In this problem set we will consider the behavior of the contaminants of concern
ID: 107667 • Letter: I
Question
In this problem set we will consider the behavior of the contaminants of concern in the sediments of Whitaker Slough, a portion of the Columbia Slough system in North and NE Portland. We will use actual field data from the Slough, and we will take into account both AVS and adsorption. On the attached sheet (end of this document) you'll find chemical and physical data for Whitaker Slough. Note that the Slough has been divided up into five reaches. The first reach begins at the downstream end of Whitaker Slough where it joins the main stem of the Columbia Slough. Subsequent segments are one mile in length, each moving upstream. The last reach is approximately 1.5 miles long and represents the upper most reach of Whitaker Slough. The chemical data for sediments include the total organic carbon (TOC) on a percent basis. AVS and metal concentrations are given in millimole/kg as indicated. Also included are data (in units of micrograms/kg) for a common form of polychlorinated biphenyls (PCB), a commercial mixture called Arochlor 4. PCBs are known cancer-causing agents and are the main risk to human health, via consumption of fish, in the Slough. Whitaker Slough receives a considerable input of groundwater. During the summer months, the groundwater is typically the only significant source of water flowing through the slough. Data is given for the groundwater flows in each segment a. [3 pts] Calculate the amount of each metal that is not bound to AVS in the sediment for each segment of Whitaker Slough. Then calculate the interstitial water concentration of each metal for each segment, taking into account adsorption onto the sediment. For each metal use the relationships described in the online document entitled Speciation of Metals in Sediments. Work up these calculations in a spreadsheet and present your results in tabular form using a spreadsheet. Make sure all rows and columns are clearly labeled b. 12 pts] For Arochlor 1254, Schwarzenbach et al. report the best correlation for Koc 1S log Ko 0.88 l Ko 0.27Explanation / Answer
(a)Acid volatile sulfide (AVS) is an operationally defined dimension of metal sulfides that naturally occur in sediments. AVS represents the iron and manganese sulfide minerals in the sediments that react with toxic metals. (Some forms of metal sulfides, such as pyrite, are unreactive and are excluded from the measurement of AVS).
Toxic metals, such as mercury or lead, form sulfide minerals less soluble than natural AVS. As a result, these toxic metals displace iron or manganese from AVS and are themselves sequestered in a very insoluble and biologically. The precipitation of metals by AVS is assumed to eliminate the mobility and toxicity of that metal, so it is important to account for this removal when computing the pore water concentration of metal from the concentration on the solids Only the residual metals present in excess of the measured AVS concentration are considered to be available for release into the pore water. This residual fraction, in turn, is corrected for adsorption by the sediment particles.
The sulfide form of each toxic metal has its own solubility. Hg sulfide (HgS) is less soluble than all of the others. Cu sulfide (CuS) is the next most soluble, and so on. The complete order of sulfide solubility is:
Hg < Cu < Pb < Cd < Zn < Ni.
AVS = 20 µmol/kg Hg = 23 µmol/kg Cu = 35 µmol/kg SEM = 58 µmol/kg > AVS
HgS = AVS = 20 µmol/kg Residual Hg(non-sulfide) = 3 µmol/kg Cu (non-sulfide) = 35 µmol/kg
If AVS is greater than the total Hg, all the Hg will precipitate and then Cu will also begin to precipitate.
If the AVS exceeds the total Hg and Cu, then PbS will precipitate, and so on through the sequence. We can calculate this by subtracting the total Hg concentration from the AVS, then subtracting the Cu from the residual AVS (if any) and then subtracting the Pb from that residual AVS (if any) and so on for each of the other metals, in the order of their sulfide solubility. Note that as you calculate each metal in sequence (for SEM > AVS) , there eventually is a point where the AVS runs out in the “middle” of a metal. (In the last example above, AVS runs out in the Cu step.) The residual metal is assumes to remain isn non-sulfide form. And, the residual (non-sulfide) concentrations of all metals more soluble than the metal in the current iteration are equal to the total concentrations of those metals.
Ctot Meu mol/kg
Cr2 meu mol / kg
AVS
100
0
Hg
10
0
Cu
75
0
Pb
25
10
Cd
15
15
Zn
80
80
Ni
60
60
(b) Formulas for the values of Kd (adsorption coefficient) Some depend on the percent organic content (%OC) of the sediment. Example values calculated using %OC value shown at the bottom as 2%.
Metal
Kd
Value
Hg
109
109
Cu
8710
10*(0.33%OC+3.28)
Pb
3162
10*(0.21%OC+2.34)
Cd
575
10*(0.21%OC+2.34)
Zn
3274
3274
Ni
150
150
Therefore OC % = 2. Toxic metals in sediments often are not completely available for uptake by organisms. Metals bound to sediment solids in various forms are largely unavailable to organisms and therefore are far less toxic than metals dissolved in the pore water. Chemists use the term speciation to describe ways of calculating the various forms or species of metals that may be present in the sediments.
Ctot Meu mol/kg
Cr2 meu mol / kg
AVS
100
0
Hg
10
0
Cu
75
0
Pb
25
10
Cd
15
15
Zn
80
80
Ni
60
60
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