Opposition lawmakers in Colombia have proposed a bill that aims to cut the use of mercury and other toxic chemicals in illegal gold mining, that has polluted rivers and blighted the ancestral lands of indigenous tribes. Colombia has one of the highest rates of mercury contamination in the Americas. The chemical – widely used to extract gold – seeps into the food chain and causes soil erosion and health problems. The bill, proposed by two congressmen this week, aims to impose stricter penalties on those who use mercury and other chemicals such as cyanide in mining operations with prison sentences of up to 12 years and tougher fines. "The bill aims to adopt provisions that allow the various state authorities to combat criminal mining causing environmental destruction in a transparent and effective way," congressman Edward Rodriguez, one of the bill's sponsors, said in a statement. For every gram of gold mined illegally, one ton of soil is contaminated with cyanide, sulphuric acid and mercury, according to the government. Record gold prices and a government crackdown on cocaine trafficking have prompted the rebel Revolutionary Armed Forces of Colombia (FARC) and new criminal gangs linked to former far-right paramilitaries to seek new revenue sources and expand into illegal mining, the Colombian police say. Illegal gold and silver mining now yield bigger profits than returns from drug trafficking, making it the main source of revenue for many organised crime groups in Colombia, they say. A study this year by Colombia's Externado University found 60 percent of all mining operations in Colombia are illegal and said 180 tons of mercury are dumped every year. Health Risks For centuries miners have used mercury to separate gold from rock or soil. Mercury seeps into soil, rivers and the food chain and can cause serious health problems, like kidney failure and acute anemia, which may not appear for years, according to the United Nations Environment Program (UNEP). Most affected by mercury contamination are Colombia's indigenous tribes and Afro-Colombian communities whose ancestral lands are rich in gold reserves, particularly those living in the rainforest province of Choco on Colombia's Pacific coast. Along with health problems, high levels of mercury pollution are one reason why indigenous groups are being driven off their lands, according to the United Nations refugee agency in Colombia. Tribal rights group Survival International has said mercury poisoning is also threatening the health and survival of Amazon tribal peoples living in Peru, Brazil and Venezuela. Colombia's President Juan Manuel Santos last year declared a war on the $2.5 billion illegal mining industry. The government says it is shutting down and confiscating equipment used in hundreds of mines operating without a license every year. Army helicopters and police patrol remote jungle areas targeting illegal mines run by organized crime networks. Rights groups and indigenous tribes say the government has been too slow to react to the armed groups' intrusion on the mining boom and that as soon as illegal mines are shut down, new ones spring up within weeks.