A history of upheaval: 150 years of environmental change: Ahuriri, Hawke's Bay, New Zealand

Abstract

Ahuriri Estuary lies on the northern outskirts of the City of Napier in the Hawke's Bay region, New Zealand. The present day estuary is all that remains of the once vast Ahuriri Lagoon (Te Whanganui-a-Orotu), which had formed at the mouth of the Tutaekuri River with an outlet into Hawke Bay. Since European settlement in the 1850's the swampy margins of the lagoon had been reclaimed to accommodate the growing Napier Township. In 1931 a large earthquake uplifted the lagoon bed and subsequent drainage, reclamation and the diversion of the Tutaekuri River out of the lagoon transformed the lagoon into the present day Ahuriri Estuary. The aim of this study was to investigate and explain the range of influences that have helped to shape and control the Te Whanganui-a-Orotu/Lower Ahuriri Estuary environment over the past 150 years. Using a multi proxy approach combining historical observation with sedimentology and geochemical analysis the impact of these influences can be identified as changes in the form, shape, depth, extent, as well as the character of sedimentation and anthropogenic pollution of Ahuriri. Events, processes and conditions affecting the environment of Te Whanganui-a-Orotu/Lower Ahuriri Estuary have been recorded or inferred from numerous existing sources. Information used to compile the historical record was collected from traditional sources such as local and national libraries and nontraditional sources including oral histories and personal accounts. Evidence presented in court proceedings regarding the ownership of the estuary was also a valuable source of information. From the historical record it has been established that the Te Whanganui-a-Orotu/Lower Ahuriri Estuary experienced substantial environmental change over the past 150 years. Due to their often-high rates of sediment accretion and stabilised vegetated nature, wetlands are considered to be highly efficient recorders of environmental change The characteristics of sediment accumulated within the lagoon/estuary can be interpreted to indicate the environmental conditions at the time of their emplacement. The sedimentological information was collected through fieldwork and laboratory analysis. The analysis of the sedimentological record provided possible evidence of several historical environmental changes. A pebble layer observed in the a core was interpreted to either be evidence of the former bed of the Tutaekuri River or/ of the construction of the Westshore Embankment Bridge. An increase in the mud content was observed in the majority of the sediment cores collected for this study. The mud increase was interpreted to be evidence of the post earthquake drainage and reclamation of the lagoon, or the post earthquake changes in the Tutaekuri River. Possible evidence of a 1987 Tanalith NCA spillage into the lower estuary was observed in two cores which contained high levels of Cu, Cr, and As. High concentrations of Cr were observed in several surface sediment samples and a core sample. The high levels of Cr were attributed to discharges by the Hawke's Bay Hide Processors into the Tyne Drain, which discharges into the Lower Ahuriri Estuary. Evidence of other environmental changes identified in the historical record was not found. Despite the well-documented record of environmental change affecting Te Whanganui-a-Orotu/Lower Ahuriri Estuary the frequency of these changes is not well represented in the sediment record. It appears that tidal activity within the lagoon/estuary environment has obliterated or obscured much of the evidence of past surfaces and environmental change. This has reduced the ability of the sediment record of the lower estuary to preserve past surfaces and environmental changes efficiently. It can therefore be concluded that the sedimentological record does not reflect and can not be fully explained by the historical record of the environmental changes that have occurred in the Te Whanganui-a-Orotu/ Lower Ahuriri Estuary within the past 150 years.