The sediment-laden freshwater of the Presumpscot River that is known as “brown tea” floats on top of dense saltwater at the mouth of Casco Bay after a heavy rain near Portland’s Eastern Prom in May. Photo courtesy of Friends of Casco Bay

Last year’s heavy rains flushed nearly 374 million gallons of raw storm and sewer water into Casco Bay, including from the devastating Dec. 18 storm that flooded western Maine communities at the upper edge of the watershed.

It was the highest annual overflow into the 200-square-mile bay in five years – and that total does not even include the back-to-back January storms that inundated coastal communities with heavy rains, storm surge and damaging waves.

The overflow that sloshed into the bay in 2023 alone was enough to fill more than 566 Olympic-sized swimming pools. If lined up end to end, it would stretch for almost 18 miles. Using a normal garden hose, it would take nearly 30 years to fill that many up.

“It’s really hard to talk about the weather this winter and not have an emotional response to the storms and the impact they are having on our state, on the people of the state, and on the water resources we all love and want to protect,” said Baykeeper Ivy Frignoca, the chief advocate of Friends of Casco Bay.

Yet as recently as 30 years ago, the city was discharging 1.8 billion gallons of sewage-tainted water into Portland’s scenic waterways – a 79% difference that shows how far the city has come, but also how difficult it can be to fix an underground infrastructure built generations before environmental laws were created.

No one can say exactly what is in the floating streams of what’s called “brown tea” that jut out into the bay after big storms because nobody monitors the contents of overflows. But it gets its color from sediment, sewage and naturally occurring tannins that can leach out of plant debris from riverbeds.

But Friends of Casco Bay, a nonprofit founded in 1989 to protect the bay’s health, is pretty sure that it’s not good. Runoff and wastewater already contain high amounts of phosphorous, nitrogen, soot, pesticides and heat – not to mention sewage – that can harm water quality, aquatic life and the local economy.

Heavy rains drive up the volume of the overflow, as well as the volume of pollutants washed out into the bay, drawn from the air, streets, yards and sewer systems of the watershed, which begins 60 miles away in Bethel and travels through the most densely populated parts of the state.

Friends of Casco Bay tracks a range of runoff characteristics, including acidity that impairs commercial shellfish harvests and carbon dioxide that contributes to global warming. Frignoca singles out nitrogen and chloride to show how the impact of these pollutants changes with volume and seasonality.

The rain absorbs nitrogen from the atmosphere as it falls and picks up more from fertilizer, manure and plant residue as it washes over land and into stormwater systems, Frignoca said. In the right amount, it feeds the algae that make up the base of the ocean food web; too much will lead to harmful algal blooms.

Winter rainstorms also pick up the salt applied to roads parking lots and sidewalks to prevent slipping and wash it into freshwater streams and brooks, turning them into chloride stews that can’t sustain many forms of aquatic life and can be extremely difficult to restore, Frignoca said.

As the back-to-back-to-back winter storms drove home, the era of extreme weather is already here. The number of 2-inch rain days Portland has racked up a year over the last two decades is now twice its long-term historical average, according to National Oceanic and Atmospheric Administration weather data.

Frignoca is concerned about increasing storm intensity, which makes precipitation more likely to overwhelm stormwater systems, and changing seasonal patterns that lead to winter rainfall instead of snow. Heavy rain washes over frozen ground in sheets, while snowmelt is usually easier to absorb.

Since high-precipitation events are projected to increase in a warmer future – as the air warms, it will be able to hold more water and draw more moisture from the Gulf of Maine – Friends of Casco Bay is renewing efforts to tighten up Maine’s outdated stormwater regulations.

The Maine Department of Environmental Protection has formed a committee of stakeholders including Frignoca to review Maine’s stormwater treatment and runoff prevention regulations that apply to large developments and construction sites. The group met this month for the second time.

The DEP wants to incorporate the Maine Climate Council’s latest rainfall projections and the regulatory flexibility to craft site-specific plans that could, for example, make sure coastal plans control nitrogen to avoid a saltwater algal bloom while inland systems focus on phosphorus to prevent a freshwater bloom.

Frignoca’s group would like amended rules to apply to smaller projects because current regulations miss a lot of the development and redevelopment in the Casco Bay watershed. It wants to turn little-used, low-impact development incentives like stream buffers into regulatory mandates.

It’s also urging municipalities to collect and manage stormwater and wastewater separately. Some cities such as Portland carry the two streams in the same aging network of often-undersized pipes, which means 2 inches of rain can send untreated stormwater and sewage spilling into the bay.

The Dec. 18 storm that dumped as much as 7 inches of rain in parts of the state and caused widespread flooding across western and central Maine caused 47 sewage overflows in 23 districts, according to DEP data. Data from January is not yet available. and combined systems only post their data once a year.

Despite an increase in extreme rain events, the number of combined overflows is on the decline because municipalities like Portland are disentangling their mingled systems, often with the help of state grants said Brian Kavanah, head of the Maine Department of Environmental Protection’s water quality bureau.

But even the best stormwater practices can’t turn a residential housing complex or shopping mall back into a forest, according to Cody Obropta, an environmental engineer with DEP’s land bureau. As the watersheds are developed, some stormwater pollution will escape even the best-managed sites.

“We just can’t capture it all,” Obropta said. “There are certain scientific limits to what we can do.”