Crude Oil Metabolites in Groundwater at Two Spill Sites

Two groundwater plumes in north central Minnesota with residual crude oil sources have 20 to 50 mg/L of nonvolatile dissolved organic carbon (NVDOC). These values are over 10 times higher than benzene and two to three times higher than Diesel Range Organics in the same wells. On the basis of previous work, most of the NVDOC consists of partial transformation products from the crude oil. Monitoring data from 1988 to 2015 at one of the sites located near Bemidji, MN show that the plume of metabolites is expanding toward a lakeshore located 335 m from the source zone. Other mass balance studies of the site have demonstrated that the plume expansion is driven by the combined effect of continued presence of the residual crude oil source and depletion of the electron accepting capacity of solid phase iron oxide and hydroxides on the aquifer sediments. These plumes of metabolites are not covered by regulatory monitoring and reporting requirements in Minnesota and other states. Yet, a review of toxicology studies indicates that polar metabolites of crude oil may pose a risk to aquatic and mammalian species. Together the results suggest that at sites where residual sources are present, monitoring of NVDOC may be warranted to evaluate the fates of plumes of hydrocarbon transformation products.     

Sediment Yield During Late Glacial and Holocene periods in the Lac Chambon Watershed,Massif Central,France

A sediment budget for the Late Glacial and Holocene periods was calculated for the Lac Chambon watershed which is located in a formerly glaciated temperate crystalline mountain area. It appears that over 15 500 years: (1) 69 per cent of eroded particles have been displaced by gravity processes and then stored within the watershed, compared to 31 per cent that have been displaced by running water and evacuated outward; (2) the mean mechanical erosion due to gravity processes on the slopes amounted to 16·1 ±6 m and only developed on a quarter of the watershed surface, whereas the mean mechanical erosion due to running water amounted 1·24 ± 0·37 m and involved the whole watershed surface. The mean sediment yields due to gravity processes on slopes were 2300 ± 1360, 1770 ± 960 and 380 ± 100 m³ km⁻³ a⁻¹, respectively, for basalts, and basic and acidic trachyandesites. Values of sediment yield due to running water were 49±15, 120±36 and 79±24 m³ km⁻² a⁻¹, respectively, during the Bôlling–Allerôd, the Younger Dryas and the Pre-Boreal–Boreal periods. They were 56±17 and 166±50 m³ km⁻² a⁻¹ during the Sub-Atlantic period before and after 1360 a BP , respectively. These values reflect variations in the natural environment and the impact of human-induced deforestation. © 1997 by John Wiley & Sons, Ltd.     

Characterization of site conditions (soil class,V<Subscript>S30</Subscript>, velocity profiles) for 33 stations from the French permanent accelerometric network (RAP) using surface-wave methods

Data provided by accelerometric networks are important for seismic hazard assessment. The correct use of accelerometric signals is conditioned by the station site metadata quality (i.e., soil class, V_S30, velocity profiles, and other relevant information that can help to quantify site effects). In France, the permanent accelerometric network consists of about 150 stations. Thirty-three of these stations in the southern half of France have been characterized, using surface-wave-based methods that allow derivation of velocity profiles from dispersion curves of surface waves. The computation of dispersion curves and their subsequent inversion in terms of shear-wave velocity profiles has allowed estimation of V_S30 values and designation of soil classes, which include the corresponding uncertainties. From a methodological point of view, this survey leads to the following recommendations: (1) perform both active (multi-analysis surface waves) and passive (ambient vibration arrays) measurements to derive dispersion curves in a broadband frequency range; (2) perform active acquisitions for both vertical (Rayleigh wave) and horizontal (Love wave) polarities. Even when the logistic contexts are sometimes difficult, the use of surface-wave-based methods is suitable for station-site characterization, even on rock sites. In comparison with previous studies that have mainly estimated V_S30 indirectly, the new values here are globally lower, but the EC8-A class sites remain numerous. However, even on rock sites, high frequency amplifications may affect accelerometric records, due to the shallow relatively softer layers.     

Biodegradation in contaminated aquifers: incorporating microbial/molecular methods

In order to evaluate natural attenuation in contaminated aquifers, there has been a recent recognition that a multidisciplinary approach, incorporating microbial and molecular methods, is required. Observed decreases in contaminant mass and identified footprints of biogeochemical reactions are often used as evidence of intrinsic bioremediation, but characterizing the structure and function of the microbial populations at contaminated sites is needed. In this paper, we review the experimental approaches and microbial methods that are available as tools to evaluate the controls on microbially mediated degradation processes in contaminated aquifers. We discuss the emerging technologies used in biogeochemical studies and present a synthesis of recent studies that serve as models of integrating microbiological approaches with more traditional geochemical and hydrogeologic approaches in order to address important biogeochemical questions about contaminant fate.     

Hydrochemical appraisal of ice‐ and rock‐glacier meltwater in the hyperarid Agua Negra drainage basin,Andes of Argentina

The Agua Negra drainage system (30 12′S, 69 50′ W), in the Argentine Andes holds several ice‐ and rock‐glaciers, which are distributed from 4200 up to 6300 m a.s.l. The geochemical study of meltwaters reveals that ice‐glaciers deliver a HCO₃^?? Ca²⁺ solution and rock‐glaciers a SO₄^2?? HCO₃^?? Ca²⁺ solution. The site is presumably strongly influenced by sublimation and dry deposition. The main processes supplying solutes to meltwater are sulphide oxidation (i.e. abundant hydrothermal manifestations), and hydrolysis and dissolution of carbonates and silicates. Marine aerosols are the main source of NaCl. The fine‐grained products of glacial comminution play a significant role in the control of dissolved minor and trace elements: transition metals (e.g. Mn, Zr, Cu, and Co) appear to be selectively removed from solution, whereas some LIL (large ion lithophile) elements, such as Sr, Cs, and major cations, are more concentrated in the lowermost reach. Daily concentration variation of dissolved rare earth elements (REE) tends to increase with discharge. Through PHREEQC inverse modelling, it is shown that gypsum dissolution (i.e. sulphide oxidation) is the most important geochemical mechanism delivering solutes to the Agua Negra drainage system, particularly in rock‐glaciers. At the lowermost reach, the chemical signature appears to change depending on the relative significance of different meltwater sources: silicate weathering seems to be more important when meltwater has a longer residence time, and calcite and gypsum dissolution is more conspicuous in recently melted waters. A comparison with a non‐glacierized semiarid drainage of comparable size shows that the glacierized basin has a higher specific denudation, but it is mostly accounted for by relatively soluble phases (i.e. gypsum and calcite). Meltwater chemistry in glacierized arid areas appears strongly influenced by sublimation/evaporation, in contrast with its humid counterparts. Copyright © 2007 John Wiley & Sons, Ltd.     

Improving Kirchhoff migration with repeated local plane-wave imaging? A SAR-inspired signal-processing approach in prestack depth imaging

A local plane-wave approach of generalized diffraction tomography in heterogeneous backgrounds, equivalent to Kirchhoff summation techniques when applied in seismic reflection, is re-programmed to act as repeated synthetic aperture radar (SAR) imaging for seismic prestack depth migration. Spotlight-mode SAR imaging quickly provides good images of the electromagnetic reflectivity of the ground via fast Fourier transform (FFT)-based signal processing. By calculating only the Green's functions connecting the aircraft to the centre of the illuminated patch, scattering structures around that centre are also recovered. SAR technology requires us to examine seismic imaging from the local point of view, where the quantity and quality of the available information at each image point are what are important, regardless of the survey geometry. When adapted to seismics, a local image of arbitrary size and sampling is obtained by FFT of seismic energy maps in the scattering wavenumber domain around each node of a pre-calculated grid of Green's functions. These local images can be used to generate a classic prestack depth-migrated section by collecting only their centres. However, the local images also provide valuable information around the centre, as in SAR. They can therefore help to pre-analyse prestack depth migration efficiently, and to perform velocity analysis at a very low cost. The FFT-based signal-processing approach allows local, efficient and automatic control of anti-aliasing, noise and resolution, including optimized Jacobian weights. Repeated local imaging could also be used to speed up migration, with interpolation between local images associated with a coarse grid of Green's functions, as an alternative to interpolation of Green's functions. The local images may, however, show distortions due to the local plane-wave approximation, and the velocity variations across their frame. Such effects, which are not necessarily a problem in SAR, should be controlled and corrected to further enhance seismic imaging. Applications to realistic models and to real data show that, despite the distortion effects, the local images can yield similar information to prestack depth migration, including common-image-point gathers for velocity analyses and AVO/AVA effects, at a much lower cost when a small target is considered.     

Analyse conjointe des régimes pluviométriques et hydrologiques dans le bassin de la Tafna (Algérie Occidentale)

Résumé

La variabilité du climat, une gestion non optimale de la ressource en eau et l'intensification des activités anthropiques pourraient être les trois facteurs majeurs menaçant les écoulements et l'état de la ressource en eau dans les sous-bassins algériens. C'est le cas du bassin de la Tafna dans la partie occidentale de l'Algérie. Ce bassin est situé dans une région au climat semi-aride. Outre l'influence des précipitations, nous examinerons dans cet article la contribution possible d'autres facteurs (géologiques, topographiques et anthropiques) à la variabilité des écoulements dans le bassin de la Tafna. Pour cela, en plus de l'analyse du régime pluviométrique et des débits, nous avons choisi d'analyser le baseflow index ou BFI et les débits de base. Cinq sous-bassins de la Tafna, où des données journalières de pluie et de débit étaient disponibles sur la période 1976–2006 ont été étudiés. L'analyse, qui a distingué deux sous-régions déterminées par la topographie: les bassins d'altitude et ceux de plaine, montre que, selon leur contexte géologique, les sous-bassins ont des comportements différents en terme d'écoulement. Les zones en altitude présentant un écoulement de base et un BFI plus importants que les bassins de plaine. Ceci peut être relié à la nature lithologique des formations, les bassins d'altitude présentant une composante karstique importante. Les analyses de tendance/rupture sur les précipitations annuelles ne montrent aucune tendance significative sur la période 1976–2006. On note une baisse des débits moyens annuels uniquement sur deux stations en altitude (significatives aux seuils 1 et 10% respectivement). Une tendance à une diminution significative des débits de base est obtenue sur deux bassins d'altitude et un bassin de plaine (au seuil 1 ou 5%). La baisse du BFI est significative au seuil 1% pour seulement une station d'altitude. Ceci serait le signe d'une baisse du stock dans les bassins d'altitude. La modification des écoulements de base sur certains sous-bassins de la Tafna serait donc imputable à des causes, notamment de nature anthropique, ne relevant pas d'une diminution de la pluviométrie.

Editeur Z.W. Kundzewicz; Editeur associé M. Lang

Citation Bakreti, A., Braud, I., Leblois, E., et Benali, A. 2013. Analyse conjointe des régimes pluviométriques et hydrologiques dans le bassin de la Tafna (Algérie Occidentale). Hydrological Sciences Journal, 58 (1), 1–19.     

Arsenic Cycling in Hydrocarbon Plumes: Secondary Effects of Natural Attenuation

Monitored natural attenuation is widely applied as a remediation strategy at hydrocarbon spill sites. Natural attenuation relies on biodegradation of hydrocarbons coupled with reduction of electron acceptors, including solid phase ferric iron (Fe(III)). Because arsenic (As) adsorbs to Fe‐hydroxides, a potential secondary effect of natural attenuation of hydrocarbons coupled with Fe(III) reduction is a release of naturally occurring As to groundwater. At a crude‐oil‐contaminated aquifer near Bemidji, Minnesota, anaerobic biodegradation of hydrocarbons coupled to Fe(III) reduction has been well documented. We collected groundwater samples at the site annually from 2009 to 2013 to examine if As is released to groundwater and, if so, to document relationships between As and Fe inside and outside of the dissolved hydrocarbon plume. Arsenic concentrations in groundwater in the plume reached 230 µg/L, whereas groundwater outside the plume contained less than 5 µg/L As. Combined with previous data from the Bemidji site, our results suggest that (1) naturally occurring As is associated with Fe‐hydroxides present in the glacially derived aquifer sediments; (2) introduction of hydrocarbons results in reduction of Fe‐hydroxides, releasing As and Fe to groundwater; (3) at the leading edge of the plume, As and Fe are removed from groundwater and retained on sediments; and (4) downgradient from the plume, patterns of As and Fe in groundwater are similar to background. We develop a conceptual model of secondary As release due to natural attenuation of hydrocarbons that can be applied to other sites where an influx of biodegradable organic carbon promotes Fe(III) reduction.     

Quantitative inter‐annual and decadal June–July–August temperature variability ca. 570 BC to AD 120 (Iron Age–Roman Period) reconstructed from the varved sediments of Lake Silvaplana,Switzerland

Annually resolved June–July–August (JJA) temperatures from ca. 570 BC to AD 120 (±100 a; approximately 690 varve years) were quantified from biogenic silica and chironomids (Type II regression; Standard Major Axis calibration‐in‐time) preserved in the varved sediments of Lake Silvaplana, Switzerland. Using 30 a (climatology) moving averages and detrended standard deviations (mean–variability change, MVC), moving linear trends, change points and wavelets, reconstructed temperatures were partitioned into a warmer (+0.3°C; ca. 570–351 BC), cooler (?0.2°C; ca. 350–16 BC) and moderate period (+0.1°C; ca. 15 BC to AD 120) relative to the reconstruction average (10.9°C; reference AD 1950–2000 = 9.8°C). Warm and variable JJA temperatures at the Late Iron Age–Roman Period transition (approximately 50 BC to AD 100 in this region) and a cold anomaly around 470 BC (Early–Late Iron Age) were inferred. Inter‐annual and decadal temperature variability was greater from ca. 570 BC to AD 120 than the last millennium, whereas multi‐decadal and lower‐frequency temperature variability were comparable, as evident in wavelet plots. Using MVC plots of reconstructed JJA temperatures from ca. 570 BC to AD 120, we verified current trends and European climate model outputs for the 21st century, which suggest increased inter‐annual summer temperature variability and extremes in a generally warmer climate (heteroscedasticity; hotspot of variability). We compared these results to MVC plots of instrumental and reconstructed temperatures (from the same sediment core and proxies but a different study) from AD 1177 to AD 2000. Our reconstructed JJA temperatures from ca. 570 BC to AD 120 showed that inter‐annual JJA temperature variability increased rapidly above a threshold of ～10°C mean JJA temperature. This increase accelerated with continued warming up to >11.5°C. We suggest that the Roman Period serves with respect to inter‐annual variability as an analogue for warmer 21st‐century JJA temperatures in the Alps. Copyright © 2011 John Wiley & Sons, Ltd.