strong/weak site concept in the 2SPNE SC/CE sorption model

The 2 site protolysis non electrostatic surface complexation and cation exchange (2SPNE SC/CE) sorption model has been used over the past decade or so to quantitatively describe the uptake of metals with oxidation states from II to VI on 2:1 clay minerals; montmorillonite and illite. One of the main features in this model is that there are two broad categories of amphoteric edge sorption sites; the so called strong (S^SOH) and weak (S^W1OH) sites. Because of their different sorption characteristics, it was expected that the coordination environments of the surface complexes on the two site types would be different. Zn isotherm data on two montmorillonites, Milos and STx-1, were measured and modelled using the 2SPNE SC/CE sorption model. The results were used to define the most favourable experimental conditions under which Zn sorption was either dominated by the strong (S^SOH, ∼2 mmol kg⁻¹) or by the weak sites (S^W1OH, ∼40 mmol kg⁻¹). Highly oriented self-supporting films were prepared for polarised extended X-ray absorption fine structure (P-EXAFS) investigations.Montmorillonites often contain Zn incorporated in the clay matrix. The Zn bound in this form was quantified and the results from the analysis of the P-EXAFS spectra were taken into account in the interpretation of the spectra measured at low Zn loadings (∼2 mmol kg⁻¹) and medium Zn loadings (∼30 mmol kg⁻¹). The Zn spectra on the “strong sites” exhibited a pronounced angular dependency and formed surface complexes in the continuity of the Al-octahedral sheets at the montmorillonite edges. In contrast, the Zn “weak site” spectra showed only a weak angular dependency. The spectroscopic evidence indicates the existence of two distinct groups of edge surface binding sites which is consistent with a multi-site sorption model and in particular with the strong/weak site concept intrinsic to the 2SPNE S/CE sorption model.     

A synthesis of post-glacial diatom records from Lake Baikal

The biostratigraphy of fossil diatoms contributes important chronologic, paleolimnologic, and paleoclimatic information from Lake Baikal in southeastern Siberia. Diatoms are the dominant and best preserved microfossils in the sediments, and distinctive assemblages and species provide inter-core correlations throughout the basin at millennial to centennial scales, in both high and low sedimentation-rate environments. Distributions of unique species, once dated by radiocarbon, allow diatoms to be used as dating tools for the Holocene history of the lake. Diatom, pollen, and organic geochemical records from site 305, at the foot of the Selenga Delta, provide a history of paleolimnologic and paleoclimatic changes from the late glacial (15 ka) through the Holocene. Before 14 ka diatoms were very rare, probably because excessive turbidity from glacial meltwater entering the lake impeded productivity. Between 14 and 12 ka, lake productivity increased, perhaps as strong winds promoted deep mixing and nutrient regeneration. Pollen evidence suggests a cold shrub — steppe landscape dominated the central Baikal depression at this time. As summer insolation increased, conifers replaced steppe taxa, but diatom productivity declined between 11 and 9 ka perhaps as a result of increased summer turbidity resulting from violent storm runoff entering the lake via short, steep drainages. After 8 ka, drier, but more continental climates prevailed, and the modern diatom flora of Lake Baikal came to prominence. On Academician Ridge, a site of slow sedimentation rates, Holocene diatom assemblages at the top of 10-m cores reappear at deeper levels suggesting that such cores record at least two previous interglacial (or interstadial?) periods. Nevertheless, distinctive species that developed prior to the last glacial period indicate that the dynamics of nutrient cycling in Baikal and the responsible regional climatic environments were not entirely analogous to Holocene conditions. During glacial periods, the deep basin sediments of Lake Baikal are dominated by rapidly deposited clastics entering from large rivers with possibly glaciated headwaters. On the sublacustrine Academician Ridge (depth = 300 m), however, detailed analysis of the diatom biostratigraphy indicates that diastems (hiatuses of minor duration) and (or) highly variable rates of accumulation complicate paleolimnologic and paleoclimatic reconstructions from these records.     

Paleoecological studies at Lake Patzcuaro on the west-central Mexican Plateau and at Chalco in the basin of Mexico

A 1520-cm sediment core from Lake Patzcuaro, Michoacan, Mexico, is 44,000 yr old at the base. All parts of the core have abundant pollen of Pinus (pine), Alnus (alder), and Quercus (oak) with frequent Abies (fir). The interval dated from 44,000 to 11,000 yr ago has a homogeneous flora characterized by abundant Juniperus (juniper) pollen and frequent Artemisia (sagebrush). It is believed to represent an appreciably drier and colder climate than at present. The Holocene at Lake Patzcuaro is characterized by a moderate increase in Pinus pollen and the loss of Juniperus pollen, as the modern type of climate succeeded. Alnus was abundant until about 5000 yr ago; its abrupt decrease with the first appearance of herbaceous weed pollen may reflect the cutting of lake-shore and stream-course alder communities for agricultural purposes, or it may simply reflect a drying tendency in the climate. Pollen of Zea (corn) appears at Lake Patzcuaro along with low peaks of chenopod and grass pollen at 3500 yr B.P. apparently recording a human population large enough to modify the natural environment, as well as the beginning of agriculture. A rich aquatic flora in this phase suggests eutrophication of the lake by slope erosion. In the most recent period corn is absent from the sediments, perhaps reflecting a change in agricultural practices. The environment changes at Lake Patzcuaro are similar to and correlate with those in the Cuenca de Mexico, where diatom stratigraphy from the Chalco basin indicates fluctuations in lake levels and lake chemistry in response to variations in available moisture. Before 10,000 yr ago climates there were cool and dry, and the Chalco basin was occupied by a shallow freshwater marsh that drained north to Lake Texcoco, where saline water accumulated by evaporation. Increases in effective moisture and possible melting of glaciers during the Holocene caused lake levels to rise throughout the Cuenca de Mexico, and Lake Texcoco flooded the Chalco basin with brackish water. After 5000 yr ago such flooding decreased, and shallow freshwater ponds and marshes were restored in the Chalco basin. This environmental change coincides with the appearance of Zea pollen and suggests cultural control of lake levels and salinity.     

Viruses as Groundwater Tracers: Using Ecohydrology to Characterize Short Travel Times in Aquifers

Viruses are attractive tracers of short (<3 year) travel times in aquifers because they have unique genetic signatures, are detectable in trace quantities, and are mobile in groundwater. Virus “snaphots” result from infection and disappearance in a population over time; therefore, the virus snapshot shed in the fecal wastes of an infected population at a specific point in time can serve as a marker for tracking virus and groundwater movement. The virus tracing approach and an example application are described to illustrate their ability to characterize travel times in high‐groundwater velocity settings, and provide insight unavailable from standard hydrogeologic approaches. Although characterization of preferential flowpaths does not usually characterize the majority of other travel times occurring in the groundwater system (e.g., center of plume mass; tail of the breakthrough curve), virus approaches can trace very short times of transport, and thus can fill an important gap in our current hydrogeology toolbox.     

The late Cenozoic diatom stratigraphy and paleolimnology of Tule Lake,Siskiyou Co. California

Lacustrine diatoms are diverse, well preserved and abundant in cores of lake sediment to 334 m depth near the town of Tulelake, Siskiyou County, northern California. The cores have been dated by radiometric, tephrochronologic and paleomagnetic techniques, which indicate a basal age of about 3 million years (Ma) and a nearly continuous depositional record for the Tule Lake basin for the last 3 million years (My). Fossil diatoms document the late Cenozoic paleolimnologic and paleoclimatic history for the northwestern edge of the Basin and Range Province. During the last 3 My, Tule Lake was typically a relatively deep, extensive lake. The Pliocene is characterized by a diatom flora dominated by Aulacoseira solida suggesting more abundant summer precipitation and warmer winters. Increases in Fragilaria at 2.4 Ma and between 2.0 and 1.7 Ma imply cooler summers that correlate to glacial environments recorded elsewhere in the world. Stephanodiscus niagarae and Fragilaria species dominate the Pleistocene. Benthic diatoms of alkalineenriched, saline waters occur with S. niagarae between 100 and 40 m depth (0.90–0.14 Ma). Tephrochronology indicates slow deposition and possible hiatuses between about 0.6 and 0.2 Ma. Overall, the Pleistocene diatom flora reflects cooler and sometimes drier climates, especially after major glaciations began 0.85 Ma. The chronology of even-numbered oxygen isotope stages approximately matches fluctuations in the abundance in Fragilaria species since 1 Ma, suggesting that glacial periods at Tule Lake were expressed by relatively cool summers with enhanced effective moisture. Interglacial periods are represented by variable mixtures of freshwater planktonic and benthic alkaline diatom assemblages that suggest seasonal environments with winter-spring precipitation and summer moisture deficits.Glacial-interglacial environments since 150 ka were distinct from, and more variable than, those occurring earlier. The last full glacial period was very dry. Aulacoseira ambigua characterizes the late glacial and early Holocene record of Tule Lake. Its distribution indicates that warmer and wetter climates began about 15 ka in this part of the Great Basin.Fluctuations in diatom concentration suggests a 41000-yr. cycle between 3.0 and 2.5 Ma and 100000-yr. cycles after 1.0 Ma. In the late Pliocene and early Pleistocene, Aulacoseira solida percentages wax and wane in an approximately 400000-yr. cycle. The apparent response of Tule Lake diatom communities to orbitally induced insolation cycles underscores the importance of this record for the study of late Cenozoic paleoclimate change.The diatom stratigraphy records the evolution and local extinction of several species that may be biochronologically important. Stephanodiscus niagarae first appeared and became common in the Tule Lake record shortly after 1.8 Ma. Stephanodiscus carconensis disappeared about 1.8 Ma, while Aulacoseira solida is rare in the core after about 1.35 Ma. Cyclotella elgeri, a diatom characteristic of some outcrops referred to the Yonna Formation (Pliocene), is common only near the base of the core at an age of about 3 Ma. Detection of local extinctions is complicated by reworking of distinctive species from Pliocene diatomites surrounding Tule Lake.A new species, Aulacoseira paucistriata, is described from Pliocene lake deposits in the Klamath Basin.     

Paleolimnology and Paleoclimate Studies in Upper Klamath Lake, Oregon

The subsiding Upper Klamath Lake Basin contains sediments that were continuously deposited in a shallow, freshwater lake for more than 40 000 years. Well dated by radiometric methods and containing volcanic ashes of known age, these sediments constitute a valuable paleoclimate record. Sediment constituents and properties that reflect past climatic conditions in the area include pollen, diatoms, sediment geochemistry, and sediment magnetic properties. Many of these proxy measurements are also useful for comparing natural conditions in the lake to conditions following human settlement. Because of its location, the paleoclimate record from Upper Klamath Lake is valuable for comparisons to offshore marine records and as part of latitudinal transects of paleoclimate records along the west coast of the Americas.     

Reverse Water‐Level Fluctuations Associated with Fracture Connectivity

Reverse water‐level fluctuations (RWFs), a phenomenon in which water levels rise briefly in response to pumping, were detected in monitoring wells in a fractured siliciclastic aquifer system near a deep public supply well. The magnitude and timing of RWFs provide important information that can help interpret aquifer hydraulics near pumping wells. A RWF in a well is normally attributed to poroelastic coupling between the solid and fluid components in an aquifer system. In addition to revealing classical pumping‐induced poroelastic RWFs, data from pressure transducers located at varying depths and distances from the public supply well suggest that the RWFs propagate rapidly through fractures to influence wells hundreds of meters from the pumping well. The rate and cycling frequency of pumping is an important factor in the magnitude of RWFs. The pattern of RWF propagation can be used to better define fracture connectivity in an aquifer system. Rapid, cyclic head changes due to RWFs may also serve as a mechanism for contaminant transport.     

Delineation of capture zones for municipal wells in fractured dolomite, Sturgeon Bay, Wisconsin, USA

A wellhead protection study for the city of Sturgeon Bay, Wisconsin, USA, demonstrates the necessity of combining detailed hydrostratigraphic analysis with groundwater modeling to delineate zones of contribution for municipal wells in a fractured dolomite aquifer. A numerical model (MODFLOW) was combined with a particle tracking code (MODPATH) to simulate the regional groundwater system and to delineate capture zones for municipal wells. The hydrostratigraphic model included vertical and horizontal fractures and high-permeability zones. Correlating stratigraphic interpretations with field data such as geophysical logs, packer tests, and fracture mapping resulted in the construction of a numerical model with five high-permeability zones related to bedding planes or facies changes. These zones serve as major conduits for horizontal groundwater flow. Dipping fracture zones were simulated as thin high-permeability layers. The locations of exposed bedrock and surficial karst features were used to identify areas of enhanced recharge. Model results show the vulnerability of the municipal wells to pollution. Capture zones for the wells extend several kilometers north and south from the city. Travel times from recharge areas to all wells were generally less than one year. The high seasonal variability of recharge in the study area made the use of a transient model necessary. Electronic Publication     

Sorption modelling on illite Part I: Titration measurements and the sorption of Ni, Co, Eu and Sn

In this study the physico-chemical, titration and sorption characteristics of Na-illite du Puy (Na-illite) have been measured and modelled. Samples of illite, collected in the region of le Puy-en-Velay, France, were purified and conditioned to the Na-form and physico-chemically characterised. Potentiometric titrations on suspensions of the Na-illite were carried out using a batch backtitration technique in 0.01, 0.1 and 0.5 M NaClO₄ background electrolytes from pH∼3 to ∼11.5 in an inert atmosphere glove box. The supernatant solutions from each titration experiment in each series were analysed for K, Mg, Ca, Sr, Si, Al, Fe and Mn. The titration data were modelled in terms of the protolysis of two amphoteric edge sites (S^W1OH and S^W2OH) without an electrostatic term. Sorption edges (solid/liquid distribution ratios versus pH at trace sorbate concentrations and constant ionic strength) were determined for the transitions metals Ni(II) and Co(II), the lanthanide Eu(III), and the heavy metal Sn(IV) on Na-illite with NaClO₄ as the background electrolyte under anoxic conditions (CO₂ ? 2 ppm, O₂ ? 2 ppm). The study thus encompasses a broad range of metals with different thermodynamic characteristics and with valence states ranging from II to IV. The results from the modelling of the titration data, in combination with a non electrostatic surface complexation and cation exchange sorption model were applied to quantitatively describe the uptake characteristics of the metals listed above on Na-illite. Since sorption edges were measured at trace concentrations, metal uptake was modelled as occurring on strong type sites (S^SOH) only. This sorption model, the two site protolysis non electrostatic surface complexation and cation exchange model (2SPNE SC/CE model) had been previously developed and used to describe metal uptake on montmorillonite.