|Sediment Sampling||2005 Results||2006 Results||2007 Results|
Sediment samples were collected as part of the 2005 and the 2006 Snapshot Day events as part of a two part sediment analysis. Part 1 involves carefully describing the sediments for color, grain size, composition and moisture content, and Part 2 involves sending the sample to Lamont-Doherty Earth Observatory for X-Ray Fluorescence (XRF) scanning to determine the amount of lead and other metal concentrations found in the river sediments.
Why are we looking at sediments? Trace metals can bind to sediments. Many of these metals are naturally occuring, but increased or heightened levels of some metals can suggest anthropogenic (human) influences. So the level or amount of metal bound to the sediments in an area can be used as an indicator of outside "inputs". For example, lead occurs naturally at rates of approximate 20 ppm in the sediments, but large increases in lead levels are associated with industrialization and leaded gasoline, paint pigments and incinerators. If we find a sample with high levels of lead we would assume it was deposited in the river after 1900, while samples with just background levels of lead would be believed to have been deposited prior to 1900. Zinc is another industrially connected metal, and it would not be surprising to see similar trends in the lead and zinc readings.
We also look at the type of sediment that exists in each area. Sediments are often described by their grain size - from fine grain to sandy or course grain. The larger the grain size the lower the surface area in the sample! While that might not sound logical think of it this way - if you are only comparing surface area in two grains of sediment, one small and one large, the larger grain would have more surface area. BUT if you were to collect two equal amounts of fine grain and large grain sediments in a container, the fine grain sample would have more overall surface area since more of the fine grains could be collected in the same overall sample size. Therefore the grain size in an area is important to consider when looking at the results.
These Snapshot Day sediment samples are from different locations in the river and have different color, grain size and overall composition.
The samples were collected by push cores with collection sleeves 14 inches long (35.56 cms). The actual samples collected were a third to half this length. Prior to anaysis with the XRF the samples were visually reviewed for the:
After recording the physical description of the core, as defined above, XRF measures were made of each of the cores for a wide range of metals. We selected Lead, Zinc, Manganese and Iron to record and discuss.