SOUTH PORTLAND — As April showers gave way to May showers, I couldn’t help thinking about the effect on Casco Bay.

During and after a rainstorm, a brown stain spreads far out from the shore. The rain-water runoff carries soil, gas and oil from city streets and fertilizers and pesticides from yards into marine habitats.

For a day or two, the expanse of brown water is all too visible, but its impact is quickly forgotten as the seawater returns to its normal color.

Many people don’t make a connection between a big rainstorm and our coastal waters, but it always worries me.

Once, after a particularly heavy rainstorm, scientists at Friends of Casco Bay measured 15 feet of fresh water floating on top of the seawater near Fort Gorges. Fresh water kills marine organisms.

But that is not the worst part. Whenever there is a measurable rainfall, millions upon millions of gallons of polluted water flow into Casco Bay.

This toxic soup can sicken swimmers, make seafood unsafe to eat and harm marine life.

Big rainstorms cause productive clam flats in Casco Bay to be closed because of widespread pollution.

Why does this happen? To keep sewage from backing up into homes or spurting up through manhole covers when it rains, many Maine communities have built systems of underground pipes that divert storm-water runoff and untreated sewage into streams, rivers and the ocean.

These systems are called combined sewer overflows, and they can disgorge several million gallons of raw sewage, industrial waste and polluted storm water.

More sewage is flushed into the ocean after only primary treatment, where screening and settling tanks remove less than half the pollutants. Untold millions of gallons of polluted runoff flows into the bay, as the whole landscape is washed by inches of rain.

Roads, drains and storm-water pipes bypass the sewage treatment system entirely and shoot polluted rain water straight into streams, rivers and the bay.

Two hundred and fifty years ago, when there were only 12,000 people in Maine, the rain that fell on the land was mostly absorbed by forests and undergrowth.

The rain that ran off into rivers and streams carried nutrients that nurtured aquatic life. The ecosystem was in balance.

Now, with 1.2 million people, Maine has changed dramatically. This has been incremental change that hasn’t imposed itself on our day-to-day consciousness.

Along the way, we’ve built homes, factories and roads where trees used to be. The internal combustion engine changed our state in a fundamental way.

Earth Day and Rachel Carson remembrances remind us that earlier in most of our lifetimes, air pollution and water pollution went relatively unchecked.

Until four decades ago, there was no Clean Water Act, no Clean Air Act and no Environmental Protection Agency.

So, are things better now? I don’t think the bay would agree.

While much is better, the toxic mix made from rainwater and all the chemicals that go into the air and onto the land is getting worse. More people, more vehicles, more pollution.

Scientists estimate that two-thirds of the pollution of water bodies comes from runoff from land.

The worst part is that a majority of people sit like lobsters in a pot of tepid water, unaware that it will come to a boil.

At Friends of Casco Bay, we have to battle to convince the EPA and the Maine Department of Environmental Protection that we are not Chicken Little, that serious consequential changes are happening to our bay right now, that we can’t continue to treat each new insult as one more incremental, negligible change.

I’m not against progress; what I’m against is the momentum of progress without the commitment to protect our water resources in equal measure.

Days of rain depress me. It’s not the gloomy weather; it’s the realization that we are sickening our coastal waters.

I wonder how much more the bay can take. How much longer will the communities that depend on the bay be able to depend on the bay?

Joseph E. Payne, a marine biologist, is the Casco Baykeeper on the staff of Friends of Casco Bay. He can be reached at keeper@cascobay.org.

— Special to the Press Herald