The Great Barrier Reef (GBR) is the world’s most extensive coral reef ecosystem and has critical environmental and economic importance. However, the GBR catchment is one of the most intensively cultivated agricultural areas in Australia. This has led to elevated concentrations of nutrients flowing to the Reef, resulting in the degradation of this vulnerable environment and a shift from coral- to macroalgae-dominated communities. (1−3) Previous investigations have revealed long-term increases in river nitrogen inputs to GBR coastal waters. (4,5) As a result, mitigating surface water nutrient inputs has been a major goal with other potential sources of nutrients largely ignored. However, river-derived nutrients were previously reported to explain only 13–30% of the nitrogen (N) and 2–5% of the phosphorus (P) necessary to support primary productivity in the GBR, (6) suggesting a possible unaccounted source of nutrients. More recent assessments have attempted to balance the GBR nutrient budget by attributing the large proportion of unaccounted nutrients to remineralization, but this is yet to be empirically quantified. (7)

Submarine groundwater discharge (SGD) is defined as the flow of any water at coastal margins from the seabed to the ocean. It often consists of both nearshore groundwater discharge (both fresh and recirculated seaward of the intertidal area) and shelf porewater exchange (PEX) in shelf sediments. (8−10) On a global scale, SGD nutrient inputs to the coastal ocean may be comparable to input from the world’s rivers. (11) Although fresh groundwater discharge is considered a minor contributor to oceanic freshwater inputs globally, (12) in coral reef systems, total SGD can be a major source of nutrients through processes like tidal pumping, (13) recirculated seawater, (14) and paleochannels. (15,16) Since nutrient concentrations in groundwater and porewater can be many times higher than those of surface waters, even relatively small volumetric contributions can potentially deliver large quantities of nutrients (17) which are often unaccounted for in traditional budgets. Importantly, because there can be a potential lag of decades between excess nutrients reaching groundwater and their discharge to coastal waters, (18) the problem can go unrecognized until long after effective management strategies could have been implemented. (16) To date, there are no regional-scale estimates of SGD and associated nutrient fluxes into the GBR. The few SGD studies that have taken place in the GBR have been qualitative or focused on local scales. For example, regional-scale radon maps along the GBR coast revealed potential SGD hotspots (19,20) and local-scale radon time series observations were used to quantify local SGD flows in reef lagoons. (13)

Radioactive tracer techniques have increasingly been used to quantify SGD rates at large scales, including in the Yellow Sea, (21) the Atlantic Ocean, (22) global oceans, (23) and coral reef systems around the globe. (14) Radium is an ideal tracer of SGD at regional shelf scales because it is highly enriched in any brackish water in contact with sediments, largely nonreactive in seawater, and its isotopic half-lives (3.7–1600 years) can provide insight into processes taking place on a range of time scales. (24) Radium has previously been used in the GBR to calculate cross-shelf mixing... (25)

...Steep declines away from the coast suggest a significant coastal source of radium isotopes (Figure 2). This is also supported by the steeper decline in ²²⁴Ra concentrations compared to ²²³Ra. With the half-life of ²²⁴Ra (3.63 days) being about a third of that of ²²³Ra (11.43 days), ²²⁴Ra concentrations would decrease much faster than 223Ra once removed from the source. Concentrations of ²²³Ra and ²²⁴Ra were elevated even at the transects’ seaward extent, indicating active benthic PEX on the continental shelf. Where vertical mixing is slow relative to radioactive decay, higher radium concentrations in samples from the bottom of the water column may also indicate benthic PEX. (39) However, there was no evidence of stratification in salinity samples nor a clear trend between the top and bottom radium samples (Figure 2), suggesting a well-mixed water column likely driven by mixing in the relatively shallow GBR waters or from the strong East Australian Current, which forms large cyclonic eddies along most of the length of the GBR. (40)...