DENITRIFICATION BELOW THE ROOT ZONE

Vibeke ErnstsenGeological Survey of Denmark and

Greenland (GEUS)

Plantekongres 2006, Herning, 10. januar 2006

Content:

• Nitrate reduction and the aquatic environment

• The processes

• The unsaturated zone - distribution, and potential for reduction

• Saturated zone - potentiale for reduction

• The water flowpattern and distribution of nitrate

Reduction by organic matter: 5 C + 4 NO3

- + 2 H2O 2 N2 + 4 HCO3- + CO2

Reduction by pyrite: 5 FeS2 + 14 NO3

- + 4 H+ 7 N2 + 10 SO42- + 5 Fe2+ + 2 H2O

Reduction by ferrous iron - N2: 5 Fe2+ + NO3

- + 12 H2O 5 Fe(OH)3 + 0.5 N2 + 9 H+

Reduction by ferrous iron - NH4: 8 Fe2+ + NO3

- + 21 H2O 8 Fe(OH)3 + NH4+ + 14 H+

Reduction med methane: 5 CH4 + 8 NO3

- + 3 H+ 4 N2 + 5 HCO3- + 9 H2O

Nitratereduction - possible processes

• biological or abiotic process - anoxic environment

Reduce compounds:

• bioavailable organic matter• reduced sulfur (pyrite & hydrogen sulphide)• available ferrous iron (exchangeable/structural)• manganese• methane• ?

SOME ARE USED UP - OTHER ARE RENEWABLE

Nitratreduction - unsaturated zone

• biological or abiotic process - anoxic environment• potential of nitrate reduction relates to the regeneration of reduced compounds - the inherited pools are used up

Vegetation - renewable source of organic matter• leaching from the surface• in the root zone - roots etc.• fauna and flora incl. bacteria

Organic matter retension:• sorption• compleks-binding • used up• precipitate• etc.

Maximum 3 meters

0

5

10

15

0 2 4

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBV

0

5

10

15

0 2 4 6

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBVI

0

5

10

15

0 2 4 6

Nitrate

(mg NO3-N kg-1)

De

pth

(m

)

SBIII

0

5

10

15

0 5 10

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBIV

0

5

10

15

0 1 2

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBXII

0

5

10

15

0 2 4

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBXI

0

5

10

15

0 1 2

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBX

0

5

10

15

0 2 4

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBIX

0

5

10

15

0 8 16

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBVII

0

5

10

15

0 8 16

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBVIII

0

5

10

15

0 8 16

Nitrate (mg NO3-N kg-1)

Dep

th (

m)

SBI

Ramsømagle

Ramsølille

Viby

SBISBIII

SBVIII

SBIX

SBIV

SBVI

SBX

SBXI

SBV

SBXIISBVII

Nitrate reduction - below the usaturated zone

15

20

25

30

0 0,1 0,2

Alkalinity

(meq L-1)

De

pth

(m)

15

20

25

30

0 50 100

Nitrate

(mg NO3 L-1)

De

pth

(m)

15

20

25

30

0 50 100

Sulfate

(mg SO4 L-1)

De

pth

(m)

15

20

25

30

0 5 10

Ferrojus iron

(mg Fe L-1)

De

pth

(m)

15

20

25

30

4 5 6 7

pHD

ep

th (

m)

Kristiansen et al., 1991

GWT

Reduction in anoxic environments by the inherited reduced compundsat different depths. Considerable differences related to geology.

Clayey samples: 15 % clay• Organic matter: 0,05 % C• Pyrite: 0,003 % S• Ferroous iron: 0,9 % Fe

Capacity: 90-110 mol nitrate pr. m3

Pr. meter: 250 - 300 years

Sandy samples: 1-2 % clay• Organic matter: 0,03 % C• Pyrite: 0,005 % S• Ferrous iron: 0 % Fe

Capacity: 30 mol nitrate pr. m3

Pr. meter: 95 years

Clayey samples: 30-40 % clay• Organic matter: 0,10 % C• Pyrite: 0,005 % S• Ferroous iron: 1,2 % Fe

Capacity: 150 -180 mol nitrate pr. m3

Pr. meter: 420 - 500 years

Sandy samples: 1-2 % clay• Organic matter: 0,15 % C• Pyrite: 0,012 % S• Ferrous iron: 0 % Fe

Capacity: 160 mol nitrate pr. m3

Pr. meter: 465 years

Calculation of redox capacity - exsamples

THIS KIND OF CALCULATION DOES NOT TAKEINTO ACCOUNT THE REACTIVITY OF THE COMPOUNDS

LITERATURE:

• Miljøstyrelsen. 2001. Arbejdsrapport nr. 24 • Miljøstyrelsen. 2005. Miljøprojekt nr. 1023, 1024 og 1025

• Ernstsen, V., H.J. Henriksen og F. von Platen. 2001. Principper for beregning af nitratreduktion i jordlagene under rodzonen. Arbejdsrapport nr. 24.

• Ernstsen, V. 2005. Nitratreduktion i den umættede zone. Miljøprojekt 1023.

• Ernstsen, V., Jørgensen, N., og Lynge, C.R. 2005. Metode til analyse af reducerende stoffer i sedimenter. Miljøprojekt 1024.

• Ernstsen, V. 2005. Undersøgelse af reaktiviteten af reducerende stoffer i nogle

danske sedimenter - et pilotstudie. Miljøprojekt 1025.