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Geochemical fingerprinting to target sediment sources in the Oreti catchment

Sediment fingerprinting is a technique for understanding where sediment entering our streams and rivers comes from. The technique utilizes properties of soils and sediments, like their geochemistry, to characterize different erosion sources and determine their relative contributions to the sediment transported through rivers.

Figure 1 Oreti catchment (Southland), showing spatial spread of sediment sampling to characterize the different sources.

The STEC programme is utilizing this technique in several catchments around the country to understand how differing erosion sources deliver sediment to rivers during storm events. One of the focus catchments is the Oreti River, which drains an area of ≈ 2,200 km2 in Southland. We collected 115 samples of different erosion sources at the end of 2019 to represent a combination of 5 erosion processes overlaid on 6 geologies throughout the catchment (Fig. 1). These sources include, for example, channel bank erosion in alluvium occurring at upper, middle and lower reaches of the catchment (Fig. 2), and gully or landslide erosion occurring in greywacke hill country in the upper reaches of the catchment (Fig. 3). We are coupling the sediment fingerprinting with characterization of erosion source properties – such as particle size, shape, and organic matter – that affect sediment-related water quality, such as visual clarity. This information on the contributions of erosion sources and their properties influencing water quality will support the development of an event-scale erosion and sediment model within STEC.

A large storm occurred early in February 2020 that caused widespread flooding in Southland. We collected suspended sediment samples at multiple stages of the flood event (21 samples) across 4 nested monitoring sites in the Oreti catchment (Fig. 4). This provides a comprehensive set of suspended sediment samples that will allow us to track the sediment dynamics occurring during the rising and falling limb of the storm hydrograph. These samples are being processed and analysed to determine the contribution of different sediment sources and the pattern of sediment delivery over time and space throughout the catchment. This represents possibly the first sediment fingerprinting dataset to capture a flood event of this magnitude and at this spatial and temporal resolution.

Further sediment fingerprinting work is being carried out in the upper Tiraumea catchment (Manawatu) as part of our instrumented erosion and sediment process research catchment.

Contact

Simon Vale and Hugh Smith (MWLR)

Arman Haddadchi (NIWA)

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