Nearly half a million Mainers are drinking tap water containing amounts of cancer-linked chemicals that, while complying with state and federal limits, still pose a public health danger, according to a new study from a national public health advocacy group. The Environmental Working Group is calling for stricter regulatory standards for hexavalent chromium, arsenic and nitrate, and urging public water systems to invest in technology that could filter them all out at once. The Sanford Water District had traces of all three chemicals highlighted in the report — higher than Environmental Working Group’s public health goal but well under state and federal standards, said David Parent, the water district’s superintendent. Those public water systems serve more than 400,000 residents, according to the Environmental Working Group report and analysis. The Environmental Working Group report has the district marked as 2.1 times the “public health goal” at 0.041 parts per billion.