Abstract
This study investigates the role of land use and flow variation on the water quality of the Leith catchment, and the interrelationship between key water quality indicators, including total nitrogen (TN), nitrate-nitrite-nitrogen (NNN), ammoniacal-nitrogen (NH4-N), total and dissolved heavy metals Cu, Zn, Pb, Cd, Fe, Mn and total suspended sediment (TSS). Baseflow samples were collected from different sites along the Leith River and its major tributary, Lindsay’s Ck, which have varying land uses including forestry, agriculture and urbanization. The spatial variability in key water quality indicators indicates that land use does impart an influence on the concentration of different forms of nitrogen, but not necessarily on TSS or heavy metals. There were hotspots of NNN contamination in the Leith River, particularly at Poulters Rd and Helensburgh. Lindsay’s Ck was enriched with NNN and NH4-N mainly due to cumulative impacts from a range of potential sources, with Norwood St generally having the most elevated concentrations of NNN and NH4-N. Agricultural activities, landfill leachate, seepage of wastewater from sewerage, stormwater and septic tanks were among the potential sources of nitrogen contamination at the Leith catchment.
Stormflow samples were also collected from the lower catchment of the Leith River during four storm events, and the water quality during baseflow and stormflow conditions indicates that storm events potentially contribute high contaminant loads via the Leith River into Otago Harbour. These concentrations increase with higher peak flow and longer antecedent dry periods between events. Hydrographic and hysteretic analyses indicate that heavy metals and TSS were mainly derived from surface event flow, and first flush effect of these contaminants during the onset of the events was attributable to the buildup of easily mobilized materials during antecedent dry periods. Nitrate-nitrite-nitrogen contamination was mainly derived from groundwater and soil water sources. During high flow conditions, NNN decreased in proportion although TN increased, and this was presumably due to the importance of particulate/organic nitrogen being mobilized during storm events. Besides groundwater and plant organic materials, there was concern of overflow of combined sewage and stormwater pipes as potential sources of nitrogen contamination during events.
The spatial and temporal data show variation in the intensity of the correlation between different water quality indicators. High correlations were observed between TSS and particulate/organic nitrogen, and between TSS and particulate metals. This suggests that, due to the physical and chemical characteristics of sediments to retain other contaminants on its surfaces, particles may be an important transport agent of particulate-associated contaminants. Overall, the spatial and temporal data on the water quality of the Leith River could be used to assist water resource management at the Leith catchment, in order to protect the intrinsic habitat values, cultural values and contact recreation at the Leith River and Lindsay’s Ck. The spatial analysis technique is useful to illuminate potential pollutant hotspots, and the temporal assessment using both hydrographic and hysteretic analyses are effective in determining the potential implication of storm events on water quality degradation of the Leith River.