Sediment trace elements in lake cores as indicators of rural land use change in six selected New Zealand lakes : A thesis submitted in partial fulfilment of the requirements for the Degree of Master of Water Resource Management at Lincoln University

Abstract

Ongoing concerns of water quality degradation in New Zealand lakes in agricultural catchments, mean that it is important to develop an understanding of both past and present conditions of lake water quality. Lake sediments can provide reliable natural records of catchment land use change, and can be used to assess the long term impacts of anthropogenic activities on lakes. Large scale studies of lake sediment cores, such as the New Zealand-wide Lakes380 research programme, can help to inform future water management policies or strategies to improve water quality in lakes. Such studies involve comprehensive collection of chemical, biological and physical parameters, which are collectively very expensive to determine. However, a subset of key environmental land use indicators, which can be more affordable to determine, may be identified from this work. The purpose of this study was to assess the potential for using trace elements signatures in lake sediment cores, as indicators of rural land use change. Sediment cores were collected from each of the six New Zealand lakes found in various agricultural land use settings: two lowland lakes (Lake Tutaeinanga and Lake Ngāpouri), two coastal lakes (Lake Moawhitu and Lake Forsyth/Wairewa) and two highland lakes (Lake Pearson and Lake Heron). The cores were dated using pollen analysis methods to identify the prehuman period and major human settlement periods (Māori and European settlements). The study determined the concentrations of trace elements that are commonly used in New Zealand agriculture (arsenic (As), cadmium (Cd), copper (Cu) and zinc (Zn)), and other elements that may provide supporting information on the sources and distribution of the agriculturally-relevant trace elements. The trace elements were digested and analysed using the inductively coupled plasma - optical emission spectrometry (ICP-OES) technique. The study also determined the enrichment factors to differentiate trace elements originating from anthropogenic activities and those from prehumen conditions. The trace elements concentrations and enrichments were linked to the history of land use changes in the respective lake’s catchments. The results of the study has shown that P, Cu and Zn showed similar enrichment in recent sediments across most of the lakes studied, and they related to rural land use activities (application of fertilizer and agrichemicals) in the period of European settlement. Cd showed similar recent enrichment in Lake Tutaeinanga, Lake Pearson and Lake Heron, but not in super-eutrophic lakes (Ngāpouri and Moawhitu and some cores in Lake Forsyth/Wairewa), where Cd concentrations may have been affected by biogeochemical processes. Arsenic showed similar enrichment to P, Cu and Zn in Lake Forsyth, Lake Pearson and Lake Heron. The distribution of arsenic in Lake Tutaeinanga and Lake Ngāpouri was affected by volcanic and geothermal activities prevalent in the catchment. Other trace elements, such as Pb, were more indicative of road runoff affecting recent sediments, and Fe and Mn were determined largely to understand their effects on the distribution of other elements. The studied trace elements also identified impacts related to land use activities associated with Māori settlement in New Zealand, which were largely attributed to enhanced soil erosion due to clearing land. This effect was also evident, and generally to a great degree, in the European settlement era in some of the lakes.