When it rains, it floods — or so it seemed after last Sunday’s heavy rainstorm.

In my more than 20 years of living here, it’s been a long time since I’ve seen that much rain.

By some accounts, the storm dumped as much as 3 inches of rain per hour on parts of Oahu. The Honolulu Star-Advertiser reported that the Honolulu Airport received 5.3 inches of rain over a 24-hour period, breaking the previous rainfall record of 5.28 inches set in 1955.

As I glanced out over my porch that Sunday, I saw a strange sight: Runoff water had created a dark brown line in the ocean that looked like the horizon.

In other words — it was as good a time as any to do some water quality testing.

Six months ago, Civil Beat tested the waters off Magic Island and the Ala Wai canal after a small sewage spill. We found the water was pretty darn clean.

So on Monday morning, I revisited the same test locations and dipped sterilized bottles into what were now brown, muddy-looking waters.

This time there were no jolly fishermen hanging around on the benches near the harbor (it was still drizzling out). Just three surfers had braved the brown swell while a pair of outrigger boats sped through the surf in the distance.

We tested for three bacterium: Fecal coliform, enterococcus and clostridium perfringens. The tests were done by Food Quality Labs, which trained me the last time to gather the samples. The results this time were far from bland.

All results are in CFU per 100 milliliters:

As it turns out, there’s no standard for fecal coliform for marine waters. The state standard for a single reading of enterococcus is 89. But the presence of enterococcus alone isn’t enough to mean that there’s human sewage in the water, because the bacteria is present in animal feces. Clostridium perfringens is used as a secondary indicator to detect human sewage.

The Hawaii Revised Statutes’ State Water Standards, Section 11-54-8(b) lists the standards for marine recreational waters.

During heavy rainstorms, the state often gives “brown water advisories” that can last anywhere from three to five days. The one the state put out for Kauai and Oahu islands on Sunday was lifted Thursday.

“When it floods it’s basically a cleaning of the land,” said Watson Okubo, supervisor of monitoring and analysis for the Clean Water Branch of the Hawaii Department of Health. “You’ve got all kinds of stuff that washed out. Trash, dead animals, herbicides, pesticides, unknown chemicals. You don’t want to be in the wash water.”

It’s not unusual to have tiger sharks trolling nearshore areas for snacks — namely the dead animals, he said.

Some of Okubo’s advice seems like common sense: “If it’s brown, stay out. Wait another day.”

On Monday at Magic Island, Christopher Vinca, 18, took one look at the brown ocean and headed the other direction with his boogie board. “That over there is pretty nasty,” he said.

His friend, Reyn Sugihara, 17, agreed.

“It looked disgusting because it was brown and dirty and there’s trash floating around,” Sugihara said, clutching a surfboard. What was he worried about most? “Disease and getting dirty.”