Although nutrients are a fundamental component of living organisms, excessive concentrations of nutrients in marine ecosystems can result in a range of negative consequences. Carbonate sand-dominated islands, like those found throughout the South Pacific, may be particularly susceptible to the nutrient over-enrichment of their surrounding waters due to the high porosity of carbonate sands and the perceived lack of biological activity in the sediments. To contribute to closing the knowledge gap of nutrient dynamics in carbonate sand systems, this thesis examines the movement, biogeochemistry and mitigation of nutrients in a coral sand lagoon system.
In order to then determine the flow of nutrient into the surrounding lagoon, the flux of submarine groundwater discharge (SGD) was determined using three different but commonly used radon mass-balance models. There was an approximately eight-fold difference in the SGD flux rates calculated by the models. The study highlighted, through the use of time series, survey and electric resistivity techniques, that the applicability of each model is based on the hydrological and geological features of the study area.
To determine the contribution of groundwater derived nutrient to the surrounding lagoon, a nutrient mass balance was then constructed. The mass balance accounted for 92% of dissolved nitrogen (N) leaving the lagoon, and for approximately 36% of dissolved phosphorus (P) inputs. SGD accounted for 81% of dissolved inorganic N (DIN) inputs and 47% of DIN exported form the lagoon. Analysis of tritium concentrations showed that old, deep groundwater (10 – 93 years old) was the main source of SGD derived nutrients to the lagoon indicating there may be a large time lag between nutrient infiltration on land and discharge into the receiving waters.
A series of long term column experiments showed that the TN removal rate in the wastewater-saturated treatments was approximately six times higher than the unsaturated treatments. The addition of a carbon source and biochar facilitated a drop in both nutrient ii and greenhouse gas concentrations. Sampling of groundwater below an in situ wastewater treatment system showed an almost complete removal of TN and TP within two meters of the aquifer surface, even with limited bioavailable carbon to facilitate denitrification. Modelling of NO3 - and N2 concentrations for an idealized treatment system indicated reduced NO3 - concentrations may be due to a combination of dilution with high volumes of groundwater and denitrification.
Lastly, in order to mitigate groundwater nutrient flow into the lagoon, a low cost, low technology wastewater treatment system adapted to the constraints of South Pacific islands was developed and monitored. The system removed 63% of total P and 43% of total N over 17 months of monitoring. The system showed complete attenuation of NH4 + as wastewater moved through the system, and simultaneous nitrification/denitrification in both aerobic and anaerobic zones.
Overall, the study endeavoured to not only make a significant contribution to the knowledge of nutrient cycling in carbonate sand system, but also provide valuable understanding of how nutrient moves and is processed in these environments so that community leaders can make informed decisions.