Water column chemical concentrations

Reaction rates of nonconservative chemicals in marine sediments can be estimated from porewater concentration profiles using a mathematical model similar to the onedimensional advection-diffusion model for the water column presented in Section 4.3.4. As with the water column, horizontal concentration gradients are assumed to be negligible as compared to the vertical gradients. In contrast to the water column, solute transport in the pore waters is controlled by molecular diffusion and advection, with the effects of turbulent mixing being negligible. 

Abiotic oxidation of sulfide by oxygen cannot supply sulfate at rates comparable to rates of sulfate reduction. Unless high concentrations of sulfide develop and the zone of oxidation is much greater than 1 cm, rates of chemical oxidation of sulfide by oxygen will be much less than 1 mmol/m2 per day (calculated from rates laws found in refs. 115-118). Such conditions can exist in stratified water columns in the Black Sea water column chemical oxidation rates may be as high as 10 mmol/m2 per day (84). However, in lakes in which sulfide is undetectable in the water column and oxygen disappears within millimeters of the sediment-water interface (e.g., 113), chemical oxidation of sulfide by oxygen is unlikely to be important. 

While the water column contaminant concentrations in the 1980s began to meet and exceed applicable standards, many benthic communities continued to be severely stressed. The most sensitive species were often missing and much of their biomass consisted of opportunistic species (2-4). Recognition of the direct and indirect ecological and human health risks posed by contaminated sediment prompted the EPA to embark on the development of defensible chemical-specific concentration limits in sediments (5). 

Many sea trials of dispersant chemicals to demonstrate the effectiveness of specific products or to elucidate the processes of oil dispersion into the water column have been described. Most tests have proved inconclusive, leading many to believe that dispersant chemicals are only marginally effective. Tests in a wave basin have been conducted to measure dispersant effectiveness under closely controlled conditions [261]. These tests show that dispersed oil plumes may be irregular and concentrated over small volumes, so extensive plume sampling was required to obtain accurate dispersant effectiveness measurements. In large-scale sea trials, dispersants have been shown effective, but only when sufficient sampling of the water column was done to detect small concentrated dispersed oil plumes and when it was known that the dispersant was applied primarily to the thick floating oil. 

The data in Table V indicate that runoff of CGA-72662 from 12 applications would result in extremely low concentrations of CGA-72662 in ponds and lakes. The water column in all cases would contain all of the chemical, the sediment little or no CGA-72662. It follows from these data that exposure of CGA-72662 to aquatic organisms would be low. The data in Table V also shows that CGA-72662 would be persistent only in eutrophic lake environments. After the load is removed, the half-life of CGA-72662 in ponds, eutrophic lakes and oligotrophic lakes was 13, 62, and 5 days respectively. Self purification times were 9, 12, and 3 months respectively. 

The elements deposited within the sediment matrix show that mobilization processes may be occurring in the upper layers. At Station SIN 3, figure 4d for example, the element deposited (pg-cm-2) in the topmost layers decreases, often much more than in the concentration (Mg g 1). This may be due to organic matter decomposition and/or to environmental chemical reactions of solubility and precipitation of the given element. The metal must have been removed rapidly from the water column since the sediment concentration is shown to decrease rapidly with distance from the shipyard (Stations SIN 3 and SIN 2). Lead may not be mobilized significantly after deposition since any diffusion in the pore water would tend to "smooth" the concentration profile with time. 

When released to surface waters, mirex will bind primarily (80-90%) to the dissolved organic matter in the water with a small amount (10-20%) remaining in the dissolved fraction, because mirex is a highly hydrophobic compound (Yin and Hassett 1989). Mean mirex concentrations in sediments, collected at four basins in Lake Ontario between 1982 and 1986, ranged from 30 to 38 pg/kg in three of the basins within the water circulation pattern of the lake. A fourth basin outside the pattern showed much lower concentrations (6.4 pg/kg), indicating that mirex was being transported with the lake water (Oliver et al. 1989). The residence time for mirex in Lake Ontario water was estimated to be 0.3 years. This indicated that mirex was either scavenged by particles or was chemically reactive and, therefore, was rapidly removed from the water column (Arimoto 1989). 

Experimental measurements in each lake included particle concentration and size measurements in the water column, sedimentation fluxes in sediment traps, and chemical and size characteristics of materials recovered from sediment traps. The colloidal stability of the particles in the lake waters was determined with laboratory coagulation tests. Colloidal stability was described by the stability ratio (a). For a perfectly stable suspension, a = 0 for a complete unstable one, a = 1.)... 

In Fig. 1.1 we already encountered a scheme of some of the chemical, biological, and physical processes which regulate the concentration of trace elements in the water column of lakes and oceans. When these trace elements are introduced into a lake by riverine and atmospheric input, they interact... 

Normal-phase HPLC has also found application in the analysis of pigments in marine sediments and water-column particulate matter. Sediments were extracted twice with methanol and twice with dichloromethane. The combined extracts were washed with water, concentrated under vacuum and redissolved in acetone. Nomal-phase separation was performed with gradient elution solvents A and B being hexane-N,N-disopropylethylamine (99.5 0.5, v/v) and hexane-2-propanol (60 40, v/v), respectively. Gradient conditions were 100 per cent A, in 0 min 50 per cent A, in 10 min 0 per cent A in 15 min isocratic, 20 min. Preparative RP-HPLC was carried out in an ODS column (100 X 4.6 mm i.d. particle size 3 jum). Solvent A was methanol-aqueous 0.5 N ammonium acetate (75 25, v/v), solvent B methanol-acetone (20 80, v/v). The gradient was as follows 0 min, 60 per cent A 40 per cent A over 2 min 0 per cent A over 28 min isocratic, 30 min. The same column and mobile phase components were applied for the analytical separation of solutes. The chemical structure and retention time of the major pigments are compiled in Table 2.96. 

Sampling sites are also referred to as station locations. For water column work, depth profiles are constructed from seawater samples collected at representative depths. Temperature and salinity are measured in situ with sensors. Remote-closing sampling bottles deployed from a hydrowire are used to collect water for later chemical analysis, either on the ship or in a land-based laboratory. The standard chemical measurements made on the water samples include nutrients (nitrate, phosphate, and silicate), dissolved O2, and total dissolved inorganic carbon (TDIC) concentrations. 

The results of concentration measurements are presented as vertical profiles similar to those for the water column, with the vertical axis representing increasing depth below the sediment-water interfece. Depth profiles of concentrations can be used to illustrate downcore variations in the chemical composition of pore waters or in the solid particles. Dissolved concentrations are typically reported in units of moles of solute per liter of pore water. Solid concentrations are reported in mass/mass units, such as grams of carbon per 100 grams of dry sediment (%C) or mg of manganese per kg of dry sediment (ppm Mn). 

As we saw with the steady-state water-column application of the one-dimensional advection-diffusion-reaction equation (Eq. 4.14), the basic shapes of the vertical concentration profiles can be predicted from the relative rates of the chemical and physical processes. Figure 4.21 provided examples of profiles that exhibit curvatures whose shapes reflected differences in the direction and relative rates of these processes. Some generalized scenarios for sedimentary pore water profiles are presented in Figure 12.7 for the most commonly observed shapes. 

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