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Whangapoua Harbour Sediment Sources


Report: TR 2006/42
Author: Max Gibbs (NIWA)


Monitoring programmes have determined that the benthic community in the Whangapoua Harbour is changing and it is suggested that the changes are associated with catchment sediment inputs. Environment Waikato requested NIWA to undertake a study to determine the most likely sources of soil from within the catchment in the impacted harbour areas and to provide an estimate of the relative proportions of each soil source present. This report presents the results of the study.

This study used a recently developed technique, using compound specific isotopes (CSI), that can identify and apportion the contribution of soils from several different land use sources in a single catchment, at a number of locations within the harbour.

It was found that native forest and scrub contributed around 20-30% of the soil to the sediments at most sites in the harbour. The proportion was highest (~26%) in the eastern estuarine arm (Mapauriki) where native forest comprised a major proportion of the catchment land use relative to undisturbed pine forest and pasture. In the other two eastern estuarine arms (Otanguru and Owera), a substantial proportion of the sediment came from local pasture (~42% and ~25%, respectively). At all three eastern estuarine arms, there was a substantial contribution of soil from recent exotic pine forest logging, with proportions ranging from about 54% to 75% at those sites in the harbour.

The soil contribution from native forest and scrub to the western side of the harbour was relatively consistent at around 20% - 30%. The pasture land use contribution to the harbour sites along this transect were low (<10%) but there was a contribution of soil from exotic pine forest land use increasing in proportion from <10% to more than 50% with distance from the Opitonui River mouth. The signatures of this material indicate that it was associated with the leached exotic pine forest soils in the Opitonui catchment rather than recent logging operations in the Owera catchment.

Whangapoua Harbour Sediment Sources
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Table of contents

Executive summary i
1 Introduction 1
1.1 Background 1
1.2 Study rationale 2
2 Methods 3
2.1 Site 3
2.2 Sampling 4
2.3 Processing and analysis 5
2.4 Source identification and apportionment 8
2.4.1 IsoSource operation 9
2.4.2 Assumptions 10
3 Results 13
4 Modelling and data interpretation 19
4.1 Scatter plots for IsoSource modelling 19
4.2 Model output 22
5 Discussion 26
5.1 Transport mechanisms 28
5.1.1 Freshwater dispersion over the estuarine salt wedge 28
5.1.2 Delayed bed-load transport from the rivers and streams 30
6 Summary and conclusions 35
7 Acknowledgements 36
8 References 37
9 Appendix - site photos 39