Temporal variations of colloidal carrier phases and associated trace elements in a boreal river

Elemental size distributions, from truly dissolved through colloidal to particulate, have been studied in a subarctic boreal river. The measurements, carried out during 2002, ranged from winter to summer conditions, including an intense spring flood event. Results are reported for a total of 42 elements. Size distributions were characterised using a combination of cross-flow (ultra)filtration (CFF), flow field-flow fractionation (FlFFF), and diffusive gradients in thin-films (DGT). The three techniques showed similar trends, but quantitative comparisons reveal some important differences that warrant further investigation.Previous work has identified two colloidal carrier phases in fresh waters, dominated by iron and carbon, respectively. The majority of the elements studied are associated with one or both of these colloidal carrier phases. The exceptions are the alkali metals and several anions that are only very weakly associated with colloidal material, and which therefore occur mainly as truly dissolved material (<1 kDa in molecular weight). We discuss the likely origin for the two colloidal carrier phases and consider how associated trace elements fit into the geochemical framework. The relative affinities of the elements for iron and carbon colloidal carrier phases are related to their chemistries, and are compared with earlier data from the Delsjö Creek in southern Sweden.Elemental colloidal concentrations show strong seasonal variations related to changes in the colloidal carrier phase(s) with which they associate. In particular, many elements show a strong spring maximum in colloidal concentrations associated with the strong maximum in colloidal carbon concentration during the spring flood.     

Improving fluorometry as a source tracking method to detect human fecal contamination

In a continuing effort to develop inexpensive source tracking methods to detect human fecal contamination in environmental waters, targeted sampling was combined with fluorometry. Targeted sampling works by identifying hot spots of fecal contamination through multiple samplings over ever-decreasing distances. Fluorometry identifies human fecal contamination by detecting optical brighteners, primarily from laundry detergents. Because organic matter fluoresces and interferes with fluorometry, two locations were chosen for sampling: waters relatively low in organic matter at Mayagüez Bay, Puerto Rico, and waters relatively high in organic matter at St. Simons Island, Georgia. In Puerto Rico, targeted sampling and fluorometry quickly and easily identified two hot spots of human fecal contamination in the Yagüez River, which flows through the city of Mayagüez. Another source tracking method, detection of theesp gene, confirmed their human origin. On St. Simous Island, targeted sampling and fiuorometry identified two hot spots of potential human fecal contamination. Detection of theesp gene confirmed the human origin of one site but not the other, most likely because background organic matter fluorescence interfered with fhiorometry. A separate experiment showed that adding a 436-um emission filter to the fluorometer reduced this background fluorescence by > 50%. With the 436-nm Filter in place, another sampling was conducted on St. Simons Island, and the second hot spot was identified as fecal contamination from birds. As long as the fluorometer was equipped with a 436-nm filter and organic matter concentrations were considered, targeted sampling combined with fluorometry was a relatively inexpensive method for identifying human fecal contamination in water.     

Natural variations of δSi ratios during progressive basalt weathering, Hawaiian Islands

Silicon stable isotopes can be used to trace the biogeochemical pathways of Si as it moves from its continental sources to its sink in ocean sediments. Along the way, Si is incorporated into clay minerals, taken up by plants where it forms plant opal, and leached into rivers, the major land-to-ocean conduit. Compared to igneous rocks, the waters that drain continents are enriched in heavy Si isotopes, but the mechanisms that control fractionation have not been elucidated. We studied Si isotope fractionation along a 4 million yr basaltic soil chronosequence on the Hawaiian Islands. Using the natural context of these samples in combination with laboratory experiments, we demonstrate that the isotopic composition of dissolved Si in weathering systems is determined by the combined effects of rock disintegration, clay mineral neosynthesis, and Si biocycling. Weathering preferentially releases ²⁸Si into solution, whereas secondary mineral formation preferentially removes ²⁸Si from solution. In humid environments, leached soils have lost large amounts of this soluble Si, thus creating a net loss of ³⁰Si from the entire soil system. As soils develop and greater fractions of Si reside in neoformed clay minerals, δ³⁰Si_{bulk soil} values change progressively toward more negative values; basalt δ³⁰Si values are about −0.5‰, but older soils have δ³⁰Si values up to −2.5‰. The difference between the solid and solution δ³⁰Si values remains more or less constant with progressive weathering, and therefore, soil water from older soils has a more negative δ³⁰Si composition. In the upper horizons of the Hawaiian soils, this weathering-driven δ³⁰Si shift is modified by the addition of unweathered primary minerals via dust, carrying δ³⁰Si values of about −0.5‰, and by biocycling of Si via plants, producing negative δ³⁰Si values in phytoliths and positive δ³⁰Si values in soil solutions derived from upper horizons. Due to the high concentrations of dissolved Si in these near-surface layers, rivers have more positive δ³⁰Si values than predicted based on the weathering status of the lower horizons. When combined with published δ³⁰Si values from large rivers worldwide, we find that the results from Hawaii point to weathering control of Si isotopes delivered to the oceans, and thus, to an important continent-ocean linkage that warrants further investigation.     

An Analytical Model for Simulating Step-Function Injection in a Radial Geometry

An analytical model is developed for analyzing underground injection of wastes that undergo advection, dispersion, sorption, and first-order degradation. The model uses a time-dependent, step-function source that simulates intermittent waste injection in a continuous fluid injection well. The governing equations for a cylindrically symmetrical system are cast in nondimensional form and then transformed and solved in Laplace space. The Laplace space solution is inverted with the Crump algorithm, which uses the real and imaginary parts of a Fourier series. The numerical solution is verified by replicating the step-function source at the point of injection, and the behavior of the model is demonstrated in a series of figures. The model is recommended for quick, scoping calculations in which there is little site-specific information and periods of discontinuous radial injection.     

Determination of the Optimum Sampling Intervals in Sediment Pore Waters Using the Autocovariance Function

An approach to scaling that has found considerable utility inthe field of physical oceanography is based on theauto-covariance function (ACF). It is demonstrated to alsobe useful in determining the characteristic length scale of thedominant variance of dissolved sulfide and ferrous iron inanoxic pore waters. The method begins with the analysisof a data series in which the measurement sampling intervalis smaller than the expected length scales. A least squarespolynomial fit to the data representing a background field isthen removed from the data series making possible the analysisof the variability superimposed on the background field. Thecharacteristic length scale is then defined as the distancein which the measurements become uncorrelated. This usuallyoccurs at a significance-level or zero-crossing of the ACF.Characteristic lengths obtained for sediments from a varietyof coastal environments are not distinguishable betweensecond and third order polynomial fits with average valuesand standard deviations of, respectively, 7.0 ± 2.7 mm and6.1 ± 2.3 mm. These values are close to the typical burrowdiameter of 7.5 mm used by Aller (1978) in his bioirrigationmodel for sediment diagenesis. These results indicate thatmacrofauna dwelling in the sediment are probably responsiblefor the variability, although in the seagrass meadow rhizomesmay also play a role.     

Meeting the Need for GIS Skills in Developing Countries: The Case of Informal Settlements

Informal settlements are a chronic issue in developing countries. Management and formalisation processes require good spatial data and analytical tools. While software such as geographic information systems (GIS) are widely available, access to good data and skilled users may be limited. In this paper we suggest that digital multimedia learning tools, accessible via the World Wide Web (WWW) may be part of the solution. We present the rationale for, and structure of, a suite of prototype learning modules for neighbourhood operations. We illustrate the use of these operations by converting shack locations into population density maps for the Imizamo Yethu settlement near Cape Town, South Africa. This prototype forms a part of a larger web site that also includes background information, data sources and software.     

The scaling characteristics of remotely-sensed variables for sparsely-vegetated heterogeneous landscapes

With increasing interest in airborne and satellite-based sensors for mapping regional and global energy balance, there is a need to determine the uncertainty involved in aggregating remotely-sensed variables [surface temperature (T_k) and reflectance (π)] and surface energy fluxes [sensible (H) and latent (λE) heat flux] over large areas. This uncertainty is directly related to two factors: (1) the nonlinearity of the relation between the sensor signal and T_k, π, H orλE; and (2) the heterogeneity of the site. In this study, we compiled several remotely-sensed data sets acquired at different locations within a semi-arid rangeland in Arizona, at a variety of spatial and temporal resolutions. These data sets provided the range of data heterogeneities necessary for an extensive analysis of data aggregation. The general technique to evaluate uncertainty was to compare remotely-sensed variables and energy balance components calculated in two ways: first, calculated at the pixel resolution and averaged to the coarser resolution; and second, calculated directly at the coarse resolution by aggregating the fine-resolution data to the coarse scale. Results showed that the error in the aggregation of T_k and π was negligible for a wide range of conditions. However, the error in aggregation of H and λE was highly influenced by the heterogeneity of the site. Errors in H larger than 50% were possible under certain conditions. The conditions associated with the largest aggregation errors in H were:

• sites which are composed of a mix of stable and unstable conditions;

• sites which have considerable variations in aerodynamic roughness, especially for highly unstable conditions where the difference between surface and air temperature is large; and

• sites which are characterized by patch vegetation, where the pixel resolution is less than or nearly-equal to the diameter of the vegetation ‘element’ (in most cases, the diameter of the dominant vegetation type or vegetation patch).

Thus, knowledge of the surface heterogeneity is essential for minimizing error in aggregation of H and λE. Two schemes are presented for quantifying surface heterogeneity as a first step in data aggregation. These results emphasized the need for caution in aggregation of energy balance components over heterogeneous landscapes with sparse or mixed vegetation types.     

Activité du gaz radon dans l'air du sol et sismicité locale : exemple du bassin de l'Arax (Arménie)Radon soil-gas concentrations and local seismicity: case of the Arax basin (Armenia)

A survey of soil gas radon concentrations has been carried out at three sites, in the seismic area of Armenia, from 1996 to 1999. The seismicity generates opposed behaviours at the different sites. This heterogeneity is related to the sites locations in the tectonic frame. An increase of radon concentration occurs inside the tectonic micro-blocks during local seismic activity. On the contrary, an abrupt co-seismic decrease is recorded close to the fault area. These variations can be due to ‘pore-collapse’ phenomena which expulse the pore-gas and increases Rn concentration in soil at intra-blocs sites, but also to pore-elastic deformations and microfracturing modifications in the fault area. To cite this article: K. Kharatian et al., C. R. Geoscience 334 (2002) 179–185.     

“There is No Program Without Farmers”: Interactive Radio for Forest Landscape Restoration in Mount Elgon Region,Uganda

Increase in ownership and use of radios and mobile phones in Uganda may present opportunities for interactive and efficient agricultural extension services. Yet the impact of interactive radio on rural development has rarely been evaluated. In a participatory project, the International Union for the Conservation of Nature together with Farm Radio International and stakeholders from local government, radio, and community-based organizations promoted the increased use of forest landscape restoration activities though interactive rural radio programs, including innovative methods to reach populations not covered by radio. An evaluation found that 98% of people who listened to most or all of the broadcasts carried out one of the practices, whereas 84% of those who had listened to only one episode had used one of the practices. Inclusion of a wide range of community stakeholders from project inception was vital for presenting the project and embedding it within local institutional contexts.     

Characterisation and evaluation of reference materials for black carbon analysis using elemental composition, colour, BET surface area and C NMR spectroscopy

A recent comparative exercise found that different black carbon (BC) quantification methods produced widely varying results for a set of BC reference materials that included three laboratory-produced BC-rich materials, five environmental BC matrices and four samples of non-BC organic matter. This variation was attributed to a range of physical and chemical properties of the samples that could, in various ways, result in the over- or under-detection of BC in some or all of the techniques. Here the most pertinent chemical and physical characteristics of the samples are presented, including elemental analysis, lightness measurements, BET surface area measurements and ¹³C NMR analysis. Amongst the BC-rich materials, soot and char could be distinguished from one another mainly on the basis of H/C and O/C ratios, NMR observability and BET surface area. The results indicate that the aromatic structures in the soot are more highly condensed, and this explains why some BC quantification techniques detect these two materials differently. The non-BC potentially interfering materials were shown to share properties with the BC-rich materials (high C content, low lightness values and high aromaticity) that are used for certain BC quantification methods. This may lead to overestimation of BC unless these interfering materials are removed during pre-treatment. The environmental matrices were found to have relatively high amounts of metal oxides that have the potential to catalyse or inhibit thermal and chemical reactions during BC analysis.