Sea-level rise (SLR) is the greatest future threat that intact Pacific island mangroves face. The loss of mangroves on Pacific Islands will have huge consequences on human populations that live on these islands as they rely heavily on mangroves for food, fiber, and fuel. Mangroves also protect human lives and coastal infrastructure from typhoons and tsunamis as well as flooding from king tides. The large amounts of carbon stored in mangrove sediments provide an important nature-based solution for climate change mitigation and adaptation. Mangroves have survived past increases in SLR by maintaining their forest floor elevation relative to sea level through root growth and sediment accumulation. Will mangroves continue to keep up with increased SLR rates predicted to occur over the next century and in the presence of human activities (e.g., deforestation, altered hydrology) that impede mangroves’ ability to pace SLR? Working together to monitor mangrove responses to rising seas in existing and new sites across PACMAN will provide this information, which can then be used by resource managers to more effectively conserve or restore mangroves that are resilient to SLR. More resilient mangroves can then continue to provide the goods and services vital for Pacific Islander existence.

While sea-level rise (SLR) is frequently associated with surficial flooding and beach erosion, its impact to groundwater is also important. SLR can lead to rising groundwater levels through groundwater inundation, aquifer salinization, and poor coastal water quality. In this study, we investigated the potential for SLR-driven groundwater inundation to compromise coastal wastewater infrastructure such as cesspools and fractured sewer lines. This was accomplished through a field-based geochemical study in Honolulu, Hawaiʻi using spring tides as a proxy for future sea levels. We focused on two potential pathways for this to occur: (1) direct groundwater inundation of wastewater infrastructure, which subsequently flows to the coastal ocean, and (2) indirect inundation of wastewater infrastructure through storm drain backflow. Groundwater discharge was monitored using radon, a naturally occurring groundwater tracer, over half tidal cycles. In addition, pharmaceuticals and dissolved nutrients were collected from groundwater, surface water, and storm drains at low, mid, and high tides. Groundwater discharge and pharmaceutical concentrations fluctuated with tides indicating tidally driven groundwater inundation and wastewater discharge. This study presents some of the first field-based evidence for groundwater inundation of coastal wastewater infrastructure, demonstrating that SLR is already leading to negative impacts to coastal water quality and environmental health.

This work has been published in Limnology and Oceanography Letters:

Citation: McKenzie, T., Habel, S., Dulai, H., (2021). Sea-level rise drives wastewater leakage to coastal waters and storm drains. Limnology and Oceanography Letters. doi: 10.1002/lol2.10186. 

Datasets:

McKenzie, T., S. Habel, H. Dulai (2020). Honolulu King Tide Study: Raw sample dataset, HydroShare, 

McKenzie, T., S. Habel, H. Dulai (2020). Honolulu King Tide Study: Radon Time Series, HydroShare,