Resolving groundwater sources to a coastal lagoon using major ions, nutrients and stable isotopes

Coastal lagoons are important for ecological, cultural, economic and recreational reasons. Globally, they are subject to significant anthropogenic pressures. Our understanding of the importance of groundwater discharge into coastal lagoons for water and solute budgets is evolving, yet key gaps remain. This study resolves sources of groundwater seepage and estimates nutrient loads from direct groundwater discharge into a large hypertrophic coastal lagoon in New Zealand. We analysed major ions, stable water isotopes and nutrients in lagoon surface water, porewater, groundwater wells and springs. Groundwater and porewater samples split into two distinct groups: (1) inland samples that were MgHCO3 dominated with more negative δ2H:δ18O ratios and lower ion concentrations, and (2) permeable barrier samples that were NaCl dominated with more positive δ2H:δ18O ratios and higher ion concentrations. Porewater entering the lagoon is sourced from alpine river and rainfall recharge on the plains. Barrier porewater appears to be sourced from infiltration from the lagoon through the barrier and local rainfall. Despite higher nitrate in deeper groundwater wells, low nitrate in shallow porewater indicates potential denitrification before groundwater discharges to the lagoon. Our observations support efforts to restore and construct wetlands around the lagoon to remove nutrients. However, wetland restoration will need to be carried out by maintaining a balance between enhancing denitrifying conditions while preventing phosphorus release from sediments. Nutrient load calculations revealed that direct groundwater seepage to the lagoon provides ~ 3% of dissolved inorganic nitrogen and ~ 30% of dissolved reactive phosphorus compared to river inputs, indicating that groundwater discharge may play an important role in phosphorus transport to the lagoon.