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Submarine groundwater discharge and associated nutrient and carbon inputs into Sydney Harbour (Australia)
Journal article   Peer reviewed

Submarine groundwater discharge and associated nutrient and carbon inputs into Sydney Harbour (Australia)

Rogger E Correa, Douglas R Tait, Christian J Sanders, Stephen R Conrad, Daniel Harrison, Michael J Reading, James P Tucker and Isaac R Santos
Journal of hydrology (Amsterdam), Vol.580, p.124262
01/2020
DOI: https://doi.org/10.1016/j.jhydrol.2019.124262

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Abstract

alkaline earth metals aquifers Australasia Australia carbon coastal aquifers concentration discharge embayments estuaries fresh water ground water Hydrogeology isotopes mass balance metals New South Wales Australia nutrients Oceania pollution pore water Ra-223 Ra-224 Ra-226 radioactive isotopes radium sea water Sydney Australia Sydney Harbour urban environment urbanization water pollution Isotope geochemistry
The effects of urbanization and scales on submarine groundwater discharge (SGD) remain poorly understood. Here, we used radium isotopes to quantify SGD-derived fluxes of nitrogen, phosphorus and dissolved organic carbon (DOC) into Sydney Harbour estuary, Australia. Sydney is the most populated city of Oceania, with several localised cases of historical groundwater pollution. We sampled top and bottom waters at the harbour scale ( approximately 20 km) and also at four small scale embayments ( approximately 2 km). A decreasing gradient in radium isotope concentrations from upstream to downstream was observed. Mass balances constructed with (super 224) Ra, (super 223) Ra, and (super 226) Ra revealed that total SGD ranged from 42 to 121X10 (super 4) m (super 3) d (super -1) depending on assumptions and isotope. These fluxes were related mostly to saline SGD (recirculated seawater), and are equivalent to >60 times the mean annual freshwater river discharge into Sydney Harbour (0.68X10 (super 4) m (super 3) d (super -1) ). The estimated SGD rates (2.2+ or -1.5 cm d (super -1) ) were comparable to the global average radium-derived-SGD in other urban estuaries ( approximately 3.1 cm d (super -1) ). No obvious relationships were observed between SGD and scale in Sydney Harbour. However, higher SGD rates estimated for embayments closer to the ocean indicate that a combination of waves, tides and urbanization control SGD. SGD derived fluxes exceeded maximum riverine nutrient fluxes by a factor of 2 for DOC, 6 for PO (sub 4) (super 3-) , 40 for NH (sub 4) (super +) and 1.3 for NO (sub X) . Previous work has suggested that nutrients enter Sydney Harbour primarily through rivers or stormwater following episodic rain events. Our results imply that diffuse saline SGD can also be an important but overlooked source of nutrients, potentially sustaining primary productivity in times of no river flow.

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