Water sediment interface

Determine whether there is a significant difference between the concentration of Zn + at the air-water interface and the sediment-water interface at a = 0.05. 

Joly observed elevated "Ra activities in deep-sea sediments that he attributed to water column scavenging and removal processes. This hypothesis was later challenged with the hrst seawater °Th measurements (parent of "Ra), and these new results conhrmed that radium was instead actively migrating across the marine sediment-water interface. This seabed source stimulated much activity to use radium as a tracer for ocean circulation. Unfortunately, the utility of Ra as a deep ocean circulation tracer never came to full fruition as biological cycling has been repeatedly shown to have a strong and unpredictable effect on the vertical distribution of this isotope. 

Finally, it must be stressed that diffusion of dissolved species in solutions is a key physicochemical process for the sea/sediment interaction and energy exchange at the sediment-water interface. The reader is referred to Cussler (1984) for a more comprehensive presentation of diffusion in fluid systems. 

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... 

Benthic invertebrates are macroscopic animals that live at or near the sediment/water interface. Some benthic invertebrates, particnlarly mnssels, readily accnmnlate metals, prompting their use as biological indicators of mercnry contamination (Smith... 

Figure 7. Sediment eontains derived both seavenged from the water eolumn during particle settling and contained in solid material. Ra produced in the sediments is highly soluble in pore waters and diffuses into the overlying water or is advected across the sediment-water interface by discharging groundwater. Rn is produced within the water column from dissolved Ra and within the underlying sediments.

Figure 7. Excess activity versus depth (left) and X-radiograph (right) in a sediment core collected from the New York Bight, showing the importance of mixing by benthic fauna in the upper part of the seabed. Abundant individnals of the small bivalve Nucula proximo may be seen in the X-radiograph near the sediment-water interface, and the light-colored areas represent bnrrows of Nephtys sp. and Ceriantheopsis sp. Reprinted from Estuarine Coastal and Shelf Science (formerly Estuarine and Coastal Marine Science) Vol. 9, Cochran and Aller, pp. 739-747, 1979, with permission from Elsevier Science.

Smith [25] has described a device for sampling immediately above the sediment water interface of the ocean. The device consists of a nozzle supported by a benthic sled, a hose, and a centrifugal deck pump, and is operated from a floating platform. Water immediately above the sediment surface is drawn through the nozzle and pumped through the hose to the floating platform, where samples are taken. The benthic sled is manipulated by means of a hand winch and a hydrowire. 

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

The maximum deposition of Zn, Cu, Ni, and Cr appear to be at about the mid 1960 s, whereas the maximum for Pb appears to be just following World War II. The shape of the deposition profile curve for Zn, Ni, Cr, and possibly Cu indicates that these elements may be migrating downward in the sediments, possibly in the pore water. The decrease in deposition at the sediment-water interface indicates diffusion of the metals out of the sediment... 

The 239j240pu can (je accounted for as being from nuclear weapons-produced fallout over the past 30 years, and not from the waste disposal canisters. The mechanism must be by transport and redistribution of the fine floculent sediment at the sediment-water interface. Because of the significant amount of sediment which is being transported down-canyon, any leakage from the canisters would probably be scavenged locally by the environmental sediment material. 

All models available to date rest on two basic assumptions, viz. (i) the flux of the radionuclide to the sediment water interface has remained constant with time (or if the flux has varied, then its time variations are known) and (ii) the tracers and sediment particles are not independently mobile in the sediment pile. If these conditions are met then the temporal variations in the radionuclide concentration at depth, z, below the moving sediment-water interface would be given by [63,64,65,66] ... 

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