In 2016 the city briefly declared a water emergency and started using Browns Pond, its backup supply, when levels of perfluorooctane sulfonate (PFOS) near the federal Environmental Protection Agency guidelines of 200 parts per trillion (ppt) were found in the lake. It is not known yet where the PFOS came from; a pond on the Stewart Air Guard Base from which Silver Stream, one of Lake Washington's tributaries, rises, had levels of 5,900 ppt. If Brown's Pond runs out before the contamination can be remediated, the city would have to pay to draw from the Catskill Aqueduct, part of the New York City water supply system.[2]