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Maui could be just hours away from confronting its first major rain event since the Aug. 8 wildfires that ripped through Lahaina and portions of the Upcountry region, elevating concerns over potentially toxic runoff polluting nearshore waters.

The expanded its flash flood watch to include all the Hawaiian Islands from Wednesday morning through Thursday afternoon, with Kauai and Oahu expected to get the brunt of the southerly winds, heavy rains and thunderstorms from a Kona low system starting Tuesday.

Among those anxiously awaiting the potential drenching are a handful of groups that have been monitoring nearshore waters along the West Maui shoreline.

“There are storm drains there, and we know some steps have been taken to prevent runoff from getting into the coast through those storm drains, but obviously we’re not going to know what’s going to happen until it does,” said Nick Hawco, part of a team of University of Hawaii researchers that has been taking water samples to assess the reef health and any levels of pollutants such as copper, lead and organic contaminants associated with burned materials.

A powerful Kona low in early December 2021 walloped South Maui and Kula, as stormwater raced down the south- and west-facing slopes of Haleakala, damaging homes and washing away several vehicles in the Maui Meadows subdivision in Kihei.

Roads in Kula were closed due to flooding, and winds of 60 mph and greater toppled trees and power lines in the area.

Due to concerns over ash getting into the ocean from wind and rain erosion, the U.S. Environmental Protection Agency sprayed a soil tackifier over the ash footprints on fire-impacted properties in Kula in late September, and in October began applying the Soiltac product to nearly two-thirds of the parcels in Lahaina burn zones.

EPA spokesperson Rusty Harris-Bishop said the stability of the soil tackifier on some of the properties was compromised by “intensive sifting” when residents were allowed to return to search for keepsakes and valuables. In those instances, the EPA has been reapplying Soiltac.

“If the soil tackifier was not used to stabilize ash and debris, the worst outcome would be large uncontrolled release of windblown ash to adjacent unburned properties or runoff into the ocean during a heavy rain event,” he said.

To further mitigate potential runoff, the U.S. Army Corps of Engineers, which is clearing fire debris on nearly two dozen properties in Kula, had contractors put in place straw wattles, which are tubes of compressed straw, and other erosion control methods on parcels whose owners have granted the agency a right of entry for the work.

The wattles allow water to flow but keep ash and debris from entering storm drains, according to USACE spokesperson Shannon Bauer, who said the debris removal work in Kula will stop if the rain gets too intense.

For its part, Maui County has deployed workers to inspect and clear culverts in flood-prone areas of South Maui and Upcountry, and heavy equipment will be staged where appropriate based on updated information from the National Weather Service on the areas most likely to be hard hit, according to . 

In preparation for a $40 million project to install an emergency stormwater capture system throughout Lahaina, the county Department of Public Works, the state Department of Transportation and the Federal Highway Administration worked with local contractors in late August to place fences and Envirosoxx, a pollutant filter system that resembles long, green “socks,” to safeguard coastal waters along the shoreline.

The larger project, funded through the FHA Emergency Relief program, runs the entire length of Front Street and a portion of Honoapiilani Highway from Front Street to Wahikuli Wayside Park, including stormwater inlets in the area, according to .

If the rains do come, Hawco said a team led by graduate students will be wading out into the water to collect samples at key sites at Mala Wharf and Mala Stream to see what’s flowing into the ocean.

The researchers from UH Manoa’s , working under a yearlong rapid response grant from the National Science Foundation, took their first samples in early October but Hawco said it was too soon to discuss the results.

“At this stage we’ve been mostly trying to get ready to observe the rains when they do come. So that’s been our real priority so far,” he said.

“One of the things we don’t necessarily know is if things make it through those defenses, where do they end up,” he added. “We think that some of the suspended solids might settle out on the reef but some of the other ones might be transported out to sea. Transported out to sea might be the best thing that could happen for things that escape those barriers.”

Hui O Ka Wai Ola also is on high alert and will be conducting additional sampling of coastal waters “from the first flush of heavy rains into Lahaina,” according to Program Manager Liz Yannell.

The organization has been sampling three sites off Lahaina since 2016 and added four new sites – Mala Tavern, Mala Ramp, the Papalaua and Front Street intersection, and Lahaina Harbor – since the August fire.

“There are still a lot of unknowns and questions about how those devastating fires will impact our coral reefs and coastal ecosystems. It’s really important for us to get out there and document what we find,” Yannell said.

Civil Beat’s coverage of Maui County is supported in part by a grant from the Nuestro Futuro Foundation.