Porewaters

There are two main sources of Rn to the ocean (1) the decay of sediment-bound "Ra and (2) decay of dissolved "Ra in a water column. Radon can enter the sediment porewater through alpha recoil during decay events. Since radon is chemically inert, it readily diffuses from bottom sediments into overlying waters. The diffusion of radon from sediments to the water column gives rise to the disequilibrium (excess Rn) observed in near-bottom waters. Radon is also continuously being produced in the water column through the decay of dissolved or particulate "Ra. 

FIGURE 3.3 Pearson correlation coefficients between fish (Gambusia) Hg concentration and MeHg concentrations in various environmental media sediment, porewater, surface water, and suspended particulate matter (SPM) from the Florida Everglades (1995-1998). 

Porewater Toxicity Testing Biological, Chemical, and Ecological Considerations Carr and Nipper, editors 2003... 

It is desirable to calculate new bulk phase Z values for the four primary media which include the contribution of dispersed phases within each medium as described by Mackay and Paterson (1991) and as listed earlier. The air is now treated as an air-aerosol mixture, water as water plus suspended particles and fish, soil as solids, air and water, and sediment as solids and porewater. The Z values thus differ from the Level I and Level II pure phase values. The necessity of introducing this complication arises from the fact that much of the intermedia transport of the chemicals occurs in association with the movement of chemical in these dispersed phases. To accommodate this change the same volumes of the soil solids and sediment solids are retained, but the total phase volumes are increased. These Level III volumes are also given in Table 1.5.2. The reaction and advection D values employ the generally smaller bulk phase Z values but the same residence times thus the G values are increased and the D values are generally larger. 

Chin, Y.-R, Gschwend, P.M. (1992) Partitioning of polycyclic aromatic hydrocarbons to marine porewater organic colloids. Environ. Sci. Technol. 26, 1621-1626. 

McGroddy, S.E., Farrington, J.W. (1995) Sediment porewater partitioning of polycyclic aromatic hydrocarbons in three cores from Boston Harbor, Massachusetts. Environ. Sci. Technol. 29, 1542-1550. 

Bishop [75] determined barium in seawater by direct injection Zeeman-modulated graphite furnace atomic absorption spectrometry. The V203/Si modifier added to undiluted seawater samples promotes injection, sample drying, graphite tube life, and the elimination of most seawater components in a slow char at 1150-1200 °C. Atomisation is at 2600 °C. Detection is at 553.6 nm and calibration is by peak area. Sensitivity is 0.8 absorbance s/ng (Mo = 5.6 pg 0.0044 absorbance s) at an internal argon flow of 60 ml/min. The detection limit is 2.5 pg barium in a 25 ml sample or 0.5 pg using a 135 ml sample. Precision is 1.2% and accuracy is 23% for natural seawater (5.6-28 xg/l). The method works well in organic-rich seawater matrices and sediment porewaters. 

The freeze-dried sediments were subjected to both a total dissolution and a selective extraction. The latter, as described in Chester Hughes (1967), is carried out in a hydroxylamine hydrochloride and acetic acid (HA) solution and designed to isolate reactive phases. With the exception of total Se, extracted metals were determined by the method described for porewaters. Total solid Se concentrations were measured by AAS with HG-FIAS (analysis ongoing). 

Whereas total concentrations of solid Fe and As remained relatively constant over time (not shown), dissolved Fe and As in the porewaters have increased substantially since 1982 (Fig. 1). The concentrations in the suboxic zone have doubled or tripled. Similarly, HA-extractable Fe and As concentrations increased significantly at stations 23 and 24 since 1982 (Fig. 2). This increase is concomitant with an accumulation of AVS in the sediment over the last 10 years (G. Chaillou, unpublished). 

Dissolved humic substances (DHS) are the main constituents of the dissolved organic carbon (DOC) pool in surface waters (freshwaters and marine waters), groundwaters, and soil porewaters and commonly impart a yellowish-brown color to the water system. Despite the different origins responsible for the main structural characteristics of DHS, they all constitute refractory products of chemical and biological degradation and condensation reactions from plant or animal residues and play a crucial role in many biogeochemical processes. 

Mayer, P Vaes, W.H.J. Wijnker, F. Legierse, K.C.H.M. Kraaij, R. Tolls, J. Hermens, J.L. 2000, Sensing dissolved sediment porewater concentrations of persistent and bioaccumula-tive pollutants using disposable solid-phase microextraction fibers. Environ. Sci. Technol. 34 5177-5183. 

The meehanism of Mo removal in suboxie systems is unelear, and so the fundamental nature of this fraetionation requires further study. However, the effeet may be rmderstood in terms of a two layer diffusion-reaetion model in whieh a reaetion zone in the sediment (where Mo is ehemieally removed) is separated from seawater by a purely diffusive zone in which there is no chemical reaction (Braudes and Devol 1997). The presence of a diffusive zone is likely because Mo removal presumably occurs in suMdic porewaters that lie a finite distance L below the sediment-water interface (Wang and van Cappellen 1996 Zheng et al. 2000a). If HjS is present in the reactive zone such that Mo is removed below this depth, then Mo isotope fractionation in the diffusive zone may be driven by isotope effects in the reactive zone. 

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. 

Over longer time scales, clay minerals can undergo more extensive reactions. For example, fossilization of fecal pellets in contact with a mixture of clay minerals and iron oxides produces an iron- and potassium-rich, mixed-layer clay called glauconite. This mineral is a common component of continental shelf sediments. Another example of an authigenic reaction is called reverse weathering. In this process, clay minerals react with seawater or porewater via the following general scheme ... 

LC50 for goldfish 1.8 mL/kg (quoted, Verschueren, 1983), 89.7 and 164 mg/L (soil porewater concentration) for earthworm Eisenia andrei) and 252 and 482 mg/L (soil porewater concentration) for earthworm Lumbricus rubellus) (Van Gestel and Ma, 1993). 

LC50 639 to 1,518 g/L (soil porewater concentration) for earthworm Eisenia andrei) and 3.5 to... 

---