PHILADELPHIA (AP) – Treading through a morass of revised data, the city’s water officials will attempt to relieve water-quality worries at a City Council hearing over findings of pharmaceutical traces in the local drinking water supply.

The City Council’s committees on public health and the environment were expected Monday to hear from water department officials, as well as environmental advocates. Some Council members were upset to learn from an Associated Press investigation, instead of the Philadelphia Water Department, that the city’s water supply carries minute concentrations of many pharmaceuticals.

Water officials will arrive at the hearing bearing corrected test results, saying that their department accidentally inflated the number of detected pharmaceuticals when it first released data to the AP earlier this year. The department made another correction to its corrections in a follow-up statement to the AP on Friday.

The changes now put the number of pharmaceuticals or their byproducts found in the city’s drinking water at 17, instead of 56; and 32 in its watershed, instead of 63. A department spokesman, Ed Grusheski, said the mistakes occurred during the preparation of a spreadsheet.

The first numbers appeared March 10-12 in an AP investigative series based on survey responses from the Philadelphia Water Department, along with information from other water providers around the country.

Even with the downward revisions, more drugs turned up in Philadelphia than in any of the 24 major metropolitan areas where detections were reported in drinking water.

On Friday, the department made its second correction to the number of pharmaceuticals tested for. It first told the AP that 73 pharmaceuticals had been screened for in its drinking water and watershed; it recently said the correct number was 70. In its latest statement Friday, in response to an AP request for clarification on all of its corrections, department officials said the actual number was 75.

The department was besieged with calls from alarmed residents in the days following the AP’s disclosure that pharmaceuticals had turned up in the drinking water systems of at least 41 million Americans, including those in Philadelphia.

The AP also reported that water utilities, including Philadelphia’s, rarely inform the public when drug traces are found.

To be sure, the drug concentrations in the drinking water for Philadelphia, and elsewhere, are minute, typically in parts per billion. Any risk is poorly understood. Drug companies, water utilities, and some scientists say any risk to humans is probably negligible, but they acknowledge that much is unknown.

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