Environmental hydrogeology of the southern sector of the Samborombon Bay wetland,Argentina

The Samborombon Bay wetland is located on the west margin of the Rio de la Plata estuary, in the Province of Buenos Aires, Argentina. This paper analyses the geological, geomorphologic, soil and vegetation characteristics of the southernmost sector of this wetland and their influence on surface water and groundwater. The study area presents three hydrologic units: coastal dunes, sand sheets and coastal plain. Coastal dunes and sand sheets are recharge zones of high permeability with well-drained, non-saline soils, and a few surface water flows. Changes in the water table are related to rainfall. Groundwater in coastal dunes is Ca–Mg–HCO₃ to Na–HCO₃, and of low salinity (590 mg/l). Groundwater in sand sheets is mainly Na–HCO₃ with a salinity of about 1,020 mg/l. The coastal plain exhibits medium to low permeability sediments, with submerged saline soils poorly drained. Groundwater is Na–Cl with a mean salinity of 16,502 mg/l. A surface hydrological network develops in the coastal plain. Surface water levels near the shoreline are affected by tidal fluctuations; far from the shoreline water accumulates because of poor drainage. Both sectors have Na–Cl water, but the former is more saline. Human intervention and sea level rise may affect the wetland severely.     

Survival of organic phases in porous IDPs during atmospheric entry: A pulse‐heating study

Abstract— In this study, we have performed pulse‐heating experiments at different temperatures for three organic molecules (a polycyclic aromatic hydrocarbon [PAH], a ketone, and an amino acid) absorbed into microporous aluminum oxide (Al₂O₃) in order to imitate the heating of the organic molecules in interplanetary dust particles (IDPs) and micrometeorites (MMs) during atmospheric entry and to investigate their survival. We have shown that modest amounts (a few percent) of these organic molecules survive pulse‐heating at temperatures in the 700 to 900 °C range. This suggests that the porosity in IDPs and MMs, combined with a sublimable phase (organic material, water), produces an ablative cooling effect, which permits the survival of organic molecules that would otherwise be lost either by thermal degradation or evaporation during atmospheric entry.     

Coastal processes and environmental hazards: the Buenos Aires (Argentina) and Venetian (Italy) littorals

The Buenos Aires (Argentina) and Venice (Italy) coastlands have experienced significant saltwater contamination of the phreatic aquifer, coastal erosion, hydrodynamic changes and relative sea level rise processes due to natural and man-induced factors. These factors expose coastal areas to morpho-hydro-geological hazards, such as soil desertification, frequency and degree of flooding, littoral erosion, and the silting of river mouths and channels. Man-made interventions and actions, such as beach mining, construction of coastal structures and exploitation of aquifers without an adequate knowledge of the hydrology setting and an adequate management program, worsen these natural hazards. Uncontrolled human activity induces environmental damage to the overall coastal plains. The coastal plains play an important role in the social/economic development of the two regions based on land use, such as agriculture, horticulture, breeding, and tourism, as well as industry. Results of investigations on saltwater contamination, sea level rise and morphological changes recently performed in these two coastal areas are presented here.     

Imaging Offsets in the Moho: Synthetic Tests using Gaussian Beams with Teleseismic Waves

We carry out a sequence of numerical tests to understand conditions under which rapid changes in crustal thickness can be reliably imaged by teleseismic body waves. Using the finite-difference method over a 2-D grid, we compute synthetic seismograms resulting from a planar P-wavefield incident below the grid. We then image the Moho using a migration scheme based on the Gaussian beam representation of the wavefield. The use of Gaussian beams for the downward propagation of the wavefield is particularly advantageous in certain geologically critical cases such as overthrusting of continental lithosphere, resulting in the juxtaposition of high-velocity mantle material over crustal rocks. In contrast to ray-based methods, Gaussian beam migration requires no special treatment to handle such heterogeneities. Our results suggest that with adequate station spacing and signal-to-noise ratios, offsets of the Moho, on the order of 10 km in height, can be reliably imaged beneath thickened crust at depths of about 50 km. Furthermore, even sharp corners and edges are faithfully imaged when precise values of seismic wave speeds are available. Our tests also demonstrate that flexibility in choices of different types of seismic phases is important, because any single phase has trade-offs in issues such as spatial resolution, array aperture, and amplitude of signals.     

Influence of earthquakes on the stability of slopes

Earthquakes are a major trigger for instability of natural and man-made slopes. Often the instability of slopes due to an earthquake causes more destruction and kills more people than the actual earthquake itself. A comparison is made between different methodologies to analyze the potential stability of slopes during earthquakes. Theoretically, it seems simple to calculate the stability of a slope during an earthquake. In reality, however, the stability is influenced by so many parameters that are either not known or which influence is so poorly known that a decent estimation of stability cannot be made. Offshore the situation is worse because proper data required for stability calculations are even less available than onshore. On- and offshore, erosion and weathering create continuously slopes that may become unstable during a future earthquake, offshore also sedimentation creates continuously new slopes. Another fundamental problem in stability analysis is the complicated and largely unknown behavior of seismic waves in three-dimensions in natural materials. The lack of accurate data and the unknown behavior of seismic waves in three-dimensions make estimations of slope stability during an earthquake unreliable.     

Assessing selected natural and anthropogenic impacts on freshwater lens morphology on small barrier Islands: Dog Island and St. George Island, Florida, USA

Abstract The freshwater lens morphologies of the barrier islands Dog Island and St. George Island on the panhandle coast of Florida (FL), USA, are controlled to varying degrees by both natural and anthropogenic factors. Variable-density groundwater flow models confirm that spatial variability of recharge values can account for the observed lens asymmetry on these islands. The depth to the base of the lens does not vary significantly seasonally. Human development has altered recharge patterns in some areas, locally thinning the freshwater lens. Aqueduct water supply to St. George Island represents 7–25% of natural recharge; higher recharge rates are required to simulate the lens on St. George Island than on Dog Island. On both islands, coastal erosion rates are sufficiently rapid that the freshwater lens may not be in equilibrium with current boundary conditions.

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Resumen Las morfologías de los lentes de agua dulce en las islas de barrera: Islas Dog y St. George, en la costa alargada y estrecha de Florida, EUA, están controladas en diferentes grados por factores antropogénicos y naturales. Los modelos de flujo con densidad variable del agua subterránea, confirman que la variabilidad espacial de los valores de recarga, pueden ser la causa de la asimetría observada en los lentes de estas islas. La profundidad hasta la base de los lentes no varía significativamente con las estaciones climáticas. El desarrollo humano ha alterado las tendencias de recarga en algunas áreas, causando adelgazamiento local de los lentes de agua dulce. El abastecimiento de agua del acueducto en la Isla St. George, representa entre 7% y 25% de la recarga natural; por otro lado se necesitan cantidades mayores de recarga para simular los lentes en la Isla St. George que en la Isla Dog. En ambas islas, las tasas de erosión costera son tan rápidas, que las lentes de agua dulce podrían no estar en equilibrio con las condiciones actuales de frontera.

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Résumé La morphologie des lentilles deau douce des îles de Dog et de St. George, situés dans le district côtier de Florida-Etats Unis est contrôlé dans une mesure variable par des facteurs naturelles et humaines. Les modèles découlement à densité variable des eaux souterraines confirment que la variation spatiale de la recharge peut expliquer les asymétries observées des lentilles des cettes îles. La profondeur des lentilles ne présente pas des variations saisonnières importantes. Les activités humaines ont modifié la recharge dans certaines zones en amincissant localement les lentilles deau douce. Lalimentation en eaux de lîle de St. George représente 7–25% de la recharge naturelle; pour simuler les lentilles de lîle de Dog il a été nécessaire dintroduire des valeurs plus élevées de la recharge par rapport aux ceux utilisés pour lîle de St. George. Pour les deux îles lérosion côtière est assez rapide ainsi que les lentilles de eau douce ne sont pas en équilibre avec les conditions à la frontière.

     

A Cluster Space Calibration Scheme for Synthetic Wind Statistics Calculated with a Mesoscale Model and its Application to Regional Wind Climate Studies

Summary ?A scheme for calibrating an ensemble of wind fields computed by a mesoscale model in order to generate synthetic wind statistics is described. It is based on two main points. The first is to exploit the power of a mesoscale model to determine wind fields over complex terrain for different weather situations classified by cluster analysis. The second is to use all the information in the cluster analysis, i.e., the centroid values as well as the internal standard deviations of the clusters, to determine a cluster space distribution at each grid point in the model domain. The latter makes calibration possible if reliable measurements are available at the position of one of the grid points. The accuracy of the calibration is increased by splitting the cluster spaces into several parts. Combining both the modelled mesoscale wind fields and the method of split cluster spaces leads to a spatial transformation of the calibration from the calibration point to each grid point in the model domain. A validation of the scheme is carried out with measurements at grid points other than the calibration point and reveals remarkable improvements in the accuracy of the model wind statistics, especially with regard to wind speed distributions. Received October 8, 1998/Revised March 19, 1999     

On the Importance of Time for GIS View Measures and Their Use in Hedonic Property Models: Does Being Temporally Explicit Matter?

Progress in GIScience has advanced the ability to represent and analyze view characteristics. GIS‐derived view measures requiring digital elevation surface models are used in hedonic property models to quantify the amenity value of view for parcel sales transactions. Ideally models should represent surface elevations that are temporally synchronized with parcel sale dates. Temporal synchronization for studies spanning multiple years may require significant effort. Few studies have undertaken this effort, leading us to investigate in this research the need to be temporally explicit. We evaluate two competing surface model approaches based on: (1) a single year 2000 LiDAR surface product; and (2) annual‐specific surface products for 1995–2002. Two competing view measures based on the different surface approaches are constructed for 561 parcel transactions during 1995–2002 in a coastal North Carolina county and are input into hedonic regression models. Results showed that being temporally explicit did matter in terms of finding significantly different view measures but did not matter in terms of finding significantly different effects of view on parcel sales prices. Despite mixed results for our case study, we advise that future research involving GIS‐based view measurement should consider the spatial and temporal contexts of study area development patterns when evaluating the need to be temporally explicit.     

Environmental impacts and simultaneity of positive and negative storm surges on the coast of the Province of Buenos Aires, Argentina

The Argentine shore of the Rio de la Plata estuary and its southwards adjacent maritime front are normally affected by extratropical positive and negative storm surges that affect human activities seriously. Positive surges can raise the water level in the estuary by more than 3 m over the predicted tide; thus, flooding the coastal plain where over 13 million people live and causing extensive property damage. Sometimes, there has been loss of life too. Although less populated than the coastal plain, the maritime front has many important tourist resorts and also undergoes severe beach erosion processes and loss of property owing to positive surges. Negative surges are particularly troublesome in the Rio de la Plata because they critically affect navigation safety and drinking water supply by lowering the predicted water level in an amount that sometimes reached more than 4 m. A remarkable point is that the same storm event can simultaneously give rise to a positive surge on the maritime front and a negative one in the Rio de la Plata. The environmental impacts of positive storm surges are strongly aggravated by human intervention. At the same time, sea level rise due to global climatic change has also its influence.