Franklin & Marshall College Franklin & Marshall College

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Sediment Sampling Methods

Trenching

  1. Excavate trenches with backhoe, adjacent to the stream and perpendicular to stream flow, through the floodplain terrace sediments down to bedrock.
  2. The ideal trench, or series of trenches, extends from valley wall to channel edge, which gives a comprehensive, cross-sectional view of the sediment stack, which is useful to determine paleo-geomorphic changes in the fluvial system.
  3. Measure stratigraphic sections, delineate sedimentary units, describe sediments, and collect samples for geochemical and geophysical studies, including nutrient and trace metal contents, radiocarbon dating and magnetic susceptibility.
  4. Sampling Methodology -- Beginning at the upper (topmost) surface, cut a vertical channel or mini-trench with a trowel in approximately 15 cm long (along the face of the cutbank), by 10 cm wide (perpendicular to face) by 5 cm deep increments.  If the sediment is moist and consolidated, the sediment can be extracted in oriented bricks (which has some advantages in subsequent analyses), but for most cases simply scoop the material into a zip-lock bag one trowel tip at a time until a representative sampling of each increment has been achieved (ca. 300 – 500 g). Continue sampling down section in 5x10x15 cm increments until bedrock or groundwater table is reached
  5. Although trenches are the best way to get full stratigraphic exposures, trenches must be pumped as bedrock is below the local water table.
  6. In the lab, open ziplock bags and allow samples to air dry (usually 2-3 weeks).
  7. During the first week, magnetic susceptibility measurements are made using a probe through the ziplock bag pressing against the surface of the sample. We routinely do three measurements to check reproducibility and homogeneity of sample.
  8. For the remaining geochemical analyses, disaggregate the air dried samples in a large ceramic mortar and pestle using a pounding motion to pulverize the sample and break up material along grain boundaries. Disaggregted material is passed through a 2 mm mesh sieve and split with a geochemical splitter into enough fractions for subsequent studies.
  9. Texture and color estimates, and grain size and density measurements can be made on one of these fraction
  10. Geochemical methods used [“F&M” denotes in-house analytical capabilities]:
    1. ICP-OES (using the EPA 3051 method for detection of environmentally available trace metals and phosphorus) [F&M]
    2. 137Cs and 210Pb analyses for age controls covering the last 250 years.
    3. 14C (radiocarbon) dating, for age controls covering ca. 300 year BP to pre-Holocene timescales.
    4. Injection flow analysis for the analyses of various species of phosphorus and nitrogen [F&M].
    5. Combustion Analyzer for total nitrogen and total carbon measurements [F&M].
    6. Less frequently used analyses include: pollen, x-ray fluorescence [F&M}, x-ray diffraction [F&M], electron and optical microscopy [F&M].
  11. Data are plotted on binary diagrams, plotted against depth from surface
  12. Digital photographs, GPS coordinates and detailed notes are recorded for each sediment profile.

Stream Bank Sampling

  1. Cut face back 10 to 20 cm to assure a clean, uncontaminated surface (using a pick, hoe, shovel or trowel).
  2. Measure stratigraphic sections, delineate sedimentary units, describe sediments, and collect samples for geochemical and geophysical studies, including nutrient and trace metal contents, radiocarbon dating and magnetic susceptibility.
  3. Sampling Methodology -- Beginning at the upper (topmost) surface, cut a vertical channel or mini-trench with a trowel in approximately 15 cm long (along the face of the cutbank), by 10 cm wide (perpendicular to face) and 5 cm deep increments.  If the sediment is moist and consolidated, the sediment can be extracted in oriented 5x10x15 cm bricks (which has some advantages in subsequent analyses), but for most cases simply scoop the material into a zip-lock bag one trowel tip at a time until a representative sampling for each 5x10x15 cm increment has been achieved (ca. 300 – 500 g).
  4. Typically, the stream surface is intersected before bedrock is reached, so this will be an incomplete stratigraphy unless it is possible to reach below the water line and extract sediment samples by hand.
  5. In the lab, open ziplock bags and allow samples to air dry (usually 2-3 weeks).
  1. During the first week, magnetic susceptibility measurements are made using a probe through the ziplock bag on the surface of the sample. We routinely do three measurements to check reproducibility and homogeneity of sample.
  2. For the remaining geochemical analyses, disaggregate the air dried samples in a large ceramic mortar and pestle using a pounding motion to pulverize the sample and break up material along grain boundaries. Disaggregted material is passed through a 2 mm mesh sieve and split with a geochemical splitter into enough fractions for subsequent geochemical studies.
  3. Texture and color estimates, and grain size and density measurements can be made on one of these fractions.
  4. Geochemical methods used:
    1. ICP-OES (using the EPA 3051 method for detection of environmentally available trace metals and phosphorus)
    2. 137Cs and 210Pb analyses for age controls covering the last 250 years.
    3. 14C (radiocarbon) dating, for age controls covering ca. 300 year BP to pre-Holocene timescales.
    4. Injection flow analysis for the analyses of various species of phosphorus and nitrogen.
    5. Combustion Analyzer for total nitrogen and total carbon analyses.
    6. Less frequently used analyses include: pollen analyses, x-ray fluorescence, x-ray diffraction, electron microscopy and optical microscopy.
  5. Data are plotted on binary diagrams, plotted with respect to depth from surface.
  6. Notes: Digital photographs, GPS coordinates and detailed notes are recorded for each sediment profile.

 

GeoProbe Cores

  1. GeoProbe cores are pneumatic cores that are pushed through soft sediments via a rig mounted on the back of a pick-up truck or mounted on an ATV. The sediment is extracted in a ca. 1-m-long clear plastic sleeve.
  2. Magnetic susceptibility measurements can be made using an O-ring analyzer, drawing the core through the ring in 3 – 5 cm increments.
  3. The plastic casing can be slit on each side with a box cutter and the core extracted and analyzed using the methods described above.
  4. Note 1: Sediments cores typically are compressed by 30-50% of their in situ volumes.
  5. Note 2: This is an excellent tool for determining depth to bedrock across the floodplain when trenching is not an option.