Pollution spreading mechanisms

1. Pollution spreading mechanisms Basic question: Where will the pollutant go, how will it behave?

2. Where will the pollutant go, how will it behave? • What kind of an answer can be satisfactory for this question? • What should I know to answer this question?

3. Today’s presentation • Solute transport= the proper term for the spreading/movement of contaminant in water • Three mechanisms contribute to the spreading of contaminant in (ground)water: diffusion, advection,dispersion • Familiarity with graphical representations of the solutions of the transport equations • which is the key dependent variable* in transport problems?

4. DIFFUSION! ~7:30 contaminant spill, nonhomogeneous distribution – variable contaminant concentrations ~9:05 same contaminant concentration everywhere

5. Diffusion • Molecular phenomenon • Due to the random movement of the contaminant molecules in the solution • Modelin one dimension: random walk • for every time step Δt, a contaminant molecule may move left or right by Δx • let’s follow the movementof 500 molecules • http://ocw.mit.edu/courses/civil-and-environmental-engineering/1-061-transport-processes-in-the-environment-fall-2008/lecture-notes/1-anim/

6. What we saw Att=0, 500 particles appear at x=0 t=1 t=2 t=21 t=48

7. What we saw (cont’d) σ function of the square root of time Nepf, Heidi. 1.061 Transport Processes in the Environment, Fall 2008. (MIT OpenCourseWare, http://ocw.mit.edu/courses/civil-and-environmental-engineering/1-061-transport-processes-in-the-environment-fall-2008)

8. Diffusion: the mechanism • Movement of every molecule: random • Aggregate movement ofmany molecules: movement from areas of high concentrations to areas of low concentrations Diffusion: the results • Spreading of the contaminant mass • Dilution (reduction of high concentrations) • reduction of concentration differences

9. Diffusion in a glass of water(1/4) 7:27 clean water 7:29 contaminant spill

10. Diffusion in a glass of water(2/4) 7:29 7:30

11. Diffusion in a glass of water(3/4) 7:35 7:45

12. Diffusion in a glass of water(4/4) 8:37 9:06 Diffusion (through air) in everyday life: the perfume of the person sitting next to us, the smells from cooking that gofrom the kitchen to other rooms

13. Nature of a substance in gaseous or aqueous solution • Be careful! The solid, liquid or gas state is an emerging property that shows the relationship among the same molecules – it is not a property of each individual molecule. Nescafé (instant coffee): suitable model/analogy for an aqueous solution Greek coffee: inappropriate model/analogy for an aqueous solution

14. ADVECTION!Go with the flow, fast... https://commons.wikimedia.org/wiki/File:Rafting_26.JPG

15. ADVECTION! (cont’d)…or slow https:https://commons.wikimedia.org/wiki/File:Rogue_WSR_%2818947144518%29.jpg

16. Advection • Mechanical phenomenon • Contaminant movement with the average water velocity • Analogyfor diffusion: a ball drops in the river and gets carried away

17. Advection: the results • The contaminant moves in the direction of the movement of (ground)water flow • There is no dilution, the contaminant moves as a “parcel”, as a “rigid body”, if the water velocity has everywhere the same magnitude and direction • it is as if Ihave placed all the contaminant molecules on a raft • Be careful! An experiment with advection only is not possible in practice

18. DISPERSION! Search the internet for rubber duck derby

19. Many ducks start together... https://commons.wikimedia.org/wiki/Category:Rubber_duck_races#/media/File:The_start_of_The_Manchester_Duck_Race_2014_at_Spinningfields,_Manchester_in_aid_of_children%27s_charity_Brainwave.jpg

20. ... but what happens along the way? http://www.greenvilledailyphoto.com/wp-content/uploads/2010/05/20100502_ducks_4_900x600.jpg

21. Dispersion • Mechanical phenomenon • Advection in microscale that follows the difference in velocity from point to point (full term: mechanical dispersion) • Analogy • Many particles get dropped at the same time in a river (not just a ball, like in advection)

22. Dispersion: the results • Whereas the center of contaminant mass moves at the average water velocity, some molecules move faster, some slower • With time, differences increase (like in diffusion) • Same contaminant mass in bigger area dilution (like in diffusion) • Because the phenomena of diffusion and dispersion produce similar results, in transport equations they are described with a common term

23. Question for understanding No 1 11:36 contaminant spill 11:36+ uniform concentration distribution“in timedt”! What did we achieve with the spoon... Attention! Significant difference between the movement in water we stir with a spoon and the movement of groundwater that typically moves very slowly with laminar flow.

24. The results of three mechanisms together: instantaneous release at the source • http://ocw.mit.edu/courses/civil-and-environmental-engineering/1-061-transport-processes-in-the-environment-fall-2008/lecture-notes/5-anim/

25. Αποτελέσματα τριών μηχανισμών μαζί: πηγή σταθερής συγκέντρωσης • http://ocw.mit.edu/courses/civil-and-environmental-engineering/1-061-transport-processes-in-the-environment-fall-2008/lecture-notes/6-anim/ σταθερή ταχύτητα σταθερός συντελεστής διάχυσης-διασποράς

26. Question for understanding Νο 2 • Thought experiment. Consider the picture in the video of the previous slide showing the spreading of the contaminant. How would it be different without diffusion and dispersion? Complete the concentration-time graph for each one of the points Α, Β και C.

27. Which difference would you pick to describe as the most significant at Α? Instantaneous release of contaminant mass at the source (example?) Constant contaminant concentration at the source (example?)

28. Relative contribution of phenomena, source of constant concentration (1/4) http://hydrolab.illinois.edu/gw_applets//?q=gw_applets/ I start the simulation with default τιμές, I see what is happening with time at a particular distance from the source (blue line)

29. Relative contribution of phenomena, source of constant concentration (2/4) I reduce diffusion-dispersion by a factor of 10 (red)... http://hydrolab.illinois.edu/gw_applets//?q=gw_applets/

30. Relative contribution of phenomena, source of constant concentration(3/4) ... I reduce diffusion-dispersion by a factor of 100 (green)... http://hydrolab.illinois.edu/gw_applets//?q=gw_applets/

31. Relative contribution of phenomena, source of constant concentration (4/4) ... I reduce diffusion-dispersion by another factor of 100 (yellow)... http://hydrolab.illinois.edu/gw_applets//?q=gw_applets/

32. What did I achieve with the yellow line? http://hydrolab.illinois.edu/gw_applets//?q=gw_applets/ To perform a numerical experiment of transport practically with advection as the only transport mechanism!

33. The effect of time Concentration profiles at different distances from the contaminant source 10m 50 m 100m 250m http://hydrolab.illinois.edu/gw_applets//?q=gw_applets/

34. Συμπεράσματα για την επίδραση της ροής στην μεταφορά ρύπων • The contaminant front (constant concentration source) or the center of contaminant mass (instantaneous release & source of finite duration) will move at the average velocity of water = advection • Advection dives a satisfactory approximation of transport: • when the contribution ofdiffusion-dispersion is small • when the we are interested in transport at short distances and short times

35. Sources of figures Note: the non-original material used in this presentation either it has been uploaded on the internet with a Creative Commons license, or it is publicly available on the internet (e.g. US policy for reports etc. by public agencies) and can be used with suitable attribution, or is used with license from the publisher. Slides 6, 7, 24, 25, 27.Nepf, Heidi. 1.061 Transport Processes in the Environment, Fall 2008. (MIT OpenCourseWare: Massachusetts Institute of Technology), http://ocw.mit.edu/courses/civil-and-environmental-engineering/1-061-transport-processes-in-the-environment-fall-2008 (Ημερομηνία πρόσβασης 16 Oct., 2015). License: Creative Commons BY-NC-SA Slide 14. Waynestock (Own work), License: Creative Commons BY-SA 4.0, https://commons.wikimedia.org/wiki/File:Rafting_26.JPG Slide 15. Bureau of Land Management (Rogue WSR), License: Creative Commons BY 2.0, https://commons.wikimedia.org/wiki/File:Rogue_WSR_%2818947144518%29.jpg Slide 19. Billh35 (Own work), License: Creative Commons BY-SA 3.0, https://commons.wikimedia.org/wiki/Category:Rubber_duck_races#/media/File:The_start_of_The_Manchester_Duck_Race_2014_at_Spinningfields,_Manchester_in_aid_of_children%27s_charity_Brainwave.jpg Slide 20. Used by permission from the photographerDenton Harryman, http://www.greenvilledailyphoto.com/wp-content/uploads/2010/05/20100502_ducks_4_900x600.jpg Slides 28-33.Results from the educational applets developed byValocchi, A. J., C. J. Werth, J. J. Decker, G. Hammond, P. Zhou & M. Hafiz, 2014, Interactive Models for Groundwater Flow and Solute Transport, http://hydrolab.illinois.edu/gw_applets//?q=gw_applets/ (AccessedOctober 16, 2015).