Effects of turbidity on water quality - NY Times

Logging in a watershed can increase the amount of turbidity in drinking water supplies. The following article in the New York Times highlights the problems New York City is facing as a result of turbidity. The article makes mention of the fact that San Francisco, Seattle and Portland are major cities that do not need to filter their drinking water. Not coincidently, these cities also forbid commercial logging in their watershed.


New York’s Water Supply May Need Filtering

New Yorkers are endowed with certain inalienable rights, among them bragging about the city’s water — so pure it doesn’t need to be filtered, so delicious it is better than bottled.

So it may surprise, perhaps even insult, proud residents to hear that federal officials are worried that the fabled water — coming from the largest unfiltered system in the country — is getting muddier and may have to be completely filtered, at a cost of billions of dollars, if it cannot be kept clean.

For much of the last year, the century-old water system that delivers 1.3 billion gallons a day to the city has been clouded by particles of clay, washed into upstate reservoirs by violent storms in quantities that make the water look like chocolate Yoo-hoo.

To keep the tap water running clear, the city has been dumping 16 tons of chemicals a day, on average, into the water supply as an emergency measure to meet federal water quality standards. The treatment does not change the taste of the water, but the city cannot rely on this stopgap approach forever.

Turbidity — the condition that makes water cloudy and interferes with chlorination to eliminate contaminants — appears to be getting worse because of changing weather patterns and increasing runoff from land development upstate.

If the city cannot find a permanent solution to the silt, it may not be able to avoid building a huge filtration plant that could cost about $8 billion.

Because its water has historically been so pure, New York has largely been exempt from federal rules created in the late 1980’s that require all water systems to be filtered. (A small part of the system, in Westchester, will be filtered in a few years.)

But as federal officials review the city’s five-year exemption, which expires at the end of this year, they have openly expressed concern about the water quality.

“The single most important item we’re looking at, and the one that could be a problem for the city, is turbidity,” Walter Mugdan, a local director of the Environmental Protection Agency, testified at a City Council hearing this spring. His office, the Division of Environmental Protection and Planning, will decide early next year whether the city’s water is clean and clear enough to avoid filtration for another five years. (Only four other major cities — Boston, San Francisco, Seattle and Portland, Ore. — are also exempt.)

The city is confident that it will win renewal. Emily Lloyd, commissioner of the city’s Department of Environmental Protection, which runs the water system, said that the department was working on plans to reduce turbidity without chemicals, particularly in two big reservoirs in the Catskill Mountains.

Steven C. Schindler, director of the department’s division of drinking water quality control, said, “I don’t consider turbidity a serious problem as long as we are able to operate the system the way it was designed.”

The city’s early engineers designed a system that only on rare occasions would have to rely on a chemical, aluminum sulfate, to reduce turbidity. Alum, as it is called, is used in most public drinking water systems in the United States to keep water clear because it draws together small particles, causing them to clump up and settle before the water enters the distribution system.

But some people see the prolonged use of alum as a sign that turbidity has become more severe. James M. Tierney, an assistant state attorney general who has special responsibility over the city’s 2,000-square-mile upstate watershed, has criticized the city for waiting too long to correct the problem.

In a letter to state environmental officials in April, Mr. Tierney said the continued use of alum “would appear to indicate seriously deficient conditions in the Catskill portion of the New York City Watershed.”

Mr. Tierney also said in the letter that the city’s alum use violated state water quality standards and effectively turned one of its reservoirs, where the alum clumps accumulate, into “a chemical sludge settling pond” that smothered aquatic life and would at some point need to be dredged at considerable expense.

Ms. Lloyd said the city had used alum from time to time over the last century without any impact on water quality. Mr. Tierney has called on the city to limit alum use and to hasten its efforts to reduce murkiness in the upstate reservoirs.

The city’s complex system — with 19 reservoirs bringing mountain water to New York from as far as 125 miles away through a gravity-fed web of aqueducts — is divided into three separate segments. In the 1990’s, the city agreed to filter the water coming from the Croton segment, the oldest and smallest section, which sits in Westchester and Putnam Counties, because it would be impossible to meet clean-water standards there. A $1.2 billion filtration plant is under construction in the Bronx.

The second oldest is the Catskill segment. In the early years of the 20th century, the city — with the help of special state laws — condemned thousands of acres in the eastern Catskills to build two reservoirs that more than doubled the city’s capacity.

In the 1950’s and 1960’s, the city expanded again, tapping the east and west branches of the Delaware River and other tributaries to create the newest and largest of its three segments.

The turbidity problem stems largely from conditions that have been present in the Catskill system from the beginning. Engineering studies in 1903 recognized that the clay of the steeply sloped Eastern Catskills turned the sweet waters of the Schoharie and Esopus Creeks into mudholes after storms.

Engineers decided to go ahead anyway, devising a two-reservoir system with built-in turbidity controls. Water from the farthest of the 19 reservoirs, the Schoharie, flows 18 miles through a tunnel under a mountain into the Esopus Creek, which then winds its way into the west end of the gigantic Ashokan Reservoir, 12 miles long and up to a mile wide.

The Ashokan is actually two reservoirs, separated by a dam, or weir. The turbid water from the Schoharie enters the west basin and is kept there until, in theory, the sediments drop. Then a gate in the dividing weir is lowered, allowing the cleanest surface water to flow into the east basin, where it is kept for several weeks longer to settle before making the long trip to New York City.

The system worked well for decades, with alum being used only rarely. But over the century, development in the Catskills, the building of roads, clearing of land and paving over of ground, all increased soil erosion, contributing to more runoff, federal officials said.

Water quality standards also got tougher; scientists have found that the clay particles hampered purification by providing nutrients for microbial pathogens and shielding them from decontaminants. Weather patterns over the last decade have brought more frequent and heavier rain. And the city has been draining murky water from the Schoharie Reservoir in order to repair its dam.

In 1998, water from the Schoharie Reservoir had become so muddy that environmental and fishing groups sued the city, claiming the sediment violated the Clean Water Act and impaired trout fishing along Esopus Creek, which is famous for it. A federal court ruled against the city in 2003, and in June an appeals court upheld the decision and the $5 million fine.

Federal officials raised concerns about turbidity in granting the filtration avoidance permit in 2002.

Since then, the city has studied several engineering and operational options for restoring the city’s water supply to its former glory.

Among the most likely fixes is the construction of a multilevel intake at the Schoharie Reservoir. Like a huge straw with openings at different levels, the new intake, which would cost more than $100 million according to city officials, would allow operators to draw off the clearer surface water, while giving the turbid waters at lower depths more time to clear. Right now, the Schoharie is equipped with a single valve on the bottom of the reservoir, which acts like a bathtub drain, allowing the lowest-quality water to exit first.

Other options include raising the weir at the Ashokan dam by five feet. This would increase its capacity by five billion gallons, and give turbid water flowing into the west basin more time to decant.

Another option is to build a baffle around the intake in the east basin to slow down the water before it enters the aqueduct to New York. Ms. Lloyd said cost estimates for these projects had not yet been prepared.

To avoid being forced to build a filtration plant for the Catskill and Delaware systems — which supply up to 90 percent of the city’s water — New York will also have to undertake other projects.

As part of its latest filtration avoidance permit, it agreed to build a new plant in Westchester County that will use ultraviolet light to purify water.

The project, under construction in Mount Pleasant and Greenburgh, will be the largest in the world when completed in 2010.

However, turbidity in the water would reduce its effectiveness because sediment deflects ultraviolet rays.

The city will also have to continue protecting stream banks and controlling development, and buy additional land in the watershed.

Over the last decade, the city has bought 70,000 acres at a cost of $168 million, and it expects to match that over the next decade. The property tax bill for upstate land costs the city over $100 million a year.

Which raises the question of whether building a filtration plant is inevitable in the long run, and if so, wouldn’t it make more sense to simply go ahead and build it now?

City, state and federal officials don’t think so. Mr. Mugdan, the federal official, calculates that the city has spent about $1 billion over the last decade to protect the water supply, compared with $6 billion to $8 billion to build a plant, along with hundreds of millions of dollars in operating costs.

"Even if, 75 years from now, some accountant asks how much has it cost the city to avoid filtration versus how much we would have spent to build it," Mr. Mugdan said, “we’ll still be ahead.”

Kevin Flynn
July 20, 2006