Transcript Slide 1

Some Uses of Channel Bed
Sediment Concentration Data
• Determine the spatial distribution of trace metals
– Identify point and non-point sources of pollution
– Assessment of the rates and patterns of contaminant dispersal
• First approximation of potential ecological and human health
effects (and regional water quality surveys)
• Monitoring of potential impacts of waste waters from industrial
or municipal sites
• Geochemical exploration (surveys)
Downstream Trends in Channel Bed
From Salomons
& Forstner, 1984
• Where point sources are present the concentrations
generally decline from the point of input.
Concentrations of Cu and Ni in the <63 um fraction of channel bed sediments from the Po River, Italy. Samples were collected in
the summer (grey bars) and winter (black bars). Acronyms along x-axis represent successive downstream sampling sites. Note
minimal variations in concentration between seasons.
Viganò, L., and 14 others, 2003. Quality assessment of bed sediments of the Po River (Italy). Water Research, 37:501-518. (from 2
of 8 graphs from figure 3, page 507)
Downstream Trends in Channel Bed
From Salomons
& Forstner, 1984
• Where point sources are present the concentrations
generally decline from the point of input.
Scale
Transitory deposits
Micro-forms
Meso-forms
Alluvial bars
Macro-forms
Mega-forms
Characteristic
Bedload temporarily at rest
Coherent structures such as ripples
with λ ranging from 10-2 to 100 m
Features with λ from 100 to 102 m;
includes dunes, pebble clusters and
transverse ribs
From by lag deposition of coarsegrained sediment
Structures with λ from 101 to 103 m
such as riffles, point bars, alternate
bars, and mid-channel bars
Structures with λ > 103 m such as
sedimentation zones
Characteristics of channel deposits (adapted from Knighton 1998; Church and Jones 1982; Hoey 1992)
(A)
Bedload
Straight
Mixed Load
Suspended Load
Straight
Straight
Meandering
Meandering
Meandering Braided
Braided
(B)
Anastomosing
Island Braided
Laterally Active Channels
Laterally Inactive Channels
Straight-Island Form
Meandering
Straight-Ridge Form
Braided
Sinuous-Stable
Single-Channel
Rivers
Anabranching
Rivers
Single-Channel
Rivers
Figure from Huggett, J.R., 2003. Fundamental of Geomorphology, Routledge
Fundamentals in Physical Geography, Routledge, London, fig. 7.7, p. 185.
Anabranching
Rivers
Straight Channel
Point Bars
(A)
Pools
Riffle
Meandering Channel
(B)
Pools
Riffle
Point
Bar
Point Bar
Point
Bar
Point
Bar
Point Bar
(C)
Longitudinal Profile
Riffle
Riffle
Pool
Riffle
Pool
Distance Downstream
Super Elevated
Water Surface
Secondary
Flow Directions
From Markham, A.J. and Thorne, C.R., 1992. Geomorphology of gravel-bed river
bends. In: P. Billi, R.D. Hey, C.R. Thorne, and P. Tacconi, (eds.), Dynamics of
Gravel-bed Rivers, pp. 433-456, New York, Jonh Wiley and Sons, Ldt., figure 22.2, p.
436.
From Thompson, A., 1986. Secondary Flows and the Pool-Riffle, Earth Surface
Processes and Landforms, 11:631-641., Figure 4, p. 636.
Reading, H.G., 1978. Sedimentary Environments and Facies, Blackwell Publications,
New York, Fig. 3.26, page 34. Company may have been purchased by Elsevier?
Figure 20. Laremie River, Wyoming (photo by J.R. Balsley); obtained from USGS Photo Library
Knighton, D., 1998. Fluvial Form and Processes: A New Perspective, Arnold, London.
Fig. 5.23, p. 233.
Grain-Size & Compositional
Variations
• Ladd et al. 1998
– Examined trace metal concentrations in 7
morphological units in Soda Butte Creek,
Montana
– (lateral scour pools, eddy drop zones, glides,
low gradient riffles, high gradient riffles,
attached bars, and detached bars)
– Highest concentrations in eddy drop zones
and attached lateral bars with largest amount
of fine sediment
Density-Dependent Variations
•Slingerland and Smith (1986) define a placer as “a deposit of
residual or detrital mineral grains in which a valuable mineral has
been concentrated by a mechanical agent,”
•A contaminant placer is defined here as a concentration of metal
enriched particles by the hydraulic action of the river. Where
they occur, trace metal concentrations will be locally elevated in
comparison to other areas (Miller & Orbock Miller, 2007)
Guilbert, J.M. and Park, C.F., Jr., 1986. The Geology of Ore Deposits. New York, W.H.
Freemand and Company, figures 16-1, p. 746 and 16-4b p.749.
Bateman, A.M., 1950. Economic mineral deposits, 2nd edition. New
York, Wiley and Sons.
Lahontan
Reservoir
Virginia
City
11
Dayton
3
7C 7D
7B
7
6
5
9
Table
Mtn.
10
12
15
14
13Mineral
Canyon
95
Fort
Churchill
16
17
Gaging
Station
4
Pyramid
Lake
2B
Carson
City
18
2
1B
Carson
Playa
Stillwater
Wildlife
Refuge
1
Reno
395
0
Gaging
station
Fallon
Lahontan
Reservoir Carson
Lake
Lake
Tahoe
Carson
City
0 1 2 3 4 5
0
1
2
3
Miles
4
Km
Carson River
Watershed Boundary
Eureka Mill, Brunswick Canyon
C o n c e n t r a t
Hg Concentrations In Channel Bed
12
M e r c u r y
10
Mineral
Canyon
Sixmile
Canyon
8
Gold
Canyon
6
Lahonton
Reservoir
4
Carso n
City
2
0
0
20
40
60
80
Distance Downstream from 395 (km)
Pool
Riffle
Point Bar
Channel
100
Variations Dependent on Time and
Frequency of Inundation
• Examples
– Queens Creek, Arizona
– Rio Pilcomayo, Bolivia
Graf, W.L., Clark, S.L., Kammerer, M.T., Lehman, T., Randall, K., Tempe, R., and Schroeder, A., 1991. Geomorphology of heavy
metals in the sediments of Queen Creek, Arizona, USA. Catena, 18:567-582, figures 2, p. 572 and 6, p. 578.
Study
Area
“Modern Mine”
Ball Mill
Floatation Process
Pb & Zn Concentrate
Sampling Site RP-1
~1.5 miles from Mills
Floatation Mill, Potosi
Effluent
20 miles from Mills
You want me to
live where?
~60 miles from Mills
20 miles from Mills
Yikes!
High-Water
Channel Deposits
Low-Water
Channel Deposits
Rio Pilcomayo, southern Bolvia near Uyuni. Photo taken in July during the dry
season.
Implications to Sampling
• Local variations – referred to as small scale or
field variance (Birch et al. 2001)
– Can be on the order of 10 to 25 % relative standard
deviation and may be significantly greater than
analytical variation (error)
– May hinder ability to decipher differences in
contaminant levels between sample sites
• Reconnaissance level surveys and sample stratification by
morphological units ?
• Sampling of specific units only?
– Composite sampling to minimize within unit variations
Changes in Sediment Composition Can:
• Influence the spatial and temporal concentration
patterns observed in aquatic systems
• Hinder the determination of localized inputs of trace
metals from either natural sources (e.g., ore bodies)
or anthropogenic sources (e.g., mining operations or
industrial complexes).
• Changes in grain-size have a particularly significant
influence on metal concentrations.
Types of Mathematical Manipulations
Commonly Applied to Bulk Metal Data
After Horowitz, 1991
• Corrections for Grain-size differences
• Normalization to a single grain-size range
• Carbonate content corrections
• Recalculation of concentration data on a carbonatefree basis
• Normalization to a conservative elemental
• Use of multiple Normalizations
Methods of Handling the Grain Size
Effect
• Analysis of a specific grain-size fraction which is
considered to be the chemical active phase
– Does not provide for an understanding of the actual
concentrations that exist in the bulk sample
– Inhibits the calculation of total trace metal transport rates
• Normalize the metal concentration data obtained for
the bulk (< 2mm or sand) sized fraction using some
form of mathematical equation and grain size data
obtained from a separate sample
– Provides actual concentration found in bulk sample
– Poorly documents the concentrations that would actually be
measured in the finer-grain size fractions
Designation of Chemical Active
Sediment Phase
• Numerous size fractions have been used as the
chemical active phase including <2 µm, <16 µm, <20
µm, <63 µm, <70 µm, <155 µm, <200 µm (Horowitz,
1991)
• Argument for using < 63 µm fraction
– It can be extracted from the bulk sample via sieving, a
process which does not alter trace metal chemistry
– It is the particle size most commonly carried in suspension
by rivers and streams and may therefore be the most readily
distributed through the aquatic environment
Grain Size Normalization
Normalized
=
Concentration
(DF *Bulk Metal Concentration)
Where,
DF = Dilution Factor
= 100/(100 - % of sediment > size range of Interest)
Concentration vs. Quantity of Fine Sediment
Sizes
Frequently Used
•2 μm
•16 μm
•62.5 μm
•63 μm
•70 μm
•125 μm
•200 μm
Data from deGroot et al., 1982
Data from Horowitz and Elrik, 1988
Differences between Measured and
Normalized Values
• Selected chemical active phase (grain size
fraction) may not contain all of the trace
metals
• Differences in concentration are not solely
due to grain size variations
• Data contain analytical errors associated
with grain size or geochemical analyses
Percent Contribution
Fractional Contributions
of Selected Metals in
Suspended Sediments
Concentration
Constituent
(mg/kg)
<63 μm
fraction
>63 μm
fraction
Total
Sample
<63 μm
fraction
>63 μm
fraction
Arkansas River (sampled 5/11/87)a,b
Mn
1100
600
800
50
50
Cu
51
22
33
58
42
Zn
325
110
190
63
37
Pb
52
25
35
54
46
Cr
56
44
49
43
57
Ni
32
16
22
55
45
Co
15
11
12.5
45
55
Cowlitz River (sampled 4/20/87)a,c
Mn
650
670
660
40
60
Cu
63
33
46
57
43
Zn
62
68
59
42
58
Pb
12
10
10.8
45
55
Cr
35
19
25
56
44
Ni
25
16
19
53
47
Co
14
14
14
41
59
aThe
(modified from Horowitz et al., 1990)
represents the mean of the initial and final composite samples
obtained at these sampling sites. b<63 μm fraction equaled 37 %, >63 μm
fraction equaled 63 %, c <63 μm fraction equaled 41 %, >63 μm equaled
59 %.
Carbonate Correction
• Assumes: Carbonate does not contain substantial quantities of
trace metals and, thus, acts as a diluent. May not be true of Cd
and Pb.
• Generally applied to streams in calcareous terrains, particularly
those in areas with karst.
Normalized
Concentration = (DF *Bulk Metal Concentration)
Where,
DF = Dilution Factor
= 100/(100 - % of carbonate in sample)
Conservative Element Corrections
• Assumes that some elements have had a uniform
flux from crustal rocks. Thus, normalization to these
elements provides a measure (or level) of dilution
that has occurred.
• Elements most commonly used are Al, Ti, and to a
lesser extent, Cs and Li.
• Normalized
value =
(Concentration of Trace Metal)
(Concentration of conservative element)
Note: this generates a ratio, not a concentration as did the previous
procedures