The standard SeaWiFS atmospheric correction algorithm, designed for open ocean water, has been extended for use over turbid coastal and inland waters. Failure of the standard algorithm over turbid waters can be attributed to invalid assumptions of zero water-leaving radiance for the near-infrared bands at 765 and 865 nm. In the present study these assumptions are replaced by the assumptions of spatial homogeneity of the 765:865-nm ratios for aerosol reflectance and for water-leaving reflectance. These two ratios are imposed as calibration parameters after inspection of the Rayleigh-corrected reflectance scatterplot. The performance of the new algorithm is demonstrated for imagery of Belgian coastal waters and yields physically realistic water-leaving radiance spectra. A preliminary comparison with in situ radiance spectra for the Dutch Lake Markermeer shows significant improvement over the standard atmospheric correction algorithm. An analysis is made of the sensitivity of results to the choice of calibration parameters, and perspectives for application of the method to other sensors are briefly discussed. © 2000 Optical Society of America OCIS codes: 010.1290, 010.4450, 120.0280.

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Atmospheric correction of SeaWiFS imagery for turbid coastal and inland waters

This chapter presents a general overview of the major controls of trace metals in seawater, developed from the extensive research on trace metals over the last few decades. The reader should be given a first-order understanding and insight into trace metal biogeochemistry in the oceans, rather than presented with a comprehensive review of the distribution of each trace metal. Each of the trace metals discussed will undoubtedly prove to have unique characteristics and subtle differences from this version, yet the comparison with these characteristics will serve as a good springboard to a more complete understanding.

Controls of Trace Metals in Seawater

Suspended particles are instrumental in controlling the reactivity, transport and biological impacts of substances in aquatic environments, and provide a crucial link for chemical constituents between the water column, bed sediment and food chain. This article reviews the role of suspended particles in the chemical and biological cycling of trace constituents (trace metals, organo-metallic compounds and hydrophobic organic micropollutants; HOMs) in estuaries, with particular emphasis on the effects of and changes to particle reactivity and composition.

The partitioning (or distribution coefficient,KD ) and bioavailability of chemical constituents, and assimilation efficiency (AE) of such by bivalve suspension feeders, are identified as key parameters requiring definition for accurate biogeochemical modelling, and the discussion centres around the determination of and controls on these parameters. Particle-water interactions encompass a variety of physical, biological, electrostatic and hydrophobic effects, and are largely dependent on the character and concentration of suspended particles and salinity. The salinity-dependence results from the competing and complexing effects of seawater ions for trace metals, and the compression of water in the presence of dissolved seawater ions and consequent salting out of neutral solute (HOMs, organo-metallic compounds and some trace metal complexes). The extent of biological solubilization of chemical constituents from suspended particles is dependent on the nature of chemical components of the gastro-intestinal environment and their interactions with ingested particles, and the physiological (e.g. gut passage time) and chemical (e.g. redox conditions and pH) constraints imposed on these interactions. Generally, chemicals that associate with fine, organic-rich particles (or, for some HOMs, fine inorganic particles), and desorb at pH 5–6 and/or complex with digestive enzymes or surfactants are most readily solubilized in the gut. The extent of assimilation of solubilized chemical is then determined by its ability to pass the gut lining and partition into cytosolic material.

In practice, KD and AE are determined experimentally by means of radiotracers added to contained suspensions or mesocosms, while operational measurement of bioavailability relies on in vitro chemical or biological (enzymatic) extraction of particles. What is lacking, however, and is identified as an ultimate goal of future research, is the ability to predict these parameters from theoretical principles and thermodynamic constants. Since many of the inherent interactions and mechanisms are controlled by particle composition and reactivity, a more immediate objective would be better characterization of the biogeochemical properties of suspended particles themselves. This includes chemical resolution of the bulk organic matter, definition of the abundance and synergistic effects of component sorbent phases, and determination of the effects of particle-seawater ion interactions on the reactivity of the particle surface.

Suspended Particles: Their Role in Estuarine Biogeochemical Cycles

More than 2.5 billion people on the globe rely on groundwater for drinking and providing high-quality drinking water has become one of the major challenges of human society. Although groundwater is considered as safe, high concentrations of heavy metals like arsenic (As) can pose potential human health concerns and hazards. In this paper, we present an overview of the current scenario of arsenic contamination of groundwater in various countries across the globe with an emphasis on the Indian Peninsula. With several newly affected regions reported during the last decade, a significant increase has been observed in the global scenario of arsenic contamination. It is estimated that nearly 108 countries are affected by arsenic contamination in groundwater (with concentration beyond maximum permissible limit of 10 ppb recommended by the World Health Organization. The highest among these are from Asia (32) and Europe (31), followed by regions like Africa (20), North America (11), South America (9) and Australia (4). More than 230 million people worldwide, which include 180 million from Asia, are at risk of arsenic poisoning. Southeast Asian countries, Bangladesh, India, Pakistan, China, Nepal, Vietnam, Burma, Thailand and Cambodia, are the most affected. In India, 20 states and 4 Union Territories have so far been affected by arsenic contamination in groundwater. An attempt to evaluate the correlation between arsenic poisoning and aquifer type shows that the groundwater extracted from unconsolidated sedimentary aquifers, particularly those which are located within the younger orogenic belts of the world, are the worst affected. More than 90% of arsenic pollution is inferred to be geogenic. We infer that alluvial sediments are the major source for arsenic contamination in groundwater and we postulate a strong relation with plate tectonic processes, mountain building, erosion and sedimentation. Prolonged consumption of arsenic-contaminated groundwater results in severe health issues like skin, lung, kidney and bladder cancer; coronary heart disease; bronchiectasis; hyperkeratosis and arsenicosis. Since the major source of arsenic in groundwater is of geogenic origin, the extend of pollution is complexly linked with aquifer geometry and aquifer properties of a region. Therefore, remedial measures are to be designed based on the source mineral, climatological and hydrogeological scenario of the affected region. The corrective measures available include removing arsenic from groundwater using filters, exploring deeper or alternative aquifers, treatment of the aquifer itself, dilution method by artificial recharge to groundwater, conjunctive use, and installation of nano-filter, among other procedures. The vast majority of people affected by arsenic contamination in the Asian countries are the poor who live in rural areas and are not aware of the arsenic poisoning and treatment protocols. Therefore, creating awareness and providing proper medical care to these people remain as a great challenge. Very few policy actions have been taken at international level over the past decade to reduce arsenic contamination in drinking water, with the goal of preventing toxic impacts on human health. We recommend that that United Nations Environment Programme (UNEP) and WHO should take stock of the global arsenic poisoning situation and launch a global drive to create awareness among people/medical professionals/health workers/administrators on this global concern.

Arsenic contamination of groundwater: A global synopsis with focus on the Indian Peninsula

European Union Risk Assessment Report

Measurements of both dissolved and particulate trace metals (Cd, Co, Cu, Fe, Mn, Ni, Pb and Zn) were made on samples collected simultaneously in the central and southern North Sea (below 56°N), during four cruises covering the seasonal cycle. Analyses of trace metals in both phases were undertaken using similar analytical protocols, and the data subjected to rigorous quality controls. Clean techniques were used throughout sampling and analysis. Trace metal concentrations ranged from those characteristic of waters entering the shelf region from the North Atlantic Ocean to values which in some cases were two orders of magnitude higher. Marked increases in concentrations were often associated with waters directly influenced by riverwaters, but the results showed that the distributions could be influenced by processes other than fluvial inputs. The redox-sensitive metals, Fe and Mn, were influenced at some sites, including those overlying areas of fine-grained sediments, by benthic inputs associated with seasonal changes in oxidation-reduction conditions in the benthic interfacial zone. The behaviour of metals which are not redox-sensitive (Cd, Zn) in some cases paralleled those of Fe and Mn, suggesting an association with Mn-or Fe-rich solid phases, or with organic matter. However, seasonal variations in concentrations of Cd and Zn were not directly linked to the cycle of biological utilization and regeneration of nutrients, suggesting that their geochemistry is more influenced by boundary inputs. Resuspension of Pb-rich sediments, long-range transport of Pb-rich suspended particulate matter (SPM), and enhanced, though diffuse, atmospheric inputs of lead were considered to have contributed to the elevated concentrations of particulate Pb (mass metal/mass sediment) in the water column during winter. Estimates of partitioning (K D s) were derived from the dissolved and particulate data for each of the metals over the seasonal cycle. For the more particle-reactive metals (Co, Fe, Mn, Pb), transport in the suspended particulate phase is a significant, sometimes dominant factor. In contrast, for Cd, Cu and Zn, transport in solution will dominate under almost all conditions in the southern North Sea. Furthermore, the values of K D , for a particular metal, varied widely, partly in response to seasonal differences in the relative contributions of lithogenous and biogenous particles to the particles in suspension. Only for Cu and Zn during the winter could the partitioning be described by a narrow range of K D values covering the observed concentrations of SPM. The data from this study give unique information concerning the contemporaneous distribution of dissolved and particulate trace metals in the southers North Sea and they provide a baseline against which the effects of reductions of inputs into the North Sea may be more accurately assessed. Viewed in combination, the dissolved and particulate data have yielded new insights into mechanisms affecting trace metal cycling and transport, essential in the development of coupled hydrodynamic-geochemical models.

Trace metals in the central and southern North Sea

Trace Metals in the Central and Southern North Sea

Coupling of physical, biological and chemical processes associated with particle resuspension and seston flux was investigated at three sites in the North Sea with contrasting water column (mixed/stratified) and seabed (cohesive/non-cohesive) characteristics.

Resuspension processes and seston dynamics, southern North Sea

The role of Callianassa subterranea (Montagu) (THALASSINIDEA) in sediment resuspension in the North Sea

Measurements were made in the southern North Sea of trace metals (dissolved and particulate Mn, Fe, Ni, Co, Cu, Zn, Cd, Pb) with contrasting biogeochemical behaviours. The extent to which distributions of dissolved metals over the survey area were determined by the mixing of river waters with seawater varied among the metals, and also seasonally; even for nickel and copper, which behaved most conservatively, other processes were significant. Specific instances of the effects of scavenging by particles and of inputs from benthic recycling are evident from the data and these processes lead generally to modifications of fluxes of dissolved metals in estuarine plumes. While the effect of atmospheric inputs on dissolved concentrations of metals is, on the basis of other evidence, a factor contributing to spatial and temporal variability, specific features arising from them were not identified; they probably account, however, for increased concentrations of dissolved lead observed at high salinities in the Humber plume. Seasonal variations in concentrations of dissolved metals, paralleling those of micronutrients, were not identified but manganese showed a pronounced seasonal variability related to its reduction-oxidation chemistry. Wide differences occurred in the partition between dissolved and particulate fractions, although for copper and zinc the partition could be described by a narrow range of distribution coefficients under certain conditions.

A.W. Morris

Papers

Trace metals in the central and southern North Sea

Trace Metals in the Central and Southern North Sea

Resuspension processes and seston dynamics, southern North Sea

The role of Callianassa subterranea (Montagu) (THALASSINIDEA) in sediment resuspension in the North Sea

Processes influencing the fate of trace metals in the North Sea