One of New York City’s most valuable assets -our drinking water supply- continues to serve us well. Drinking water quality “remains high,” the City said last month, and meets all health-related State and federal standards.

New York City’s water supply system provides about a billion gallons of drinking water daily to over nine million metro-area residents and visitors. And our water takes a long journey before it reaches our faucets, coming from reservoirs as far as 125 miles to the north.

In 2013, the city’s Department of Environmental Protection collected over 30,000 samples from its water distribution system and 17,000 samples from upstate reservoir watersheds. The City tests for a “broad spectrum of microbiological, chemical, and physical measures of quality.”

The results of tens of thousands of water quality tests conducted last year were summarized in a recent DEP report.

The City says it monitors continuously for certain “water quality parameters” –such as evidence of microscopic pathogens- as water enters the distribution system. And DEP regularly tests water quality at nearly 1,000 water quality sampling stations throughout New York City.

According to the DEP, of all the substances that were detected in the city’s water supply, none were at levels that exceeded State or federal standards. For instance, traces of chloride, from road salt, and nitrate, caused by runoff from fertilizer use and leaching from septic tanks, were found in some samples.

What else was picked up in the City’s tests?

Relatively low levels of aluminum, barium and chromium, which are caused by erosion; iron and sulfate, which are naturally occurring; fluoride and chlorine, which the City adds for public health purposes; and coliform bacteria, also naturally occurring.

The City tested for -but could not find evidence of- a number of organic compounds, such as benzene and toluene, which are hazardous to human health. Benzene and toluene, known human carcinogens, can be released into the environment through activities like gas and oil drilling.

While most New Yorkers expect consistently high-quality and safe drinking water, our water supply is unique, both in terms of purity and volume. Most of the city’s water supply is unfiltered, coming from upstate reservoirs fed by mountain streams. It is one of the largest unfiltered municipal water supply systems on the planet.

New York City’s water supply network has three major components, the Catskill, Delaware and Croton watersheds. The city drew only from the Catskill and Delaware systems in 2013, both of which are unfiltered.

The Croton watershed, located in Westchester and Putnam counties, much more densely populated areas and closer to pollution sources, now requires additional filtration. Under a Consent Decree between New York City, the State, and the federal government, the City is constructing a multi-billion dollar filtration plant for Croton water.

While the massive facility under construction in the Bronx’ Van Cortlandt Park is nearing completion, the City noted that it had “missed several milestones in the Consent Decree”.

But the City has recently opened an ultraviolet water treatment facility, the world’s largest, it says. The facility serves the Catskill and Delaware systems, providing treatment for Cryptosporidium and Giardia, “naturally occurring microorganisms that can be found in surface waters and can cause gastrointestinal ailments in humans”.