Sedimentation mass balance

The profile of Mg2+ in Figure 8.25 indicates downward diffusion of this constituent into the sediments. Mass balance calculations show that sufficient Mg2+ can diffuse into the sediments to account for the mass of organogenic dolomite formed in DSDP sediments (Baker and Bums, 1985 Compton and Siever, 1986). In areas of slow sedimentation rates, the diffusive flux of Mg2+ is high, and the pore waters have long residence times. Dolomites form under these conditions in the zone of sulfate reduction, are depleted in 13c, and have low trace element contents. With more rapid sedimentation rates, shallowly-buried sediments have shorter residence times, and dolomites with depleted 13C formed in the sulfate-reduction zone pass quickly into the underlying zone of methanogenesis. In this zone the DIC is enriched in 13C because of the overall reaction... 

Bokuniewicz, H. J., Gebeit, J., and Gordon, R. B. (1976). Sediment mass balance of a large estuary, Long Island Sound. Estuarine Coastal Mar. Sci. 4, 523-536. 

We begin by finding the amount of power from the tide, wind, and river flow available to drive sedimentary processes in the Sound. The temporal variability of these power sources is characterized. Next we identify the sediment sources and the processes by which sediment received by the estuary is altered in form. Sediment transport and the stability of sediment deposits is related to the power dissipated in the estuary. Deposition rates and the sediment mass balance of the estuary are examined. Finally, a set of parameters that characterize the estuarine sedimentary system are defined and evaluated. It is suggested that these parameters may be used to compare the sedimentation characteristics of different estuaries. 

Resuspension may be conveniently defined as the net transfer of sediment from bottom sediment into the water column. Since the gross transfer of sediment is called erosion, resuspension is the net result of bottom erosion and deposition of suspended sediment. Transfer of sediment changes the wave-mean-suspended sediment concentration with elevation 2 (= z + h, conveniently chosen as the coordinate with respect to the rigid bed as datum) and time t. This choice of datum means that, referring to Fig. 27.3 the rigid-bed level, the static-bed level and the mean position of the fluid mud-water interface are one and same. Fluid mud, if it occurs above this level is taken to be within the water layer (above z = 0). The concentration C z, t) is obtained from the simple sediment mass balance equation in the vertical direction ... 

The failure to identify the necessary authigenic silicate phases in sufficient quantities in marine sediments has led oceanographers to consider different approaches. The current models for seawater composition emphasize the dominant role played by the balance between the various inputs and outputs from the ocean. Mass balance calculations have become more important than solubility relationships in explaining oceanic chemistry. The difference between the equilibrium and mass balance points of view is not just a matter of mathematical and chemical formalism. In the equilibrium case, one would expect a very constant composition of the ocean and its sediments over geological time. In the other case, historical variations in the rates of input and removal should be reflected by changes in ocean composition and may be preserved in the sedimentary record. Models that emphasize the role of kinetic and material balance considerations are called kinetic models of seawater. This reasoning was pulled together by Broecker (1971) in a paper called "A kinetic model for the chemical composition of sea water."... 

An important aspect in the preceding discussion is the need to separate the fluid and sediment components spatially and (as we will see) also temporally. Quantitative mass balance estimates (e g., McCulloch and Gamble 1991 Stolper and Newman 1994 Ayers 1998) often conclude that there is as much, or even more, Th and U in the bulk slab componenf (i.e., sediment plus fluid from the altered oceanic crust). However, if the sediment component added is in U-Th isotope equilibrium (or returns to this state prior to fluid addition see Section 5.3), then addition of only 0.02 ppm U in the fluid will result in significant U-excesss in the composite source (e.g., Condomines and Sigmarsson 1993 Turner et al. 1997). 

SEDIMENT ADDITION, MASS BALANCE FOR Th AND TIME SCALES... 

The short4ived particle reactive radionuclides of the U/Th series also have enormous potential for tracking particle source and transport in ocean margins. Mass balances comparing inventories in sediments with supply can be used to determine import or export of particles to an area. Such approaches are increasingly important in understanding the fates of particle-reactive contaminants whose sources are often enhanced in the coastal ocean. Studies of especially when supplemented by other... 

In the Delaware and Chesapeake estuaries (USA), uranium shows distinctly nonconservative behavior at salinities <5 (Sarin and Church 1994 Church et al. 1996). This was suggested to be due to sedimentary redox processes in the extensive salt marshes in the Delaware and Chesapeake bays. From mass balance calculations it was concluded that almost two-thirds of the uranium in the tidal waters were retained in the sediments. It was also suggested that, extrapolated globally, uranium removal in salt marshes and marine wetlands, including mangroves, are important sinks for U that may responsible for up to 50% of the total marine removal (Church et al. 1996). Removal of U is also observed within the Baltic Sea, related to the association of U with colloids (see Section 2.5). 

Export processes are often more complicated than the expression given in Equation 7, for many chemicals can escape across the air/water interface (volatilize) or, in rapidly depositing environments, be buried for indeterminate periods in deep sediment beds. Still, the majority of environmental models are simply variations on the mass-balance theme expressed by Equation 7. Some codes solve Equation 7 directly for relatively large control volumes, that is, they operate on "compartment" or "box" models of the environment. Models of aquatic systems can also be phrased in terms of continuous space, as opposed to the "compartment" approach of discrete spatial zones. In this case, the partial differential equations (which arise, for example, by taking the limit of Equation 7 as the control volume goes to zero) can be solved by finite difference or finite element numerical integration techniques. 

As a final note, a variant of the calculation is useful in many cases. Suppose a chemical analysis of a groundwater is available, giving the amount of a component in solution, and we wish to compute how much of the component is sorbed to the sediment. We can solve this problem by eliminating the summations over the sorbed species (the over q terms) from each of the mass balance equations,... 

Only the formulas for KLa by Parkhurst and Pomeroy (1972), Taghizadeh-Nasser (1986) and Jensen (1994) have been developed for sewer pipes. Taghizadeh-Nasser (1986) performed the investigation in a pilot sewer, whereas the formulas developed by Parkhurst and Pomeroy (1972) and Jensen (1994) were based on measurements in real sewers. Parkhurst and Pomeroy (1972) made investigations based on an oxygen mass balance in sewers that were cleaned for sediments and biofilm. Jensen (1994) based his formula on the one developed by Pomeroy and Parkhurst (1972) and measurements of the reaeration by a direct methodology using krypton-85 as radiotracer (cf. Chapter 7). 

Figure 9 gives the simplified mass balance for heavy metals in a catchment, including both a soil compartment in the catchment area and the aquatic system with water and sediment compartments. A complete steady-state mass balance of heavy metals for a catchment equals ... 

Water flows into the water column at a volumetric flow rate Q containing a total metal concentration Cin. It is assumed that water flows out from the column at the same rate and at the same concentration as in the water column. Mass balances on the water column and top sediment layer give... 

The simplest approach considers a perfectly mixed bioturbated layer of thickness L and homogeneous concentration C. If v is the sedimentation rate, the mass balance condition for element i reads... 

To assess the relative importance of the volatilisation removal process of APs from estuarine water, Van Ry et al. constructed a box model to estimate the input and removal fluxes for the Hudson estuary. Inputs of NPs to the bay are advection by the Hudson river and air-water exchange (atmospheric deposition, absorption). Removal processes are advection out, volatilisation, sedimentation and biodegradation. Most of these processes could be estimated only the biodegradation rate was obtained indirectly by closing the mass balance. The calculations reveal that volatilisation is the most important removal process from the estuary, accounting for 37% of the removal. Degradation and advection out of the estuary account for 24 and 29% of the total removal. However, the actual importance of degradation is quite uncertain, as no real environmental data were used to quantify this process. The residence time of NP in the Hudson estuary, as calculated from the box model, is 9 days, while the residence time of the water in the estuary is 35 days [16]. 

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