Officials now say that 46 of 143 children aged 11 or younger who were tested in the area have been found with 10 or more microgrammes of arsenic per gramme of creatine in urine samples.
Testing began this summer after residents became alarmed about contamination risks when the former Salsigne mine in southern France flooded during heavy rains last October.
The ARS health agency for the Occitanie region also said Wednesday that it had re-tested 11 children first tested in June.
Of the 11 children, one still had more than 10 microgrammes, “which can indicate long-term exposure,” Jean-Jacques Morfoisse, deputy director of the ARS Occitanie, told AFP.
Two others saw their test results rise above 10 microgrammes, Morfoisse said, while the arsenic readings fell to under the recommended level for the eight others.
The Salsigne mine in the Aude valley, near Carcassonne, was the world's biggest source of the element, as well as Europe's largest gold mine, before it was closed in 2004.
Millions of tons of toxic waste were then stocked at five sites nearby, and local associations say some have begun to leak.
Several parents called on local authorities to take urgent measures, and officials closed off access to some playgrounds and also began soil and atmospheric testing for arsenic.
Officials also prohibited swimming or fishing in the nearby Orbiel river and banned the eating of fruits and vegetables produced in 12 nearby communes for up to four months.
Acute exposure to arsenic can occur after eating certain foods, like shellfish or meat. The ARS acknowledges that “in the large majority of cases” high levels are the result of ingesting contaminated food or water.
Chronic arsenic poisoning can lead to discolouration and hardening of the skin, and eventually cause a variety of cancers.
President Emmanuel Macron acknowledged a “worrying” situation in a written response to two senators from the region seeking an inquiry, and assured that “everything will be done to protect residents”.