Submarine Groundwater Discharge is a Source of Contaminants of Emerging Concern to the Coastal Ocean (Sydney, Australia)

Coastal water pollution derived from sewage, industrial, or agricultural sources pose problems to most urban areas globally. While this is commonly associated with surface runoff, submarine groundwater discharge (SGD) offers another potential pathway for pollutants to reach the coastal ocean. SGD has previously been established as a major source of nutrient pollution in many coastal areas, yet it has not been extensively studied as a source of contaminants of emerging concern (CECs) such as pharmaceuticals, industrial chemicals, pesticides, and other anthropogenic compounds. We investigated SGD as a source of CECs in two tidally dominated estuaries in Sydney Harbour, Australia, a highly urbanized area with suspected leaking sewer pipelines. This study had two primary goals: (1) to investigate SGD dynamics using radium isotopes, (2) to quantify CEC concentrations in both SGD and coastal surface water to assess SGD-derived fluxes into the coastal ocean. We sampled shore-perpendicular transects for radium, CECs, dissolved nutrients, and dissolved organic carbon during a perigean spring tide. Radium-based water residence times ranged from less than 4 days to greater than 10 days. Of the CECs studied, 94% of samples (n = 48) had concentrations greater than the detection limit for carbamazepine (10 ng/L) and caffeine (50 ng/L), with concentrations ranging up to 325 and 3,200 ng/L, respectively. The antibiotic group fluoroquinolones were also detected in some locations, but all below the quantification limit. The detection of these CECs implies that SGD is a source of wastewater to the estuary. Overall, we compare the impacts of tidal influence and hydrogeology on coastal water quality downstream of urban land-use and offer insight into how SGD can act as a vector for wastewater to reach coastal waters in areas with aging or insufficient wastewater infrastructure.