The compound, potassium thiocyanate, was used to scale up the production of one of Merck's experimental vaccines, but the scale-up was abandoned, and instead of discarding the material properly, operators at the vaccine research pilot plant simply poured it down the drain.
As a result, the discharge is believed to have entered the Upper Gwynedd wastewater treatment plant through its sewage collection system, interacted with chlorine and emerged in the Wissahickon Creek, where the released cyanide killed more than 1,000 fish.
An investigation by the US Environmental Protection Agency (EPA) is underway to establish the extent of the pollution, estimated to be between two to eight miles downstream, however health advisories to avoid recreational contact with the creek have been lifted.
The discharge, which took place on the 13th of June, was not in accordance with Merck's protocols for proper waste disposal, as normally the discarded liquid would be sent to a pretreatment plant at Merck's West point site for minimal processing followed by more processing at the municipal facility and then release to the sewage.
What prompted the plant's employees to simply get rid of the chemical straight in the sewage is not known, but Merck spokeswoman Connie Wickersham described the incident to In-PharmaTechnologist.com as "a human error of fairly good-sized proportions."
"Disciplinary action against some staff is pending and we expect our internal investigations to lead to such action being taken," she said.
"The creek is back in its former status but there will probably be penalties imposed by the EPA, this is under review."
An EPA spokeswoman told In-PharmaTechnologist.com it is possible that, when investigations are completed, Merck can be fined up to $32,500 (€25,000) per day per violation in civil penalties.
Merck's West Point campus, where the pilot plant is located, spans a massive 397 acres and has the size of a small town, with its own post office, power supply and water.
Comprising dozens of buildings, it shelters Merck's sole vaccine manufacturing facility, making vaccines for diseases like chickenpox, Hepatitis A and B, shingles, and human papillomavirus (HPV).
It also produces drugs such as Crixivan for AIDS, Fosamax for osteoporosis and Zocor for high cholesterol, and even contract manufactures for the likes of AstraZeneca and Johnson & Johnson.
Thus, it is not uncommon to have an entire building on campus dedicated to the development of a single drug or vaccine, so if a product is pulled from production, its building has to be completely retrofitted for a new product or use.
Whether the abandonment of the undisclosed vaccine scale up operation, which led to the creek's environmental damage, will cost Merck dearly now depends on the EPA.