Other Random Transport Processes in the Environment

Now you are ready to estimate the relevant diffusion distances. For the deepest depths in which PCNs appear, they are present at 120/18 000 = 0.0067 of the peak concentration at 24—25 cm. As discussed with respect to Eq. 18-23, this implies that you are interested in the argument of the complementary error function where the erfc(y0 0067) = 0.0067. In Appendix A, you find that y0,oo67 is about 1.9. Thus, you can solve  

it appears reasonable to expect a small portion of the PCNs (about 1% of the peak concentration) to have diffusively migrated from a layer at about 24—25 cm down to one at about 35 cm. This is similar to the deepest layers in which PCNs are found, and given the uncertainties of the various estimated parameters, it is not too farfetched to argue that release to the lake occurred around 1960 (+10 years). 

IJtS Other Random Transport Processes in the Environment 

On the large side, the concept of diffusion also can be applied to macroscopic transport. This process is called turbulent diffusion. Turbulent diffusion is not based on thermal molecular motions, but on the mostly irregular (random) pattern of currents in water and air. 

Laminar flow is defined by a set of well-defined, distinct streamlines along which fluid elements flow without exchanging fluid with neighboring elements. Currents 

---