Impacts of Subway Tunnels on Goundwater QualityMike Huffington Dan MontonyeNorth 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 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)

What is happening?BEFORE TUNNEL WATER LEAKS IN

Water Level Well

Tunnel

Pump

Stream

Water Collected and pumped out of tunnel

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)

Summary of water quality data

In Comparison to Initial Data

(Chae et al 2008)

What is happening?

Before tunnel After tunnel

Sewer Pipe

Water Level

Construction Materials

Continued….

Sewer Pipe

NH4+ and organic matter

Ground water and soil

Generates reducing conditions

Tunnel Water

Fe and Mn dissolve

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

amporphous Mn(OH) for Mn minerals

(Chae et al 2008)

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)

Model 2

Add sewage water to model 1 to produce reducing conditions

Simplified sewage to CH2O (6.25 mmol added)

(Chae et al 2008)

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)

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)

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)

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)

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)

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)

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)

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

1 2 4 70

50

100

150

200

Fe Precipitation vs Organic Carbon Concentration

Organic Carbon (mmol)Perc

ipit

ate

d F

e (

um

ol L-1

)

Cont.

SI Values Before and After Mixing With River Water

Phase SI log IAP log KT

Manganite -15.29 10.05 25.34 MnOOH

Hematite -2.48 -6.49 -4.01 Fe2O3

Manganite -3.20 22.14 25.34 MnOOH

Hematite 10.85 6.84 -4.01 Fe2O3

1:1 ratio

Fe and Mn containing solids in the presence of high organic solid concentration

Same solids after solution mixed with Mississippi River water

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)

Questions??