Anoxic water samples

Sampling. Monthly samplings were performed at the deepest point of the water column (depth = 31 m) over a period of 2 years. Because a number of labile reduced species occur in anoxic water samples, special attention has to be given to the sampling and sample pretreatment methods to obtain reliable results. Several approaches have been used to circumvent sampling and storage problems. Rapid field analysis of labile species has been recommended (e.g., ref. 9). This rapid analysis is particularly important in measuring the very labile species of Fe(II) and S(-II). 

An alternative interference removal step was developed by Ledo de Medina et al. who developed an IC method for phosphate in natural waters in the presence of high concentration of sulphates. This interference was avoided by first precipitating sulphate as lead sulphate prior to 1C analysis. Samples with high iron content were investigated by Simon. Interferences caused by the precipitation of iron hydroxides from air oxidation of ferrous iron in anoxic water samples and from the alkaline eluent used in IC, were found to affect the determination of phosphate and other inorganic anions in riverine sediment interstitial water samples with high concentrations of dissolved iron (0.5 to 2.0 mmol/1). To eliminate this interference the complexation of iron with cyanide was used prior to IC analysis. ... 

Anoxic water samples, because they contain little in the way of particles, are far easier than aquifer materials to develop radioassays for the measurement of arsenate reduction. Arsenic speciation quantitatively changes from arsenate to arsenite with vertical transition from the surface oxic waters to the anoxic bottom depths of stratified lakes and fjords (55,56). This also occurs in Mono Lake, California (57), a transiently meromictic, alkaline (pH = 9.8), and hypersaline (salinity = 70-90 g/L) soda lake located in eastern California (Fig. 11). The combined effects of hydrothermal sources coupled with evaporative concentration have resulted in exceptionally high ( 200 fiM) dissolved arsenate concentrations in its surface waters. Haloalkaliphilic arsenate-respiring bacteria have been isolated from the lake sediments (26), and sulfate reduction, achieved with... 

In the analysis of seawater, the only significant interference arises from turbidity caused by particles in the sample. Prior filtration of the sample is therefore necessary. For anoxic waters, however, sulfide concentrations over 2 pm were found to decrease the absorbance. This was overcome by adding an excess of either Cd2+ or Hg2+ to the sample [171,172],... 

Airey et al. [195] have described a method for the removal of sulfide prior to the determination of phosphate in anoxic estuarine waters. Mercury (II) chloride was used to precipitate free sulfide from samples of anoxic water. The sulfite-free supernatant liquid was used to estimate sulfide by measuring the concentration of unreacted mercury (II), as well as to determine phosphate by the spectrophotometric method in which sulfide interferes. The detection limit for phosphate was 1 ig/l. 

In anoxic hypolimnion samples collected from Lower Mystic Lake, MA, hexachloroethane was abiotically transformed into tetrachloroethylene via reductive elimination and to pentachloro-ethane via hydrogenolysis. Tetrachloroethylene accounted for 70% of hexachloroethane in unaltered lake water and 62% in filter-sterilized water after 10 d. Trichloroethylene and pent-achloroethane accounted for <1 and 2% in unaltered lake water and filter-sterilized water, respectively. Disappearance rate constants for hexachloroethane were 0.33/d for unaltered water and 0.26/d for filter-sterilized water. At least 80% of the hexachloroethane disappearance in unaltered water was abiotic in origin due to the reactions with naturally occurring aqueous polysulfides, H2S and (Miller et al, 1998a). 

Inorganic arsenic species, As111 and Asv, in natural water and anoxic seawater samples were not stable (Cutter et al., 1991). Rapid freezing and storage at —4°C was recommended as a means of preservation. Particulate samples were collected in acid-cleaned plastic bags, and then frozen. 

It might make sense, in case of anoxic sediments, to load the extractor in an inert atmosphere, or even better, to carry out the whole procedure in the glove box under these conditions. This becomes inevitable if, for instance, divalent iron is to be analyzed. The question of how fast this pore water sample needs to be analyzed for single constituents, will be discussed further in Section 3.4. 

Fig. 9.4 Distribution of carbonate species (a and c) and calcium species (b and d) in seawater after Nordstrom et al. 1979 (a and b) and in an anoxic pore water (c and d). The pore water sample was extracted from the core previously shown in Figure 3.1 and was taken from a depth of 14.8 m below the sediment surface. The calculation of species distributions was performed with the program PHREEQC (Parkhurst 1995)...

The pore water composition of lanthanides is poorly characterized. Low concentrations and small volumes of pore water samples (typically 10-50 ml) combine to make detection difficult. Only the lanthanide-enriched pore waters of anoxic coastal and estuarine sediments have been measured at a few sites (sect. 6). Currently, the pore water chemistry of lanthanides in non-coastal marine sediments is unknown. The field of lanthanide pore water chemistry remains an important challenge. 

Biological. Microbial degradation of trichloroethylene via sequential dehalogenation produced cis- and /ra/3s-l,2-dichloroethylene and vinyl chloride (Smith and Dragun, 1984). Anoxic microcosms in sediment and water degraded trichloroethylene to 1,2-dichloroethylene and then to vinyl chloride (Barrio-Lage et al., 1986). Trichloroethylene in soil samples collected from Des Moines, lA anaerobically degraded to 1,2-dichloroethylene. The production of 1,1-dichloroethylene was not observed in this study (Kleopfer et al., 1985). 

Equilibrium calculations suggested that Hg complexation varies greatly among redox and pH levels typical of the regions of lakes sampled during this study. In an oxic lake, pore water, and groundwater, Hg complexation with organic matter most likely dominates. Under anoxic conditions in the hypolimnion and pore waters, Hg most likely forms soluble bisulfide and polysulfide complexes. 

---