NITROGEN BIOGEOCHEMISTRY OF SUBMARINE GROUNDWATER DISCHARGE: A SYNTHESIS

Coastal groundwater discharge (including submarine groundwater discharge—SGD) is often an important and even dominant pathway for nitrogen (N) transport from land to sea, and plays an important role in estuarine eutrophication. In addition to fresh groundwater, SGD commonly includes substantial quantities of saline groundwater, carrying N with a marine sediment source, and brackish groundwater produced by mixing of fresh and saline groundwater. There is therefore a need to understand processes controlling: N loads carried by fresh groundwater; N loads carried by saline groundwater; and biogeochemical transformations in discharge zones that alter the N load carried to sea. Many SGD zones share some common features in terms of salinity structure, and it is useful to discuss processes in the context of three aquifer zones: a fresh groundwater zone, a shallow salinity transition zone (STZ), and a deep STZ. Careful examination and clear separation of water and N sources within SGD zones is critical because N concentrations and discharge rates of marine and terrestrial groundwater commonly differ, and because only the terrestrial source represents new N loaded to coastal waters related to human activity on watersheds. N concentrations and loads in fresh groundwater are largely controlled by watershed land uses, but there is evidence that biogeochemical conditions and N speciation may be more influenced by watershed geological setting rather than by land uses. Saline groundwater in the deep STZ is commonly chemically reducing due to recharge through estuarine sediments. Within the nearshore aquifer, N concentrations and biogeochemical conditions are often highly variable, and show particularly steep vertical gradients. The vertical gradients are maintained in part by limited vertical dispersion within aquifers, but relatively rapid microbial transformations (such as denitrification) in mixing zones may also help to maintain the gradients. The variability and vertical gradients necessitate high spatial resolution sampling to observe the range of concentrations and to capture such “hotspots” of biogeochemical activity. In this review of N cycling in SGD zones, we will draw on our own published and unpublished data, as well as the published works of others to synthesize the current state of knowledge.