Well-mixed estuaries

Static die-away tests were performed by Potter et al. with an A9PEO7 24 mixture in water from a vertically well-mixed estuary in Florida [36]. Lag times of 0—12 days were observed, and after 4—24 days, primary degradation was complete. These rates are similar to those reported by Kvestak and Ahel [6]. It is likely that the microorganisms in these experiments were pre-acclimated to biodegrade nonionic surfactants, as a municipal sewage treatment plant discharge is present a few kilometres upstream. 

Figure 3.7 The general categories of estuarine circulation identified as Type A, well-mixed estuaries, where there is minimal vertical stratification in salinity Type B, partially mixed estuaries, where the vertical mixing is inhibited to some degree Type C, highly stratified with lower freshwater discharge than the salt wedge system and Type D, salt wedge estuary and many fjords. (Modified from Bowden, 1980.)...

Hansen and Rattray (1966) introduced a general classification scheme for estuaries based on stratification/circulation that is divided into the following four estuarine types Type 1 estuaries well-mixed estuaries with mean flow in the seaward direction and the salt balance being maintained by diffusive processes—via tidal transport Type 2 estuaries partially mixed estuaries where the net flow reverses at depth and the salt flux is maintained by both diffusive and advective processes Type 3 estuaries these estuaries include fjords with two distinct layers and advection accounting for the majority of the salt flux Type 4 estuaries these are salt-wedge estuaries where freshwater flows out over a stable more dense bottom layer. 

Type 1 estuary well-mixed estuaries with mean flow in the seaward direction and the salt balance being maintained by diffusive processes, via tidal transport. 

Well-mixed estuaries minimal vertical stratification in salinity. 

Dyer, K.R., and Taylor, P.A. (1973) A simple segmented prism model of tidal mixing in well-mixed estuaries. Estuar. Coastal Shelf Sci. 1, 411—418. 

Kelly, R.P., and Moran, S.B. (2002) Seasonal changes in groundwater input to a well-mixed estuary estimated using radium isotopes and implications for coastal nutrient budgets. Limnol Oceanogr. 47, 1796-1807. 

Nunes, R.A., and Simpson, J.H. (1985) Axial convergence in a well-mixed estuary. Estuar. Coastal Shelf Sci. 20, 637-649. 

Kjerfve, B. Proehl, J.A. (1979) Velocity variability in a cross-section of a well mixed estuary. Journal of Marine Research 37, 409-18. 

Leendertse, J. J. (1970). A Water-Quality Simulation Model for Well-Mixed Estuaries and Coastal Seas, Vol. I, No. RM-6230-RC. The Rand Corporation, Santa Monica. 

C. T. Friedrichs and D. G. Aubrey, Non-linear tidal distortion in shallow well-mixed estuaries A synthesis, Est. Coast. Shelf Sci. 27(5), 521-545 (1988). 

The environmental sampling of waters and wastewaters provides a good illustration of many of the methods used to sample solutions. The chemical composition of surface waters, such as streams, rivers, lakes, estuaries, and oceans, is influenced by flow rate and depth. Rapidly flowing shallow streams and rivers, and shallow (<5 m) lakes are usually well mixed and show little stratification with... 

Let US return to the discussion of computational transport routines, where each computational cell is the equivalent of a complete mix reactor. If we are putting together a computational mass transport routine, we could simply specify the size of the cells to match the diffusion/dispersion in the system. The number of well-mixed cells in an estuary or river, for example, could be calculated from equation (6.44), assuming a small Courant number. Then, the equivalent longitudinal dispersion coefficient for the system would be calculated from equation (6.44), as well, for a small At (At was infinitely small in equation 6.44) ... 

Batch Technique. As with river reaeration measurements, tracers can also be put into lakes, estuaries, and oceans to measure the influence of wind on liquid film coefficient. If we have a volatile tracer in a lake with a well-established mixed layer, for example, we can apply the same batch reactor equation from Section 6.A, as though we had a well-mixed tank ... 

Two of the key assumptions of the thin-film model (see Section 6.03.2.1.1) are that the main bodies of air and water are well mixed, i.e., that the concentration of gas at the interface between the thin film and the bulk fluid is the same as in the bulk fluid itself, and that any production or removal processes in the thin film are slow compared to transport across it. It is quite likely that there are near-surface gradients in concentrations of many photochemically active gases. Little research has been published, although the presence of near-surface gradients (10 cm to 2.5 m) in levels of CO during the summer in the Scheldt estuary has been reported (Law et al., 2002). Gradients may well exist for other compounds either produced or removed photochemically, e.g., di-iodomethane, nitric oxide, or carbonyl sulfide (COS). Hence, a key assumption made in most flux calculations that concentrations determined from a typical sampling depth of 4-8 m are the same as immediately below the microlayer may well often be incorrect. 

For models of the phytoplankton populations in coastal oceanic waters and in lakes, the sinking rate of phytoplankton cells is an important contribution to the mortality of the population. The cells have a net downward velocity, and they eventually sink out of the euphotic zone to the bottom of the water body. This mechanism has been investigated and included in phytoplankton population models (5,12). However, for the application of these equations to a relatively shallow vertically well mixed river or estuary, the degree of vertical turbulence is sufficient to eliminate the effect of sinking on the vertical distribution of phytoplankton. 

Figs. 5.8b and c show the variation in DIN concentrations in a vertical direction in the ebb and flood tides respectively. The results indicated that both in the northern area and southern area of the estuary, the variation was not apparent in a vertical direction, while the concentration in the surface layer was higher than that in the bottom layer in the middle area. The reason is that the middle area is influenced by the invasion of the briny wedge, the freshwaters from nmoff mainly concentrate on the surface, and the seawater is distributed at the bottom, while the water system in the southern and northern areas is well mixed in a vertical direction. This also indicated that the runoff of the Pearl River was the chief source of the DIN from another point of view. 

Figure 9.16 Basic circulation and salinity distribution in salt wedge, partially mixed, well-mixed and fjord-type estuaries. Numbers and shading show salinity values [34]...

FIGURE 2.9 (a) An idealized estuary in cross section. In this well-stratified estuary, a distinct salt wedge extends upstream beneath fresher water at the surface. The freshwater/saltwater interface moves upriver at high tide and seaward at low tide. Data from Silver Bay, Alaska, USA show (b) a steep average salinity gradient (salinity is shown in parts per thousand) and (c) upstream advection of saltwater at depth. More strongly mixed estuaries exhibit weaker vertical stratification. Data from Rattray (1967). 

In Section 20.3 the analogous situation in surface water bodies is discussed. Rivers usually have longitudinal (upstream-downstream) concentration gradients, but are usually well mixed vertically because of relatively high current velocities and shallow depths. Wide rivers may have horizontal (shore to shore) gradients. Ponds, lakes, estuaries, and oceans may be less well mixed or even stratified vertically, thus the well mixed box assumption can be inapplicable. Stratification can be enhanced by stable density gradients derived from temperature or salinity differences in the water column. Inputs or outputs from the atmosphere may then primarily affect near-surface layers and effects on deeper layers can be damped or delayed. Likewise interactions with bottom sediments may fail to penetrate to surface layers. In temperate regions, these effects will vary seasonally. 

India (Borole et al. 1982) and the Forth estuary in the UK (Toole et al. 1987), nonconservative behavior of uranium was also demonstrated. In the Amazon estuary, uranium showed elevated concentrations compared to simple mixing (McKee et al. 1987). Release of uranium from bottom sediments on the shelf was suggested to be a source of dissolved (<0.4 im) uranium. However, subsequent studies in the Amazon also demonstrated that U removal (Fig. 3) occurred at salinities <12 (Swarzenski et al. 1995, Swarzenski et al. 2003). Overall, it was established that the behavior of U is highly variable examples have been found of conservative behavior as well as both additions and removal of U by interaction with sediments. 

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