Atrazine in a Stream-Aquifer System: Transport of Atrazine and Its Environmental Impact Near Ashland, Nebraska

Lincoln's municipal wellfield consists of 44 wells located adjacent to the Platte River near Ashland, Nebraska. The herbicide atrazine was monitored in the river and two transects of monitoring, wells. The amount of atrazine transported down the Platte River in 1989, 1990, and 1991 was shown to increase each year. Induced recharge from the Platte River results in movement of atrazine from the river into the aquifer. A 21-day lag time was determined for the movement of atrazine from the river to a transect of monitoring wells 10 feet West of the bank. The role that colloids play on the transport of atrazine was determined to be insignificant. A small percentage of atrazine found in the river was determined to come from rain water. The infiltration of agrichemical-contaminated river water was shown to significantly reduce the quality of raw water and finished water being produced by the adjacent aquifer.     

Brevetoxin abundance and composition during ECOHAB-Florida field monitoring cruises in the Gulf of Mexico

This project was undertaken to provide information about the composition and fate of brevetoxins in concert with the multidisciplinary study, ECOHAB-FL, of Karenia brevis blooms in the Gulf of Mexico. Brevetoxin composition was provided for water samples collected during and in the absence of K. brevis blooms from November 1998, through September 2002. The identity and concentration of the most abundant brevetoxins were determined using high performance liquid chromatography with ultraviolet diode array detection (HPLC-DAD). The analytical methods changed in 2002 to the use of a mass spectrometer for brevetoxin identification and quantitation. The most abundant brevetoxins observed during blooms were PbTx-1, -2 and -3. PbTx-2 was the most abundant toxin observed in viable bloom situations with an abundance of K. brevis cells. Starting with the 2000 cruises, a distinction was made between intra-cellular toxins (inside viable K. brevis cells) and extra-cellular brevetoxins (dissolved brevetoxins outside of the cell). An important observation was the change in composition of the major brevetoxins from intra-cellular to extra-cellular toxins. The most abundant intra-cellular toxin was PbTx-2, whereas the most abundant brevetoxin recovered from the extra-cellular (dissolved) fraction in the water was PbTx-3. The abundance of PbTx-3 relative to PbTx-2 generally increased as a bloom aged, indicating the conversion of PbTx-2 to -3 as cells lysed, and the persistence of PbTx-3 in the water after cell death.     

Resolving the contributing factors to Mississippi Delta subsidence: Past and Present

To date, quantification of individual components that contribute to shallow and deep‐seated subsidence in passive margin deltas worldwide has proven problematic. A new, regional gridded chronostratigraphic dataset for the Lower Mississippi Delta region, derived from 80,928 well reports across the northern Gulf of Mexico (GOM), has bridged the disparity between geodetic mean rates measuring total land surface subsidence across annual‐to‐decadal timescales and the deep‐seated stratigraphic subsidence rates that record isostatic response over timescales of >10⁴ years. Through a quantitative assessment of gridded chronostratigraphic surfaces, sections, and subsidence rates extending from the Middle Pleistocene (0.58 Ma) to the Late Pliocene (3.85 Ma), we identify both temporal and spatial variability in deep‐seated subsidence across the northern GOM. Targeted deep‐seated subsidence data extracted across prior GOM Holocene sea‐level sample locations have revealed more than an order of magnitude greater rates of isostatic compensation in the Mississippi depocentre versus similar GOM sea‐level control sites in Florida and Alabama, casting doubt on efforts towards a representative Holocene sea‐level curve. Spatial variability in subsidence was also assessed locally in both the strike and dip directions to assess the contributions of growth faults. Fault throw displacement magnitude was discovered to decrease with depth, accounting for less than half of the total deep‐seated subsidence record of the Middle Pleistocene. Temporal subsidence complexities were also revealed including a direct, inverse logarithmic relationship between subsidence rate and time indicating variable subsidence component controls across different timescales. Despite the spatial and temporal complexities, this dataset serves as the first regional baseline for deep‐seated subsidence rates across the northern GOM.     

Groundwater temperature evolution in the subsurface urban heat island of Cologne,Germany

Long‐term heating of shallow urban aquifers is observed worldwide. Our measurements in the city of Cologne, Germany revealed that the groundwater temperatures found in the city centre are more than 5 K higher than the undisturbed background. To explore the role of groundwater flow for the development of subsurface urban heat islands, a numerical flow and heat transport model is set up, which describes the hydraulic conditions of Cologne and simulates the transient evolution of thermal anomalies in the urban ground. A main focus is on the influence of horizontal groundwater flow, groundwater recharge and trends in local ground warming. To examine heat transport in groundwater, a scenario consisting of a local hot spot with a length of 1 km of long‐term ground heating was set up in the centre of the city. Groundwater temperature‐depth profiles at upstream, central and downstream locations of this hot spot are inspected. The simulation results indicate that the main thermal transport mechanisms are long‐term vertical conductive heat input, horizontal advection and transverse dispersion. Groundwater recharge rates in the city are low (<100 mm a^?1) and thus do not significantly contribute to heat transport into the urban aquifer. With groundwater flow, local vertical temperature profiles become very complex and are hard to interpret, if local flow conditions and heat sources are not thoroughly known. Copyright © 2014 John Wiley & Sons, Ltd.     

The dissolution kinetics of a granite and its minerals—Implications for comparison between laboratory and field dissolution rates

The present study compares the dissolution rates of plagioclase, microcline and biotite/chlorite from a bulk granite to the dissolution rates of the same minerals in mineral-rich fractions that were separated from the granite sample. The dissolution rate of plagioclase is enhanced with time as a result of exposure of its surface sites due to the removal of an iron oxide coating. Removal of the iron coating was slower in the experiment with the bulk granite than in the mineral-rich fractions due to a higher Fe concentration from biotite dissolution. As a result, the increase in plagioclase dissolution rate was initially slower in the experiment with the bulk granite. The measured steady state dissolution rates of both plagioclase (6.2 ± 1.2 × 10⁻¹¹ mol g⁻¹ s⁻¹) and microcline (1.6 ± 0.3 × 10⁻¹¹ mol g⁻¹ s⁻¹) were the same in experiments conducted with the plagioclase-rich fraction, the alkali feldspar-rich fraction and the bulk granite.Based on the observed release rates of the major elements, we suggest that the biotite/chlorite-rich fraction dissolved non-congruently under near-equilibrium conditions. In contrast, the biotite and chlorite within the bulk granite sample dissolved congruently under far from equilibrium conditions. These differences result from variations in the degree of saturation of the solutions with respect to both the dissolving biotite/chlorite and to nontronite, which probably was precipitating during dissolution of the biotite and chlorite-rich fraction. Following drying of the bulk granite, the dissolution rate of biotite was significantly enhanced, whereas the dissolution rate of plagioclase decreased.The presence of coatings, wetting and drying cycles and near equilibrium conditions all significantly affect mineral dissolution rates in the field in comparison to the dissolution rate of fully wetted clean minerals under far from equilibrium laboratory conditions. To bridge the gap between the field and the laboratory mineral dissolution rates, these effects on dissolution rate should be further studied.     

Central Andean magmatism can be constrained by three ubiquitous end‐members

The geochemical character of primary magmas and the nature of crustal melts remain poorly constrained at the active continental margin of the Central Andes, where the crustal thickness is >70 km. We identify three end‐members and constrain their full major and trace element compositions by polytopic vector analysis (PVA). The end‐members include two mafic parent magmas: (1) a slightly evolved calcalkaline basaltic andesite derived from the fluid‐fluxed mantle wedge that is typical of the Andes in general and (2) a strongly LREE‐enriched basalt akin in its trace element pattern to back‐arc shoshonites. (3) The third end‐member is a variably HREE‐depleted rhyodacite representing partial melts of lower to middle crustal lithologies. We propose that these end‐members are ubiquitous and typical of such thick‐crust settings, and can explain – in highly variable mixing proportions – the entire compositional range of Central Andean magmatism.     

Strong climate and tectonic control on plagioclase weathering in granitic terrain

Investigations to understand linkages among climate, erosion and weathering are central to quantifying landscape evolution. We approach these linkages through synthesis of regolith data for granitic terrain compiled with respect to climate, geochemistry, and denudation rates for low sloping upland profiles. Focusing on Na as a proxy for plagioclase weathering, we quantified regolith Na depletion, Na mass loss, and the relative partitioning of denudation to physical and chemical contributions. The depth and magnitude of regolith Na depletion increased continuously with increasing water availability, except for locations with mean annual temperature < 5 °C that exhibited little Na depletion, and locations with physical erosion rates < 20 g m^? 2 yr^? 1 that exhibited deep and complete regolith Na depletion. Surface Na depletion also tended to decrease with increasing physical erosion. Depth-integrated Na mass loss and regolith depth were both three orders of magnitude greater in the fully depleted, low erosion rate sites relative to other locations. These locations exhibited strong erosion-limitation of Na chemical weathering rates based on correlation of Na chemical weathering rate to total Na denudation. Sodium weathering rates in cool locations with positive annual water balance were strongly correlated to total Na denudation and precipitation, and exhibited an average apparent activation energy (Ea) of 69 kJ mol^? 1 Na. The remaining water-limited locations exhibited kinetic limitation of Na weathering rates with an Ea of 136 kJ mol^? 1 Na, roughly equivalent to the sum of laboratory measures of Ea and dissolution reaction enthalpy for albite. Water availability is suggested as the dominant factor limiting rate kinetics in the water-limited systems. Together, these data demonstrate marked transitions and nonlinearity in how climate and tectonics correlate to plagioclase chemical weathering and Na mass loss.     

A new natural gamma radiation measurement system for marine sediment and rock analysis

A new high-efficiency and low-background system for the measurement of natural gamma radioactivity in marine sediment and rock cores retrieved from beneath the seabed was designed, built, and installed on the JOIDES Resolution research vessel. The system includes eight large NaI(Tl) detectors that measure adjacent intervals of the core simultaneously, maximizing counting times and minimizing statistical error for the limited measurement times available during drilling expeditions. Effect to background ratio is maximized with passive lead shielding, including both ordinary and low-activity lead. Large-area plastic scintillator active shielding filters background associated with the high-energy part of cosmic radiation. The new system has at least an order of magnitude higher statistical reliability and significantly enhances data quality compared to other offshore natural gamma radiation (NGR) systems designed to measure geological core samples. Reliable correlations and interpretations of cored intervals are possible at rates of a few counts per second.     

Arsenic in sediments, groundwater, and streamwater of a glauconitic Coastal Plain terrain, New Jersey, USA—Chemical “fingerprints” for geogenic and anthropogenic sources

Glauconite-bearing deposits are found worldwide, but As levels have been determined for relatively few. The As content of glauconites in sediments of the Inner Coastal Plain of New Jersey can exceed 100 mg/kg, and total As concentrations (up to 5.95 μg/L) found historically and recently in streamwaters exceed the State standard. In a major watershed of the Inner Coastal Plain, chemical “fingerprints” were developed for streambed sediments and groundwater to identify contributions of As to the watershed from geologic and anthropogenic sources. The fingerprint for streambed sediments, which included Be, Cr, Fe and V, indicated that As was predominantly of geologic origin. High concentrations of dissolved organic C, nutrients (and Cl⁻) in shallow groundwater indicated anthropogenic inputs that provided an environment where microbial activity released As from minerals to groundwater discharging to the stream. Particulates in streamwater during high flow constituted most of the As load; the chemical patterns for these particulates resembled the geologic fingerprint of the streambed sediments. The As/Cr ratio of these suspended particles likely indicates they derived not only from runoff, but from groundwater inputs, because As contributed by groundwater is sequestered on streambed sediments. Agricultural inputs of As were not clearly identified, although chemical characteristics of some sediments indicated vehicle-related inputs of metals. Sediment sampling during dry and wet years showed that, under differing hydrologic conditions, local anthropogenic fingerprints could be obscured but the geologic fingerprint, indicating glauconitic sediments as an As source, was robust.