department of science, information technology, innovation and the arts application of geochemical...
DESCRIPTION
Burdekin Floodplain AquiferBurdekin River Nitrogen discharge (tonnes/yr)0 > 5,5002,200TRANSCRIPT
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)
Department of Science, Information Technology, Innovation and the Arts
Department of Science, Information Technology, Innovation and the Arts
Burdekin Floodplain Aquifer Burdekin River
Nitrogen discharge (tonnes/yr) 0 > 5,500 2,200
Department of Science, Information Technology, Innovation and the Arts
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
Department of Science, Information Technology, Innovation and the Arts
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
Department of Science, Information Technology, Innovation and the Arts
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
Department of Science, Information Technology, Innovation and the Arts
Department of Science, Information Technology, Innovation and the Arts
Piezometer Installation:
Barratta: 4 bores
Haughton: 5 bores
Burdekin 4 bores
Department of Science, Information Technology, Innovation and the Arts
Riparian Zone Geochemistry
Department of Science, Information Technology, Innovation and the Arts
Barratta Creek
Burdekin & HaughtonRivers Riparian Transects
Electron donors increase toward channel
Electron receivers decrease toward channel
High potential nitrogen attenuation
Department of Science, Information Technology, Innovation and the Arts
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+
Department of Science, Information Technology, Innovation and the Arts
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
Department of Science, Information Technology, Innovation and the Arts
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
)(
Department of Science, Information Technology, Innovation and the Arts
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, )
Department of Science, Information Technology, Innovation and the Arts
Acknowledgements
NWCDr Alistair Usher
Qld GovMaria HarrisBob BennettAmy BeckeWendy Miller Ray McGowan
CSIRORex KeenJoseph Kemei
Department of Science, Information Technology, Innovation and the Arts
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?
Department of Science, Information Technology, Innovation and the Arts
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?
Department of Science, Information Technology, Innovation and the Arts
Phosphate Attenuation