# Homework Help: Concentration of C6Cl6 in a sediment core w/ diffusion coefficient

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1. Dec 18, 2015

### SansaStark

1. The problem statement, all variables and given/known data
Exercise: Hexachlorobenzene (C6Cl6) is a highly toxic waste product of pesticide manufacturing. It is resistant to biodegradation. Sediments at the bottom of a reservoir in the Upper Mississippi River catchment have been found to contain high C6Cl6 concentrations. The sedimentation rate is largely unknown, but an effective diffusion coefficient had been determined for C6Cl6 from core samples of the reservoir sediments for hexachlorobenzene and these sediments: D=1.2∙10‐9 m2∙s‐1. When did the spill occur? (Do a least square fit of the Gaussian distribution to the measured concentrations)

2. Relevant equations

3. The attempt at a solution
I am looking for a way to calculate the time difference for when the concentration was 0.1 and when it was at it's maximum level at 2.5 g/m³, like: how long did it take the C6Cl6 to diffuse to reach a level of 2.5.

At 2.5 the partial dericative of time when concenttration meets its max is 0.

Then of course to this time interval the time for the reverse process ,the decrease of the concentration level has to be added.

The thing is I have no x-value.

And I kinda don't know if I'm on the right path anyway. Could someone evtl. help me with this? Regards, Vera

2. Dec 22, 2015

### haruspex

Doesn't the curve fit give you values for M and Dt? You are given D, so t follows.