The ARS health agency for the Occitanie region said it tested 103 children aged 11 or younger after residents became alarmed when the former Salsignes mine was flooded during heavy rains last October.
Of those, 38 had arsenic readings above the reference level of 10 microgrammes per gramme of creatine in urine samples, with 10 children showing levels above 15 microgrammes.
- Area around Notre-Dame closed as Paris authorities begin decontamination
- What we know about the mystery of babies born without arms in rural France
The mine has been closed since 2004. Photo: AFP
The ARS cautioned that a further round of testing in two months was needed to determine if the arsenic resulted from chronic exposure, as opposed to acute exposure that can occur after eating certain foods like shellfish or meat.
The Salsignes 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 stored at five sites nearby, and local associations say some have begun to leak.
Last month, the health agency expanded monitoring to include tests for all children 11 and younger in the area near the Orbiel river.
The move came after media reports in June said three boys aged four, seven and nine had arsenic levels ranging from 12 to 20 microgrammes per gramme of creatine.
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 the element.
They also prohibited swimming or fishing in the Orbiel and banned the eating of fruits and vegetables produced in 12 nearby communes for up to four months.
Arsenic poisoning from long-term exposure can lead to discolouration and hardening of the skin, and eventually cause a variety of cancers.