Back in the 1970s, before climate change had entered the nation’s vocabulary, The Sunday Times engaged its readers in a bit of dirty work. People were invited to visit a beach, anywhere in Britain, and walk from the high tide line down to the water’s edge. Along the way they were asked to collect all the plastic bottles within a metre-wide strip and to write down the serial numbers they would find on the base — the manufacturers’ codes that would reveal where each one was made.

The mass of data that flowed into the paper’s office conclusively settled a long-running dispute. Who was to blame for the rim of garbage that was not just disfiguring the coastline but, as it abraded into plastic sand, actually changing its chemistry? Did it come from land, or was it dumped overboard from ships? The answer came loud and as clear as a foghorn. Overwhelmingly, the junk was coming from ships, and the proof of it made more than just a good headline. It helped the Department of the Environment (as it then was) ratify an international law against dumping at sea, which duly came into force in 1988.

Twenty years later we might expect to find pristine beaches unscarred by anything worse than seashells or worm-casts. Well, we might, but we would be disappointed. Every September since 1993, the Marine Conservation Society (MCS) has run a Beachwatch survey, and it is still waiting for good news to report. The contribution of the shipping industry may have declined (though the precise amount is open to question), but the UK’s scum-line now is thicker, more persistent and more dangerous than ever.

Last year’s survey clocked a total of 2,195 pieces of garbage for every kilometre of coast — that’s one for every human stride, more than double what it was in 1994, and 7% more than 2007. You would expect plastic bags to be a nuisance, and so they are, but at 46.5 per kilometre they reached only 14th in the litter league. Those above them included dog-ends (60 per kilometre), plastic drinks bottles (71), pieces of glass (76.8), cotton-bud sticks (100.5) and polystyrene (142). The chart-toppers, as always, were plastic fragments. Pieces ranging from less than a centimetre to half a metre in length totted up to an astonishing 487.3 per kilometre — very nearly one every two metres.

The offensiveness of all this is not just aes-thetic. Glass, drinks cans and the all-too-accurately labelled “surgical sharps” are obvious risks to human feet. Plastics are promiscuous killers of wildlife. According to the MCS, more than 170 species of marine wildlife are known to mistake litter for food, resulting in starvation, poisoning and fatal stomach blockages, and at least 144 species including turtles, whales, dolphins, seals, sea lions, seabirds and fish, have been found entangled in debris. Leatherback turtles, for example, feed on jellyfish, which closely resemble plastic bags. No surprise: 34% of leatherbacks washing up dead have plastic in the gut.

On the beaches in 2008, plastic accounted for 59.6% of all litter. Much of it will never completely break down. If the environment survives that long, the plastic will persist for hundreds and maybe thousands of years (lower temperatures mean plastics degrade much more slowly in water than they do in air). The Hellenic Marine Environ-ment Protection Association reckons a plastic bottle at sea will take 450 years to disintegrate fully. As with so many other manifestations of 21st-century recklessness, it will fall to future generations to see if this is right.

Six years ago the US National Academy of Sciences figured that 6.4m tonnes of litter entered the oceans every year — a total hardly likely to have reduced in the meantime. Closer to home, we’re told that 20,000 tonnes of waste goes into the North Sea every year. Seventy per cent of this sinks to the bottom, 15% stays in the water and 15% hits the beaches. What this means in terms of pollution is largely, though not wholly, visible to us all. Plastics never disappear — they simply break down into smaller and smaller fragments. When a series of 45 seemingly clean sand samples was taken from the Northumbrian coast, every one of them was found to contain microscopic plastic fibres at densities of up to 10,000 per litre of sand. More have been discovered in plankton samples dating back to the 1960s. Already, there may be no such thing as a clean beach.

Britain’s affection for the seaside depends heavily on nostalgia for a golden age of deckchairs and donkey rides that most people are too young to remember. Even nostalgia will drain away if future generations are not given something more to feel nostalgic about. A survey by Natural England last year revealed that many people “are put off by the seas around England because they appear cold, dirty and potentially dangerous (especially for their children)”. The nature of this perceived danger is not specified, but is unlikely to be confined to drowning, hypothermia and broken bottles.

After visitors and the fishing industry, the third most prolific source of rubbish on the beach is what the jargon calls SRD — sewage-related debris. Two phenomena contribute to this: the public’s unshakable belief that solid objects somehow dematerialise if flushed down a lavatory, and the inadequacy of drainage systems built in the age of the carriage horse. Nobody in the mid-19th century could have imagined a tripling of the UK population or visualised the roaring currents of effluent that would result. Neither would they have had any clue about climate change, though it had already started. It would have taken more imagination than anyone possessed to see a connection between a Victorian industrial chimney and a 21st-century beach clogged with faeces.

Ironically, the cause of the problem is a piece of engineering designed to prevent fouling by sewage. More than 460 British bathing beaches are served — and served all too literally — by combined sewer overflow pipes (CSOs). There are many more elsewhere along the coast, in estuaries and along rivers. Trouble arises mainly after floods or periods of heavy rainfall, when the CSOs act as a kind of rudimentary safety valve for over-full sewerage systems and prevent them backing up into people’s homes or spilling into streets. The “safety”, however, is relative. The torrent may be diverted away from overloaded sewage treatment plants, but it hardly counts as a gold standard for public hygiene.

For the effluent, journey’s end is wherever the pipe stops. If it were not for the accompanying flotsam of modern sanitary conveniences — the condoms, tampons, cotton-buds and other bearers of bodily fluids flushed from the nation’s bathrooms — the result could be called medieval. Whatever passes through the human gut passes into the sea. Even in concentrations too low for nose and eye to detect, raw sewage carries the risk of gastroenteritis, acute respiratory illness and infections of the ear, nose and throat.

Some of the spills are minor and infrequent; others are massive and regular. In the Thames Tideway, for example, the Environment Agency’s estimate of how much untreated sewage is discharged from CSOs swings between 30 billion and 50 billion cubic metres a year, with 36 outfalls offending at least once a week. After heavy rain in August 2004, 600,000 tonnes went into the river in a single day, killing 10,000 fish. Diversion into the CSOs wasn’t the only problem. Under this kind of pressure, the sewage-works themselves couldn’t cope, and squirted what the EA delicately describes as “poor-quality effluent” into the Tideway. Though not quite the Great Stink of 1858, it was not the kind of portent a more fastidious century is looking for.

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