1. 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 Lenahan Kiran Bajracharya, Keith L. Bristow (CSIRO)

2. Department of Science, Information Technology, Innovation and the Arts

3. Burdekin Floodplain AquiferBurdekin River Nitrogen discharge (tonnes/yr)0 > 5,5002,200

4. Department of Science, Information Technology, Innovation and the Arts Nitrogen attenuation reactions –Reducing agents Organic Carbon Mn 2+ Fe 2+ H 2 S –Oxidizing agents O 2 NO 3 - MnO 2 FeO(OH) SO 4 2- Oxidation is the loss of electrons Reduction is the gain of electrons Example of nitrogen reduction (gaining electrons) Nitrate Nitrogen gas

5. 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

6. Department of Science, Information Technology, Innovation and the Arts Groundwater Sampling (August-September) Groupn Coastal Nested Sites48 Riparian Zone10 Floodplain4 Barratta Creek1 Burdekin River1 Haughton River1 Analyses: Salinity, Nutrients, Metals, Oxygen, CO 2, pH

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8. Piezometer Installation: Barratta: 4 bores Haughton: 5 bores Burdekin 4 bores

9. Department of Science, Information Technology, Innovation and the Arts Riparian Zone Geochemistry

10. Department of Science, Information Technology, Innovation and the Arts Barratta Creek Burdekin & Haughton Rivers Riparian Transects Electron donors increase toward channel Electron receivers decrease toward channel High potential nitrogen attenuation

11. Department of Science, Information Technology, Innovation and the Arts Fe 2+ + 0.25O 2(aq) + 1.5H 2 O  FeOOH + 2H + Fe 2+ + 0.2*NO 3 - + 1.4*H 2 O -> FeOOH + 0.1*N 2 (aq) + 1.8*H +

12. 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

13. 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

14. Department of Science, Information Technology, Innovation and the Arts Summary High Nutrient Attenuation Potential Nutrient discharge form lower Burdekin STE potentially ammonium (non-anthropogenic) riparian zone potentially nitrate Nutrient dynamics dependant on geochem + transport condit ions 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,  )

15. Department of Science, Information Technology, Innovation and the Arts Acknowledgements NWC Dr Alistair Usher Qld Gov Maria Harris Bob Bennett Amy Becke Wendy Miller Ray McGowan CSIRO Rex Keen Joseph Kemei

16. Department of Science, Information Technology, Innovation and the Arts Subterranean Estuary no detectable NO 3 - (or PO 4 3- ) abundance of electron donors Fe 2+ and DOC shallow/fresh Mn 2+ deep/saline elevated NH 4 + Implications High nutrient attenuation capacity NO 3 - loss through STE unlikely NH 4 + loss possible Palaeochannels by-pass STE?

17. Department of Science, Information Technology, Innovation and the Arts Subterranean Estuary Nutrients: low NO 3 - (no PO 4 3- ) shallow/fresh no NO 3 - (or PO 4 3- ) deep/saline elevated NH 4 + in deep/saline salinity controls on nitrogen? Electron Donors elevated Fe 2+ and Mn 2+ Fe 2+ in shallow/fresh Mn 2+ in deep/saline salinity controls on Fe + Mn? Implications High nutrient attenuation capacity NO 3 - loss through STE unlikely NH 4 + loss possible Palaeochannels by-pass STE?

18. Department of Science, Information Technology, Innovation and the Arts Phosphate Attenuation