Department of Science, Information Technology, Innovation and the Arts Application of geochemical...

Department of Science, Information Technology, Innovation and the Arts

Application of geochemical modelling to assess groundwater discharge of nitrogen from the lower Burdekin coastal plain aquifer

Matthew LenahanKiran Bajracharya, Keith L. Bristow (CSIRO)


Burdekin Floodplain Aquifer Burdekin River

Nitrogen discharge (tonnes/yr) 0 > 5,500 2,200


Nitrogen attenuation reactions

– Reducing agents• Organic Carbon• Mn2+

• Fe2+

• H2S

–Oxidizing agents• O2

• NO3-

• MnO2

• FeO(OH)• SO4

2-

4223 NHNNONO

• Oxidation is the loss of electrons

• Reduction is the gain of electrons

• Example of nitrogen reduction (gaining electrons)

Nitrate Nitrogen gas


Objective:

• assess current nutrient attenuation capacity• determine geochemical conditions in coastal and riparian zones

• delineate zones of high and low nutrient mobility

• apply geochemistry to modelling of flow and transport

• identify knowledge gaps needed to quantify aquifer discharge of nutrients


Groundwater Sampling (August-September)

Group n

Coastal Nested Sites 48

Riparian Zone 10

Floodplain 4

Barratta Creek 1

Burdekin River 1

Haughton River 1

Analyses:

Salinity, Nutrients, Metals, Oxygen, CO2, pH


Piezometer Installation:

Barratta: 4 bores

Haughton: 5 bores

Burdekin 4 bores


Riparian Zone Geochemistry


Barratta Creek

Burdekin & HaughtonRivers Riparian Transects

Electron donors increase toward channel

Electron receivers decrease toward channel

High potential nitrogen attenuation


K

NOFeHN

mmmknr ONOFetwFe

5/12

5/910/12

2 123

23

eqOHOFetwFe K

OFeH

ammknr4/1

22

2

2 12

23

Fe2+ + 0.25O2(aq) + 1.5H2O FeOOH + 2H+

Fe2+ + 0.2*NO3- + 1.4*H2O -> FeOOH + 0.1*N2(aq) + 1.8*H+


Riparian Nitrogen Transport

transport + mixing + reactions

Assumptions

Geochem:•riparian chem constant•riparian reactants constant

• reactants replaced faster than consumed

T-port:•flow rate constant •transport rate constant


Assumptions

•Geochem conditions

Constant:•specific discharge = 10 m / yr•porosity = 30%•dispersivity = 25 cm•conservative t-port ~ 35 m / yr

iixi

L

i RCvxx

CDxt

C

)(


SummaryHigh Nutrient Attenuation Potential

Nutrient discharge form lower Burdekin• STE potentially ammonium (non-anthropogenic)• riparian zone potentially nitrate

Nutrient dynamics dependant on geochem + transport conditions• marine discharge possibly insignificant

o preferential flow paths unknown• riparian discharge uncertain

Attenuation uncertainty• reaction rates

• temporal variation in geochem conditions?• variation in attenuating reactant supplies?

• flow rates• temporal variation in hydraulic gradient underway• unknown aquifer properties (e.g. K, )


Acknowledgements

NWCDr Alistair Usher

Qld GovMaria HarrisBob BennettAmy BeckeWendy Miller Ray McGowan

CSIRORex KeenJoseph Kemei


Subterranean Estuary• no detectable NO3

- (or PO43-)

• abundance of electron donors

Fe2+ and DOC shallow/fresh

Mn2+ deep/saline

• elevated NH4+

ImplicationsHigh nutrient attenuation capacity

NO3- loss through STE unlikely

NH4+ loss possible

Palaeochannels by-pass STE?


Subterranean Estuary

Nutrients:

• low NO3- (no PO4

3-) shallow/fresh

• no NO3- (or PO4

3-) deep/saline

• elevated NH4+ in deep/saline

• salinity controls on nitrogen?

Electron Donors• elevated Fe2+ and Mn2+

• Fe2+ in shallow/fresh• Mn2+ in deep/saline• salinity controls on Fe + Mn?

Implications

High nutrient attenuation capacity

NO3- loss through STE unlikely

NH4+ loss possible

Palaeochannels by-pass STE?


Phosphate Attenuation

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