Back Cove is one of Portland's most popular public spaces. The 3.5-mile trail around it is ideal for a walk or a jog. And it's pretty.

Just don't look too close.

Hypodermic needles, the kind used to inject insulin or vaccines, keep showing up around the edges of Back Cove. It's been a dirty little secret known only to the few brave souls who sift through the debris around the high-tide line in boots and gloves, trying in vain to keep up with the never-ending surge of trash that ends up here.

"I was mortified," said Sandra Wachholz, remembering when she first started finding the needles about four years ago.

Wachholz is a criminology professor at USM. But her hobby is picking up litter, which she does with the diligence of an archaeologist.

She regularly comes to Back Cove for about an hour, more than enough time to fill a trash bag with plastic wrappers, straws, tampon applicators and other cast-offs the rest of us would rather ignore. And she usually finds at least one or two hypodermic needles.

"My record is seven," she said.

At first, she guessed the needles were left by people using illegal drugs. But it was soon clear that this was something bigger. They show up all around the cove.

And there are a lot of them.

Although finding one is, literally, like pulling a needle out of haystack, a volunteer clean-up crew collected 42 during one day in June.

The needles usually have plastic caps covering their points, but not always.

Wachholz decided against leading student clean-ups here because she's worried about someone getting pricked by a dirty needle. And she worries about the risk of younger kids straying a few feet off the walking path to explore the water line.

"I would love to know where they are coming from because it's so disgusting to find them," she said.

The source, for now, is a mystery. The delivery mechanism, on the other hand, seems more obvious.

Back Cove sits at the discharge end of about a dozen sewer overflow pipes. During rainstorms, when storm drains fill and flood the city's sewers, the pipes discharge backed-up sewage and stormwater, along with whatever was flushed down toilets or washed into storms drains.

It can be a fairly disgusting body of water.

Portland is in the midst of a 6-year, $61 million effort to reduce sewer overflows around the city, although it will need to spend even more to eliminate the problem entirely.

In the meantime, there is new interest in finding the source of the syringes.

Are they getting flushed down toilets by people who inject their own insulin? Are they getting dumped into the sewers or storm drains by someone who doesn't want to pay for the proper disposal of medical waste? Is it an ongoing disposal problem or are all the needles already out there and simply recirculating around the confined bay?

Peter Milholland, stewards coordinator for Friends of Casco Bay, led the cleanup last June that turned up 42 needles. Another group found 12 more in July.

"It's in the last couple years that we've noticed it," he said. Along with being an obvious public health issue, the needles are a symbol of all the bad stuff that flows into the bay, he said.

The group has begun asking city officials about tracing the source of the hypodermics and is considering using screens or nets to capture debris at each overflow point. That could narrow it down to one part of the city.

Fortunately, there are people willing to take a closer look and see things as they are, not just as they appear to be.

The rest of us probably ought to stay on the path and take in the view.

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