In this problem set we will consider the behavior of the contaminants of concern

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 andNE 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 1254. 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 ptsl 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 Ko is log Koc 0.88 log Kow 0.27

Explanation / Answer

Note: The question has sub-parts within the sections, thus, as per the regulations, only four sub-parts, namely -
a. i. Calculation of amount of metal for each segment
a. ii. Calculation of interstitial water concentration of each metal, taking adsorption onto sediment
b. Determination of Kow

Usage - 1 mile = 1.61 km; Volume = length of segment x width x length in m3 and 1 m3 = 1000 L, moreover 1 L of water is approximately 1 kg.

Multiplying the molar concentration/kg with the volume for each metal in the segment, we find out the above data in mmol for individual metals.

Online document mentioned in the question - Speciation of Metals in Sediments - cannot be found.
In that case, the answer can be calculated by multiplying the percentage that remains in the water with the molar concentration of the segments obtained in the above table, for each of the metals.

b. Koc is calculated with the organic material in mg/g upon the total organic amount in g/ml. Using PCB as the organic material in concern and then, TOC in % as g/100 ml, we find out the Koc for each segment -

Now, using the given equation Kow can be calculated as -

c.
i. Now, the groundwater flow has been given in L/(m2*s), which needs to converted days from seconds by multiplication of 86400 s/d, and the calculation also requires the multiplication of the respective atomic weights of the metals since the unit requires is mg m-2 d-1 -

ii. To calculate the mass flow from each segment as g/d, we need to multiply with the width and the length given for each segment -

iii. Total mass flow is a sum of all these metals and PCB, i.e. 40228747.96 g/d.

Segment River Mile Length (mile) River Mile Length (km) Width (m) Length (m) Volume (L) G/W Flux (L/m2/s) 1 1 1.61 26 1609 67352.74 0.0046 2 1 1.61 28 1609 72533.72 0.0051 3 1 1.61 30 1609 77714.7 0.0044 4 1 1.61 29 1609 75124.21 0.0041 5 1.5 2.42 27 2719 177659.46 0.0046

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