Abstract:
Restricted Item. Print thesis available in the University of Auckland Library or may be available through Inter-Library Loan. Suspended particulate matter (SPM) is an important transport agent for metal contaminants in streams and rivers. Trace metal adsorption to suspended particulate matter (SPM) influences bioavailability and toxicity of trace metals in natural waters. Although the nature of SPM from large non-urban and rural catchments has been characterised, urban catchment SPM has received less attention. In this study, for highly contaminated urban catchments in the greater Auckland area, trace metal partitioning between the dissolved phase and SPM is determined, and SPM characterised in terms of its Si, Al, Fe, Mn, Zn, Cu, Pb, TOC, TON and PO4 concentrations, as well as particle size, abundance, type and surface area. The degree of trace metal adsorption onto the SPM is assessed by way of adsorption edge experiments. Data are compared to similar data from representative non-urban catchments in the Auckland region; the Kaipara River and Waikato River catchments, to identify any significant differences in the SPM, and any difference in the ability of the SPM to adsorb Cu, Pb and Zn. Finally, a model is developed for predicting trace metal partitioning in catchments of the Auckland region.
Auckland urban SPM collected from three catchments comprised quartz, feldspars and clay minerals, with Fe-oxides and minor Mn-oxides. No particles of anthropogenic origin, other than glass shards' were observed. No change in urban SPM particle size or specific surface area was observed with seasonal change in temperature, but the nature of the SPM was observed to change with flow regime. The abundance of finer particles, the surface area and the Al content of the SPM increased under moderate flow conditions. However, the Si:Al ratios remained constant, confirming the importance of aluminosilicate detrital minerals in surface run-off. The SPM Fe content was observed to decrease with increased flow and was attributed to dilution of Fe-oxide rich SPM particles, of groundwater origin.
The urban SPM was found to be mineralogically, chemically and biologically similar to the Kaipara River SPM. However, major differences between urban catchment SPM and SPM from the much larger (non-urban) Waikato River were observed, and attributed to a higher abundance of diatoms in the Waikato River. The Fe content of the Waikato River SPM was consistently lower (<5%), and the Si:Al ratio and Mn content was higher than in SPM from the urban catchments and the Kaipara River.
It was found that the ability of the Auckland urban SPM to adsorb trace metals decreased in the order Pb > Cu > Zn. Little difference in adsorption was observed between the non-urban Waikato and Kaipara River SPM and urban SPM, or between urban SPM from different flow and seasons, despite some compositional differences in the SPM. This suggests that, on the basis of a single site, metal adsorption onto SPM could be readily predicted across a range of urban and non-urban catchments in the Auckland region.
Adsorption edges were modelled with a diffuse layer, surface complexation model to assess the role of Fe-oxide in adsorption. The MINTEQA2 model was used, assuming Fe-oxide (as HFO) was the only adsorbing surface. There was generally good agreement between observed and modelled adsorption for Pb, confirming the importance of Fe-oxide surfaces for Pb adsorption. However, the model did not predict Zn or Cu adsorption as well. The TOC content of the SPM, and presence of dissolved ligands and organic matter in the water column, appeared to play an important role in Cu adsorption to the SPM. For Zn, the presence of adsorbing surfaces other than HFO appeared to influence adsorption.
In situ trace metal partitioning in the urban and non-urban catchment waters were assessed by way of a distribution coefficient: Kd = [MeSPM] / [MeDiss]. Differences in the nature of the SPM observed between urban and non-urban SPM did not appear to affect the partitioning of Zn and Cu, however, Pb in the urban catchments was found to have a stronger association with SPM, most likely reflecting higher particulate Pb inputs to urban systems. The strong particulate association for Pb means that toxic effects may be inhibited in urban waters, but Pb will have a tendency to accumulate in estuarine and coastal sediments. A significant proportion of the total Zn and Cu concentration was transported through urban aqueous environments in dissolved form. The dissolved concentrations were high enough in both base and storm flow waters to potentially lead to persistent chronic or acute toxic effects on aquatic life, particularly for Zn.
A simple partition coefficient model, K*Fe= [MeAs] - [MeDiss] / [MeDiss]([ FePart] / 0.0317 + l.l), based on a positive relationship between SPM and the particulate Fe concentration in the column, was developed to estimate trace metal partitioning in waters of the Auckland region using only readily measurable water quality variables.
To prevent further degradation of Auckland region estuarine and coastal areas, stormwater treatment needs to be considered in Auckland urban catchments. The dissolved and finer-sized particulate fraction was found in this study to have a major influence on trace metal transport and fate in non-urban and urban catchment waters. However, these fractions are more difficult to control by traditional stormwater methods.