Just below the soaring Scarborough Bluffs, 17-month old Piper Clark scoops the fine sand into gloppy pies. Her brother Reed, 4, bravely ventures deeper into the water.

Around them on this hot summer day, bikini-clad girls frolic and tease boys in the waves, while the lifeguard warns them from straying too far out.

Colonies of swallows, warily eyeing the hawks soaring above, cling to the sandy face of the cliff.

Colleen Clark, a nurse, stands watch over her toddlers, her feet in the water, her dress hitched up above her knees.

“Oh sure, I let them go in,” she says, pointing to the green flag indicating the water is safe. “But I wouldn’t let them drink it.”

A few kilometres west of Bluffer’s Park, just below the Gardiner, two adult geese paddle around Keating Channel with their four fluffy goslings

That’s where the Don River spills into Toronto Harbour, spewing sewage as it flows.

It’s also where heavy trucks rumble on their way to the Leslie Spit to dump their loads of asphalt and rusty steel, bricks and rebar — what the city calls “clean fill.”

A boom spreads just beneath the trees where the geese shelter, there to catch the “floatables,” the used condoms, plastic tampon applicators and hypodermic needles that bob among the mini-explosions of methane bubbles.

Unlike Colleen Clark, Mother Goose can’t read the menace in the soupy black water.

Mark Mattson, president of Lake Ontario Waterkeeper, a criminal lawyer turned, appropriately enough, environmental lawyer, surveys the scene and says, “I’ve been investigating the channel for 20 years, and this is as bad as it’s ever been.

“These carp go all over the lake, the birds migrate,” continues Mattson, as one of the geese elegantly dips its beak into the water. “They’re still part of the diet of northern communities. I wouldn’t want to be the hunter who shoots one of these geese and feeds it to his children.”

This, folks, is your water, what comes out of your tap, what you drink, what you bathe in and, if you aren’t lucky enough to have a cottage, what you swim in.

Some 4 1/2 million humans who have made their homes around Lake Ontario depend on this water — as does the wildlife on, in, above and around it.

“It’s our only source of drinking water,” says Mattson. “We’re very fortunate because, unlike so many other cities, Boston, New York, Vancouver, they don’t have their drinking water at the bottom of their street.

“But think about it: if Lake Ontario became undrinkable, if we had a Fukushima disaster at one of Ontario’s 21 reactors, there would be no alternative potable drinking water. We’d have to build a pipeline to Lake Huron or James Bay or something.”

In the Huron language, Lake Ontario means Lake of Shining Waters.

Stand on the crest of the slope on Jones Avenue, just below the Danforth, and, when the sun hits the water it’s like a mirror.

But of all the Great Lakes, Lake Ontario, which is furthest downstream, is almost certainly the most polluted.

“Your fish warnings are more ominous in Lake Ontario,” says Ontario’s Environmental Commissioner, Gord Miller. “If you look at the guidelines for eating fish, you’ll find that the highest level of contaminants is in Lake Ontario.”

Just check out the Environment Ministry’s 2011-2012 “Guide to Eating Ontario Sport Fish.” No kids under 15 and no women considering getting pregnant should even think of a fish fry.

Here’s an appetizing bit: “In the various species of trout and salmon found in Lake Ontario, dioxins, furans, dioxin-like PCBs, mirex, photomirex, toxaphene and chlordane can be elevated in the same fish . . .

“Consumption of species such as walleye, pike, bass and perch is usually restricted because of mercury. In total, 58.6 per cent of the advice given for sport fish from Lake Ontario results in some level of consumption restriction.”

Lake Ontario is the 14th largest lake in the world: 19,529 square kilometres, 1,146 kilometres of shoreline, 244 metres at its deepest.

Which is one reason it’s not a total cesspool, experts say.

Its size and depth help dissipate the bacteria, which our drinking-water filtration plants kill off with chlorine.

But what about the agricultural runoff? The sewage? The industrial sludge? The nuclear waste? The pesticides? Herbicides? Road salt? The toxic by-products of burning medical and municipal waste? The engine oil you poured down the storm sewer? The leftover prescription pills you flushed down the toilet?