Urban sprawl and climate change have combined to put yet another comforting certainty of American urban life at risk. New York City's tap water, held by admirers to be the "champagne of drinking waters", may now have to be filtered to protect its legendary purity.
Until now, New York has been able to avoid hugely expensive filtration plants thanks to a string of remarkably farsighted decisions during the 19th century. Instead of relying on dubious local water sources, the city fathers of the era invested in an elaborate system of reservoirs and aqueducts to collect water from pristine watershed areas upstate, and let gravity carry the pristine liquid south to the city itself.
Every day the network delivered 1.2 billion gallons of drinking water fit for the gods (at least in the opinion of boastful New Yorkers) to the eight million inhabitants of America's largest city.
The H20 that flowed out of taps in Queens, the Bronx and Brooklyn would beat established mineral waters in blind tasting tests. It used to be shipped to other cities to make bagel and pizza dough - even to England, it is claimed, as an ingredient in the perfect cup of tea. But that may be true no longer.
In the early 1980s, the Croton segment - smallest and closest of the three segments into which the system serving New York is divided - had started to suffer from pollution caused by rapid suburban development, and a decision was taken to build a filtering plant, due for completion in 2009. Now, however, the two main systems - the Catskill and the Delaware - are also under threat. The culprit again is land development, but also the increasingly severe storms that many scientists attribute to climate change and global warming.
The storms have washed mud and clay into reservoirs and rivers, turning them a muddy brown in many places. In order to keep the tap water clear, the city authorities have had to add 16 tons of aluminium sulphate, known as alum, to the daily water delivery to keep down turbidity and meet federal government standards.
The added chemical does not change the taste of the water, but works by drawing tiny particles of impurity into heavier clumps. These settle before the water is distributed.
This stopgap solution cannot go on for ever, officials say. Storms grow steadily fiercer and sprawl continues to advance. Sooner or later, New York may be forced to join most other big cities and construct a filtration plant - costing anything up to $8bn (£4.3bn).
City officials say this should not be necessary. Plans are afoot to lower turbidity in two large upstate reservoirs. This was "not a problem as long as we are able to operate the system as it was designed", Steven Schindler, of the city's environmental protection department, told The New York Times. Among the measures envisaged are a new multilevel draw-off facility at one large reservoir. This would allow pure surface water to be fed into the system, while giving muddier water at lower depths more time to clear. In other places dams may have to be heightened, while the city will have to make land purchases to keep development at bay.
Over the past decade, New York has spent $1bn to protect water, to avoid being forced to build filtration plants. Officials insist this is still the cheaper solution.Reuse content