When it comes to oceanic pollution, underwater seepage from coastal aquifers trumps runoff from rivers, a new study says. Coastal aquifers are pockets of permeable earth—gravel, sand, or silt—that trap water. Much like rivers, these reservoirs absorb ground contaminants, such as fertilizer, subterranean carbon, and metals. These chemicals can subsequently leak from the seabed into the ocean, through a process known as submarine groundwater discharge (SGD), but the scale of this seepage remains in question.
Kwon et al. created a mathematical model to estimate SGD of radium-228, a radioactive isotope that serves as a readout for terrestrial leaching. Prior models made indirect estimates of SGD by subtracting the recorded levels of radium-228 deposited by rivers, dust blown from land, and diffusive fluxes from coastal sediments from the steady state of radium-228 found in marine water. This earlier method is more accurate near shore, according to the authors, but becomes problematic for open seas, where accurate recordings at depth are sparse.
To circumvent this issue, the team posited a new model to get accurate recordings at depth based on tactics used to estimate the global mixing of ocean acidification. When combined with radium-228 data, this circulation model inferred SGD for the global ocean, excluding the polar seas because of insufficient available recordings of isotopes.
They estimate that SGD from coastal aquifers contributes 3 to 4 times more terrestrial contaminants to the ocean than river water. Also, the bulk—70%—of SGD occurs in the Indian and Pacific oceans, suggesting these regions are more susceptible to natural and man-made pollutants that are stored in coastal aquifers. The findings may also inform future research and policy decisions with regard to biogeochemical changes, pollution cycles, and ecosystem dynamics. (Geophysical Research Letters, doi:10.1002/2014GL061574, 2014)
—Nsikan Akpan, Freelance Writer
Citation: Akpan, N. (2015), Aquifers spew more pollution into oceans than rivers, Eos, 96, doi:10.1029/2015EO027151. Published on 31 March 2015.