Glacial suspended particulate matter : character, composition and adsorption potential in freshwater environments
Thesis DisciplineWater Resource Management
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
Suspended particulate matter (SPM) plays an important role in regulating the transport and bioavailability of toxic trace elements and nutrients in natural waters. This research addresses the absence of data on glacier-fed catchments, which are likely to undergo accelerated changes in water quality and SPM concentration as glacial recession proceeds into the future.
The glacier-fed Waitaki catchment has been subjected to intensive agricultural development in recent decades. The ability of the glacial SPM to adsorb phosphorus (P), cadmium (Cd) and copper (Cu), contaminants that are commonly associated with agrichemical applications, was determined with laboratory and modeling experiments. The SPM was first characterised in terms of its mineralogy and morphology, particle size, trace element content and degree of particle weathering. High concentrations of SPM occurred close to the glacial source, characteristically dominated by extremely fine inorganic particles (quartz, mica, feldspar) with a low degree of chemical weathering compared to the source rock. Major evolutions in SPM character occurred down-catchment. SPM concentrations declined significantly, becoming progressively weathered and enriched in organic matter, diatoms and aggregates, with corresponding reductions in the concentrations of reactive surface oxides and specific surface area (SSA).
The adsorption capability of the Waitaki SPM was determined by adsorption edge and isotherm experiments. The fresh glacial SPM was found to have the greatest capacity to adsorb P, which was attributed to the relatively high SSA and greater concentrations of reactive iron (Fe) oxides associated with the fine sediments. Down catchment, the SPM was found to have a greater capacity to adsorb Cu, which was attributed to the significant influence of organic matter. The low affinity of Cd for SPM throughout the catchment limited its adsorption generally. The novel measure of ‘adsorption potential’ conceived in this study, a function of the variable SPM concentrations and its adsorption capacity, indicated that SPM will readily adsorb Cu throughout the catchment. However, the adsorption of P and Cd will be highly constrained by the concentrations of SPM that remain in solution. Down catchment, the adsorption potential of the SPM declined by up to three orders of magnitude. As such, the SPM may not play an appreciable role in the attenuation of these contaminants as land-use modification and climate change proceed into the future. Instead, the biogeochemical cycling of these elements is predicted to be largely controlled by adsorption onto surface sediments and uptake by aquatic biota.
The role of glacial SPM as a vector for the nutrient P was determined in the polar Onyx River catchment, Antarctica. The Onyx River is the largest meltwater stream in Antarctica, flowing inland and into the ultra-oligotrophic Lake Vanda. The system is generally considered to be P-limited and is one of the least biologically productive and clearest lakes in the world. Suspended particulate matter was collected from the Onyx River during the 2016/17 K802 sampling campaign with Antarctica New Zealand. The SPM was dominated by fine, inorganic particles (mica, smectite, chlorite, quartz) with a high degree of roundness. Sequential extractions indicated that the majority of the SPM P was associated with primary phosphorus-containing minerals (such as apatite) and is not expected to be readily bioavailable. Less than 20% was found to be associated with reactive metal oxides (Fe and Mn), clay minerals and organic matter. However, this may be liberated for metabolic uptake when incorporated into benthic mats, or exposed to the highly reducing conditions present in the bottom waters of Lake Vanda. In oxic waters, the SPM is predicted to be an efficient scavenger of dissolved P. The flux of SPM in the Onyx River catchment was predicted over a 20 year period with the generation of an SPM-discharge rating curve. This was applied to flow data collected by the McMurdo Long Term Ecological Research Network (LTER). The predicted SPM loads for the annual melt seasons were highly variable, ranging from 1.0 – 908.7 tonnes. So too were the predicted quantities of P, with loads varying from 0.6 – 515.2 kg of P. High flow events, such as those during the 2001/02 and 2008/09 austral summers, resulted in SPM loads up to three orders of magnitude greater than typical years. As such, the dominant source of P to the catchment is likely to be during these episodic events, which are predicted to have a long-lasting effect on the ecological productivity of the lake. An increase in the frequency of flood events caused by anthropogenic climate change may initiate a profound shift in the physicochemical characteristics of Lake Vanda, and its biological structure and diversity.