Rising Seas Compromise Coastal Cesspools, Deliver Sewage to Nearshore Ecosystems

posted in: April 2021 | 0

Contamination of coastal waters caused by sewage from cesspools and other onsite sewage disposal systems (OSDS) as well as sewage infrastructure will only increase with the rise in sea level over the next few decades.

In fact, it is already occurring, especially at spring and king tides.

Two University of Hawaiʻi researchers have been studying just how this contamination occurs: Dr. Shellie Habel, a Sea Grant agent with the University of Hawaiʻi working with the state Department of Land and Natural Resources’ Office of Conservation and Coastal Lands, and Dr. Trista McKenzie, a post-doctoral researcher at UH’s School of Ocean and Earth Science and Technology.

Both described their recent work at a webinar hosted by the university’s Water Resource Research Center and the ʻIke Wai project.

Habel has modeled the impact that rising sea levels will have on cesspools in the Honolulu urban core. Although the area is served by the city’s Sand Island wastewater treatment plant, the largest in the state, processing an average of 65 million gallons a day of sewage, hundreds of homes and businesses continue to dispose of wastewater by means of cesspools or other OSDS. 

(Asked why there remain so many unsewered buildings in Honolulu, Habel said, “Lots of folks in the primary urban core area might not even know what their system is.” In addition, digging new laterals – connections between buildings and sewer mains – is difficult in Waikiki especially, given the likelihood of disturbing ʻiwi bones, other cultural assets, and buried infrastructure.)

Habel presented a map of OSDS from Kapiʻolani Park in the east to the airport in the west, with each known OSDS color-coded red, yellow, or green as to whether it was already flooded at the ground surface, partially submerged in groundwater, or whether it was presumed to meet Department of Health guidelines, with at least 15 feet of unsaturated soil between the surface and the water table, based on current sea levels and the assumed elevation of groundwater head.

Nearly all – 86 percent – of the identified systems are already compromised (yellow), with a few already completely inundated (red). Only those furthest mauka were presumed to be compliant with DOH guidelines.

With sea level rise and eroding coasts, even more cesspools and other OSDS will fail, Habel said. To model this, Habel overlaid rising sea level scenarios – of 32, 60, 98, 122, and 152 centimeters, representing reasonable predictions of increasing sea levels from 2030 up to the end of the century – on the OSDS maps. 

By century’s end, the map was dominated by red symbols.

Arbitrary Setbacks

Outside the urban core of Honolulu, coastal erosion and sea level rise are already exposing cesspools on beaches next to homes that are themselves being threatened by encroaching seas.

Habel gave an example of a house in Pupukea, on Oʻahu’s North Shore, where oceanfront homes have often resorted to extreme measures – sand burritos or illegal and often jerry-rigged defenses – in an effort to halt erosion.

In December, the house was relocated to an area further from the shore on the same lot, the kind of shoreline retreat that generally is praised by planners. In connection with the move, the cesspool that had served the house was closed, replaced by a septic system with a leach field, which requires a minimum 50-foot setback from the coast.

“But there are two problems with relying on a 50-foot setback,” Habel noted. “One is evident here: as the distance between the [onsite sewage disposal system] and shoreline decreases with erosion, systems can end up very near to the shoreline or directly on the shoreline, on the public beach.”

“The second issue is the distance used to measure the setback when authorizing an OSDS. It isn’t measured from the actual shoreline but from the tax-map-key boundary,” a boundary that rarely reflects actual shorelines in areas where erosion has occurred, as in Pupukea.

“For major changes, counties require shoreline surveys to be made, but authorization for installation of OSDS might not follow that same requirement,” she said.

In the case of the Pupukea house, the distance of the new septic tank from the shore was 69 feet, as measured from the TMK boundary. However, as aerial photos clearly show, the actual distance is much less than the 50-foot setback, Habel said.

“When you look at how erosion is predicted to affect an area, both the leachfield and septic tank, as well as the relocated house itself, will be directly affected by erosion with as little as half a foot to one foot of sea level rise,” she said.


“It’s one thing to run models and simulate these issues… but it’s another to actually confirm it,” Habel said as she introduced McKenzie. 

To determine the ways in which rising sea levels compromise sewage infrastructure, McKenzie looked at how groundwater flows shift in response to seawater. There are two ways in which rising seas can lead to sewage contamination of coastal areas, McKenzie explained: the direct way, with inundated cesspools or fractured sewer lines releasing contaminants as the water table rises.

Then there’s the indirect means, with the flooding of storm drains that flow to the coast. “This pathway becomes a vector for wastewater transport when OSDS are in the vicinity or compromised sewer lines,” they explained.

To confirm direct and indirect transport of contaminants, McKenzie sampled water in two low-lying areas of Honolulu, Waikiki and Mapunapuna, at spring, high, mid-, and low tides, with spring tides used as a proxy for conditions under higher sea levels.

Waikiki was marshland until the Ala Wai canal was dug. There, the suspected contamination in the Ala Wai canal is direct, coming from cesspools that are compromised as sea levels rise. Here McKenzie took samples at two sites along the Ala Wai and one near the small-boat harbor.

At Mapunapuna, near the Honolulu airport, the area regularly floods at high tides and in heavy rains as seawater rushes in through manholes, an example of the indirect means of contamination. Here, the samples were taken from water rising in three storm drains and again at a nearby coastal site.

To detect the extent of groundwater in the samples, McKenzie tested them for radon (a naturally occurring element in groundwater but not seawater), nutrients (signs of fertilizers and sewage), and other chemicals (including caffeine and pharmaceuticals). 

As expected, storm drains in Mapunapuna quickly overflowed with seawater at spring tides, carrying into the streets contaminants from compromised sewage infrastructure – “which is not great,” McKenzie said.

Samples from the Ala Wai also showed tidally influenced changes in groundwater and pharmaceutical signals, confirming that as sea levels rise, contamination from wastewater will be an increasing issue.

Among the disturbing findings were the high levels of unregulated contaminants, called contaminants of emerging concern (CECs), in the sampled water.

More than 94 percent of the samples had at least one CEC, with caffeine and carbamazepine, an anticonvulsant drug, the most frequently detected, McKenzie said. Again, the presence of CECs and the fluctuation in their concentrations with the tides “provides concrete evidence of tidally influenced inundation of wastewater infrastructure,” they added.

At Waikiki, CEC scores increased with rising tides, showing increased infiltration of groundwater. The opposite occurred at Mapunapuna, where seawater intrusion through the storm drain network, which already has contaminated water in it, leads to dilution of the sewage effluent.

“CECs are really interesting,” McKenzie said, providing “a lot of potential to use in research.” Yet, they added, “even in trace quantities, they’re demonstrated to have negative impacts on the ecosystem.”

The risk quotients for CECs are calculated based on the extent to which they exceed the Predicted No-Effect Concentration, or PNEC, and anything greater than a risk quotient above 1 poses a high risk for the ecosystem.

Sixty-two percent of the samples had risk quotients exceeding 1 for carbamazepine and caffeine. Samples taken at the Ala Wai and in both coastal sampling areas showed no presence of fluoroquinolones (antibiotics, including Cipro), which, McKenzie explained, degrade quickly in the presence of sunlight. At two of the storm drain sites in Mapunapuna, however, the fluoroquinolone risk quotients were 21 and 26.

McKenzie wrote up the results of the study in an article published in Limnology and Oceanography Letters, co-authored with Habel and Henrietta Dulai (“Sea-level rise drives wastewater leakage to coastal waters and storm drains”). In recognition of the high quality of the research, McKenzie was awarded the L&O Letters Early Career Publication Honor.

— Patricia Tummons

Learn More:

The webinar featuring Habel and McKenzie’s work is available through the Water Resources Research Center website: www.wrrc.hawaii.edu.

For more information, see McKenzie, Habel, and Dulai: “Sea-Level rise drives wastewater leakage to coastal waters,” at https://doi.org/10.1002/lol2.10186

For more on sewage-related contaminants of emerging concern in Hawaiʻi’s environment, see our June 2016 cover story, “Several Common Drugs Are Apt to Leach Into Oʻahu’s Groundwater, Study Finds,” and sidebar, “Emerging Contaminants in the Ocean,” available on our website, environment-hawaii.org.

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