Salinity gradients

Figure 4 Measurements of (A) uranium aetivity ratios, UARs ( U U) and U eoneentrations (B) aeross a salinity gradient off the Amazon River mouth (1996). UARs were determined by thermal ionization mass speetrometry (TIMS) at Calteeh (D. Poreelli) U eoneentrations by ICPMS...

Theoretical concepts for generating electricity from ocean currents such, as the Gulf Stream, and salinity gradients (differences in salt content) are being investigated. More research and development is required before these concepts reach the stage of demonstration power plants. 

Estuaries exhibit physical and chemical characteristics that are distinct from oceans or lakes. In estuaries, water renewal times are rapid (10 to 10 years compared to 1 to 10 years for lakes and 10 years for oceans), redox and salinity gradients are often transient, and diurnal variations in nutrient concentrations can be significant. The biological productivity of estuaries is high and this, coupled with accumulation of organic debris within estuary boundaries, often produces anoxic conditions at the sediment-water interface. Thus, in contrast to the relatively constant chemical composition of the... 

PIGMENT COMPOSITION. IN ig/L. ALONG THE SALINITY GRADIENT. AS DETERMINED FROM HPLC ANALYSES... 

M. Estrada, P. Henriksen, J.M. Gasol, E.O. Casamayor and C. Pedros-Alio, Diversity of planktonic photoautotrophic microorganisms along a salinity gradient as depicted by microscopy, flow cytometry, pigment analysis and DNA-based methods. FEMS Microbiol. Ecol. 49 (2004) 281-293. 

Note that plots, such as Figure 5.1, provide information only on the net outcome of chemical reactions. In the case of iron, a small addition does take place in estuaries as a result of desorption of Fe from the surfaces of riverine particles. As these solids move through the estuarine salinity gradient, the major cation concentrations increase and effectively displace the iron ions from the particle surfeces. Since this release of iron is much smaller than the removal processes, the net effect is a chemical removal of iron. Sedimentation of these iron-enriched particles serves to trap within estuaries most of the riverine transport of reactive iron, thereby preventing its entry into the oceans. 

Halocline A region in which a strong salinity gradient exists. 

PRO power generation is technically feasible, but not economically viable with currently available RO membranes and a seawater/fresh water salinity gradient resource. 

The economics of PRO systems using brines and fresh water sources and current membranes are more favorable, with estimated power outputs as high as 200 watt/m. However, surface brines exist in deserts where there is limited fresh water, and brines that might be produced from salt domes pose a difficult effluent disposal problem. If PRO systems can be produced at an installed cost of 100/m2 of membrane, the projected economics are competitive with other power-generating techniques. This appears to be the only salinity gradient resource worthy of further study. 

The sample locations are shown on the map in Figure 1. Euromonding (Port of Rotterdam) and the North Sea Canal (Port of Amsterdam) are well-known polluted areas. The sites in the North Sea are situated at the end of the estuarine harbour areas, except for Noordwijk (site 6), which is a t5 ical coastal location. Two relatively clean sites, the Oesterput in the Eastern Scheldt estuary (site 14) and Ussehneer near Enkhuizen (site 12), were also sampled and used as saltwater and freshwater reference sites respectively. In the Port of Rotterdam transect the salinity gradient is continuous whilst in the Port of Amsterdam transect the salinity gradient is interrupted and more complex due to canalisation. The numbers of the sampling sites correspond with the site numbers reported previously for the results of the chemical sediment and flounder liver analyses (De Boer 2001). The transects were sampled from 19 to 26 September 1996. 

Shiaris, M. P. (1989b). Phenanthrene mineralization along a natural salinity gradient in an urban estuary, Boston Harbor, Massachusetts. Microbial Ecology, 18, 135—46. 

Changes in the electrophilicity of the acceptor enormously affects the rate of exchange (69), This trend is illustrated by Fig. 4, which shows the rates of transmethylation of some methylmetals toward chloromercury species in certain cases there is a 107-fold dimunition of rate constant as salinity increases. This strongly suggests that the frequent salinity gradients found in natural waters will markedly influence the rates of naturally occurring transmethylations (vide infra). 

Cifuentes, L.A., 1982. The character and behavior of organic nitrogen in the Delaware Estuary salinity gradient. M.S. Thesis, University of Delaware, Newark, DE, 81 pp. 

Before discussing the chemical dynamics of estuarine systems it is important to briefly review some of the basic principles of thermodynamic or equilibrium models and kinetics that are relevant to upcoming discussions in aquatic chemistry. Similarly, the fundamental properties of freshwater and seawater are discussed because of the importance of salinity gradients and their effects on estuarine chemistry. 

The solubility of mineral salts may be enhanced with the formation of ion pairs, a salting-in effect, requiring the inclusion of ion speciation effects. Conversely, there is commonly a salting-out effect of dissolved constituents across a salinity gradient. This can be particularly important when examining more hydrophobic organic compounds (HOC), such as aromatic hydrocarbons in estuaries. 

The reactivity of a particular estuarine constituent has been traditionally interpreted by plotting its concentration across a conservative salinity gradient. The simplest distribution pattern, in a one-dimensional, two end-member, steady-state system, would be for a conservative constituent to change linearly with salinity. For a nonconservative constituent, there is net loss or gain in concentration across a salinity gradient. 

Figure 6.9 Theoretical diagram of fluid mud formation and disruption at the frontal zone showing (a) neap tide stratified fluid mud formation—with strong vertical salinity gradient and (b) spring tide mixed or mobile fluid mud formation—with strong horizontal salinity gradient. (Modified from Kineke et al., 1996.)...

As repulsive forces decrease with increasing salinity, van der Waals forces will dominate and flocculation will proceed. Strong salinity gradients in estuaries make them ideal environments for coagulation processes. 

Although mechanisms of DOM removal by physical/chemical processes in the mixing zone of estuaries are not well understood, they are believed to be important processes affecting the composition of riverine/estuarine DOM. Some of the earliest work on DOM removal processes noted that iron is important in the initial steps of flocculating humic substances across an estuarine salinity gradient and that much of this humic material was composed of humic acids (Swanson and Palacas, 1965 Eckert and Sholkovitz, 1976 ... 

Figure 9.1 Freshwater-marine mixing ratios of dissolved inorganic carbon (DIC) and isotopic composition (DI13C) across three different salinity gradients. Bottom isotopic change between — 10%e at the freshwater end-member, and +2%c at the marine end-member, both end-member values are based on concentration-weighted averages (data sources Spiker and Schemel, 1979 Spiker, 1980). (Modified from Fry, 2002.)...

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