Coastal lagoons provide important wetland and aquatic habitat and food resources. These valuable cultural and recreational sites are also home to some of the world’s most developed coastal communities. Yet they are often significantly influenced by anthropogenic activities and face many pressures such as habitat degradation, eutrophication, draining and land development. Ongoing research has shown that groundwater discharge plays important roles in coastal biogeochemical processes and water quality. However, groundwater input to coastal water bodies has frequently been discounted as an important component of water and nutrient budgets, largely because groundwater seepage is difficult to measure. This thesis addresses this gap by investigating groundwater processes in a large, nutrient-rich coastal lagoon in New Zealand: Te Waihora (Lake Ellesmere).

I first identified where discharge locations occur. Traditional conceptual models of groundwater seepage distribution place most seepage near the margins of lakes and lagoons. The first part of this thesis research set out to test the validity of this model in a geologically heterogeneous lagoon. I carried out an airborne thermal infrared imaging survey and spatial surveys (measuring 222Rn—a naturally occurring groundwater tracer and other physicochemical parameters) by boat in two seasons. The initial survey concentrated on the margins of the lagoon. I found evidence of diffuse seepage, as well as point-source seepage (i.e., springs) on mudflats and on the lagoon shore. Signs of groundwater seepage were concentrated on the northern and western sides of the lagoon. I later conducted a more comprehensive spatial survey with a high-density sampling grid to identify artesian aquifers discharging to the lagoon. However, I found no significant signs of freshwater inputs away from the lagoon shore. I did find new evidence of freshwater along the lagoon-barrier interface, which I hypothesised is either sourced from upwelling inland-sourced groundwater from underneath the lagoon or seepage from the surface aquifer on the mixed sand and gravel barrier.

Previous work based on seepage meter measurements estimated that groundwater discharge to Te Waihora was a small component of the lagoon’s water budget. I built on earlier work using a method at a broader scale than seepage meters—radon mass balances. These models showed that groundwater seepage to the lagoon was 1-2 orders of magnitude greater than previous estimates. Groundwater discharge estimates to the lagoon ranged from 5.2 ± 5.8 m3/s to 18.7 ± 19.6 m3/s during summer and 0.9 ± 2.2 m3/s to 8.1 ± 10.5 m3/s during winter. Wind-driven radon degassing to the atmosphere was the most influential variable in the model. I carried out an in-depth uncertainty analysis and found that the most sensitive parameters in the model were radon degassing, as well as lagoon surface area and volume; the radon in groundwater endmember; and the average radon concentration in the lagoon surface water.

Finally, I carried out a hydrogeochemistry survey to distinguish groundwater sources to Te Waihora and shed light on their contribution to nutrient transport to the lagoon. I analysed major ions, stable water isotopes, trace metals and nutrients in lagoon surface water, porewater (shallow nearshore groundwater), existing groundwater wells and springs. Groundwater seepage split into two groups: (1) Inland samples with low ion concentrations, dominated by Ca2+ and HCO - ions and more negative oxygen-18 and deuterium ratios, and (2) permeable barrier samples with higher ion concentrations, dominated by Na+ and Cl- ions and more positive oxygen-18 and deuterium ratios. The ion and stable isotope chemistry imply that inland seepage is sourced primarily from alpine river recharge and inland rainfall recharge, while barrier porewater is comprised of mainly mixing with lagoon surface water and localised rainfall recharge on the barrier. The study did not find evidence in the barrier porewater of freshwater inputs from upwelling artesian groundwater from under the lagoon. Analysis of porewater samples showed evidence of potential denitrification on the lagoon margins attenuating nitrogen inputs. In contrast, dissolved reactive phosphorus was elevated in porewater, suggesting phosphorus mobilisation in nearshore anoxic groundwater. Seepage-derived nitrogen inputs were only 3% of dissolved inorganic nitrogen river inputs, but the phosphorus groundwater load was 30% of river inputs.

While previous studies assumed groundwater discharge played a minor role in the hydrology and water quality of Te Waihora, the results of this thesis highlight that groundwater is an important component of both the water and nutrient budget at this site. This research underscores that groundwater discharge in coastal environments should not be discounted. These results also highlight the value in studying lagoon types that do not feature as prominently in the international literature, such as those in temperate climates with gravel barriers and managed openings to the sea.