Information from stream sediments

1. Information from stream sediments • Measurement of concentration of metals present (as solids) in sediment Gives information on: • Background levels of contamination • Comparison of [M] between sites and between depths in sediment at same site • transport of sediment away from site of contamination

2. Background levels of contamination Supposing we want to know how much pollution has resulted from industrial activity around the River Aire in Leeds. We could sample the sediment and analyse for metals such as Pb, Cd, Cu, etc But we need to know the natural levels of these elements. So sample in another area of similar Geology- get back ground level for that environment.

3. Measuring variation in metal concentration at a particular site Mine site • Sampling above and • below the mine site • provides information on • Background [M] • Degree of contamination • Decay of [M] with distance Map of a river

4. Sediment sampling- factors affecting design of field technique • Effect of Particle size of sample All fine sediment Fine sediment plus cobble Fine sediment represents all local rock types The cobble represents a single rock type Need to control particle size of sample for analysis Need to sieve sample prior to collection

5. Sediment sampling technique : 2 • Where should we take a sediment sample? • Does it matter? • It only matters if the nature of the sediment varies between sampling sites – over a short distance • Why should we get a variation in river sand at different points?

6. Sedimentary environments Cobbles larger boulders midstream The river sorts the sediment according to size

7. Formation of gravel bars Section- gradient change Inside of bends River widens All changes from high- low energy

8. Sorting by density • Minerals of different s.g. are segregated by water action Segregation occurs in 3 dimensions…

9. Distribution of heavy minerals in fluvial sediments Bedrock % of heavy mineral grains in fine sediment increases

10. Implications for sampling Bulk chemistry of river sediment changes with : • Place in the river • Depth in sediment Need to standardise our sampling technique If possible select: • Gravel bar on inside of bend or at edge of river • Standard depth: 0.5 m. Record details of the sampling site in your field note book

11. How are the metals present? In one of two forms: • As particulate minerals • Coatings on gravel/silt 1- assess through panning exercise 2- Measure through chemical analysis So need to collect sample and split it to enable us to do both.