Cmax XCi xCo1122

The ratio x, given by the diffusion coefficient of metal fulvic acid complexes, Do, divided by the diffusion coefficient of simple inorganic metal species, Di, has a value of 0.2 to a good approximation. For the hard-water Furtbach stream, metals Ni, Cu and Cd, dynamic speciation measurements agreed well with model predictions of cdmyanx calculated from total dissolved concentrations (Fig. 11.5).

In general, GIME measured less metal than DGT, consistent with the smaller time window available for metal uptake. For Pb in the River Wyre, a soft-water with high dissolved organic carbon (DOC), DGT and GIME agreed well, but were an order of magnitude lower than model predictions. These results suggest that most metal complexed by humic substances in the environment is labile within the timescale of DGT.

11.4.4 Bioavailability

Webb and Keough [36] have investigated whether DGT could be used as a surrogate for the bioaccumulation of trace metals (Cu, Cd, Pb and Zn)

Fig. 11.5. Concentrations measured by different speciation techniques compared to total measurements and speciation calculations by WHAM VI and Visual MINTEQ (reprinted from Ref. [27]. Copyright (2006), in part with permission from American Chemical Society).

Fig. 11.5. Concentrations measured by different speciation techniques compared to total measurements and speciation calculations by WHAM VI and Visual MINTEQ (reprinted from Ref. [27]. Copyright (2006), in part with permission from American Chemical Society).

by mussels. Over the 1 year study period, mussel concentrations of Zn and Cd were similar at the four stations (two inside and two outside an enclosed marina), while Cu and Pb showed significant temporal and spatial variations. While DGT also showed significant temporal variations, there were more defined differences between sites. It was suggested that bio-fouling effects on the DGT sampling devices need to be quantified or avoided by using multiple short-duration deployments.

The accumulation of Cu by the gills of Rainbow Trout (Oncorhynchus mykiss) was compared with measurements of Cu speciation by DGT and ion selective electrodes (ISE) [38]. Natural organic matter (NOM) decreased the uptake of Cu by trout gills and the concentrations measured by both DGT and ISE. The source of NOM also seemed to affect the free ion concentration and the amount of Cu taken by the trout gills and DGT, with allochthonous (terrestrially derived) NOM decreasing uptake more effectively than autochthonous (algal-derived) NOM.

Tusseau-Vuillemin et al. [39] used DGT to evaluate the toxicity of Cu to Daphnia magna in solutions containing EDTA, NTA and glycine. DGT could not predict the influence of NTA, as Cu-NTA complexes were completely labile. However, as Cu-EDTA complexes were completely inert, Cu toxicity could be predicted by the DGT measurement. Cu-glycine complexes were both partially measurable and toxic. Humic acids and aged and fresh algal exudates all appeared partially labile and non-toxic when using APA2 gels. The contribution to the DGT-measured mass was greatly reduced when a restricted gel was used, as complexes with humic material and aged algal exudates were largely excluded. It was concluded that DGT with a RG is a powerful tool for assessing the bioavailable fraction of Cu in natural waters. A bioavailable fraction was calculated in English and Italian rivers by Garofalo et al. [37] as the DGT-measured concentration divided by the total dissolved concentration. The percentage of bioavailable fractions of Ni, Cu, Zn, Cd and Pb were 14-51%, 5-52%, 2-66%, 0.4-16% and 9% to completely labile, respectively.

Al measured by DGT was found to predict the gill uptake of Al more accurately than conventional column measurements of Al, as evidenced by strong linear correlations with the fish physiological responses, increased blood glucose levels and decreased plasma chloride [40]. The better prediction by DGT was attributed to the measurement being in situ.

11.4.5 The use of DGT as a routine monitoring tool

As an alternative to logistically demanding and expensive discrete sampling, routine monitoring of estuarine waters was carried out using DGT [41]. Time-integrated DGT concentrations showed good correlations with composite total dissolved concentrations (< 0.45 mm), sampled at 4 h intervals over 24 h period. The following correlation coefficients were obtained: Ni, r — 0.92; Cu, r — 0.97; Zn, r — 0.91 and Pb, r — 0.80. Using free metal diffusion coefficients, corrected to the in situ temperature, DGT concentrations as a fraction of the total dissolved (< 0.45 mm) were 217 2%, 29 711%, 28 7 5% and 27 712% for Cu, Pb, Zn and Ni, respectively. Interestingly, the Cu results correspond exactly with the predicted cmax for 100% complexation by fulvic acid. While it was acknowledged that the fraction measured was operationally defined, the overall conclusion was that DGT is a very promising in situ monitoring tool for these dynamic estuarine waters. Munksgaard and Parry [42] found sufficient sensitivity, accuracy and precision for the measurement of Mn, Cu, Cd, Co and Pb in nearly pristine, but turbid coastal sea-water. The DGT measured fraction was 44-63% for Cu but was close to 100% for both Co and Cd.

A study was conducted in Queensland's Gold Coast Broadwater area, Australia, to determine whether anti-biofouling paints used on small recreational boats lead to increased levels of Cu in and around recreational boat anchorage sites [43]. While total dissolved Cu concentrations were above the 1.3 mg L-1 guideline value when the number of boats in the vicinity exceeded 30, DGT concentrations were well below this value at all of the boat numbers observed. DGT results showed a strong linear correlation (n — 14; r — 0.82; p< 0.001) with the number of recreational boats in the vicinity of the anchorage sites, while total dissolved Cu concentrations were less significantly correlated (n — 14; r — 0.70; p<0.01). This was the first study to show a clear correlation between recreational boat numbers and available Cu concentrations.

Cleven et al. [44] used DGT for routine monitoring of Ni, Cu and Pb concentrations in the rivers Meuse and Rhine. They concluded that while DGT was suitable as a robust tool for routine monitoring, the associated errors were greater (^20-30%) than those observed under controlled laboratory conditions (<10%), due partly to variations in some of the constants (e.g. temperature) used in the DGT calculations.

11.4.6 Metal remobilization from settling particles

DGT has been used to measure metal remobilization from settling particles in a lake-water column. The DGT device formed the base of a cylindrical sediment trap. Any remobilization of trace metals from particles was measured as an increase in mass taken up by DGT [73]. Laboratory tests indicated that there was negligible turbulence near the bottom of the trap, where the particles are collected and consequently uptake is likely to be diffusion controlled. Control devices, deployed upside down and thus, not in contact with settling particles, were used to measure metal uptake in the absence of settling particles. Remobilization of Al, Ba, Co and from settling particles significantly increased the DGT accumulated masses over the controls, with the release being attributed to reductive dissolution of Mn oxides.

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