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Impacts of Subway Tunnels on Goundwater Quality

Impacts of Subway Tunnels on Goundwater Quality. Mike Huffington Dan Montonye North Dakota State University. Detrimental effects of subway tunnels on groundwater levels in Seoul. Tunnels may cause significant drop of groundwater levels due to seepage of surrounding groundwater into tunnel

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Impacts of Subway Tunnels on Goundwater Quality

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  1. Impacts of Subway Tunnels on Goundwater Quality Mike Huffington Dan Montonye North Dakota State University

  2. Detrimental effects of subway tunnels on groundwater levels in Seoul • Tunnels may cause significant drop of groundwater levels due to seepage of surrounding groundwater into tunnel • Impacts both quantity and quality of the tunnel seepage water • Decreased groundwater level from 16.85 to 20.4 meters in some areas • 379 wells near tunnels were abandoned (Chae et al 2008)

  3. What is happening? Before Tunnel Water leaks IN Water Level Well Tunnel Water Collected and pumped out of tunnel Stream Pump

  4. Detrimental effects on water quality • All samples from collecting stations failed drinking water standards • Tunnel water high in Mn,Fe, and NH4+ compared to previous groundwater data from nearby areas • Some also failed turdity and color standards • Most likely result of high Fe and Mn • Most frequent problem is occurrence of pathogenic microbes (Chae et al 2008)

  5. Summary of water quality data In Comparison to Initial Data (Chae et al 2008)

  6. What is happening? • Before tunnel • After tunnel Water Level Sewer Pipe Construction Materials

  7. Continued…. Sewer Pipe Ground water and soil Tunnel Water NH4+ and organic matter Generates reducing conditions Fe and Mn dissolve

  8. Model 1 • Initial solution reacted with Fe and Mn • Used hematite (Fe2O3), geothite (FeO OH), and amorphous Fe(OH)3 for Fe minerals • Pyrulosite (MnO2), manganite (MnO OH), and amporphousMn(OH) for Mn minerals (Chae et al 2008)

  9. Results of Model 1 • Mn and Fe alone had no significant impact on water chemistry • Did not dissolve under oxidizing conditions (Chae et al 2008)

  10. Model 2 • Add sewage water to model 1 to produce reducing conditions • Simplified sewage to CH2O (6.25 mmol added) (Chae et al 2008)

  11. Model 2 Results • Electrons supplied by oxidation of organic carbon occurs preferentially via Mn reduction • Dissolved Mn and organic carbon controls the redox state of the water (Chae et al 2008)

  12. Model 3 • Effect of Varying amount of Mn solids looked at • Addition of .0001 mmol to .1 mmol • Hematite used as Fe source/pyrulosite for Mn and 6.25 mmol CH20 used (Chae et al 2008)

  13. Model 3 Results • Ph had little change • Pe decrease – because organic carbon continually provides e- to oxidize • Fe concentration decrease with increase concentration of Mn • Shows redox chemistry of water controlled by amound of Mn solids in quifer (Chae et al 2008)

  14. Model 4 • Effects of changing amount of organic carbon looked at • Vary amount from .625 mmol to 6.25 mmol • Pyrulosite kept constant at .1 mmol (Chae et al 2008)

  15. Results of Model 4 • Fe shows progressive increase – due to reductive dissolution of hematite • When organic carbon is supplied in sufficient quantities the concentration of dissolved Fe increases after the reductive consumption of Mn from all sources (Chae et al 2008)

  16. Summary of Study • The oxidation of organic carbon releases electrons that are used in the reduction of iron and maganeese bearing solids • H+ ions are also released causing the solution to become more acidic and making the reduction of Fe and Mn bearing solids even more favorable • Organic carbon • CH2O + H2O = CO2 + 4H+ + 4e− • Hematite • Fe2O3 + 6H+ = 2Fe3+ + 3H2O • Manganite • MnO∙OH + 3H+ + e− = Mn2+ + 2H2O • (Chae et al, 2008)

  17. Summary of Study • One of the controling factors for the solutions redox condition is the availability of manganese • Dissolved Mn accepts the majority of the electrons that are released via the oxidation of organic carbon • Fe only dissolves when there are enough available electrons to satisfy the Mn that is available to the system (Chae et al, 2008)

  18. Our Analyses • As the contaminated water is dumped into the river system the concentration of organic carbon is reduced, resulting in a reduction in the number of free electrons available • The free electrons that are available are accepted by the dissolved Mn, causing Fe to precipitate out of solution and hence causing the water to stain red

  19. Cont. SI Values Before and After Mixing With River Water Phase SI log IAP log KT Fe and Mn containing solids in the presence of high organic solid concentration Manganite-15.29 10.05 25.34 MnOOH Hematite -2.48 -6.49 -4.01 Fe2O3 Manganite-3.20 22.14 25.34 MnOOH Same solids after solution mixed with Mississippi River water Hematite 10.85 6.84 -4.01 Fe2O3 1:1 ratio

  20. Bacteria • Excess amounts of iron in a system result in fewer free chloride molecules • This reduction in Cl- makes the environment more suitable for bacteria to grow and survive • Excess iron also allows bacteria to conserve energy that they would otherwise be used in iron uptake (Chae et al, 2008)

  21. Questions??

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