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, 

The aerosol emissions from Kilauea volcano are affecting Hawai’i rainfall locally. Based on the analyses from 2014-2017, the days with high emissions have on  average 8 mm/day less rainfall downstream of the Kilauea. Read the to learn more

This research explores the relationship between 4-types of atmospheric disturbances and their contributions to daily rainfall on O‘ahu Hawai‘i.  On average, atmospheric disturbances account for 29% of the annual and 41% of the seasonal (Nov – April) rainfall on O‘ahu Hawai‘i. Cold, fronts are the most common disturbance type, and fronts that cross over the island are shown to bring significantly more rainfall than the fronts that track to the north of the Island chain. Understanding the relative contributions of disturbances to wet-season RF as well as the spatial distribution of RF during these events is important in the context of a changing climate in which disturbances are expected to become less frequent and more intense. This work has been published in the American Meteorological Society Journal, “Monthly Weather Review”.

Schematics showing six types of synoptic patterns that produce rainfall in Hawai‘i: (a) crossing fronts (CR), (b) noncrossing
fronts (NC), (c) upper-level low pressure systems (UL), (d) kona low storms (KL), (e) tropical cyclones (TC), and (f) nondisturbances
(ND). The 500-hPa isobars are indicated with solid black lines and the wind direction is indicated by the arrow.

Frequency of occurrence for each disturbance type from 1 Oct 1990 to 31 Sep 2010.

Percentage of rainfall that has occurred for each disturbance category over (left) the 20 water years and
(right) wet seasons that have occurred between 1990 and 2010.

In Hawaiʻi, both magnitude and occurrence dates of annual maximum daily rainfall (RFmax) and the annual stream peak flow (PFmax) have shifted from 1970 to 2005. More than half stations of both show decreasing trends across the Hawaiian Islands, except for Kauaʻi and Molokai. Comparing to the trends of RFmax, trends PFmax show stronger distinction between windward and leeward. The occurrence dates of both RFmax and PFmax have shifted from early Jan. to late Dec., likely due to the changes in ENSO. We also found that the annual peak flow rarely coincides with the annual maximum rainfall, and more studies are needed. These overall findings alert us to prepare for the flood impact differently, and likely indicate differences responses of ecosystems. This work has been published in the Journal of Hydrology. 

Figure 1. RFmax trends from water years 1970 to 2005 across five major Hawaiian Islands. The brown line indicates the physiographic division of windward (northeast) and leeward (southwest) regions on the islands. 

Figure 2. PFmax trends from the water year 1970 to 2005 for the five main Hawaiian Islands. The brown line indicates the physiographic division of windward (northeast) and leeward (southwest) regions on the islands. 

Figure 3. The occurrence date (median ± 0.5 circular deviation) of RFmax (green line) and PFmax (black line with the  phase shaded (red: warm phase; blue: cool phase, white: neutral) ) on the leeward (left) and windward (right) sides. The y-axis indicates the months from June through May (bottom to top). Occurrences under ENSO warm phase are indicated by red squares for RFmax, and a red circle for PFmax. The Oceanic Nino Index (ONI) on the top indicates the strength of the ENSO from 1970 to 2005. 

The Sen’s slope estimator for the timing shows the occurrence date of RFmax and PFmax shifted earlier. In leeward regions, the median occurrence date of RFmax and PFmax were ~1.5 days and significantly ~1.5 days earlier, respectively. On the windward side, the median occurrence date of RFmax and PFmax significantly shifted ~half day and ~6 hr later, respectively.