Sources of Sediment in Whangapoua Estuary

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 (now Waikato Regional Council) 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 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.

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

Whangapoua Harbour

2005-2006

Surface (top 2 cm) soil and sediment samples (Table 2) were collected from 22 selected locations within the Whangapoua Harbour and catchment. Top soil samples were taken from locations selected to represent the main catchment land use types — native forest (N), native scrub (NS), pasture (P), and exotic pine forest (EP) and within different sub-catchments. Additional samples of subsoil were collected from material stockpiled for road working and from natural slip-face material. Samples were analysed for organic content, bulk N and C stable isotope analysis. Compound specific isotopes (CSI) and resin and fatty acids were also extracted and analysed. Full details are given in the report referenced below. One off data collection

See report.

Report is available in .pdf format from Waikato Regional Council's website (see link below)

All results presented in report, as referenced below. Results contribute to the management of land use in estuarine catchments.

Report is freely available.