The effect of sub-catchment industrialisation on the health of Ahuriri Estuary : a thesis presented in partial fulfillment of the requirements for the degree of Master Environmental Management at Massey University, Manawatū, New Zealand
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Ahuriri Estuary is steeped in geological and cultural history. In 1931 a magnitude 7.8 earthquake uplifted the bed of the former Ahuriri Lagoon by approximately 2 metres. The drainage, diversion of the Tutaekuri River, channelisation of waterways, and urban development of the surrounding catchment which ensued formed the modern 275-hectare estuary in the centre of Napier City. To investigate the effects of sub-catchment industrialisation on the health of Ahuriri Estuary, research outlined in this thesis involved undertaking a multi-temporal land use change assessment using historic aerial photographs of the Onekawa and Pandora sub-catchments. In 2018, 41.9ha of grasslands remained within the sub-catchments compared to 254.8ha in 1936, with 223.3ha of impervious industrial development spanning the sub-catchments. The waterways which flow through the Onekawa and Pandora Industrial zones have shown elevated dissolved and sediment-bound heavy metal concentrations. Six sediment cores were retrieved from tributary discharge zones within Ahuriri Estuary. Visual observations of grain sizes and fossil macrofauna densities accompanied Itrax™ core scanning for chemical constituents. Results for marine and terrigenous sediment ratios identified the 1931 earthquake. To investigate the relationship between sub-catchment industrialisation and the health of the modern Ahuriri Estuary, heavy metal peaks were assigned approximate depositional timeframes using calculated sediment accumulation rates. The early 1970s registered across five of the six cores as a time of excessive heavy metal contamination delivered to Ahuriri via the urban tributaries. Extreme zinc and lead pollution from the Pandora Industrial zone lasting several years from 1973-1976 exceed levels found in the literature of Itrax™ XRF-scanned estuarine cores. Zinc levels are high across the cores compared to the available literature. Above identified peak zones, heavy metal trends across five of the six cores gradually reduce towards the core surface. Despite observed heavy metal reductions, recent toxicity assessments using surficial sediment suggest that Ahuriri Estuary is in a state of poor ecological health, meaning its functionality as an important spawning ground and nursery for aquatic species is undermined. Once land-based contaminant mobilisation is reduced via improvements to land and freshwater management, rehabilitation options for contaminated sediment within the Ahuriri Estuary can be investigated for feasibility.