Sedimentation in the Tamar estuary has been an ongoing issue for many decades, and is of particular concern to the community for its impact on the recreational an aesthetic values of the estuary.
This project is assessing the feasibility of applying chemical fingerprints to determine the provenance of suspended sediments delivered to the Tamar estuary from the Esk catchment and is being conducted as part of a University of Tasmania PhD study.
It is proposed that potential sources of sediment (e.g. surface soils, channel banks, river beds) within the Esk catchment be identified and characterised by chemical and physical means to enable establishment of sediment fingerprints. Collection and analysis of suspended sediment samples from major rivers within the catchment would then enable determination of the provenance of the suspended sediment using the established source sediment fingerprints.
The project is focussing on the upper South Esk catchment, including the Break O’Day sub-catchment, as a means of assessing the feasibility of applying chemical fingerprints to determine the provenance of suspended sediments to the South Esk River.
The big picture
The project will focus on surface geology and land systems as the prime determinants of sample site selection in an effort to best discriminate between potential sediment sources throughout the catchment. By considering land systems as well as geology the sampling will consider a broader, more distinctive range of potential sources, with land systems also incorporating rainfall, altitude and topography, as well as geology, into sample site selection.
Land use will not be considered in the selection of sample sites, but can be overlaid on the GIS system to see if land uses fall into particular geologies or land systems.
Why this activity is important
Sedimentation in the Tamar estuary has been an ongoing issue for many decades, and is of particular concern to the community for its impact on the recreational an aesthetic values of the estuary. A potential outcome of the project will be an improved understanding of the source(s) of sediments to the Tamar and the ability to better manage sediment inputs.
What has been solved
Initial plans to focus sampling on suspended sediments in rivers proved to be problematic for a number of reasons, with a major issue being that the low suspended sediment loads during times other than peak flow events made it difficult to acquire sufficient sediment for chemical analysis. Whilst this method would have provided a more effective means of sampling source sediments, e.g. by including natural size fractionation and weathering processes into the sampling process, the potential variation in sediment source contributions during flow events and across seasons would have also provided additional complications in differentiating sediment sources, hence the decision to sample surface geologies.
What we have achieved
After initial soil sampling in the upper South Esk catchment in December 2010, work has focussed on expanding the study area down to the confluence of, and including, the Break O’Day River. Extensive consideration has been given to the establishment of a practical source geology basis set, with four source geologies chosen (S1 = basic igneous; S2 = acid igneous; S2 = sedimentary rocks; S4 = Quaternary sediments). The area under consideration is 1018km2 with the source geologies accounting for 970km2, or approximately 95per cent of this study area (S1 = 190km2; S2 = 103km2; S3 = 503km2; S4 = 175km2). A total of 55 sample sites have been identified (S1 = 10; S2 = 10; S3 = 20; S4 = 15) with surface and sub-surface samples to be collected from each site, after which samples will be dried, sieved to less than 63 microns and chemically analysed (HF digestion and ICP-MS analysis; LOI analysis). Sample sites have been chosen to ensure that source variances due to geological and geographical differences can be identified, and then statistical tests can be undertaken to determine if significant differences in sediment chemistry exist between the proposed source geologies.
There are a total of 29 geological identities in the study basin, six are common across all five catchments (-Rsp, Czc, Jdtm, Psp, Qa, Qrc) and one geology (Czb) is found in all catchments except Brumbys, whilst another (Cz) is found only in the Brumbys, Macquarie and South Esk catchments. There are a number of geologies found only in one or two catchments (Qs in Macquarie and North Esk; Nsg in Brumbys; -COsd, -Csr, Mst, Nsoo, Qsg in Meander; ODsm in South Esk and North Esk; Dfmy, Qdc in South Esk), including the granites which are found only in the South Esk and/or North Esk. Three granites of the study basin are common to both the South Esk and North Esk catchments (Dgrh, Dgrr, Dgrt), whilst six are specific to the South Esk (Dfmy, Dggy, Dghe, Dghy, Dgyi, Dgyr) and three are specific to the North Esk (Dgru, Dgud, Dgup). In general the distribution of surface geologies is also consistent with an altitudinal distribution. Since altitude correlates positively with precipitation, the two major erosion predispositions (geology and climate) are accounted for through consideration of the geologies alone. The main exception is Jurassic dolerite (Jdtm), also known for its mineral variability, which has a Midlands plains distribution as well as montane distributions, but consideration of land systems will take into account these different altitudinal distributions, thus use of surface geology and land systems as the prime determinants of sample site selection should provide a discriminate between potential sediment sources throughout the catchment.